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Astronomy& Astrophysicsmanuscript no. ClashMetalsAstroPhJAN2016 c©ESO 2018September 12, 2018

CLASH-VLT: Strangulation of cluster galaxies inMACS J0416.1-2403 as seen by their chemical enrichment ⋆

C. Maier1, U. Kuchner1, B. L. Ziegler1, M. Verdugo1, I. Balestra2, M. Girardi2, 3, A. Mercurio4, P. Rosati5, A. Fritz6,C. Grillo7, M. Nonino2, and B. Sartoris2, 3

1 University of Vienna, Department of Astrophysics, Tuerkenschanzstrasse 17, 1180 Vienna, Austriae-mail:[email protected]

2 INAF - Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11, I-34133, Trieste, Italy3 Dipartimento di Fisica, Università degli Studi di Trieste,via Tiepolo 11, I-34143 Trieste, Italy4 INAF - Osservatorio Astronomico di Capodimonte, Via Moiariello 16, I-80131 Napoli, Italy5 Dipartimento di Fisica e Scienze della Terra, Università diFerrara, Via Saragat 1, I-44122 Ferrara, Italy6 INAF - Istituto di Astrofisica Spaziale e Fisica cosmica (IASF) Milano, via Bassini 15, I-20133 Milano, Italy7 Dark Cosmology Centre, Niels Bohr Institute, University ofCopenhagen, Juliane Maries Vej 30, DK-2100 Copenhagen, Denmark

Received ; accepted

ABSTRACT

Aims. Environmental effects gain importance as large scale structures in the Universe develop with time and have become the dominantmechanism for quenching galaxies of intermediate and low stellar masses at lower redshifts. Therefore, clusters of galaxies atz < 0.5are the sites where environmental effects are expected to be more pronounced and feasible to be observed with present-day largetelescopes.Methods. We explore the Frontier Fields cluster MACS J0416.1-2403 atz = 0.3972 with VIMOS/VLT spectroscopy from theCLASH-VLT survey covering a region which corresponds to almost three virial radii. We measure fluxes of [O II]λ3727, Hβ,[O III] λ5007, Hα, and [N II]λ6584 emission lines of cluster members enabling us to unambiguously derive O/H gas metallici-ties, and also star formation rates from extinction-corrected Hα fluxes. We compare our cluster galaxy sample with a field sample atz ∼ 0.4 drawn from zCOSMOS.Results. The 76 galaxies of our cluster sample follow the star-forming metallicity sequence in a diagnostic diagram disentanglingionizing sources. We provide and confirm the reliability of new O/H calibrations for star-forming sequence galaxies based onthe useof either the [OIII]/Hβ or the [NII]/Hα line ratio. For intermediate massses we find a similar distribution of cluster and field galaxiesin the mass vs. metallicity and mass vs. sSFR diagrams. An in-depth investigation furthermore reveals that bulge-dominated clustergalaxies have on average lower sSFRs and higher O/Hs compared to their disk-dominated counterparts. We use the location of galaxiesin the projected velocity vs. position phase-space to separate our cluster sample into a region of objects accreted longer time ago anda region of recently accreted and infalling galaxies. We finda higher fraction of accreted metal-rich galaxies (63%) compared to thefraction of 28% of metal-rich galaxies in the infalling regions. Intermediate mass (9.2 < log(M/M⊙) < 10.2) galaxies falling into thecluster for the first time are found to be in agreement with predictions of the fundamental metallicity relation (FMR). Incontrast, foralready accreted star-forming galaxies of similar masses,we find on average metallicities higher than predicted by themodels. Thistrend is intensified for accreted cluster galaxies of the lowest mass bin (log(M/M⊙) < 9.2), that display metallicities two to three timeshigher than predicted by models with primordial gas inflow. Environmental effects therefore strongly influence gas regulations andcontrol gas metallicities of log(M/M⊙) < 10.2 cluster galaxies. We also investigate chemical evolutionary paths of model galaxieswith and without inflow of gas showing that strangulation is needed to explain the higher metallicities of accreted cluster galaxies.Our results favor a strangulation scenario in which gas inflow stops for galaxies with log(M/M⊙) < 10.2 when accreted by the cluster.

Key words. Galaxies: evolution – Galaxies: clusters: individual: MACS J0416.1-2403 – Galaxies: star formation – Galaxies: abun-dances – Galaxies: structure – Galaxies: morphologies

1. Introduction

Gas-phase metallicites in galaxies are regulated by the com-plex interplay between star formation, accretion of metal-poorgas and galactic outflows of processed gas in galaxies, represent-ing their "current" state of chemical enrichment. Comprehensivestudies of large samples in the local Universe have established atight relation between stellar mass and metallicity showing thatthe brighter and hence more massive a galaxy is, the greater is

⋆ Based on observations obtained at the European Southern Obser-vatory (ESO) Very Large Telescope (VLT), Paranal, Chile; ESO largeprogram 186.A-0798

its gas metallicity, as measured by the O/H abundance (e.g., us-ing SDSS, Tremonti et al. 2004; Kewley & Ellison 2008). Oneexplanation for the origin of this relation has been summa-rized by Lilly et al. (2013, Li13 in the following), dubbed "thebathtub model". The star formation rate (SFR) is thereby inti-mately linked to the mass of gas in the galaxy which is mod-ulated by infalling gas, and outflows are driven by supernovaewinds. In that scenario a relation between metallicity, SFRand stellar mass can be derived. Several authors have studiedthe role of the SFR and of the specific SFR (sSFR=SFR/M∗)in this context and presented links between SFR and metal-licity (e.g., Ellison et al. 2008; Lara-López et al. 2010), finally

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claiming an epoch-independent fundamental metallicity rela-tion (FMR) between metallicity, mass and SFR, Z(M∗,SFR), ex-pected to be applicable at all redshifts (Mannucci et al. 2010).Recenty, Salim et al. (2014) reconciled conflicting resultsre-garding this SFR dependence showing that the metallicity isanti-correlated with sSFR for SDSS local galaxies, regardless ofthemetallicity and SFR indicators used.

The evolution of galaxies is governed internally by their stel-lar mass and externally influenced by their environment (e.g.,Peng et al. 2010, 2012) made visible through well establishedchanges of observable properties of cluster galaxies that affectthe star formation histories (SFHs), gas- and stellar compo-nents. Therefore, examining the interrelationships between stel-lar mass, environment, metallicity and stellar structure,and theirevolution with time will reveal the physical processes thatcon-trol the evolution of galaxies. Since environmental effects getstronger atz < 0.5 as large scale structure in the Universe de-velops (see, e.g., Fig. 15 in Peng et al. 2010), clusters of galax-ies atz < 0.5 are the sites where influences of the environmentshould be easier to be detected with the available present-daylarge telescopes. It is well established that specific cluster phe-nomena influence star formation and the exchange of gas, e.g.,by supressing gas inflow. Star formation and gas inflow influencegas metallicities, therefore it is expected that gas metallicities de-pend on environment.

According to previous works in thelocal Universe, usingSDSS galaxies, little effects of the environment on the mass-metallicity relation (MZR) were found. Defining environmentby local densities, Mouhcine et al. (2007) reported variation inmetallicity, at a given mass, ranging from 0.02 dex for massive,to 0.08 dex for galaxies with logM/M⊙ < 9.5, occurring overa contrast factor of 100. Another investigation of 1318 galaxiesin cluster environments found a comparable increase of 0.04dexin oxygen abundances for galaxies in clusters compared to thefield (Ellison et al. 2009). This was confirmed numerically byDavé et al. (2011), who used cosmological hydrodynamic simu-lations to investigate environmental effects on the MZR, report-ing a small enhancement of the mean MZR of galaxies in high-density regions by∼ 0.05 dex. On the other hand, Cooper et al.(2008) used the SDSS sample and claimed that there is a strongerrelationship between metallicity and environment (largely drivenby galaxies in high-density regions, such as groups and clusters),such that more metal-rich galaxies favor regions of higher over-density.

