lenticular galaxy NGC7572 - arXiv.org e-Print archiveof lenticular galaxies. Years later, two coplanar subsystems with drastically di erent dynamical and stellar population properties

MNRAS 000, 1–16 (2020) Preprint 1 April 2020 Compiled using MNRAS LATEX style file v3.0

An excessively massive thick disc of the enormous edge-onlenticular galaxy NGC7572

Anastasia V. Kasparova,1? Ivan Yu. Katkov1,2,3 and Igor V. Chilingarian1,41Sternberg Astronomical Institute, Moscow M.V. Lomonosov State University, Universitetskij pr., 13, Moscow, 119234, Russia2New York University Abu Dhabi, P.O. Box 129188, Abu Dhabi, United Arab Emirates3Center for Astro, Particle, and Planetary Physics, New York University Abu Dhabi Abu Dhabi, P.O. Box 129188,Abu Dhabi, United Arab Emirates4Smithsonian Astrophysical Observatory, Harvard-Smithsonian Center for Astrophysics, 60 Garden St. MS09, Cambridge, MA 02138 USA

Accepted 2020 February 27. Received 2020 February 27; in original form 2019 September 22

ABSTRACTGalactic discs are known to have a complex multilayer structure. An in-depth study ofthe stellar population properties of the thin and thick components can elucidate theformation and evolution of disc galaxies. Even though thick discs are ubiquitous, theirorigin is still debated. Here we probe the thick disc formation scenarios by investigatingNGC 7572, an enormous edge-on galaxy having R25 ≈ 25 kpc and Vrot ≈ 370 km s−1,which substantially exceeds the Milky Way size and mass. We analysed DECaLSarchival imaging and found that the disc of NGC 7572 contains two flaring stellardiscs (a thin and a thick disc) with similar radial scales. We collected deep long-slitspectroscopic data using the 6m Russian BTA telescope and analysed them with anovel technique. We first reconstructed a non-parametric stellar line-of-sight velocitydistribution along the radius of the galaxy and then fitted it with two kinematic com-ponents accounting for the orbital distribution of stars in thin and thick discs. The oldthick disc turned out to be 2.7 times as massive as the intermediate-age thin compo-nent, 1.6× 1011 M� vs. 5.9× 1010 M�, which is very unusual. The different duration ofthe formation epochs evidenced by the [Mg/Fe] values of +0.3 and +0.15 dex for thethick and thin discs respectively, their kinematics and the mass ratio suggest that inNGC 7572 we observe a rapidly formed very massive thick disc and an underdevelopedthin disc, whose growth ended prematurely due to the exhaustion of the cold gas likelybecause of environmental effects.

Key words: galaxies: disc – galaxies: evolution – galaxies: individual: NGC7572

1 INTRODUCTION

We present an in-depth observational study of the edge-on giant disc galaxy NGC 7572 and the vertical structureof its disc. NGC 7572 is a quiescent S0 counterpart of “su-per spirals”, the most massive star-forming disc galaxies inthe Universe (Ogle et al. 2016). Its somewhat small diame-ter compare to the galaxies in Ogle et al. (2016) is due toNGC 7572’s large mass-to-light ratio caused by its old stel-lar populations. According to our analysis, the NGC 7572disc component is five times as massive as the disc of theMilky Way (MW)1, its rotation velocity reaches 370 km s−1

and the radius is R25 = 0.51 arcmin ≈ 24.6 kpc2 (HyperLeda,

? Contact e-mail: [email protected] The total mass of the MW stellar disc (thin and thick compo-nents) is about 4 × 1010 M� (Bland-Hawthorn & Gerhard 2016).2 Value R25 is the 25 mag arcsec

−2 isophotal radius in the B-band.

Makarov et al. 2014) (see Fig. 1 and Table 1). With a totalr-band luminosity of 1.1 × 1011 L� and the corrected colourg − r = 0.80 mag, NGC 7572 is as luminous as bright clustergalaxies like Messier 87 in the Virgo cluster and it sits onthe bright end of the red sequence populated by the mostmassive ellipticals and cD galaxies3. The unique propertiesof this object and its edge-on orientation provide us withan opportunity to investigate the vertical structure of thedisc components of a galaxy having a significantly largersize than the MW.

This work is a part of our multicycle observational pro-gram using the Russian 6-m BTA telescope to study thevertical structure of edge-on galaxies in various environ-ments (see Kasparova et al. 2016). White et al. (1999) placed

3 The predicted colour at the red sequence for Mr = −23.0 magis g − r = 0.81 mag (Chilingarian et al. 2017).

© 2020 The Authors

arX

iv:1

912.

0488

7v2

[as

tro-

ph.G

A]

31

Mar

202

0

mailto:[email protected]

2 A. Kasparova et al.

NGC 7572 in the poor cluster WBL 703 at z ≈ 0.044. How-ever, at the same redshift (z = 0.040) at a projected distanceof 25 arcmin ≈ 1.3 Mpc there is the non-virialized clusterAbell 2572 with two bright merging X-ray cores. Hence, thisgalaxy lives in a fairly dense environment, which can cer-tainly affect its evolution. We adopt D = 178 Mpc as a lumi-nosity distance and D = 165 Mpc as an angular-size distanceto NGC 7572 (the same as the cosmology-corrected luminos-ity distance to Abell 2572 from NED). This corresponds toa spatial scale of ≈ 0.8 kpc arcsec−1 and a distance modulusm − M = 36.25 mag.

Decades ago Burstein (1979) and Tsikoudi (1979) dis-covered the existence of thick discs during their studiesof lenticular galaxies. Years later, two coplanar subsystemswith drastically different dynamical and stellar populationproperties were identified in the Milky Way (Gilmore &Reid 1983; Majewski 1993; Fuhrmann 1998; Prochaska et al.2000). With the increase of a vertical distance from the mid-plane, the volume density of young metal-rich stars quicklyabates and the stellar population becomes dominated by oldand metal-poor α-enhanced stars (see e.g., Bland-Hawthorn& Gerhard 2016). This phenomenon was attributed to differ-ent conditions of formation and/or evolution of the two pop-ulations, which were named thin and thick discs (Haywoodet al. 2013). Additional evidences in favour of the differentorigin of the two disc components in the MW were providedby the modern wide-field imaging and spectroscopic surveysGaia (Gaia Collaboration et al. 2016) and APOGEE (Ma-jewski et al. 2017). In particular, Mackereth et al. (2019)demonstrated the very different age–velocity dispersion re-lations (AVRs) and shapes of the σz/σR distributions forstellar populations with low and high [α/Fe], correspondingto the thin and thick discs, respectively. However, there areonly a few external galaxies having spectral and photomet-ric data with sufficient quality to clearly separate the stellarpopulation properties of thin and thick discs.

Over the last decade, several teams published resultsof the analysis of the vertical structure for large samples ofedge-on galaxies from photometric data (Yoachim & Dalcan-ton 2006; Comerón et al. 2011b, 2012; Bizyaev et al. 2014,2017; Comerón et al. 2018). However, up to now there is lessthan two dozens of objects with detailed kinematics and stel-lar population properties of thick discs from spectroscopy(Yoachim & Dalcanton 2008b; Comerón et al. 2015, 2016;Guérou et al. 2016; Kasparova et al. 2016; Sarzi et al. 2018;Pinna et al. 2019a,b). Among them, there are no massivegalaxies with circular velocities exceeding 300 km s−1 andonly three galaxies have masses comparable to that of theMW. This severely limits the possibility of comparing the re-sults with the MW disc, and it complicates the verificationof the evolutionary scenarios of galactic discs. It is still un-known, how similar the vertical structure of disc galaxies isfor various morphological types from irregular to lenticularand for different luminosities from dwarfs to giants, whetherthere is a universal scenario of their origin, and how criticalthe environmental effects are for these processes. It is worthmentioning that a direct comparison of the properties of theMW’s thick disc as defined based on its chemodynamics us-ing individual stars with thick discs in other galaxies definedby photometric analysis is difficult. This is because the twodefinitions do not quite overlap. One of the reasons for thisis that the high-[α/Fe] stars (in the Solar neighbourhood

10 arcsec

Figure 1. DECaLS RGB image of NGC 7572 retrieved from theweb-site legacysurvey.org. The dashed and dotted lines show the

slit positions for our spectroscopic observations. The offset be-

tween slits is 5 arcsec which corresponds to ≈ 4.0 kpc.

in particular) probably have two different origins, the thickdisc itself and the inner thin disc regions (Haywood et al.2013; Hayden et al. 2017). Nevertheless, we should not com-pletely reject the idea of comparing the multilayer structureof our Galaxy with other galaxies. Therefore, it is crucial toexpand a sample of galaxies with high-quality spectral datafor thick and thin stellar populations to be able to identifydisc components taking into account both chemistry andkinematics.

There are several scenarios explaining complex verticaldisc structure of galaxies. Thick discs could be formed asa result of a thin disc secular evolution (Quinn et al. 1993;Schönrich & Binney 2009; Villalobos et al. 2010). The sec-ond possibility is an external origin of thick discs from ac-creted dwarf satellites (e.g. Abadi et al. 2003; Yoachim &Dalcanton 2005). The third scenario is a two-stage modelimplying the thick disc formation at high redshift (z ∼ 2)followed by a gradual growth of a thin disc (Chiappini et al.1997; Elmegreen & Elmegreen 2006; Bournaud et al. 2009).Spatially resolved internal kinematics of thin and thick discscombined with stellar population analysis can support or de-bunk a particular scenario in a given galaxy, and this is theway we follow in our study.

The paper is organized as follows. Our photometric andspectroscopic analysis is presented in Section 2; in Section 3and Section 4 we discuss and summarize our results; in Ap-pendix A we test the stability of our photometric analysisand explore potential pitfalls, which arise when one dealswith real “non-ideal” galaxies.

MNRAS 000, 1–16 (2020)

http://legacysurvey.org

A massive thick disc of NGC7572 3

22

24

26

7, m

ag/a

rcse

c2 R = 10 arcsec zt = 1.9 arcsec

zT = 9.1 arcsec

-30 -20 -10 0 10 20 30

z, arcsec

-500

50100

resi

d., %

R = 15 arcsec zt = 2.1 arcsecz

T = 11.6 arcsec

-30 -20 -10 0 10 20 30

z, arcsec

22

24

26

R = 20 arcsec zt = 2.6 arcsecz

T = 14.2 arcsec

-30 -20 -10 0 10 20 30

z, arcsec

-50050100

Figure 2. The black dots show the vertical observed profiles at R = 10, 15, and 20 arcsec in the DECaLS g-band. The black and greenlines mark the contributions of the thin and thick components to the total model (red lines). The bottom row shows the relative residuals

“(data–model)/data” and the grey-shaded area covers ±20 per cent.

Table 1. Basic parameters of NGC 7572.

References: [1] NED (http://ned.ipac.caltech.edu); [2] LEDA

(http://leda.univ-lyon1.fr); [3] EGIS (Bizyaev et al. 2014). Weadopted the distance to NGC 7572 as the cosmology-corrected lu-

minosity distance to Abell 2572 (from NED, the cosmological pa-

rameters: H0 = 67.8 km s−1 Mpc−1, Ωm = 0.308 and Ωλ = 0.692).We took g and r magnitudes from EGIS and corrected them for

the Galactic extinction (from NED) and k-correction (Chilingar-

ian et al. 2010; Chilingarian & Zolotukhin 2012). The rotationvelocity is from our study.