The current state and gas content of galaxies is furthermoreexpected to be related to their morphologies. The importanceof structural parameters such as compactness and surface massdensities in setting the gas properties has been recognizedbefore(Saintonge et al. 2012). Galaxies with higher surface mass densi-ties have transformed more of their gas into stars over the courseof their SFH, thereby enhancing the overall gas-phase metallicity(Kauffmann et al. 2003b). The dependence of the MZR on struc-tural parameters leaves its imprint on the scatter of the relation.Tremonti et al. (2004) related higher stellar mass surface den-sities to higher metallicities at fixed stellar mass. Additionally,Ellison et al. (2008) found that at fixed stellar masses, galaxieswith larger sizes (i.e., with lower stellar mass surface densities)have lower metallicities. On the theroretical side, Caluraet al.(2009) studied the MZR by means of chemical evolution mod-els of different morphological types. Their results indicate thatthe the observed MZR is reproduced by considering differentmorphological mixes at different redshifts. The varying strengthof a central bulge governs the classification scheme introducedby Hubble (1926). While connections between the presence ofa

bulge and quenching of star-formation have been reported (e.g.,Saintonge et al. 2012, suggesting that the main driver for gasdepletion in galaxies is bulge mass), the physical mechanismsresponsible for this star-formation shutoff and the enhancementof measured gas metallicities is not yet fully understood. Con-necting the gas metallicities with mass, SFRs and morphologiesas a function of environment and redshift may therefore leadtoincreased knowledge of the temporal evolution of the chemicalproperties of the stellar populations and allow a more qualitativelook into the nature of the galaxies building the MZR.

We can now extend these investigations to intermediate red-shifts, measuring O/H abundances in a very massive cluster atz ∼ 0.4, and contrasting them to comparable measurements ofzCOSMOS field galaxies at similar redshifts. We do this usingthe two connected relations, the MZR and the mass−(s)SFR rela-tion, presenting evolutionary effects betweenz ∼ 0.4 and the lo-cal Universe. To estimate the chemical abundances, a numberofdiagnostics have been developed based on strong emission lines(ELs), [O II] λ 3727, Hβ, [O III] λ 5007, Hα and [NII]λ 6584. Athigher redshifts, these ELs move to the near-infrared, withHαand [NII]λ 6584 already shifting beyond optical above redshiftsof ∼ 0.5, requiring near-infrared spectroscopy to observe theseELs (e.g., Maier et al. 2004, 2005, 2006, 2014, 2015) . Thus, theepoch we probe is the highest redshift at which all these ELscan be observed with the same optical spectrograph, withoutthecomplication of obtaining a good relative calibration betweenspectra from optical and near-infrared spectrographs.

The paper is structured as follows: In Sect. 2 we present theselection of the cluster EL galaxies atz ∼ 0.4 and their VI-MOS spectroscopy. We also discuss the comparison field sam-ples of zCOSMOS and SDSS galaxies. In Sect. 3 we investi-gate the AGN contribution and present the derivation of SFRs,metallicities, morphological parameters, and stellar masses ofthe 76 cluster galaxies we observed. In Sect. 4 we present theMZR and mass−sSFR relation atz ∼ 0.4, and investigate theirdependence on environment and morphological parameters. InSect. 5 we discuss how the cluster environment affects the chem-ical enrichment. We investigate how the SFR impacts the MZRin the cluster environment, and how this compares with predic-tions of the FMR. Several evidences for a strangulation scenariofor intermediate and low mass cluster galaxies are discussed. Fi-nally in Sect. 6 we summarize our conclusions. Aconcordance-cosmology with H0 = 70 km s−1 Mpc−1, Ω0 = 0.25,ΩΛ = 0.75is used throughout this paper. We assume a Salpeter (Salpeter1955) initial mass function (IMF) for all derived stellar massesand SFRs and correct existing measurements used in this paperto a Salpeter IMF. Note thatmetallicity andabundancewill betaken to denoteoxygen abundance, O/H, throughout this paper,unless otherwise specified. In addition, we use dex throughout todenote the anti-logarithm, i.e., 0.3 dex is a factor of two.

2. The data: cluster and field galaxies at z ∼ 0.4

2.1. The cluster MAC0416

MACS J0416.1-2403 (MAC0416 in the following) is a mas-sive, X-ray-luminous (Ebeling et al. 2001) galaxy cluster at z =0.3972. MAC0416 is one of 25 clusters of the HST multi-cycletreasury program “Cluster Lensing And Supernova survey withHubble” (CLASH, Postman et al. 2012). The mass profile mea-sured from the galaxy dynamics (Balestra et al. 2015, B15 in thefollowing) reveals a complex dynamical state of this cluster thatcan be explained by a pre-collisional phase scenario. The inner


C. Maier et al.: MZR in cluster galaxies atz ∼ 0.4

regions of the cluster are nevertheless represented best bya sim-ple isothermal sphere (SIS) model, as described in B15.

CLASH-VLT (Rosati et al. 2014), the spectroscopic follow-up of 13 CLASH clusters at 0.3 < z < 0.6, provides spectra overan area of 25× 25 square arcminutes, using the Low ResolutionBlue (LB in the following) and Medium Resolution (MR in thefollowing) grisms of VIMOS. The details of the LB and MR VI-MOS observations and redshift determinations of MAC0416 aregiven in B15. The observations and data reduction of the MRpointings used for our SFRs and metallicities measurementsaredescribed in Sect. 2.1.2. High quality SUBARU Suprime-CamBRz imaging covers the VIMOS field of 25× 25 square arcmin-utes area (Umetsu et al. 2014). For our metallicity study we se-lected this particular cluster from the CLASH sample due to itsideal redshift (z ∼ 0.4), which places all five strong ELs in aregion not affected by night skylines, allowing measurements ofall ELs needed in one spectrum using the MR grism of VIMOS.

The cluster members identification was described in B15,adopting as the center of the cluster the position of one of thetwo brightest cluster galaxies (BCGs), namely the NE-BCG,position which coincides with the peak of the X-ray emission(see Ogrean et al. 2015). B15 obtained a velocity dispersionσ ∼ 1000 km/s, virial radiusR200 ∼ 1.8 Mpc, and a virial massof M200 ∼ 1× 1015 M⊙ for MAC0416, where the ’200’ subscriptrefers to the radius where the average density is 200 times themean density of the Universe at the cluster redshift. We consideras cluster members all galaxies within a larger range of±3σ ofthe velocity dispersion, because we want to study environmentaleffects by dividing the sample in “infalling” and “accreted longertime ago” cluster galaxies, as described later in Sect. 5.