RA (J2000.0) 23h16m50.s368 –DEC (J2000.0) 18◦28′59.′′45 –Luminosity distance 178 Mpc [1]

Angular-size distance 165 Mpc [1]Position angle 162.3◦ [2]Rotation velocity 368±8 km s−1 –R25 0.51 arcmin [2]Mr −23.0 mag [3]g − r (corrected) 0.80 mag [3]

2 ANALYSIS OF ARCHIVAL DATA AND NEWOBSERVATIONS

2.1 Photometric analysis

We used g-band DECaLS images (Dey et al. 2019) to es-timate photometric parameters of the two disc componentsin NGC 7572. We analysed individual one-arcsec wide slicesalong the vertical axis z (vertical profiles) and obtained thecontribution of thin and thick components to the total mass,the relationship between their vertical and radial scales andthe vertical scale variations with radius.

2.1.1 Light profile decomposition

We used several different techniques for 2D and 1D photo-metric data analysis while choosing the best method to ob-tain the disc characteristics of NGC 7572. It is often thoughtthat a 2D image decomposition is a more advanced techniquethan fitting individual slices along the radius. However, ouranalysis of NGC 7572 images with IMFIT functions for edge-on galaxies of the constant thickness following Erwin (2015)always left strong residuals suggesting the need to take into

account the flaring of disc components in the outer part ofthe galaxy. At the same time, adding new functions witha priori unknown thickness variations implies a significantincrease in the number of free parameters, which lead tothe ambiguity and degeneracies between parameters in thebest-fitting solution.

Recently, Comerón et al. (2018) presented an extensivephotometric study of multiple component disc structuresin edge-on galaxies (see also Comerón et al. 2011b). Theirmethod followed the formalism by Narayan & Jog (2002)and implied certain specific assumptions. In particular, thescaleheights of the thin and the thick discs are assumed to beconstant and their scalelengths similar, otherwise the line-of-sight integration in edge-on galaxies becomes non-trivial(see Sec. 3.2.2 in Comerón et al. 2018).

We aim to measure thickness variations of the disc com-ponents of NGC 7572, therefore we decided to use a simplermethod for the 1D-decomposition of vertical slices at differ-ent radial distances R, which handles the flaring properly. Inour analysis, we excluded the area of NGC 7572 affected bythe bulge or bar at |R| < 8 arcsec, which is also dramaticallydifferent in stellar velocity dispersion and stellar populationproperties from the rest of the galaxy (see below).

Spitzer (1942) and later Van der Kruit & Searle (1981)demonstrated that for an isothermal disc in an equilibriumstate, the vertical disc density profiles can be described bythe law I ∝ sech2(z/z0) where z0 is the scaleheight. In ourwork, we use the sum of two such components:

µz (R, z) = µt (R) sech2(

zzt (R)

)+ µT (R) sech2

(z

zT (R)

), (1)

where t and T correspond to the thin and thick discs. Al-though this approximation is not quite physically justified,it allows us to compare the results with other observationalstudies and numerical simulations of the vertical structureof discs (Yoachim & Dalcanton 2006; Bournaud et al. 2009;Villalobos et al. 2010; Loebman et al. 2011, and many oth-ers). However, Comerón et al. (2011b) and Comerón et al.(2012) compared a more physically motivated fitting methodwith a double–sech2 model and demonstrated that it under-estimated the ratio between thick and thin discs masses by20–30 per cent in total. Note that for a vertical structure ofthe MW, the laws of the double–exp and –sech2 can be suc-

MNRAS 000, 1–16 (2020)


cessfully applied (see review by Bland-Hawthorn & Gerhard2016).

In Appendix A we present a series of tests that predictthe behaviour of our simplified method to estimate photo-metric parameters for a widely accepted model of the struc-ture of disc galaxies, two coplanar discs with constant thick-ness along the radius and different radial scales. We suc-cessfully restore both the vertical and the radial scales ofboth components. Also we analyse the reconstruction of discproperties if a galaxy is not viewed exactly edge-on.

2.1.2 Vertical scales

In Fig. 2 we show three examples of the decomposition ofvertical profiles at R = 10, 15, and 20 arcsec. It is clear that asingle-component sech2–model cannot properly describe theobserved profiles. We also see that at least down to the sur-face brightness level of 25 mag arcsec−2, that corresponds toabout 12 kpc of the vertical distance z, the two-componentmodel describes the data quite satisfactory in all three cross-sections along the radius. However, there is an excess of fluxat at 25.5–26 mag arcsec−2 and the closer we get to thegalaxy centre, the more noticeable the excess light becomes.It cannot be a stellar halo because it plays an essential roleat much lower surface brightnesses in optical bands (see e.g.Peters et al. 2017). Thus, we probably see the deviation ofthe double–sech2 analytical form from observed profiles dueto the line-of-sight integration of regions with different thick-nesses at different radii because of projection effects or dueto the peculiarities of the formation process of a thick disc.Qu et al. (2011a) obtained a similar stellar excess in a se-ries of N-body/SPH models of thick disc formation throughminor mergers, which forms due to tidal interactions of theprimary galaxy with a satellite happening early during themerger event. Comerón et al. (2018) found this feature insome most massive galaxies and they interpreted it as aprobable sign of the third disc component.

It is worth mentioning the possible effect of the dif-fuse scattered light. According to Comerón et al. (2018) andMart́ınez-Lombilla & Knapen (2019), the point spread func-tion (PSF) can significantly change the brightness distri-butions of the disc components, and for edge-on galaxiesthis effect becomes especially important for vertical pro-files. Moreover, PSF-related effects can lead not only toincreased thickness estimates of the disc components, butalso to a counterintuitive decrease in them (see NGC 0429in Mart́ınez-Lombilla & Knapen 2019). The DECam PSF issubstantially narrower than that of the IRAC instrument ofthe Spitzer Space Telescope and also that of the SDSS. InSection A we test the possible impact of the DECam PSF onour results. Using our method, the vertical scale of the thindisc can be overestimated by 20 per cent while for the thickdisc the effect is no more than 3 per cent. The estimates ofthe radial scales and the mid-plane surface brightness val-ues do not change significantly. Also, we cannot explain theexcess of light at large heights by PSF-related effects. Ac-cording to the DECam instrument manual, the g-band PSFat a radius of 10–15 arcsec falls by 12–14 mag arcsec−2 fromthe maximum.

It is obvious from Fig. 2 that the vertical scaleheightsof both components increase towards the periphery of thegalaxy. In Fig. 3 we show the changes of the thickness

18

20

22

24

7, m

ag/a

rcse

c2

thin discthick disc

-20 -10 0 10 20R, arcsec

0

5

10

15

20

z 0, a

rcse

c

S N

Figure 3. The fitted values of the mid-plane surface bright-nesses and the vertical scales for the thin (black lines) and thick

(green lines) disc components. The values are not corrected for

the Galactic extinction and k-corrections. The slightly asymmet-ric observed axial profile of NGC 7572 (red dots) has a clear“knee”

on the Northern side of the galaxy. The grey areas mark the cen-

tral region affected by the bulge and/or bar and the regions out-side the truncation radius at 23 arcsec.

and mid-plane surface brightness with radius obtained fromthe decomposition of vertical slices. Both components havenoticeable flaring starting at 8 − 12 kpc (10 − 15 arcsec).The scaleheight of the thin disc in the inner part4 is zt =1.9 ± 0.1 arcsec and it grows to ∼ 3 arcsec in the outer re-gions. The scaleheight of the thick component grows fromzT = 7.5 ± 0.3 arcsec by a factor of two. This proves thatthe model of the constant disc thickness is not applicable toNGC 7572. Here we must keep in mind that choice of the fit-ting function and the effects of the line-of-sight integrationcertainly affect the estimate of the disc component thick-ness. In addition, the estimates of zt and possibly zT forlow-surface brightness regions are a bit overestimated dueto the PSF influence. Therefore, our method provides onlya first-order characterization of the flaring.

2.1.3 True radial scales of the thick and thin discs

While investigating the photometric properties of edge-ondisc galaxies, we certainly want to know the radial scales ofthe two components. In particular, it is important to com-pare them with the estimates of the corresponding param-

4 In the Appendix A we show that our method satisfactory ex-cludes the bulge influence.

MNRAS 000, 1–16 (2020)

https://cdcvs.fnal.gov/redmine/projects/des-sci-verification/wiki/Mapping_of_the_PSF_to_large_radii


-5 -2.5 0 2.5 5z, arcsec

6

8

10

h, a

rcse

c

Figure 4. The gray and green bands mark 1σ error ranges of thescalelengths for the thin and thick discs respectively, the red line

shows the scalelength of the composite disc at different z as fitted

using Eq. 2.

eters for the MW. However, the radial photometric profilecannot be unambiguously decomposed into thin and thickcomponents because of (i) the degeneracy between the pa-rameters, and (ii) the lack of a priori knowledge of theirrelative contributions to the total light distribution.

If we measure the exponential scalelength h in an edge-on galaxy fitting the following equation to the integral lightprofile along the disc major axis:

µR ∝Rh

K1

(Rh

), (2)

where K1 is the modified Bessel function (Van der Kruit& Searle 1981), we will obtain a value for the “composite”thin+thick disc. However, usually this value is taken as a ra-dial scale of the thin disc. It is also often assumed, that thethick component radial scale can be derived either from theouter region of the disc beyond break/antitruncation radiusof the light profile or at a certain height above the mid-plane(see e.g. Comerón et al. 2018). However, the “knees” on theradial profile of an edge-on galaxy may not be the sign of thetransition to the area of thick disc predominance in galac-tic outskirts but it can be associated with the disc flaring(Borlaff et al. 2016). And in the case of a non-constant com-ponent thicknesses, the thick disc radial scalelength derivedfrom profiles above the mid-plane will become overestimated(see below).

The light profile of NGC 7572 exhibits weak “knees” at|R| ≈ 23 arcsec better pronounced on the North side (right-hand side from the centre in Fig. 3). There seem to be athin disc truncation on the right-hand side but within therange 8 < R < 23 arcsec both layers are almost purely expo-nential and have similar scalelengths. We have derived thescalelengths within this range using Eq. 2 over the radialprofiles of the mid-plane surface brightness of both discs asobtained in our vertical surface brightness fits (Sect. 2.1.2).In Fig. 4 the gray and green-shaded areas mark the scale-lengths for the thin and thick disc correspondingly, and thered line shows the result of the decomposition of radial pro-files of the composite disc (total axial profiles including boththin and thick disc light) at different values of z.

The scalelengths of the thin and thick components nearthe mid-plane coincide within uncertainties (ht = 6.8 ±0.2 arcsec and hT = 7.0 ± 0.3 arcsec). The fitted compositedisc radial scale above the mid-plane (where a thin com-

Figure 5. The map of thick disc to the total disc (thick + thin

components) light ratio. The area affected by the bulge and/or

bar is shaded in grey. The two dotted lines mark the slit positionsfor our spectroscopic observations.

ponent contribution can be neglected) yields a value of thethick disc scalelength only in the ideal case because therecan be a complex h(z) trend due to the flaring effect and/orif a galaxy is not seen exactly edge-on (see Appendix A).We can see from the Fig. 4 that if we try to estimate theradial scale of the thick disc by fitting the surface brightnessprofiles above the mid-plane with the Eq. 2 then we alwaysoverestimate its value.

2.1.4 The relative contributions of the thick and thin discs

Our decomposition procedure for the vertical profiles de-scribed earlier yields, in addition to the scalelengths, the es-timate of the thick-to-thin disc ratio over the entire galaxyimage. In Fig. 5 we show a map of the intensity ratio of thethick disc to the total thick + thin disc component. FromFig. 3 and Fig. 5 we see that the thick disc contribution inthe main plane of NGC 7572 remains almost constant withradius at about 30 per cent and it grows up to 90 per centwhen we move away from the mid-plane to |z | = 5 arcsec,where we placed the slit for our spectroscopic observations(see below). We can therefore neglect the contribution ofthin disc stars at |z | = 5 arcsec.

Using our photometric analysis we also estimate the to-tal luminosity of the NGC 7572 disc components. Taking intoaccount the variations of the observed vertical scaleheightwith radius, the total luminosity of the thin and thick com-ponents in the g-band are 1.9 × 1010 L� and 3.5 × 1010 L�respectively. Our estimate of the bulge luminosity from the2D IMFIT model is about 9.3 × 109 L�. These values arecorrected for the Galactic extinction (from NED) and k-correction (Chilingarian et al. 2010; Chilingarian & Zolo-tukhin 2012).

Our analysis allows us to model what NGC 7572 wouldlook like seen face-on. Using our model of the vertical struc-ture of the disc we de-projected it to the face-on orientationand integrated both components in the z direction. Then wecalculated “face-on” radial scalelengths, which turned to be8.3 ± 0.5 and 11.4 ± 2.2 arcsec for the thin and thick discscorrespondingly. One should note, that these values differ

MNRAS 000, 1–16 (2020)


from those in the mid-plane due to the significant flaring.In the “face-on” view, the dynamically hot old thick discwould contribute over 60 per cent to the light profiles, whilethe estimate of the disc radius (four radial scales) becomesabout 32 kpc. In an enormous disc galaxy like NGC 7572seen non-edge-on, we would not be able to directly analyseits thin disc kinematics and stellar populations, because theintegrated light would be dominated by its thick disc.

2.2 Spectroscopic observations and data analysis

This study is a continuation of our observational programfor which we described the methodology and presented firstresults of spectroscopic data analysis in Kasparova et al.(2016). To estimate the stellar population properties of thethin and thick disc components of NGC 7572 we collecteddeep long-slit spectra using two slit positions (see Fig. 1),which cover the galaxy mid-plane and the area above themid-plane at z = 5 arcsec, where the thick disc prevails.

2.2.1 Observations and data reduction

We observed NGC 7572 using the universal spectrographSCORPIO (Afanasiev & Moiseev 2005) operated at theprime focus of the Russian 6m BTA telescope. We used thevolume phase holographic grism “VPHG2300G” which pro-vides a spectral resolution of full width at half-maximumFWHM = 2.2 Å corresponding to σinst ≈ 55 km s−1 in termsof velocity dispersion with a 1 arcsec wide 6 arcmin longslit in the wavelength range 4800 − 5600 Å. We have ob-tained both mid-plane and thick disc spectra on the nightof 07–08/09/2016 under good atmospheric conditions (see-ing FWHM of 1.4 arcsec) with the exposure times of 6000 s(mid-plane) and 9000 s (thick disc). The slit positions areshown in Fig. 1. The CCD EEV42-40 detector (2048 × 2048pixels) provided a spectral sampling of 0.37 Å pix−1 and aslit plate scale of 0.36 arcsec pix−1 with the 1×2 binning. Inaddition to the science spectra, we obtained night-time in-ternal flat fields and He-Ne-Ar arcs, and also twilight spectraand a spectrophotometric standard star.

We reduced the spectroscopic data with our own idl-based pipeline that included the following steps: bias sub-traction, flat fielding, cosmic ray hit removal using the Lapla-cian filtering technique (Van Dokkum 2001), wavelength cal-ibration and linearization, sky subtraction and flux calibra-tion. SCORPIO has an instrumental line spread function(LSF) with a complex non-Gaussian shape, which variesboth along and across the dispersion directions. To accountfor LSF variations in the subsequent analysis, we determinedits shape with a Gauss–Hermite parametrization by usinghigh signal-to-noise twilight spectra observed with the sameinstrumental setup. We have estimated the night sky back-ground from the outer slit regions not covered by our targetgalaxy and then used an optimized sky subtraction tech-nique that takes into account LSF variations along the slit(Katkov & Chilingarian 2011; Katkov et al. 2014). We com-puted flux uncertainties from the photon statistics at theinitial stage of the reduction and propagated them throughall reduction steps.

2.2.2 Full spectral fitting

To derive internal kinematics and stellar population proper-ties (mean ages, metallicities [Fe/H] and α-elements abun-dances [Mg/Fe]) of thin and thick discs, we applied theNBursts full spectrum fitting technique (Chilingarian et al.2007a,b), which we updated by including the fitting of α-element abundances. We used two different grids of sim-ple stellar population (SSP) models. To better constrain in-ternal kinematics we used the high-resolution (R = 10000)pegase.hr SSPs (Le Borgne et al. 2004). Age, metallic-ity, and α-abundance of stellar populations were derivedusing intermediate-resolution (R ≈ 2000) miles v11 mod-els (Vazdekis et al. 2015) where two SSP grids are avail-able for solar-scaled [Mg/Fe]= 0.0 dex and α-enhanced[Mg/Fe]= +0.4 dex populations with the Kroupa (2002) stel-lar mass function.

The NBursts technique implements a pixel space χ2

minimization algorithm where an observed spectrum is ap-proximated by a stellar population model broadened with aparametric line-of-sight velocity distribution (LOSVD) andmultiplied by a polynomial continuum to take into accountdust attenuation and/or possible flux calibration imperfec-tions in both observations and models. The principal compu-tational step during the χ2 minimisation is the spectrum in-terpolation from the stellar population grid according to thegiven model parameters. For the pegase.hr models we used2D spline interpolation across ages and metallicities imple-mented in the original version of NBursts. The miles v11library has two values of [Mg/Fe] (0.0 dex and +0.4 dex).To include the [Mg/Fe] dimension use this grid we modifiedthe interpolation procedure by running the 2D spline inter-polation twice for both [Mg/Fe] values and then linearly in-terpolating along the [Mg/Fe] axis. This approach providedfully consistent results with the direct scan of the χ2 spaceover an oversampled [Mg/Fe] axes presented in Chilingar-ian & Asa’d (2018). To take into account the instrumentalbroadening, we pre-convolved the grid of stellar populationmodels with the LSF derived from twilight spectra as de-scribed earlier.

Prior to the full spectrum fitting, we binned our long slitspectra in the spatial direction using bins with sizes linearlyincreasing when moving away from the galaxy centre andsimultaneously requiring a certain minimal signal-to-noise(S/N) ratio5 ((S/N)min = 16 and 11 in cases of the mid-planeand offset spectra respectively). The binning along the slitdiffers for kinematic and stellar population profiles in bothspectroscopic data sets. Our spectra in the offset slit posi-tion that covered the thick disc component have a relativelylow S/N. Therefore for stellar population analysis, we com-puted a large spatial bin (8 < |R| < 20 arcsec) in additionto the regular bins. Before co-adding the spectra within thisbin, we accounted for the galaxy rotation curve shifting eachspectrum at given unbinned pixel to the same systematic ve-locity. Examples of spectra with their best-fitting models areshown in Fig. 6. The mid-plane spectrum has been extractedat a radius of 12.3 arcsec, and the thick disc spectrum isbinned over the whole large bin. We estimated uncertaintiesof the derived parameters by running a Monte-Carlo simu-

5 We computed the S/N in the spectral range of 5270 ± 30 Å.

MNRAS 000, 1–16 (2020)


0.5

1.0

Hβ Mg1Fe5015 Mg b

Fe5270

Mid-plane11′′ < R < 15′′

4900 5000 5100 5200 5300 5400 5500

Wavelengths, Å

0.5

1.0Flu

x,

rel.

un

its

Offset plane8′′ < |R| < 20′′

Figure 6. Two examples of the full spectrum fitting of NGC 7572 data sets. The top panel shows a mid-plane spectrum (in black) for

the spatial bin at R = 12.3 arcsec; the bottom panel corresponds to the total spectrum of the offset slit position co-added in a one largebin (8 < |R | < 20 arcsec). The red lines show the best-fitting model, residuals (shifted by an arbitrary value) are shown in gray, bluelines represents the Poisson noise propagated through the data reduction steps. The shaded area shows masked region around the 5460 Å

night sky emission line.

lation for a hundred realization of synthetic spectra for eachspatial bin, which were created by adding a random noise tothe best-fitting model corresponding to the signal-to-noiseratio in that bin.

The reconstructed stellar population properties for bothslits are shown in Fig. 7. We see a clear difference in theparameters between the mid-plane and offset spectra where,respectively, the thin and thick discs dominate. Outside thebulge area (|R| > 8 arcsec), we see an intermediate age stellarpopulation TSSP ≈ 5 . . . 8 Gyr for the mid-plane, and an older10 . . . 13 Gyr population for the offset-plane. Metallicities aresolar or slightly sub-solar in the disc regions in both slits.The α-abundance values exhibit a clear difference betweenthe mid-plane [Mg/Fe]≈ 0.15 dex and offset slit positions[Mg/Fe]≈ 0.3 dex. We do not find significant radial gradientsof the stellar populations properties of the disc components.

The central area of the galaxy has an old stellar popu-lation TSSP ≈ 12 Gyr. This is similar to the age of the thickdisc which dominates the contribution to the offset slit spec-trum even close to the principal axis of the galaxy. There isa sharp metallicity peak in the nuclear part of the galaxy,where [Fe/H] reaches 0.25 dex compared to 0.05 dex a fewarcsec away, which is a sign of a chemically decoupled nu-cleus. From the available data, we cannot tell whether wesee a real bulge in the central region of NGC 7572 or it is ahuge bar in its thin disc. The latter option is favoured by rel-atively low [Mg/Fe] values, however, whether a bar can existwithin such a dynamically hot disc is a question that can beanswered only with dedicated numerical simulations. Theline-of-sight velocity in the mid-plane reaches the plateau atabout 300 km s−1, the velocity dispersion in the disc dom-inated area is about 120 km s−1. In the offset-plane, vLOSreaches 150 km s−1 and the velocity dispersion is as high asσLOS ≈ 180 km s−1.

From the stellar population properties estimated frommiles v11 SSP models we have calculated the average mass-to-light ratios (M/L)t = 3.1 and (M/L)T = 4.6 in Solar unitsfor the thin and thick components in the SDSS g-band.