2.1.1. Selection of the cluster galaxies for our metallicitystudy

The target selection for the LB VIMOS pointings was based oncolor-color diagrams (as described in B15). The spectroscopicredshifts derived from the [O II]λ 3727 ELs as delivered by theVIMOS LB grism observations allowed us to select an appropri-ate target sample, with ELs free of strong telluric emission, forthe MR grism observations which provide higher resolution andlarger (including redder) wavelength coverage than the LB spec-tra. The target sample contained 165 blue cluster member galax-ies with the four ELs Hβ, [O III] λ 5007, Hα, and [N II]λ 6584,that are necessary for the derivation of reliable metallicities, freeof contamination by night skylines. 82 galaxies of this sam-ple were observed using the MR grism, out of which 60 havea signal-to-noise (S/N) higher than two in the weakest EL de-tected (Hβ, [O III] λ 5007, or [N II]λ 6584). Additionally, somegalaxies without LB spectra were selected based on color-colorcriteria and observed with the MR grism, without being part ofthe parent sample of 165 blue cluster members. 16 galaxies outof these turned out to have five ELs with high enough S/N formetallicity studies resulting in a total sample of 76 cluster galax-ies with five ELs detected (sampleZgals).

Fig. 1 demonstrates the uniform coverage of our sampleZgals with respect to the parent sample of all spectroscopicblue cloud galaxies in both luminosities and masses. The fig-ure shows the color-magnitude and color-mass diagrams of thespectroscopic member galaxies of the blue cloud (cyan filledcir-cles) and the 76Zgals objects used for our metallicity study(blue symbols). The fractions indicate that ourZgals sample isa good representation of the overall blue (star-forming) popula-tion of cluster galaxies, especially at intermediate masses. Tab. 1lists the percentages of all spectroscopic members representative

for the blue cloud (first row) in four luminosity, mass and colorbins, as well as the percentage of theZgals objects in the samebins (second row) and their fractions (third row), as visualized inFig. 1.

2.1.2. MR VIMOS observations and data reduction

The main target galaxies were observed in four VIMOS point-ings with the MR grism and GG475 filter, with exposure times of60 min each, and masks designed with 1 arcsec slits. The coveredwavelength range was 480-1000nm with a resolutionR = 580,thereby including ELs from [O II]λ 3727 to [N II]λ 6584 ofz ∼ 0.4 galaxies.

Data reduction of the VIMOS spectra of MAC0416 galax-ies was carried out using the Vimos Interactive Pipeline Graph-ical Interface (VIPGI; Scodeggio et al. 2005) software package.VIPGI uses automated procedures for bias subtraction, flux andwavelength calibration of the spectra, identification of objectsin the slit profile, and extraction of the one-dimensional spectra.Wavelength calibration uses a HeNeAr arc lamp exposure ob-tained immediately following the science exposures. The stan-dard star LTT-1788 was used for flux calibration. The determi-nation of redshifts of target galaxies is unambiguous, due to the5 ELs observed with the MR grism of VIMOS.

2.2. Comparison sample: zCOSMOS field galaxies at z ∼ 0.4

As our comparison sample of field galaxies, we selected 93zCOSMOS-bright (Lilly et al. 2007, 2009) objects at similarred-shifts as the cluster galaxies, namely 0.373 < z < 0.413. Theredshift bin is similar to the redshifts of cluster galaxies, andensures that all ELs are observed free of strong OH night sky-lines. These objects have four ELs, Hβ, [O III] λ 5007, Hα, and[N II] λ 6584, measured with VIMOS, and stellar masses derivedfrom the SED fitting based on 30 COSMOS bands (Ilbert et al.2009). For details of the measurements of zCOSMOS EL fluxeswe refer to Maier et al. (2009), Pérez-Montero et al. (2013) andMaier et al. (2015).

2.3. Comparison local sample: SDSS

We selected a local comparison sample of SDSSgalaxies from the DR4 release, Garching repository(http://www.mpa-garching.mpg.de/SDSS/DR4/), in the red-shift range 0.04< z < 0.08. The lower redshift limit ofz = 0.04was chosen following Kewley et al. (2005), to reduce the sys-tematic and random errors in the SFRs that arise from apertureeffects due to the 3 arcsec size of the SDSS fibers. Duplicateobjects, and SDSS galaxies on problematic or special plateswere excluded from the SDSS sample; since this exclusion wasbased solely on position on the sky, it should not be related toany galaxy properties.

We excluded AGNs from the SDSS sample using the[O III] λ 5007/ Hβ versus [N II]λ 6584/Hα diagnostic diagram,excluding objects that satisfy Eq. (1) of Kauffmann et al. (2003).The evolution of the sSFR shows an average decrease by a factorof about two fromz ∼ 0.4 until the epoch of the SDSS sample, atredshifts 0.04< z < 0.08 (see also Sect. 4.1). Therefore, a lowerlimit in SFR of 0.3M⊙/yr for our Zgals MAC0416 sample atz ∼ 0.4 corresponds to a lower limit of 0.15M⊙/yr for the SDSSsample. To ensure a consistent comparison of the physical prop-erties of the SDSS andZgals samples, we applied an additionalS/N > 10 cut in Hα for SDSS galaxies like for theZgals sam-


http://www.mpa-garching.mpg.de/SDSS/DR4/


ple, and only SDSS galaxies with SFRs larger than 0.15M⊙/yrare used. Therefore, it should be noted that this sample is slightlydifferent from other local comparison SDSS samples often usedin the literature like, e.g., the samples of Tremonti et al. (2004)or Mannucci et al. (2010).

3. The Measurements

3.1. EL fluxes

The EL fluxes of MAC0416 cluster member galaxies weremeasured interactively using the packagesplot in IRAF. Theflux errors were usually dominated by systematic uncertaintiesin establishing the local continuum, which was conservativelyestimated by exploring rather extreme possibilities. We assumedthat the flux of [O III]λ 4959 is 0.34 of the flux of [O III]λ 5007.As in Maier et al. (2005) for data with similar spectral resolu-tion, we assumed an average underlying stellar absorption in Hβof 3 ± 2Å and corrected the restframe equivalent width of thisline by this amount, thereby increasing the Hβ line flux. We notethat all Hα ELs of Zgals cluster galaxies have aS/N > 10, and70 out of 76 galaxies (92% of the sample) have aS/N > 3 in[N II] λ 6584.

3.2. BPT diagramm

We use the BPT (Baldwin et al. 1981) diagram shown inFig. 2, to establish if the source of gas ionization is of stellarorigin, or rather associated with AGN activity. The MAC0416Zgals cluster galaxies and also all zCOSMOS field galaxiesat z ∼ 0.4 lie under and to the left of the theoretical (solid)curve of Kewley et al. (2001) and the empirical (dashed) curveof Kauffmann et al. (2003), which separate star-forming galax-ies from AGNs. This indicates that in all of them the domi-nant source of ionization of the gas is recent star formation.Moreover, most galaxies lie on the star-forming abundance se-quence with rough boundaries indicated by the thick dashedlines. zCOSMOS field galaxies which all have log(M/M⊙) > 9.2andZgals cluster galaxies of similar masses (blue symbols) liein a similar, lower part of the BPT diagram. Cluster galaxieswithlower masses log(M/M⊙) < 9.2 (yellow symbols) have typi-cally higher [O III]λ 5007/Hβ ratios, which correspond to lowermetallicities.

3.3. Oxygen abundances

Eq. 4 in Kewley et al. (2013), 12+ log(O/H) = 8.97− 0.32×log(([OIII] /Hβ)/([NII] /Hα)), corresponds to the Pettini & Pagel(2004) O3N2 calibration for the Kewley & Dopita (2002) metal-licity scale. The only difference to the Pettini & Pagel (2004)O3N2 calibration is that the first (normalization) factor is8.97instead of 8.73. We use this equation to consistently computemetallicities for theZgals MAC0416 cluster objects, the zCOS-MOS field galaxies atz ∼ 0.4, and for the SDSS sample.