The spectral resolution and high signal-to-noise ratioof the data allows us to estimate the coefficients h3 andh4 along the mid-plane slit. The non-zero values of h4 and,in particular, h3 coefficients indicate strongly non-Gaussianand asymmetric LOSVD shape. An anticorrelation of veloc-ities and h3 profiles (as in the case NGC 7572) is known foredge-on galaxies (Chung & Bureau 2004), but such large h3values (up to 0.2) are suggestive that it could be the result ofa noticeable contribution of the thick disc component witha low rotation velocity compared to that of the thin disc.We did not find significant deviations from the GaussianLOSVD shape in the offset spectra as evidenced by h3 andh4 having zero values within uncertainties.

2.2.3 A non-parametric LOSVD reconstruction and atwo-component model of disc kinematics

To clarify whether the thick disc influences the mid-plane kinematics, we reconstructed the LOSVD in a non-parametric way. The deconvolution can be considered as alinear inverse problem whose solution L = {l1, ..., ln} can beestimated using a least-squares technique. We used a zero-order regularization (Sec. 19.4.1, Press et al. 2007) to stabi-lize a solution, which is very sensitive to noise in the data.The regularized inverse problem can be expressed as:

minL| |Y − A · L||2 + λ | |L||2, (3)

where Y is an observed spectrum logarithmically rebinnedin the wavelength domain, A is a n × m matrix, where each

MNRAS 000, 1–16 (2020)


−20 −10 0 10 20−300

−150

0

150

300

vLOS, km s−1

Mid-plane

Offset-plane

−20 −10 0 10 20

150

200

250

σLOS, km s−1

−20 −10 0 10 20−0.2

−0.1

0.0

0.1

0.2h3, h4

h3h4

−20 −10 0 10 20R, arcsec

5

9

15

TSSP, Gyr

−20 −10 0 10 20R, arcsec

−0.2

0.0

0.2

[Fe/H]SSP, dex

−20 −10 0 10 20R, arcsec

0.1

0.2

0.3

0.4

[Mg/Fe]SSP, dex

Figure 7. Radial profiles of stellar population properties derived from the SCORPIO long-slit spectra. The blue and orange colors

correspond to the mid-plane and offset slit positions, respectively. We did not find significant deviations from a Gaussian LOSVD foroffset spectra, hence we show h3, h4 coefficients for the mid-plane spectra only. Note that the kinematics and stellar population parametersare computed for different spatial binning layouts (see Sec. 2.2.2). The offset spectrum had a relatively low S/N, therefore we measured

stellar populations in one large bin shown by the orange line. The shaded orange area around the line corresponds to the uncertainties forthe stellar population parameters in that bin. The grey shaded area shows the region where the bulge or bar has a significant contribution

to the integrated light.

−20 −10 0 10 20R, arcsec

−400

−200

0

200

400

LO

SV

D,

km

s−

1

−20′′ < R < −8′′Vrot = 357 km s

−1

σr = 116 km s−1

q = 0.7

8′′ < R < 20′′

Vrot = −350 km s−1σr = 117 km s

−1

q = 0.7

−400 0 400Velocity, km s−1

T/(t+ T ) = 0.22vt = 369 km s

−1

vT = 170 km s−1

σtr = 95 km s−1

σTr = 185 km s−1

qt = 0.7qT = 1.1

−400 0 400Velocity, km s−1

T/(t+ T ) = 0.24vt = −367 km s−1vT = −167 km s−1

σtr = 91 km s−1

σTr = 157 km s−1

qt = 0.7qT = 1.1

Figure 8. The left-hand panel shows a non-parametrically reconstructed stellar LOSVD along the mid-plane slit. The right-hand paneldemonstrates the cross-sections of the stellar LOSVD as a black stepped line for the two radial bins (8 < |R | < 20 arcsec) in the disc-dominated region of NGC 7572. The red lines on the two top right-hand panels show a one-component model of the LOSVD obtained as a

result of the line-of-sight integration of the luminosity-weighted Schwarzschild orbital distribution in the disc (see details in Section 2.2.3).Bottom right-hand panels show the result of the two-component modelling where blue and orange lines correspond to the contribution

of the thin and thick discs to the total LOSVD.

MNRAS 000, 1–16 (2020)


row contains a template spectrum of the length of m pixelsshifted in velocity corresponding to a given li element of theLOSVD L, λ is a free regularization parameter. To solve thisproblem numerically, we used the bounded-variable least-squares (BVLS) method (Lawson & Hanson 1995) imple-mented by M. Cappellari6 because we need to impose thepositivity constraint on the LOSVD. As a template, weused the best-fitting SSP models from the NBursts stel-lar population analysis which we broadened only with theinstrumental LSF. We applied a similar approach in thepast to the LOSVD reconstruction in several galaxies withcounter-rotating stellar discs (Katkov et al. 2013, 2016).Fig. 8 demonstrates the result of the stellar LOSVD recon-struction in the mid-plane spectra for the two spatial binssymmetrically placed on the different sides of the galaxy at−20 < R < −8 and 8 < R < 20 arcsec.

The reconstructed stellar LOSVD looks very asymmet-ric with extended tails towards low rotational velocities (seeFig. 8) which we attribute to the presence of a slowly ro-tating component originating from the thick disc. To quan-tify the thick disc contribution to the line-of-sight integratedspectrum we first applied a simplified approach by decom-posing the reconstructed LOSVD into two Gaussian com-ponents assuming a narrow high-velocity component corre-sponding to the thin disc and a broad slowly rotating com-ponent corresponding to the thick disc. The resulting best-fitting model did not look reliable, because it yielded almostequal contributions of the two components, which was inclear contradiction to the photometric profiles decomposi-tion (see above).

Then we applied a physically motivated approach byintegrating the luminosity-weighted LOSVD along the line-of-sight and taking into account the orbital structure of disccomponents. Namely we assumed that (i) the LOSVD fora given point along the line-of-sight is a projection of theSchwarzschild orbital distribution (a set of Gaussian distri-butions along each coordinate in velocity space) at a givengalactocentric distance; (ii) the weights of the LOSVD ex-ponentially decline with galactocentric distance with scalesderived from the photometric analysis; (iii) there is no gra-dient of radial and azimuthal velocity dispersions. The pa-rameters of our model are: the circular velocity Vrot, the ra-dial velocity dispersion σr , and the ratio of the radial toazimuthal velocity dispersion components q = σφ/σr . Ourtechnique is in fact an application of the approach describedin Zasov & Khoperskov (2003) to observational data. Be-cause of the strong degeneracy between σr and q we fixed qto the value of 1/

√2 calculated for the flat rotation curve un-

der the epicyclic approximation, which should be valid for arelatively dynamically cold thin disc. The two top panels onthe right half of Fig. 8 clearly show that the reconstructedLOSVDs (black stepped lines) are poorly fitted by a single-component model (red solid lines).

Then we used a two-component model where q was fixedfor the thin disc component and was left as a free param-eter for the thick disc, which is assumed to be too hot forthe epicyclic approximation to be valid. A two-component

6 The BVLS routine implemented by M. Cappellari is availableon his web page http://www-astro.physics.ox.ac.uk/~mxc/

software/

Table 2. The observed properties of the thin and thick disc com-ponents of NGC 7572. In two last blocks of the table correspond-

ing to the results of the spectral analysis we provide the approx-

imate averaged values for the radial range 8 < |R | < 20 arcsec.

Thin disc Thick disc

Photometry

Radial scalelength 5.4 ± 0.1 kpc 5.6 ± 0.3 kpcCentral scaleheight 1.5 ± 0.1 kpc 6.0 ± 0.2 kpcFlaring Yes Yes

Total mass 5.9 × 1010 M� 1.6 × 1011 M�

Slit positions: z = 0 kpc z = 4 kpc

vLOS 300 km s−1 150 km s−1σLOS 120 km s−1 180 km s−1TSSP 5–8 Gyr 10–13 Gyr[Fe/H] 0.0 dex −0.1 dex[Mg/Fe] 0.15 0.3(M/L)g/(M/L)�,g 3.1 4.6

LOSVD analysis of the mid-plane spectrum

Rotation velocity Vrot 370 km s−1 170 km s−1Radial dispersion 〈σr 〉 90 km s−1 170 km s−1q = σφ/σr 0.7 ≈ 1

model has one additional parameter, a relative contributionof the components. In both radial bins the reconstructedLOSVDs are very well reproduced by the model (see bottompanels on right-hand side of Fig. 8). From this modelling,we obtained the radial velocity dispersion of the thin disc ofσTr = 93±3 km s−1 and a much larger value for the thick discσTr = 171±20 km s−1. The ratio between the dispersion com-ponents of the thick disc q ≈ 1 corresponds to the isotropicstellar velocity dispersion distribution. The corrected thickdisc rotation velocity turns out to be ≈ 170 km s−1 while forthe thin disc it reaches ≈ 370 km s−1. Note that the rota-tional velocity estimates from our modelling are free fromthe asymmetric drift effect and represent the true circularvelocity of a galaxy at a given distance from the galaxy cen-tre. The two-component LOSVD decomposition yields thecontribution of the thick disc to the mid-plane spectra of23 per cent, which is fully consistent with the photometricanalysis (≈ 30 per cent, see Sect. 2.1.4).

3 DISCUSSION

There are several scenarios explaining the difference of stel-lar population properties of thin and thick discs. (i) A thickdisc could have formed as a result of the thin disc secularevolution that changed its radial and vertical structure bythe effects of the radial migration (Schönrich & Binney 2009;Minchev & Famaey 2010; Loebman et al. 2011; Minchevet al. 2015), the dynamical heating by giant molecular clouds(Spitzer & Schwarzschild 1953; Lacey 1984) and/or minormergers (Quinn et al. 1993; Villalobos et al. 2010; Qu et al.2011b). (ii) An accretion scenario implies that stars whichwe observe as a thick disc were predominantly formed indwarf satellite galaxies which merged with the host and builtup a dynamically heated discy component (e.g. Abadi et al.2003). (iii) A two-stage model describes the thick disc forma-

MNRAS 000, 1–16 (2020)

http://www-astro.physics.ox.ac.uk/~mxc/software/http://www-astro.physics.ox.ac.uk/~mxc/software/


tion at high redshift (z ∼ 2) in a high-density and high veloc-ity dispersion interstellar medium (Elmegreen & Elmegreen2006; Bournaud et al. 2009; Lehnert et al. 2014). After that,a thin disc is formed from an additional supply of gas froman external source. The new thin gas layer may come fromintergalactic filaments (Chiappini et al. 1997; Combes 2014),minor wet mergers (Sil’chenko et al. 2011) or as a result ofthe cooling of gas ejected earlier through stellar feedback(Fraternali et al. 2013) or left from the first (thick) disc for-mation phase (the “upside-down” models by Burkert et al.1992; Bird et al. 2013). The latter group of scenarios natu-rally explains the presence of an observed time delay betweenthe onset of star formation in the thin and thick discs of theMilky Way (see e.g. Lehnert et al. 2014; Kilic et al. 2017).

A thick disc formation in a major accretion event is notlikely because there is a too low fraction of counter-rotatingstars in thick discs of massive galaxies, which should be ob-served more commonly if such scenario took place (Yoachim& Dalcanton 2008a; Comerón et al. 2015, 2019; Kasparovaet al. 2016). However, for the definitive rejection of this sce-nario it is critical to obtain observational data on the verticalstructure of giant discs.