There are several reasons for this choice of metallicity cali-bration. First, the zCOSMOS sample atz ∼ 0.4 does not have[OII] measured, but only the other four ELs at redder wave-lengths. Second, it is more straightforward to compute O3N2metallicities, than using aχ2 method which is based on compar-ing five measured ELs with theoretical predictions of the fluxesfor given metallicities and extinction values. This lattermethodcan be affected by gradients and degeneracies of the theoreticalmodels (see discussion in Maier et al. 2005). Third, using O3N2the comparison with other literature data is easier, because the

(χ2) approach to simultaneously fit all the five strong ELs anduse theoretical models to generate a probability distribution ofmetallicities and statistically estimate the oxygen abundances isvery seldom used in the literature. Fourth, as discussed aboveand also below in Sect. 3.4, most galaxies in this study follow astar-forming abundance sequence in the BPT-diagram, indicat-ing that some line ratios are related to each other, a fact whichtheχ2-method does not take into account.

3.4. New O/H calibrations for galaxies on the star-formingabundance sequence

At higher redshifts galaxy samples often do not have all ELsmeasured which are ideally needed to derive O/Hs. Therefore,O/H calibrations based on less ELs have to be used. Here weprovide such calibrations based on two ELs for galaxies whichfollow the star-forming sequence in the BPT diagram, like ourZgals sample.

In the appendix of Maier et al. (2015) it was discussed howthe O3N2 calibration is related to the BPT-diagram (see theirFig.A.1). For galaxies which follow the star-forming abundancesequence we used Eqs. 3 and 4 of Kewley et al. (2013) to derivecalibrations for O/H as a function of [OIII]/Hβ:

12+ log(O/H) = 8.97− 0.32×(log([OIII] /Hβ) − 0.61/(log([OIII] /Hβ) − 1.1)+ 0.08)

(1)

and for O/H as a function of [NII]/Hα:

12+ log(O/H) = 8.97− 0.32×(1.1+ 0.61/(log([NII] /Hα) + 0.08)− log([NII] /Hα))

(2)

We point out that Eq. 2 is similar in its form to the N2-calibration of Salim et al. (2015), which re-calibrated N2 forthe SDSS sample. Our Eq. 2 corresponds to their Eq.4 witha = 8.618, b = 0.32, c = −0.1952, and d= −0.08.

Fig. 2 shows that ourZgals cluster galaxies lie on the star-forming sequence, whose rough boundaries are indicated by thedashed lines. Thus, we can use the O/H measurements of theZgals sample to test our new O/H calibrations. Fig. 3 shows agood agreement for theZgals objects between the O3N2 methodand the O/Hs obtained with Eq. 1, and also with Eq. 2, except atthe highest O/Hs where it is known that the N2-method saturates.

3.5. Stellar Masses

Stellar masses of the MAC0416 galaxies were calculated us-ing the codeLephare of Arnouts & Ilbert (2011), which fits stel-lar population synthesis models (Bruzual & Charlot 2003) totheavailable photometry (SUBARU B, R and z-bands). This codeis a simpleχ2 minimization algorithm that finds the best matchof templates for the given data. The limited number of filtersre-sults in a reduction of used templates. In particular, we do notfit templates with stellar ages of less than 1 Gyr and larger than11 Gyrs. This is a valid assumption since atz = 0.4 the Universeis ∼4 Gyrs younger than today, and a considerable fraction ofthe stellar mass assembly is already completed. In addition, wekept the redshift fixed and also limited the number of extinctionE(B−V) values (0.1, 0.3, 0.5) to avoid over-fitting. We are confi-dent in the robustness of the calculated masses since theB − R



color encompasses the redshifted 4000Å break and thus is sen-sitive to galaxy mass-to-light ratios (Kauffmann et al. 2003b).

As described in Maier et al. (2015), the stellar masses forzCOSMOS galaxies were derived by fiting photometric datapoints with the Bruzual & Charlot (2003) synthetic stellar popu-lation models, and picking the best-fit parameters by minimizingthe χ2 between observed and model fluxes. The stellar masseswere obtained by integrating the SFR over the galaxy age, andcorrecting for the mass loss in the course of stellar evolution.Thus, the stellar masses of zCOSMOS galaxies are comparableto the stellar masses of MAC0416 galaxies, which are also basedon SED fitting using Bruzual & Charlot (2003) models. We con-verted the masses from a Chabrier (2003) IMF to a Salpeter(1955) IMF (as used in this paper) by multiplying the Chabrier(2003) IMF masses by 1.7. This factor of 1.7 was found byPozzetti et al. (2007) to be a systematic median offset, with avery small dispersion, in the masses derived with the two dif-ferent IMFs, being rather constant for a wide range of SFHs.Regarding the consistency of SDSS stellar masses with stellarmasses derived from SED fitting, we note that Moustakas et al.(2013) showed the comparison between the two methods of es-timating stellar masses indicating that they are consistent withinthe uncertainties.

3.6. Morphologies

The decomposition of galaxies into their main stellar compo-nents is done by analyzing the light distribution with the sum oftwo Sérsic profiles with different indices (e.g., a DeVaucouleursbulge and an exponential disk). To quantify stellar structures ofMAC0416 galaxies, we measure parameters such as Sérsic in-dices, sizes, and luminosities, globally for the entire galaxy aswell as for bulges and disks separately for all available bands(SUBARU B, R and z) with the new galaxy profile model fittingsoftware Galapagos-2 developed within the MegaMorph project(Häußler et al. 2013). It performs a 2D modeling of the galaxyflux assuming predefined light distributions for all bands avail-able simultaneously. Galapagos-2 is constructed in a way toeffi-ciently handle modern large multi-wavelength imaging surveysand is thus ideally suited to analyze the CLASH data. It uti-lizes GALFIT-M (Bamford et al. 2011), a recently developedmulti-wavelength version of the two-dimensional fitting algo-rithm GALFIT (Peng et al. 2010b). This novel approach wassuccessfully tested on both ground-based and simulated data(Häußler et al. 2013; Vika et al. 2014) and leads to an increaseof the stability and accuracy of measured parameters down tofainter limits in comparison to the widely used single-bandfits.This is especially important for the measurements of groundbased SUBARU imaging for this study.

To ensure reliable morphologies, we compare our SUBARU-based classification step by step with HST high-resolutionCLASH data. However, we note that none of the galaxies inour Zgals sample is in the central part of the cluster covered byHST. We therefore analyze an independent sample of about 100overlapping galaxies of the central cluster region and find that10% (15%) of the galaxies are misclassified using bulge-to-diskdecompositions (single Sérsic fits). Whether we classify basedon single Sérsic fit or bulge-to-disk decomposition only changesthe outcome in 5% of the cases (based on measurements of 820galaxies in CLASH clusters covered with SUBARU, Kuchner etal. in prep.). With F-statistics following Simard et al. (2011) wecompare the residual images of one- and two-component fits todecide which model better represents the light distribution of aparticular galaxy. To classify the cluster galaxies, we expand the

decision tree of Neichel et al. (2008) (adopted by others too, e.g.Nantais et al. 2013) to our needs and include the new capabilitiesof Galapagos-2 with varying sets of bulge and disk profiles, aswell as color information and visual classifications. This leads to4 distinct classes: compact, peculiar, disk-like and objects with asmooth, regular light distribution.