Overmassive giant disc galaxies are very rare and poorlystudied (see e.g. Saburova 2018). The fact that we ob-tained high-quality spectrophotometric data for NGC 7572,an edge-on galaxy with a size comparable to those discussedin Saburova (2018), is therefore important. According toSaburova (2018), giant galaxies with R25 > 30 kpc repre-sent about 1 per cent of the total population of discs withinclination angles i > 40◦. The objects with rotation veloci-ties > 350 km s−1 are 4 per cent of the Saburova’s sample ofgiant galaxies. According to modern concepts, nearly all gi-ant galaxies with masses > 1012 M� should have experiencedat least one major merger (Rodriguez-Gomez et al. 2015).During such an event the initial disc structure is thoughtto be destroyed but a new disc could reform later in somecases (Puech et al. 2012; Peschken et al. 2019). The principalquestion is “what stellar population properties and verticalstructure peculiarities of such a newly formed disc are?”

Each thick disc formation mechanism leaves a fossilrecord in the observed kinematics and stellar populationsproperties of galaxies. Below we discuss the key observablefacts regarding NGC 7572 (Table 2) which we compare withthe Milky Way and other objects. Then we use them to testdifferent scenarios for the thick disc origin. We compare ourdata with numerical models, which were made mainly forMW-like galaxies in terms of mass and size. It would be ofinterest to have libraries of galaxy simulations covering verymassive galaxies such as the one studied in here.

3.1 The observed properties of NGC 7572

NGC 7572 is a giant lenticular galaxy with no signs of thecold gas presence or current star formation, which is con-firmed by the absence of emission lines in the mid-planespectrum and the Hi non-detection according to Springobet al. (2005). Its rotation velocity reaches 370 km s−1 andthe total disc mass is more than five times the mass of theMW disc. Such high rotation velocities are typical for “superspirals” (Ogle et al. 2016) and giant LSB galaxies (Saburovaet al. 2018) and they are extremely rare. NGC 7572 has com-plex vertical structure and to properly describe it, we need

at least two (thin and thick) flaring components with verysimilar mid-plane radial scales. The thick disc stellar massis more than double the thin disc mass, 1.6 × 1011 M� ver-sus 5.9× 1010 M�. The thick component contribution to thetotal flux increases from 30 per cent to 90 per cent when wemove from the mid-plane to |z | = 4 kpc above and it slightlyvaries with the radius.

We have not detected any significant radial gradients ofthe stellar population properties outside the bulge area. Inthe mid-plane we see intermediate age stars (tt = 5 . . . 8 Gyr)while the stellar population of the thick disc is older tT =10 . . . 13 Gyr. Despite the similarity in metallicities betweenthe thin and thick discs ([Fe/H]t ≈ 0 dex and [Fe/H]T ≈−0.1 dex) the difference in the α-abundance is statisticallysignificant ([Mg/Fe]t=+0.15 dex vs [Mg/Fe]T=+0.3 dex).This suggests that the formation epochs for the two disccomponents had different duration (see e.g. Thomas et al.2005). According to Eq. 4 in Thomas et al. (2005) the thickdisc was formed quickly in ∼ 0.25 Gyr while the thin discgrew over a much longer period of time (2 Gyr).

From the non-parametric modelling of the stellarLOSVD for the mid-plane spectra we estimated the rota-tion velocities and radial velocity dispersions of the thin andthick discs in the galactic main plane: υt ≈ 370 km s−1 and〈σtr 〉 ≈ 90 km s−1 for the thin disc and υT ≈ 170 km s−1and 〈σTr 〉 ≈ 170 km s−1 for the thick disc. The high valueof υT /σTr argues that the thick disc of NGC 7572 is a dy-namically hot subsystem where the epicyclic approximationis invalid and, therefore q = σφ/σr must be treated as afree parameter in the LOSVD modelling. For the thick discwe derived q ≈ 1 while for the thin component we fixed itto q = 1/

√2 following the epicyclic approximation, which

is valid for a relatively cold thin disc (σr � υ). Assumingthat q does not vary along the z axis and accounting thatthick disc line-of-sight velocity dispersion at 4 kpc above themid-plane is very close to that in the mid-plane value, weconclude that σr (z) for the thick disc of NGC 7572 is close toconstant (similar to the MW according to Mackereth et al.2019).

We detected extended wings of the vertical photomet-ric profiles noticeable in the inner part of the disc (the lightexcess is found at |z | > 15 arcsec) not described by the two-component model. Comerón et al. (2018) have found similarfeatures in 6 per cent of their sample (and it is preciselythe most massive galaxies) despite using the self-consistentequations to define the vertical profiles and taking into ac-count the diffuse scattered light. Qu et al. (2011a) obtaineda similar additional stellar excess in the numerical models ofthick disc formation through minor mergers. Although thisrequires numerical confirmation, we believe that this effectcan be explained in two more ways: (i) the line-of-sight inte-gration of the light from two flaring and not exactly sech2–like disc components should yield a flux excess, which willbe more noticeable at small R, as we observe in NGC 7572;(ii) the wings might represent an additional structural com-ponent of the disc if we consider a two-stage formation sce-nario of the vertical disc structure (e.g. Chiappini et al. 1997)as the simplest case of a multistage scenario, which can, inprinciple, lead to the formation of three or more disc layers(see also NGC 4013 in Comerón et al. 2011a). The likelihoodof such repeated disc formation episodes should be higher formassive galaxies. The two main questions to be answered

MNRAS 000, 1–16 (2020)


regarding this scenario are “how much time is needed foreach disc formation stage?” and “do we have the instrumen-tal sensitivity to obtain observational confirmation of thisscenario?”

3.2 The imprints of the disc build-up scenarios

Considering suitable scenarios for the NGC 7572 disc forma-tion, first of all we stress that its thick disc could not becreated from stars of accreted dwarf galaxies, because nor-mally, stellar populations in dwarfs have lower metallicitiesand [Mg/Fe] ratios (see e.g. Chilingarian 2009) than whatwe observe in NGC 7572. According to Forbes et al. (2004),stellar population properties of the MW thick disc do notagree with such a model either.

3.2.1 Kinematics of stellar disc components

Observational data indicate that stellar populations of thethin and thick discs of the MW have different kinematics,in particular the velocity dispersions and the mean orbitalradii (Lee et al. 2011; Yu & Liu 2018; Mackereth et al. 2019).Several numerical models challenge the ability of radial mi-gration alone to create a thick disc from stars with large dis-persion by moving stars born in the inner regions of the discaway from the mid-plane (see Minchev et al. 2012). How-ever, Loebman et al. (2011) and Roškar et al. (2013) havereproduced this effect in their models. In case of NGC 7572,the inner region of the disc/bulge has a velocity dispersionvalue comparable to that of a thick disc. The observed veryold ages in this area suggest that the later star formationepisode did not significantly affect that region. The high stel-lar metallicity and relatively low [Mg/Fe] exclude the possi-bility that the inner region in the mid-plane is the source ofstars observed in the thick disc of NGC 7572.

Although we do not have the data to build the AVRsfor NGC 7572 for the analysis of the thin disc heating pro-cess, some observed features allow us to reject the thin discheating scenario. The velocity dispersion of the NGC 7572thin disc is greater than that of the MW thick disc (Bland-Hawthorn & Gerhard 2016), which is inert and not subjectto additional heating. Moreover, if a thick component is theresult of a thin disc heating via the influence of giant molec-ular clouds, then the radial velocity dispersion of the high–[α/Fe] component would be lower than observed. Indeed thisheating mechanism is not sufficient to explain the observedproperties of the MW thick disc (Aumer et al. 2016; Yu &Liu 2018). Unless GMCs were much more effective at scat-tering stars in NGC 7572 than in the MW, the same musthold true in NGC 7572. Hence, it is more natural to explainthe observed ratio of velocity dispersions of NGC 7572 discsby having its thick disc formed before the thin disc.

3.2.2 The radial scalelengths of the disc components

The ratio of the radial scalelengths of the thin and thickcomponents is an important indicator of the disc forma-tion scenario. In upside-down scenarios, the radial scale of ayounger thin disc should be larger than that of a thick disc(see for example Bird et al. 2013; Lehnert et al. 2014) be-cause a thin disc is formed from gas having a higher angular

momentum than the thick disc material. According to themodels by Stinson et al. (2013) the scalelengths of the stel-lar populations with narrow [Fe/H] and [α/Fe] abundancesranges grow non-linearly with age. In a MW-like galaxy wewill be able to notice an increase in the radial scale of the disccomponent with age from observations only for a populationyounger than 5 Gyr because all older populations have thesame scalelengths (see cosmological hydro-dynamical simu-lations by Stinson et al. 2013, Fig. 8).

However, comparing these models with observations isdifficult even for the MW because it is not always possi-ble to unambiguously separate thick and thin disc stellarpopulations. For example, in the Solar neighbourhood starswith a high [Mg/Fe] content likely have diverse origins, notonly they come from the thick disc but also from the in-ner regions of the thin disc (Haywood et al. 2013; Haydenet al. 2017). Nevertheless, according to several recent stud-ies (e. g. Carollo et al. 2010; Cheng et al. 2012; Bovy et al.2016; Mateu & Vivas 2018), the radial scale of the MW thickcomponent having a high [α/Fe] ratio is similar or slightlyshorter than that of the thin disc, which is approximately2.6 kpc (Bland-Hawthorn & Gerhard 2016). On the contrary,López-Corredoira & Molgó (2014) using simple star countsconclude that the thick disc radial scale is longer than thatof the thin component.

In NGC 7572, the equal values of the mid-plane radialscales of the disc components with ages 5 . . . 8 Gyr and10 . . . 13 Gyr are in good agreement with the upside-downformation scenario. However, if we integrate the vertical dis-tributions of the disc components at every R, the scalelengthof the resulting radial distribution of the thick disc will be-come longer than that of the thin disc (see Sect. 2.1.4).

In other (edge-on) galaxies, scales of components aredefined using photometric decomposition techniques, whichmakes it difficult to compare them with the results for theMW. According to Comerón et al. (2012), the thick discsusually have larger scalelengths than thin discs (see alsoPohlen et al. 2004; Yoachim & Dalcanton 2006). These val-ues apply both to the mid-plane and to any z because theflaring is usually neglected. But, as we demonstrated above,the radial scales can be greatly overestimated in case of sig-nificant disc flaring and also due to other purely geometriceffects.

3.2.3 Disc flaring

In recent photometric studies of the vertical structure of discgalaxies (Yoachim & Dalcanton 2006; Comerón et al. 2018,and many others), the thickness variations of both disc com-ponents with radius were neglected. This assumption reliedon earlier observational studies (see e. g. Van der Kruit &Searle 1981; Bizyaev & Mitronova 2002), even though thereare hints that vertical scaleheights are constants with radiusonly in late-type galaxies (see de Grijs & Peletier (1997)and fig. 9 in Bizyaev et al. (2014)). Pinna et al. (2019a,b)demonstrated significant thin disc flaring in two out of threestudied S0 galaxies in the Fornax cluster. The thin disc ofthe MW broadens in the outskirts while there is still a vig-orous discussion about the thick disc (López-Corredoira &Molgó 2014; Bland-Hawthorn & Gerhard 2016; Bovy et al.2016). However, as we demonstrated earlier, in NGC 7572we cannot ignore the change of the thicknesses of both disc

MNRAS 000, 1–16 (2020)


components, even though because of our simplified mathe-matical approach, the exact form of the component broad-ening cannot yet be precisely calculated.