Specifically, we follow three main steps for the morpho-logical analysis of theZgals sample: i) we perform surfacebrightness profile measurements with Galapagos-2, using threeSUBARU-bands, for B/T and n; ii) we construct RGB colormaps for every GALFIT-M stamp; iii) we label each galaxy mor-phologically based on visual inspection in both the individualimages and the color map. The classification starts with divid-ing the sample of star-forming galaxies into three broad B/T bins(B/T<0.5, 0.5<B/T<0.8 and B/T>0.8), with∼3% incorrect mea-surements in the general comparison to HST in the overlap re-gion. Each stamp is then visually inspected for spiral features,color gradient and offsetting nuclei (see Nantais et al. 2013, fordetails). Due to the low resolution of the SUBARU imaging,only the most striking spiral features are seen, a smooth appear-ance is common. Therefore we opted for calling high B/T ob-jects without any visible featuressmooth. The reader should re-member however, that these are still star-forming galaxies, with aprominent bulge and no obvious signs of spiral structures. Galax-ies with effective radii re< 3 kpc (corresponding to less than0.55 arcseconds atz ∼ 0.4) are classified ascompact. Low B/T(or low Sérsic index) galaxies, and those with spiral features areclassified asdisk galaxies, galaxies with visible disturbances intheir stellar structure are calledpeculiar.

3.7. SFRs

One of the most reliable and well calibrated SFR indicators isthe Hα EL. The conversion of Hα luminosity to SFR requiresseveral steps. To correct for slit losses forZgals MAC0416 clus-ter galaxies we convolve each VIMOS spectrum with the SUB-ARU Cousins R-band filter and then compare this magnitudewith the SUBARU R-band magnitude of the respective galaxy.The difference between the two magnitudes gives the aperturecorrection factor for each spectrum. This correction assumes thatthe Hα line flux and the R-band continuum suffer equal slitlosses. Thus revised Hα line luminosities L(Hα) are correctedfor extinction based on the Balmer decrement and then trans-formed into SFRs by applying the Kennicutt (1998) conversion:SFR(M⊙yr−1) = 7.9× 10−42L(Hα)ergs/s.

The SFRs of the zCOSMOS sample atz ∼ 0.4 and of SDSSgalaxies are also computed from extinction corrected Hα lu-minosities. For the zCOSMOS galaxies an aperture correctionbased on the observed I-band magnitude was applied to eachVIMOS spectrum to take into account slit losses, as describedin Maier et al. (2009). The SDSS SFRs were corrected for fiberlosses using the aperture corrections given by Brinchmann et al.(2004), corrections which work forz > 0.04, as demonstrated byKewley et al. (2005).

4. Results: Mass-metallicity and mass-sSFRrelations in field and cluster

We discuss several aspects of galaxy evolution in differentenvironments, each presented in the framework of the relationsbetween mass and sSFR (Sect. 4.1) and mass and metallicity(Sect. 4.2). We present evolutionary effects between redshiftsz ∼ 0.4 andz ∼ 0, establishing the MZR atz ∼ 0.4. We then



sub-divide the samples to study effects of the cluster and fieldenvironments. Finally, we include a more detailed investigationof morphology dependencies of each relation. For a meaning-ful investigation and considering our relatively small sample oftargets, we divide the galaxies into three (physically motivated)mass-bins:

i) Galaxies with Mhigh (log(M/M⊙) > 10.2) lie abovea threshold invoked by chemical downsizing in the mass-metallicity plane, manifested in the flattening of the MZR athighstellar masses (c.f. Maier et al. 2015). Furthermore, this highmass-regime is controlled by early-type and bulge-dominatedgalaxies, explaining the low number statistics for our cluster star-forming galaxies (see also Fig. 1).

ii) The intermediate mass bin Mmed (9.2 < log(M/M⊙) <10.2) is well populated by both field and cluster galaxies.

iii) Galaxies below log(M/M⊙) ∼ 9.2, a stellar mass regimewhich we call Mlow, will be considered low-mass objects. We donot have comparison galaxies in the field sample for this massrange due to zCOSMOS-bright selection criteria.

4.1. The mass−sSFR relation at z ∼ 0.4

The sSFR has been found to be a tight but weak function ofmass at all epochs up toz ∼ 2 for (most) star-forming galaxies(main sequence, MS), and to evolve with time increasing by afactor of 2−3 to z ∼ 0.4 compared toz ∼ 0 (e.g., Noeske et al.2007; Elbaz et al. 2007). Peng et al. (2010) derived a formulaof the evolution of the sSFR as a function of mass and timethat we use to calculate the mean sSFR(M) atz ∼ 0.4. Forthis, we assume a dependence of sSFR on mass as observedfor local SDSS galaxies and revised by Renzini & Peng (2015),sSFR∝Mβ with β = −0.24 (Fig. 4, oblique solid black line).The local MS relation was found to have a dispersion of a fac-tor of 0.3 dex about the mean relation (e.g., Salim et al. 2007;Elbaz et al. 2007; Peng et al. 2010), indicated by the gray areaaround the mean MS relation atz ∼ 0.4 in Fig. 4.

The distribution in the mass−sSFR plane of our sampleof field and cluster galaxies atz ∼ 0.4 is comparable (Fig. 4), inparticular in the stellar mass bin Mmed where theZgals samplehas a uniform coverage with respect to the parent sample (seeFig. 1). The median values of the two samplesin this mass rangeand the scatter of the data points are also quite consistent withthe mean MS relation and scatter predicted by Peng et al. (2010)at z ∼ 0.4. In the lower mass bin Mlow there are no field galax-ies from zCOSMOS since the zCOSMOS-bright sample was se-lected to haveI < 22.5, so we cannot study environmental ef-fects on SFRs for lower masses. In contrast, in the highest massbin Mhigh there are only six cluster galaxies in our sample, mostof them showing lower sSFRs than typical MS galaxies at theseredshifts.

Investigating in detail the morphological distribution inoursample of cluster galaxies, a segregation is revealed betweenbulge-dominated on the one hand and disk and peculiar galax-ies on the other (Fig. 5). We measure a difference between thepopulations, by comparing deviations of the median residuals ofthe fits. We find that cluster galaxies with prominent bulges liesystematically lower (>1σ) than the disk star-forming galaxiesthat are well compatible with the MS. Stars in galactic bulgesat z ∼ 0.4 are dominated by an old, red and passive popula-tion (Pérez et al. 2013), accounting little for the overall ongoingstar-formation of the galaxy. As a consequence, our galaxies ofsmooth category, that have rather high B/T values, are expectedto display lower sSFRs. Peculiar galaxies on the other hand lieabove the MS indicating an increase of current star formation

with notably scatter. This type of galaxy is nearly impossible todistinguish through classifications based on single Sérsicn andtherefore not captured in some previous studies.

Numerous observations describe an increase of bulge-dominated galaxies in high-density regions, such as clusters ofgalaxies (e.g., Dressler 1980; Kormendy et al. 2009). The ob-served slope and scatter of the mass−sSFR relation reflect the in-crease in bulge-mass fractions in higher mass galaxies and there-fore a diversity in SFHs (Abramson et al. 2014). Our finding isin compliance with Whitaker et al. (2015), who link the scatterand slope of the MS to galaxy structures, using single Sérsicmeasurements as a proxy for a dominant bulge or disk. For ourwork, we perform galaxy decompositions and therefore directlymeasure the presence and strength of bulges.