If a thick disc forms in an isolated galaxy at high red-shift (z ∼ 2) in the conditions of high-velocity dispersionof the interstellar medium (Elmegreen & Elmegreen 2006;Bournaud et al. 2009; Lehnert et al. 2014), a dynamicallyhot thick disc would have a constant vertical scale. After-wards it becomes insensitive to additional heating. But ifsome mergers occurred along the rapid formation stage ofa thick disc, according to Bournaud et al. (2009) and Quet al. (2011a) the resulting thick stellar component wouldflare. Because NGC 7572 is an overmassive giant galaxy ina dense environment, having a number of minor mergers inits early formation history looks quite plausible and it willexplain the significant flaring of the thick disc that we ob-serve.

3.2.4 The mass ratio of the disc components

The mass ratio of the thick to thin disc components isthought to anticorrelate with the total mass of the system(Yoachim & Dalcanton 2006; Comerón et al. 2018). It isabout 1/3 for massive galaxies but in intermediate-mass sys-tems with rotation velocities vrot < 120 km s−1 the thick andthin component masses can be equal. We note that to cor-rectly estimate the thick and thin disc masses of externalgalaxies, in addition to accurate photometric decomposition,it is necessary to know the mass-to-light ratios of the com-ponents. The use of the “mass follows light” assumption inthe optical bands is a too big of a simplification, becauseas we demonstrated in Kasparova et al. (2016) and Katkovet al. (2019), stellar population properties of thick and thindiscs can differ significantly (see also Comerón et al. 2015;Pinna et al. 2019b).

In NGC 7572 the mass ratio of the thick and thin disccomponents is MT /Mt ≈ 2.7 and it is an unprecedentedlylarge value for such a massive galaxy. Nonetheless, we cansimply explain this fact within the scenario of the rapid for-mation of its thick disc and subsequent gradual growth ofthe thin disc. In a two-stage scenario there must be objectswhose thin discs have not yet grown or became underdevel-oped because of environmental effects. The latter situationcan happen if a cold gas reservoir (e.g. intergalactic filamentsor settling gas ejected by galactic winds) had been strippedor exhausted before the thin disc grew to the “normal” size.The proposed formation mechanism of NGC 7572 is in agood agreement with a scenario by Sil’chenko et al. (2012)explaining the general domination of lenticular galaxies overspirals in dense environment by the fact that they did nothave enough material to form a thin disc. This scenario canbe realized due to a number of effects associated with adense environment, e.g ram pressure stripping (Gunn & Gott1972), interaction with nearby galaxies (Mihos 2004), star-vation (Larson et al. 1980) and others. Comerón et al. (2016)and Katkov et al. (2019) also showed the significant influ-ence of the environment on the thin disc growth in severalcluster galaxies.

We believe that our observational data analysis forNGC 7572 provides an example on a disc formation mecha-nism similar to for the MW that acts in the extreme case ofa dense cluster environment for a very massive giant galaxy.

3.3 How a massive thick disc can affectthe interpretation of galaxy observations

The example of NGC 7572 clearly shows that it is naive toassume that for any non-edge-on oriented galaxy most of itslight comes from a thin component of the disc. When westudy galaxies with arbitrary orientation of their disc planein the sky we must keep in mind the multicomponent natureof their discs. This can significantly complicate the analysisof stellar population properties (especially if they are oldenough) in cases of non-edge-on disc galaxies because theirthick disc components will affect both kinematics and stellarpopulation measurements. In certain regions of galaxies wewill see a mixture of stellar populations from layers formedin very different conditions at different redshift and star for-mation histories. If we turn NGC 7572 to the face-on pro-jection, more than 60 per cent of the light will come fromthe old hot thick disc and its contribution will grow to theoutskirts. Then we will underestimate its rotation velocitysignificantly due to the large velocity dispersion of the thickdisc and, in fact, it might be classified as a rotating ellipticalgalaxy. In some cases, observed gradients of stellar popula-tion properties can be the result of projection effects of twosub-populations on the line-of-sight. There are similar ob-servational manifestations along the antitruncation featuresin surface brightness profiles if the flaring is insignificant butthe radial scale of the thick disc in the mid-plane is largerthan that of the thin disc (Comerón et al. 2012). These ef-fects combined with a possible thin disc truncation allow usto study stellar populations of a thick disc not only in themid-plane of edge-on galaxies (Katkov et al. 2019) but alsoin the periphery of discs at any orientation to the line-of-sight.

4 SUMMARY

In this study we first present a novel data analysis tech-niques and then apply them to deep imaging and spectro-scopic observational data for the extremely luminous andlarge lenticular galaxy NGC 7572.

Using simple 1D decomposition of vertical cross-sectionsof the disc and assuming a double-sech2 model we detectedand measured changes in the vertical scaleheights of boththick and thin discs and measured their radial scalelengths.We also obtained a map of the relative contributions of athick and thin disc required for a physically motivated in-terpretation of spectral data where we observe the overlapof stellar populations in the line of sight. We demonstratedthe importance of taking into account the individual con-tributions of the thick and thin discs to the mid-plane lightprofile required to correctly estimate radial scales of the bothcomponents. We showed that the thick disc radial scale asobtained by fitting the surface brightness profiles above themid-plane can be overestimated because of the flaring andimperfect edge-on disc orientation.

As a part of the analysis of mid-plane spectra using real-istic assumptions about stellar dynamics, we reconstructedthe stellar LOSVD in a non-parametric way and fitted itusing a two-component parametric model that took into ac-count the orbital distribution of stars on the line of sight.This is possible for NGC 7572 because of the significant dif-ference in the kinematics of its thick and thin discs.

MNRAS 000, 1–16 (2020)


We carried out the first spectrophotometric study ofthe vertical structure of a giant disc galaxy significantly ex-ceeding the size of the Milky Way. NGC 7572 has a totaldisc mass of ∼ 2.2 × 1011 M�, a circular rotation velocity of∼ 370 km s−1 and it lives in a dense environment.

Our photometric analysis of DECaLS-g images demon-strates that:

• We need at least two components to describe the verti-cal structure of the disc in NGC 7572;• Both disc components exhibit flaring. The scaleheight

of the thin disc in the inner part is zt = 1.5 ± 0.1 kpc and itgrows to ∼ 2 kpc in the outer regions. The scaleheight of thethick component grows from zT = 6.0 ± 0.2 kpc by a factorof two;• The scalelengths of the thin and thick components are

almost identical (ht = 5.4 ± 0.1 kpc and hT = 5.6 ± 0.3 kpc);• The thick disc contribution to the disc brightness

changes from 30 per cent to 90 per cent moving from themid-plane to |z | = 4 kpc above it and it only slightly changeswith the radius;• We found some excess flux noticeable at |z | > 15 arcsec,

not described by the former two components, which can beeither (i) the sign of the formation process of a thick discthrough minor mergers or (ii) the effect of the line-of-sightintegration of the light from two not exactly sech2–like flar-ing components or (iii) an additional third disc component.

We analysed deep spectroscopic long-slit observationscollected with the Russian 6-m BTA telescope. We obtainedspectra at two positions, the major axis (mid-plane) andthe region parallel to the major axis offset by 5 arcsec(≈ 4.0 kpc) above the mid-plane. We do not see statisticallysignificant radial gradients of the stellar population prop-erties (age, [Fe/H], [Mg/Fe]). The thick disc has an olderand dynamically hotter stellar population than the thin disc,the α-element abundances [Mg/Fe] correspond to +0.15 and+0.3 dex for the thin and thick components respectively. Thenon-parametric stellar LOSVD along the mid-plane showsthat the thick disc rotates 200 km s−1 slower than the thindisc. Despite significant differences in mass, scalelengths andkinematics, mean ages and metallicities of the disc com-ponents are close to those for “ordinary” giant early-typedisc galaxies: TSSP,t = 5 . . . 8 Gyr and TSSP,T = 10 . . . 13 Gyr,[Fe/H]SSP,t = 0.0 dex and [Fe/H]SSP,T = −0.1 dex for the thinand thick components respectively.

We derived the stellar mass-to-light ratios (M/L)t =3.1 (M/L)� and M/LT = 4.6 (M/L)� (SDSS g-band) for thethin and thick discs and estimated their stellar masses to beMt = 5.9 × 1010 M� and MT = 1.6 × 1011 M�. Hence, thethick-to-thin mass ratio is MT /Mt = 2.7.

Our results allow us to draw a number of important con-clusions regarding the analysis of observational data of non-edge-on oriented galaxies, at least for massive giant discs. Itis always necessary to take into account the multicomponentnature of discs due to a potential significant contribution ofa thick disc to the total light. In some regions of galaxies wewill see a mixture of stellar populations from layers formedin very different conditions at different epochs. Because ofthe flaring, “face-on” radial scales can differ from the valuesin the mid-plane (in the case of NGC 7572 it is 6.6± 0.4 kpcand 9.1 ± 1.8 kpc for the thin and thick components) whichcomplicates the analysis of stellar population gradients.

At the end, our analysis allowed us to choose the mostplausible scenario explaining the observed vertical structureof NGC 7572. At high redshift (z ∼ 2) the progenitor of agiant galaxy in the process of a wild star-forming phase suf-fers by some minor mergers and gives birth to a thick giantflaring disc with large radial and vertical scales. Then, aftersome time, the thin disc formation starts because of eitheradditional gas accretion from infalling gas-rich satellites oraccreted through filaments or the cooling of the gas remainedafter the phase of the thick disc formation. For some reasonabout 5 Gyr ago the cold gas reservoir was exhausted orremoved earlier than in most galaxies, e.g. due to the influ-ence of a dense cluster environment. This stopped the thindisc formation in NGC 7572 prematurely so that it could notgrow further.

ACKNOWLEDGMENTS

We are grateful to Anatoly Zasov and Olga Sil’chenko atSternberg Astronomical Institute, Moscow State Universityfor useful suggestions and fruitful discussions. We thankAlexey Moiseev and Dmitri Oparin at Special Astrophys-ical Observatory, Russian Academy of Sciences for their as-sistance in our SCORPIO observations. We are thankful tothe anonymous referee for the important comments. Thedevelopment of advanced data analysis techniques for deepspectroscopic observations is supported by the Russian Sci-ence Foundation project 19-12-00281. The authors acknowl-edge the support from the Program of development of M.V.Lomonosov Moscow State University for the Leading Scien-tific School “Physics of stars, relativistic objects and galax-ies”.

The 6-m telescope of the Special Astrophysical Obser-vatory of the Russian Academy of Sciences is operated withthe financial support of the Ministry of Science and HigherEducation of the Russian Federation.

This research has made use of the Lyon ExtragalacticDatabase (LEDA, http://leda.univ-lyon1.fr/).

This research has made use of the NASA/IPAC Extra-galactic Database, which is funded by the National Aero-nautics and Space Administration and operated by the Cal-ifornia Institute of Technology.