We see indication for the same trend of lower sSFRs inlog(M/M⊙) < 10.2 compact galaxies. However, decompositionsinto bulges and disks become less reliable for these small galax-ies in ground-based imaging data (as revealed by our SUBARU- HST comparisons). Therefore, we can only speculate about thepresence of a bulge in compact galaxies. In the local universe,however, Ellison et al. (2008) have previously demonstrated apotential link between galaxy sizes and bulge fractions. Theyfound that galaxies with smaller half light radii (re < 3 kpc) havehigher B/T ratios (B/T>0.4), supporting our hypothesis. Fig. 5indeed shows similar effects of lower sSFRs for smooth (bulge-dominated) and to some extent also for compact galaxies on theone hand, and of higher sSFRs for disk and peculiar types (lowbulge fractions) on the other hand.

4.2. The MZR at z ∼ 0.4 in field and cluster

Fig. 6 shows the MZR of ourZgals sample of 76 MAC0416z ∼ 0.4 cluster members (blue symbols) and 93 zCOSMOS fieldgalaxies (green symbols) compared to SDSS (gray area). Allmetallicities were computed with the same method, as describedin Sect. 3.3. For the matched SDSS sample constructed as de-scribed in Sect. 2.3, we indicate 16th, and 84th percentiles, andthe medians (50th percentiles) of the distribution of O/H valuesin the respective mass bin as solid lines and gray area. We notethat the mean SDSS O/H values in the respective mass bins differfrom the median O/Hs by less than 0.01 dex. We also extrapolatethe MZR of the matched SDSS sample to lower stellar massesby assuming that the slope of the low-mass MZR stays constantfor log(M/M⊙) < 9.2. While both the local and distant sample offield galaxies have stellar masses logM/M⊙ > 9.2, severalZgalscluster galaxies extend down to lower masses log(M/M⊙) ∼ 8.5thanks to the deep spectroscopy of CLASH-VLT.

The polynomial fits (third degree) to the data points (blue andgreen thick lines in the left panel of Fig. 6) and also the medianvalues (right panel) are very similar for cluster and field galaxiesof intermediate masses. We determine an evolution of∼ 0.1 dexon average for these Mmed galaxies. This offset for field galax-ies towards lower metallicities is in agreement with the valuegiven by Pérez-Montero et al. (2013). From their Table 3 of theirzCOSMOS sample at similar redshifts, we infer an evolution of∼ 0.09 dex.

At the highest masses Mhigh most of the zCOSMOS fieldgalaxies have lower O/Hs than the local ones, while the high-mass cluster galaxies (albeit with only a few measurements)ex-hibit higher O/H values, comparable to SDSS metallicities. Thismay indicate that the “chemical downsizing” effect describedin Maier et al. (2015), regarding the time evolution of the massthreshold below which one finds low metallicities, may have adifferent mass/time dependence in field and cluster environment.



This indicative interpretation has to be verified with a larger sam-ple of metallicity measurements for cluster star forming Mhighgalaxies.

In their environmental study of central and satellite SDSSgalaxies, Pasquali et al. (2012) report a small metallicityin-crease of 0.06 dex at logM/M⊙ ∼ 8.5, which decreases to0.004 dex at logM/M⊙ > 9.3, for satellite galaxies in comparisonto central galaxies of the same stellar mass. At higher redshiftswe do not have the benefit of such a large database of metallicitymeasurements and restrictions in mass ranges or smaller sampleslike ours cannot reproduce this accuracy.

In their publication, Pasquali et al. (2012) speculate whetherthe slight increase of the MZR at lower masses found in satellitegalaxies can be explained through strangulation scenarios. Stran-gulation prevents satellite galaxies from accreting new, low-metallicity gas which would otherwise dilute their interstellarmedium. We will further explore this scenario in Sect. 5.1, whenwe discuss the Z(M,SFR) of cluster galaxies.

In a numerical work of environmental effects on metallici-ties, Davé et al. (2011) used cosmological hydrodynamic simu-lations and compared the MZR of model galaxies subdivided bylocal galaxy density. They found that (model) galaxies in high-density regions lie above the mean MZR by∼ 0.05 dex, howeverwith the differences disappearing at the massive end in all mod-els. The general trend of both studies, numerical works and localobservations, are in accordance with our findings: the MZRs ofcluster and field galaxies only differ marginally.

Several studies in the local Universe found a correlationbetween gas-phase metallicites and galaxy morphology, suchthat more bulge-dominated galaxies are more metal rich (e.g.,Vila-Costas et al. 1992; Zaritsky 1993; Zaritsky et al. 1994).Similar trends that connect samples of high concentration(bulge-dominated) galaxies to higher fractions of solar metallic-ities in comparison to low-concentration (more disk-like)sam-ples and lower metallicities were reported by Pasquali et al.(2012). Morphologies in our sample are uniformly distributedover stellar masses and colors within the blue cloud, especiallyin our intermediate mass bin Mmed and any trends found aretherefore unlikely to purely result from the strong correlationsbetween luminosity, color and morphology. The chemical evo-lution of galaxies is connected to the SFH, as well as influencedby the environment through gas interchange with the surround-ings. We therefore expect trends to be intimately linked to the ob-served scatter of the MS and differences in sSFRs. Fig. 7 showsthe median offsets to the local SDSS MZR in mass bins forZgals MAC0416 galaxies, divided by their morphologies. Justlike in the mass−sSFR relation, we find different behaviors forsmooth/compact versus disk/peculiar types. In accordance withpredictions from the FMR the smooth and compact galaxies thathave lower sSFRs display higher metallicities with mean valuesonly slightly below, in some cases comparable (within 1σ) tothe mean local MZR. In contrast, disk-dominated and peculiarobjects (with higher sSFR) exhibit metallicities well below thelocal MZR.

We note that, as discussed in Sect. 4.1, smooth and com-pact galaxies have higher bulge fractions than disk-dominatedand peculiar objects. Simulations have shown that bulges appearto stabilize against inflows of metal-poor gas and nuclear star-bursts driving galactic outflows (e.g Mihos & Hernquist 1994;Miho & Hernquist 1996; Cox et al. 2008). Bulge-dominatedgalaxies like thesmooth galaxies in ourZgals sample there-fore possibly retain their metal-enriched gas more easily.For theother types, likedisk andpeculiar Zgals galaxies, inflowing gas

may be funneled more efficiently to their centers with low bulgefractions.

5. Discussion: Environmental effects and evidencefor starvation

The location of galaxies in projected phase-space (cluster-centric radius vs. line-of-sight velocity relative to the cluster red-shift) provides valuable information of their accretion history:on average, infalling galaxies are spatially separated from viri-alized galaxies. Simulations of mass assembly around clustershave helped to identify these regions in phase-space. Stackedcaustic (isodensity profile) plots of galaxies orbiting the30 mostmassive clusters in the Millenium simulation demonstrate howgalaxies of different accretion histories populate different areasin a characteristic trumpet shaped caustic profile (Haines et al.2015). Recently accreted galaxies (with, in general, higher line-of-sight velocities) are located outside these caustics and are thuswell separated from the dense cluster core, where galaxies weremore likely to have been accreted at earlier times.

For our analysis, we define the rest-frame velocity asvr f =

c(z− zmean)/(1+ zmean), wherezmean = 0.3972 is the mean clusterredshift, and we depict R vs.|vr f | in Fig. 8, with R being the pro-jected radial distance from the cluster center. To separatemassand environmental effects, we concentrate on a limited rangein stellar mass and consider only 53 MAC0416 cluster galax-ies with Mmed (9.2 < log(M/M⊙) < 10.2). The dashed magentaline in Fig. 8 indicates the caustic determined by B15, separat-ing the regions in phase-space of recently accreted and infallinggalaxies (outside caustics) from the region of the galaxiesac-creted longer time ago and possibly virialized population (us-ing the nomenclature of Haines et al. 2015, see also Jaffé et al(2015)). For simplicity, we will call the two classes “infalling”(above and to the right of the magenta dashed line, star symbols)and “accreted” (filled circles).

We note that only 5 out of 18 infalling Mmed galaxies (28%)have higher O/Hs consistent with the local MZR, while thisfraction is higher for accreted Mmed objects, namely 22 out of35 galaxies (63%). As discussed and shown in the following,infalling galaxies with lower metallicities than the localMZR(blue star symbols, representing 72% of the “infalling” popu-lation) can increase their O/Hs by∼ 0.2 dex in tenrich ∼ 1 Gyrreaching the local MZR while they move towards the centralregion of the cluster. This enrichment time is smaller than thecrossing timetcrossing = R200/σ of the cluster of about 2 Gyrs,and also smaller than the 2− 3 Gyrs needed by galaxies in theinfalling region to reach the central, denser part of the cluster,as derived by Haines et al. (2015). This makes a strangulationscenario a plausible mechanism, in which the gas metallicity in-creases because the reservoir of pristine gas is depleted when thegalaxies enter the cluster. We explore further evidences for thisscenario in the following sections.

5.1. The Z(M,SFR) of cluster galaxies at z ∼ 0.4

In order to explain star formation as a second parameter in theMZR, some authors have ascribed an ad-hoc inflow of gas as theresponsible driver for the dilution of metallicity and increase ofSFR (Mannucci et al. 2010; Davé et al. 2012; Dayal et al. 2013).A different explanation was put forward by Li13, who proposeda simple model of galaxy evolution in which the SFR is regu-lated by the mass of gas present in a galaxy, implying that Zdepends on both stellar mass and current SFR. They derived



a Z(M,SFR) relation that depends on the internal parametersdescribing the regulator system, specifically the star-formationefficiency ǫ = SFR/Mgas and the mass-loading of any windthat drives gas out of the system, theirλ = out f low/SFR.The local SDSS data from Mannucci et al. (2010) can be repro-duced with astrophysically plausible values ofǫ(M) andλ(M),see Eq. 40 in Li13 and also Eq. 3 in Maier et al. (2014): Zeq =

Z0 + y/(1+ λ(1− R)−1 + ǫ−1((1+ β − b)S FR/M − 0.15)), whereZeq is the equilibrium value of the metallicity,Z0 is the metallic-ity of the infalling gas,y the yield,R the returned fraction. Thepossible values of the different parametersǫ, λ, y, b are given inTab. 1 of Li13. These analytic approximations have since beenverified by more detailed calculations in Pipino et al. (2014).

Our aim is to challenge this model description of a funda-mental relation in a dense cluster environment at higher redshift,i.e., we want to investigate whether the Z(M,SFR) dependenceis similar to the one found in the local Universe so that the FMRis still valid. To this purpose we calculate the expected O/H val-ues from the Li13 model for each galaxy individually with theirrespective stellar mass M and SFR. We then compare the pre-dicted values with the measured ones and show the differencein Fig. 9. We find that 87% of the zCOSMOS field compari-son galaxies have O/H values which are in accordance with themodel expectations within 0.2 dex, the nominal maximum errorin Z determination. We conclude therefore that the majorityofzCOSMOS field galaxies atz ∼ 0.4 (with log(M/M⊙) > 9.2)obey the FMR. This is in accordance with Cresci et al. (2012)that used a small subsample of zCOSMOSz ∼ 0.4 galaxies, al-beit with a strong S/N selection:S/N > 20 andS/N > 8 in Hβflux for log(M/M⊙)> 10 and log(M/M⊙)< 10, respectively. Theyalso considered the difference between measured O/H and pre-dicted O/H measurements, but used the Mannucci et al. (2010)predictions instead of the Li13 ones to conclude that the FMRholds for field galaxies atz ∼ 0.4.

To investigate the FMR for cluster members, we show inpanel c) of Fig. 9 the difference in O/H measurements to pre-dictions using both primordial and enriched inflowing gas. 88%of Zgals cluster galaxies with log(M/M⊙) > 9.2 have O/H val-ues which are in accordance with the model expectations within0.2 dex, similar to the behaviour of zCOSMOS field galaxies.In contrast, lower mass Mlow cluster members deviate systemati-cally from the FMR predictions. For the first time we can explorethe Z(M,SFR) of star-forming cluster galaxies at higher redshiftsdown to low masses Mlow (8.5 < logM/M⊙ < 9.2) with CLASH-VLT. These Mlow cluster galaxies are not compatible with theFMR if primordial inflowing gas (Z0 = 0) is assumed (opencircles), having metallicities two to three times higher than pre-dicted. We note that most of these low mass galaxies are accretedobjects (except three infalling Mlow objects shown in blue). Thissuggests that galaxies of lower masses Mlow are more prone toenvironmental effects when they are accreted by the cluster, dueto their lower gravitational potential. Better agreement with theFMR can be achieved with infall of enriched gas with metal-licity Z0/y = 0.1 (filled symbols). This is in compliance withthe predictions of Peng & Maiolino (2014) that the inflowing gasshould get progressively metal-enriched (Z0 is higher) in denseregions, producing higher metallicities for cluster galaxies oflow masses compared to similar field galaxies. Peng & Maiolino(2014) do not give a physical explanation how (metal enriched)gas inflow can happen in the dense cluster environment. How-ever, they offer an alternative scenario based on strangulation,in which the inflow of pristine gas is stopped, as a possible ex-planation for higher O/Hs in lower mass objects. They do not

elaborate this scenario further because they find that the sSFRsare similar in different environments like we do (see Fig. 4).

It should be noted that we call strangulation/starvation a sce-nario in which the supply of cold gas onto the galaxy disk ishalted, because the halo gas is stripped due to external forces.In that case star formation can continue, using the gas availablein the disk until it is completely used up. Therefore, similar sS-FRs for star-forming cluster and field galaxies do not definitelyexclude strangulation as a plausible mechanism for the chemi-cal enrichment effects we see in the cluster galaxies. In a contextof a rich cluster, this scenario was first described by Larsonet al.(1980), who used the removal of the gas reservoir due to externalforces to explain the passive cluster galaxy population.

To explore the strangulation scenario, we divide the Mmedcluster sample in infalling and accreted objects accordingto theirposition in the phase-space diagram (Fig. 8) and depict theminblue and red in Fig. 9. The difference between measured and pre-dicted O/Hs for most of these Mmedcluster objects is very similarfor the two scenarios with inflow of primordial gas (open circles)and enriched gas (filled circles): the difference between the val-ues of the open and the respective filled circles for most Mmedgalaxies is less than∼ 0.02 dex. Therefore, the histograms ofMmedgalaxies shown in panel a) of Fig. 9 are very similar for thetwo scenarios of inflowing enriched or primordial gas. Thesehis-tograms show that the O/H distribution of Mmed infalling clustergalaxies is in quite good agreement with the FMR, while ac-creted galaxies have a distribution shifted to higher metallicities,in general higher than predicted by the Li13 model. This indi-cates that, after being accreted by the cluster, galaxies may in-crease their metallicities as a result of starvation. On theotherhand, infalling galaxies are still star-forming, in general follow-ing the FMR and behaving similar to field galaxies (panel b ofFig. 9), with their O/Hs not yet influenced by environmental ef-fects of the cluster. In the next section, we discuss furtherevi-dence that strangulation is a plausible mechanism for explainingthe metallicities of cluster galaxies.