In this study we used the data from the Dark En-ergy Camera Legacy Survey (DECaLS; NOAO Proposal ID#2014B-0404; PIs: David Schlegel and Arjun Dey) avail-able online at http://legacysurvey.org obtained at theBlanco telescope, Cerro Tololo Inter-American Observatory,National Optical Astronomy Observatory (NOAO) with theDark Energy Camera (DECam), which was constructed bythe Dark Energy Survey (DES) collaboration. The LegacySurveys imaging of the DESI footprint is supported by theDirector, Office of Science, Office of High Energy Physicsof the U.S. Department of Energy under Contract No. DE-AC02-05CH1123, by the National Energy Research ScientificComputing Center, a DOE Office of Science User Facilityunder the same contract; and by the U.S. National ScienceFoundation, Division of Astronomical Sciences under Con-tract No. AST-0950945 to NOAO.

MNRAS 000, 1–16 (2020)

http://legacysurvey.org


REFERENCES

Abadi M. G., Navarro J. F., Steinmetz M., Eke V. R., 2003, ApJ,

597, 21

Afanasiev V. L., Moiseev A. V., 2005, Astron. Lett., 31, 194

Aumer M., Binney J., Schönrich R., 2016, MNRAS, 459, 3326

Bird J. C., Kazantzidis S., Weinberg D. H., Guedes J., CallegariS., Mayer L., Madau P., 2013, ApJ, 773, 43

Bizyaev D., Mitronova S., 2002, A&A, 389, 795

Bizyaev D. V., Kautsch S. J., Mosenkov A. V., Reshetnikov V. P.,

Sotnikova N. Y., Yablokova N. V., Hillyer R. W., 2014, ApJ,

787, 24

Bizyaev D. V., Kautsch S. J., Sotnikova N. Y., Reshetnikov V. P.,

Mosenkov A. V., 2017, MNRAS, 465, 3784

Bland-Hawthorn J., Gerhard O., 2016, ARA&A, 54, 529

Borlaff A., Eliche-Moral M. C., Beckman J., Font J., 2016, A&A,591, L7

Bournaud F., Elmegreen B. G., Martig M., 2009, ApJ, 707, L1

Bovy J., Rix H.-W., Schlafly E. F., Nidever D. L., Holtzman J. A.,Shetrone M., Beers T. C., 2016, ApJ, 823, 30

Burkert A., Truran J. W., Hensler G., 1992, ApJ, 391, 651

Burstein D., 1979, ApJ, 234, 829

Carollo D., et al., 2010, ApJ, 712, 692

Cheng J. Y., et al., 2012, ApJ, 752, 51

Chiappini C., Matteucci F., Gratton R., 1997, ApJ, 477, 765

Chilingarian I. V., 2009, MNRAS, 394, 1229

Chilingarian I. V., Asa’d R., 2018, ApJ, 858, 63

Chilingarian I. V., Zolotukhin I. Y., 2012, MNRAS, 419, 1727

Chilingarian I., Prugniel P., Sil’chenko O., Koleva M., 2007a, inVazdekis A., Peletier R., eds, Proc. IAU Symp. Vol. 241, Stel-

lar Populations as Building Blocks of Galaxies. Cambridge

University Press, Cambridge, p. 175

Chilingarian I. V., Prugniel P., Sil’chenko O. K., Afanasiev V. L.,

2007b, MNRAS, 376, 1033

Chilingarian I. V., Melchior A.-L., Zolotukhin I. Y., 2010, MN-

RAS, 405, 1409

Chilingarian I. V., Zolotukhin I. Y., Katkov I. Y., Melchior A.-L.,Rubtsov E. V., Grishin K. A., 2017, ApJS, 228, 14

Chung A., Bureau M., 2004, AJ, 127, 3192

Combes F., 2014, in Seigar M. S., Treuthardt P., eds, ASP Conf.

Ser. Vol. 480, Structure and Dynamics of Disk Galaxies. As-

tron. Soc. Pac., San Francisco, p. 211

Comerón S., et al., 2011a, ApJ, 738, L17

Comerón S., et al., 2011b, ApJ, 741, 28

Comerón S., et al., 2012, ApJ, 759, 98

Comerón S., Salo H., Janz J., Laurikainen E., Yoachim P., 2015,A&A, 584, A34

Comerón S., Salo H., Peletier R. F., Mentz J., 2016, A&A, 593,L6

Comerón S., Salo H., Knapen J. H., 2018, A&A, 610, A5

Comerón S., Salo H., Knapen J. H., Peletier R. F., 2019, A&A,

623, A89

Devour B. M., Bell E. F., 2017, MNRAS, 468, L31

Dey A., et al., 2019, AJ, 157, 168

Elmegreen B. G., Elmegreen D. M., 2006, ApJ, 650, 644

Erwin P., 2015, ApJ, 799, 226

Forbes D. A., Strader J., Brodie J. P., 2004, AJ, 127, 3394

Fraternali F., Marasco A., Marinacci F., Binney J., 2013, ApJ,764, L21

Fuhrmann K., 1998, A&A, 338, 161

Gaia Collaboration et al., 2016, A&A, 595, A1

Gilmore G., Reid N., 1983, MNRAS, 202, 1025

Guérou A., Emsellem E., Krajnović D., McDermid R. M., Contini

T., Weilbacher P. M., 2016, A&A, 591, A143

Gunn J. E., Gott III J. R., 1972, ApJ, 176, 1

Hayden M. R., Recio-Blanco A., de Laverny P., Mikolaitis S.,Worley C. C., 2017, A&A, 608, L1

Haywood M., Di Matteo P., Lehnert M. D., Katz D., Gómez A.,2013, A&A, 560, A109

Kasparova A. V., Katkov I. Y., Chilingarian I. V., Silchenko O. K.,Moiseev A. V., Borisov S. B., 2016, MNRAS, 460, L89

Katkov I. Y., Chilingarian I. V., 2011, in Evans I. N., AccomazziA., Mink D. J., Rots A. H., eds, ASP Conf. Ser. Vol. 442, As-

tronomical Data Analysis Software and Systems XX. Astron.

Soc. Pac., San Francisco, p. 143

Katkov I. Y., Sil’chenko O. K., Afanasiev V. L., 2013, ApJ, 769,

105

Katkov I. Y., Sil’chenko O. K., Afanasiev V. L., 2014, Astrophys.

Bull., 69, 121

Katkov I. Y., Sil’chenko O. K., Chilingarian I. V., Uklein R. I.,

Egorov O. V., 2016, MNRAS, 461, 2068

Katkov I. Y., Kniazev A. Y., Kasparova A. V., Sil’chenko O. K.,

2019, MNRAS, 483, 2413

Kilic M., Munn J. A., Harris H. C., von Hippel T., Liebert J. W.,

Williams K. A., Jeffery E., DeGennaro S., 2017, ApJ, 837, 162

Kroupa P., 2002, Science, 295, 82

Lacey C. G., 1984, MNRAS, 208, 687

Larson R. B., Tinsley B. M., Caldwell C. N., 1980, ApJ, 237, 692

Lawson C., Hanson R., 1995, Solving Least Squares Prob-lems. Classics in Applied Mathematics, Society for Indus-

trial and Applied Mathematics, Philadelphia, PA, https:

//books.google.ru/books?id=AEwDbHp50FgC

Le Borgne D., Rocca-Volmerange B., Prugniel P., Lançon A., Fioc

M., Soubiran C., 2004, A&A, 425, 881

Lee Y. S., et al., 2011, ApJ, 738, 187

Lehnert M. D., Di Matteo P., Haywood M., Snaith O. N., 2014,

ApJ, 789, L30

Loebman S. R., Roškar R., Debattista V. P., Ivezić Ž., Quinn

T. R., Wadsley J., 2011, ApJ, 737, 8

López-Corredoira M., Molgó J., 2014, A&A, 567, A106

Mackereth J. T., et al., 2019, MNRAS, 489, 176

Majewski S. R., 1993, ARA&A, 31, 575

Majewski S. R., et al., 2017, AJ, 154, 94

Makarov D., Prugniel P., Terekhova N., Courtois H., Vauglin I.,

2014, A&A, 570, A13

Mart́ınez-Lombilla C., Knapen J. H., 2019, A&A, 629, A12

Mateu C., Vivas A. K., 2018, MNRAS, 479, 211

Mihos J. C., 2004, Clusters of Galaxies: Probes of Cosmologi-

cal Structure and Galaxy Evolution, ed. J. S. Mulchaey, A.Dressler, & A. Oemler. Cambridge University Press, Cam-

bridge

Minchev I., Famaey B., 2010, ApJ, 722, 112

Minchev I., Famaey B., Quillen A. C., Dehnen W., Martig M.,Siebert A., 2012, A&A, 548, A127

Minchev I., Martig M., Streich D., Scannapieco C., de Jong R. S.,Steinmetz M., 2015, ApJ, 804, L9

Narayan C. A., Jog C. J., 2002, A&A, 394, 89

Ogle P. M., Lanz L., Nader C., Helou G., 2016, ApJ, 817, 109

Peschken N., Lokas E. L., Athanassoula E., 2019, arXiv e-prints,p. arXiv:1909.01033

Peters S. P. C., van der Kruit P. C., Knapen J. H., Trujillo I.,

Fliri J., Cisternas M., Kelvin L. S., 2017, MNRAS, 470, 427

Pinna F., et al., 2019a, A&A, 623, A19

Pinna F., et al., 2019b, A&A, 625, A95

Pohlen M., Balcells M., Lütticke R., Dettmar R.-J., 2004, A&A,422, 465

Press W. H., Teukolsky S. A., Vetterling W. T., Flannery B. P.,

2007, Numerical Recipes 3rd Edition. The Art of ScientificComputing, Cambridge University Press

Prochaska J. X., Naumov S. O., Carney B. W., McWilliam A.,Wolfe A. M., 2000, AJ, 120, 2513

Puech M., Hammer F., Hopkins P. F., Athanassoula E., FloresH., Rodrigues M., Wang J. L., Yang Y. B., 2012, ApJ, 753,128

Qu Y., Di Matteo P., Lehnert M. D., van Driel W., 2011a, A&A,

MNRAS 000, 1–16 (2020)

http://dx.doi.org/10.1086/378316http://adsabs.harvard.edu/abs/2003ApJ...597...21Ahttp://dx.doi.org/10.1134/1.1883351http://adsabs.harvard.edu/abs/2005AstL...31..194Ahttp://dx.doi.org/10.1093/mnras/stw777https://ui.adsabs.harvard.edu/abs/2016MNRAS.459.3326Ahttp://dx.doi.org/10.1088/0004-637X/773/1/43http://adsabs.harvard.edu/abs/2013ApJ...773...43Bhttp://dx.doi.org/10.1051/0004-6361:20020633http://adsabs.harvard.edu/abs/2002A%26A...389..795Bhttp://dx.doi.org/10.1088/0004-637X/787/1/24http://adsabs.harvard.edu/abs/2014ApJ...787...24Bhttp://dx.doi.org/10.1093/mnras/stw2972http://adsabs.harvard.edu/abs/2017MNRAS.465.3784Bhttp://dx.doi.org/10.1146/annurev-astro-081915-023441http://adsabs.harvard.edu/abs/2016ARA%26A..54..529Bhttp://dx.doi.org/10.1051/0004-6361/201628868http://adsabs.harvard.edu/abs/2016A%26A...591L...7Bhttp://dx.doi.org/10.1088/0004-637X/707/1/L1http://adsabs.harvard.edu/abs/2009ApJ...707L...1Bhttp://dx.doi.org/10.3847/0004-637X/823/1/30http://adsabs.harvard.edu/abs/2016ApJ...823...30Bhttp://dx.doi.org/10.1086/171378http://adsabs.harvard.edu/abs/1992ApJ...391..651Bhttp://dx.doi.org/10.1086/157563http://adsabs.harvard.edu/abs/1979ApJ...234..829Bhttp://dx.doi.org/10.1088/0004-637X/712/1/692http://adsabs.harvard.edu/abs/2010ApJ...712..692Chttp://dx.doi.org/10.1088/0004-637X/752/1/51https://ui.adsabs.harvard.edu/abs/2012ApJ...752...51Chttp://dx.doi.org/10.1111/j.1365-2966.2009.14450.xhttps://ui.adsabs.harvard.edu/abs/2009MNRAS.394.1229Chttp://dx.doi.org/10.3847/1538-4357/aaba77http://dx.doi.org/10.1111/j.1365-2966.2011.19837.xhttp://dx.doi.org/10.1111/j.1365-2966.2007.11549.xhttp://adsabs.harvard.edu/abs/2007MNRAS.376.1033Chttp://dx.doi.org/10.1111/j.1365-2966.2010.16506.xhttp://dx.doi.org/10.1111/j.1365-2966.2010.16506.xhttps://ui.adsabs.harvard.edu/abs/2010MNRAS.405.1409Chttp://dx.doi.org/10.3847/1538-4365/228/2/14http://dx.doi.org/10.1086/420988https://ui.adsabs.harvard.edu/abs/2004AJ....127.3192Chttp://dx.doi.org/10.1088/2041-8205/738/2/L17https://ui.adsabs.harvard.edu/abs/2011ApJ...738L..17Chttp://dx.doi.org/10.1088/0004-637X/741/1/28http://ads.inasan.ru/abs/2011ApJ...741...28Chttp://dx.doi.org/10.1088/0004-637X/759/2/98http://adsabs.harvard.edu/abs/2012ApJ...759...98Chttp://dx.doi.org/10.1051/0004-6361/201526815http://adsabs.harvard.edu/abs/2015A%26A...584A..34Chttp://dx.doi.org/10.1051/0004-6361/201629292http://adsabs.harvard.edu/abs/2016A%26A...593L...6Chttp://adsabs.harvard.edu/abs/2016A%26A...593L...6Chttp://dx.doi.org/10.1051/0004-6361/201731415http://ads.inasan.ru/abs/2018A%26A...610A...5Chttp://dx.doi.org/10.1051/0004-6361/201833653https://ui.adsabs.harvard.edu/abs/2019A&A...623A..89Chttp://dx.doi.org/10.1093/mnrasl/slx021http://adsabs.harvard.edu/abs/2017MNRAS.468L..31Dhttp://dx.doi.org/10.3847/1538-3881/ab089dhttps://ui.adsabs.harvard.edu/abs/2019AJ....157..168Dhttp://dx.doi.org/10.1086/507578http://adsabs.harvard.edu/abs/2006ApJ...650..644Ehttp://dx.doi.org/10.1088/0004-637X/799/2/226http://adsabs.harvard.edu/abs/2015ApJ...799..226Ehttp://dx.doi.org/10.1086/421003http://adsabs.harvard.edu/abs/2004AJ....127.3394Fhttp://dx.doi.org/10.1088/2041-8205/764/2/L21http://adsabs.harvard.edu/abs/2013ApJ...764L..21Fhttps://ui.adsabs.harvard.edu/abs/1998A&A...338..161Fhttp://dx.doi.org/10.1051/0004-6361/201629272https://ui.adsabs.harvard.edu/abs/2016A&A...595A...1Ghttp://dx.doi.org/10.1093/mnras/202.4.1025https://ui.adsabs.harvard.edu/abs/1983MNRAS.202.1025Ghttp://dx.doi.org/10.1051/0004-6361/201628743https://ui.adsabs.harvard.edu/abs/2016A&A...591A.143Ghttp://dx.doi.org/10.1086/151605http://adsabs.harvard.edu/abs/1972ApJ...176....1Ghttp://dx.doi.org/10.1051/0004-6361/201731494http://adsabs.harvard.edu/abs/2017A%26A...608L...1Hhttp://dx.doi.org/10.1051/0004-6361/201321397http://adsabs.harvard.edu/abs/2013A%26A...560A.109Hhttp://dx.doi.org/10.1093/mnrasl/slw083http://adsabs.harvard.edu/abs/2016MNRAS.460L..89Khttp://dx.doi.org/10.1088/0004-637X/769/2/105https://ui.adsabs.harvard.edu/#abs/2013ApJ...769..105Khttps://ui.adsabs.harvard.edu/#abs/2013ApJ...769..105Khttp://dx.doi.org/10.1134/S1990341314020011http://dx.doi.org/10.1134/S1990341314020011http://adsabs.harvard.edu/abs/2014AstBu..69..121Khttp://dx.doi.org/10.1093/mnras/stw1452https://ui.adsabs.harvard.edu/#abs/2016MNRAS.461.2068Khttp://dx.doi.org/10.1093/mnras/sty3268https://ui.adsabs.harvard.edu/abs/2019MNRAS.483.2413Khttp://dx.doi.org/10.3847/1538-4357/aa62a5http://adsabs.harvard.edu/abs/2017ApJ...837..162Khttp://dx.doi.org/10.1126/science.1067524http://dx.doi.org/10.1093/mnras/208.4.687http://adsabs.harvard.edu/abs/1984MNRAS.208..687Lhttp://dx.doi.org/10.1086/157917http://adsabs.harvard.edu/abs/1980ApJ...237..692Lhttps://books.google.ru/books?id=AEwDbHp50FgChttps://books.google.ru/books?id=AEwDbHp50FgChttp://dx.doi.org/10.1051/0004-6361:200400044http://adsabs.harvard.edu/abs/2004A%26A...425..881Lhttp://dx.doi.org/10.1088/0004-637X/738/2/187http://adsabs.harvard.edu/abs/2011ApJ...738..187Lhttp://dx.doi.org/10.1088/2041-8205/789/2/L30http://adsabs.harvard.edu/abs/2014ApJ...789L..30Lhttp://dx.doi.org/10.1088/0004-637X/737/1/8http://adsabs.harvard.edu/abs/2011ApJ...737....8Lhttp://dx.doi.org/10.1051/0004-6361/201423706https://ui.adsabs.harvard.edu/abs/2014A&A...567A.106Lhttp://dx.doi.org/10.1093/mnras/stz1521https://ui.adsabs.harvard.edu/abs/2019MNRAS.489..176Mhttp://dx.doi.org/10.1146/annurev.aa.31.090193.003043https://ui.adsabs.harvard.edu/abs/1993ARA&A..31..575Mhttp://dx.doi.org/10.3847/1538-3881/aa784dhttps://ui.adsabs.harvard.edu/abs/2017AJ....154...94Mhttp://dx.doi.org/10.1051/0004-6361/201423496http://adsabs.harvard.edu/abs/2014A%26A...570A..13Mhttp://dx.doi.org/10.1051/0004-6361/201935464https://ui.adsabs.harvard.edu/abs/2019A&A...629A..12Mhttp://dx.doi.org/10.1093/mnras/sty1373https://ui.adsabs.harvard.edu/abs/2018MNRAS.479..211Mhttp://dx.doi.org/10.1088/0004-637X/722/1/112http://adsabs.harvard.edu/abs/2010ApJ...722..112Mhttp://dx.doi.org/10.1051/0004-6361/201219714https://ui.adsabs.harvard.edu/abs/2012A&A...548A.127Mhttp://dx.doi.org/10.1088/2041-8205/804/1/L9http://adsabs.harvard.edu/abs/2015ApJ...804L...9Mhttp://dx.doi.org/10.1051/0004-6361:20021128http://ads.inasan.ru/abs/2002A%26A...394...89Nhttp://dx.doi.org/10.3847/0004-637X/817/2/109https://ui.adsabs.harvard.edu/abs/2019arXiv190901033Phttp://dx.doi.org/10.1093/mnras/stx1205http://adsabs.harvard.edu/abs/2017MNRAS.470..427Phttp://dx.doi.org/10.1051/0004-6361/201833193https://ui.adsabs.harvard.edu/abs/2019A&A...623A..19Phttp://dx.doi.org/10.1051/0004-6361/201935154https://ui.adsabs.harvard.edu/abs/2019A&A...625A..95Phttp://dx.doi.org/10.1051/0004-6361:20035932http://adsabs.harvard.edu/abs/2004A%26A...422..465Phttp://dx.doi.org/10.1086/316818https://ui.adsabs.harvard.edu/abs/2000AJ....120.2513Phttp://dx.doi.org/10.1088/0004-637X/753/2/128http://adsabs.harvard.edu/abs/2012ApJ...753..128Phttp://adsabs.harvard.edu/abs/2012ApJ...753..128Phttp://dx.doi.org/10.1051/0004-6361/201015224


530, A10

Qu Y., Di Matteo P., Lehnert M. D., van Driel W., Jog C. J.,

2011b, A&A, 535, A5

Quinn P. J., Hernquist L., Fullagar D. P., 1993, ApJ, 403, 74

Rodriguez-Gomez V., et al., 2015, MNRAS, 449, 49

Roškar R., Debattista V. P., Loebman S. R., 2013, MNRAS, 433,

976

Saburova A. S., 2018, MNRAS, 473, 3796

Saburova A. S., Chilingarian I. V., Katkov I. Y., Egorov O. V.,

Kasparova A. V., Khoperskov S. A., Uklein R. I., VozyakovaO. V., 2018, MNRAS, 481, 3534

Sarzi M., et al., 2018, A&A, 616, A121

Schönrich R., Binney J., 2009, MNRAS, 399, 1145

Sil’chenko O. K., Chilingarian I. V., Sotnikova N. Y., Afanasiev

V. L., 2011, MNRAS, 414, 3645

Sil’chenko O. K., Proshina I. S., Shulga A. P., Koposov S. E.,

2012, MNRAS, 427, 790

Spitzer Lyman J., 1942, ApJ, 95, 329

Spitzer Jr. L., Schwarzschild M., 1953, ApJ, 118, 106

Springob C. M., Haynes M. P., Giovanelli R., Kent B. R., 2005,ApJS, 160, 149

Stinson G. S., et al., 2013, MNRAS, 436, 625

Thomas D., Maraston C., Bender R., Mendes de Oliveira C., 2005,ApJ, 621, 673

Tsikoudi V., 1979, ApJ, 234, 842

Van Dokkum P. G., 2001, PASP, 113, 1420

Van der Kruit P. C., Searle L., 1981, A&A, 95, 105

Vazdekis A., et al., 2015, MNRAS, 449, 1177

Villalobos Á., Kazantzidis S., Hel

Documents

lenticular galaxy NGC7572 - arXiv.org e-Print archiveof lenticular galaxies. Years later, two coplanar subsystems with drastically di erent dynamical and stellar population properties