5.2. Co-evolution of SFR and Z

To understand the combined SFR and metallicity evolution ofgalaxies, we consider four toy models of different SFHs: a con-stant SFR and primordial gas inflow, an exponentially decliningSFR with a timescaleτ of 3 Gyrs and inflow, and a strangulationscenario for these two SFHs, i.e., without any inflow. We fol-low the evolution of the model galaxy in these four scenariosfor3 Gyrs, which roughly corresponds to: i) the time between theredshift of the MAC0416 cluster,z ∼ 0.4, and the upper redshiftrange of the SDSS comparison sample,z ∼ 0.08, and ii) the timespan betweenz ∼ 0.8 andz ∼ 0.4. We can hereby follow: i) theevolution of an infalling low metallicity galaxy (blue starsym-bols in Fig. 8) which will reach more inner regions of the clusterand increase its metallicity, as expected from the observedO/Hsatz ∼ 0.4 of most accreted cluster members; and ii) the evolutionof a galaxy infalling into the cluster atz ∼ 0.8, and becoming anaccreted cluster galaxy when being observed atz ∼ 0.4, likelybecoming a higher-metallicity object (red filled circles inFig. 8).Thus, the paths of our model galaxies are assumed to start in aregion populated by infalling low-metallicity galaxies, and areexpected to end in the region of the mass-metallicity diagrampopulated by higher metallicity objects (with O/Hs consistentwith the local MZR), as indicated by the red dashed-dotted lineand red symbols in panel b of Fig. 10.

As discussed in the previous section, Fig. 9 demonstrates thatthe more massive (log(M/M⊙) > 9.2) field andinfalling cluster



galaxies atz ∼ 0.4 are generally compatible with the Z(M,SFR)relation as given by Li13 (their Eq. 40). Since this equation,which we reproduced in Sect. 5.1, is by construction also ap-plicable to local SDSS galaxies, we can assign a SFR value toevery grid point in the MZR diagram. In the following we useZ(logM,SFR) to denote the metallicity O/H as a function of stel-lar mass and SFR.

The starting point of the evolutionary tracks for the fourmodel galaxies atz = 0.4 (shown as a filled green circle witha black cross in the panels b and c of Fig. 10) was derived byextrapolating the local MS to this redshift, following the evo-lution of the sSFR, derived in Peng et al. (2010). Initial valuesfor stellar mass and SFR are chosen as log(Mbegin/M⊙) = 9.5and SFRbegin = 1.2M⊙/yr. Following considerations of theFMR (Li13), the model galaxy starts with an initial metallic-ity value of Zbegin(9.5, 1.2) = 8.55. These are typical (mean)Z(logM,SFR) values for infalling, low metallicity, MSZgalscluster galaxies with 9.2 < log(M/M⊙) < 9.8 (blue star symbolsin Fig. 10). On the other hand, the five cluster infalling galax-ies with higher metallicities show, with one exception, lowersSFRs than the MS (red star symbols in panel c of Fig. 10).The lower sSFRs (and higher O/Hs) of theseinfalling galaxiescould be due to pre-processing in the group environment (e.g.,Zabludoff & Mulchaey 1998) prior to arriving in the infall re-gion of the cluster.

We follow the tracks of our model galaxies in the MZR andin the∆sSFR−∆O/H diagrams. We chose∆sSFR to be the offsetto the MS atz ∼ 0.4 (see Fig. 5), while∆O/H is the offset be-tween the measured O/H and the local SDSS MZR value for O/Hat the respective mass (see Fig. 7). The sSFR offset from the MSat a given mass indicates the level of star formation compared towhat is typical for a galaxy of that mass.

The magenta lines in panel b of Fig. 10 show the Li13 ex-pectations of the Z(M,SFR) for galaxies on the MS at differentredshifts, with dashed, solid and dotted lines corresponding toz ∼ 0.8, z ∼ 0.4 andz ∼ 0.06 MS galaxies. A MS model galaxywith Zstart(9.5, 1.2) = 8.55 lies on the MZR of MS galaxies atz ∼ 0.4 (solid magenta line). It forms stars for 3 Gyrs with a con-stant SFR and increases its stellar mass to log(Mend/M⊙) = 9.83,and its metallicity to Zend(9.83, 1.2) = 8.78 (cyan track). Al-though the SFR is constant, the sSFR of the model galaxy withconstant SFR decreases because stellar mass is formed, reachingthe expected MZR of MS galaxies atz ∼ 0.06 (dotted magentaline in the panels a and b of Fig. 10). Thus we note that an MSz ∼ 0.4 model galaxy with constant SFR can reach the MS ofSDSS galaxies without decreasing its SFR, but simply because itincreases mass through constant SFR, which leads to a decreaseof its sSFR.

The tracks for a model galaxy forming stars with an exponen-tially declining SFR with a timescaleτ of 3 Gyrs and having aninflow of gas are shown by the green tracks in panels a and b ofFig. 10. In this case log(Mend/M⊙) = 9.73, SFRend= 0.44 M⊙/yr,and the sSFR decreases after 3 Gyrs by a larger amount than inthe case of constant SFR (panel a), and the model galaxy reachesZend(9.73, 0.44) = 8.78, the same metallicity as in the case ofconstant SFR (panel b). Any naively assumed difference in Zend(after all, galaxies with lower Mend should also have lower Zend)cancels out due to the fact that smaller SFRend implies higherZend.

In our assumption of beginning strangulation, stars continueto form, consuming the available gas in the disk, either withaconstant SFR (blue tracks in Fig. 10) or with a slowly decreas-ing SFR with a timescaleτ of 3 Gyrs (black tracks in Fig. 10).We find that a model galaxy with an exponentially faster declin-

ing SFR with aτ = 1 Gyr becomes passive byz ∼ 0.06 witha sSFR about 30 times lower than atz ∼ 0.4, therefore disap-pearing from panel a of Fig. 10. Recent results of Cantale et al.(2016) strengthen the plausibility of our toy models with ongo-ing star-formation for several Gyrs, since they found that galax-ies are able to continue forming stars for up to 5 Gyrs after beingaccreted into clusters. Additionally, Jaffé et al (2015) studied HIgas masses in cluster galaxies atz ∼ 0.2 and found a plausiblescenario in which galaxies affected by ram-pressure stripping asthey first fall into the cluster are able to continue to form stars inthe remaining gas disk.

In the strangulation scenario, our model galaxy reaches thesame values of logMend and SFRend as above, but the increase inmetallicity is higher than in the non-strangulation case, i.e., Zendis higher. After stopping the inflow of gas, the evolution of thegalaxy can be approximated with a closed-box model. This first-order assumption neglects effects of gas outflows that continueafter the supply of cold gas from the surroundings comes to ahalt. However, studies have shown (e.g., Peng et al. 2015) that inthe case of truncated gas inflows, metallicities are independent ofoutflows, because the element mix of the retained gas stays con-stant. Thus, a closed-box model is still a good approximation.Maier et al. (2006) constructed a large grid of Pégase2 modelsto explore which region of the parameter space could reproducethe constraints imposed by the local metallicity-luminosity re-lation and by the metallicities and luminosities of galaxies athigher redshifts. They explored models in which the gas inflowon the model galaxy stops after about 1 Gyr. The galaxy subse-quently forms stars in a “closed-box”-like scenario. The tracksof these closed-box models show that, after gas inflow stops,the model galaxies increase their metallicities by∼ 0.2 dex intenrich ∼ 1 Gyr, and by more than a factor of two, namely by∼ 0.35 dex, intenrich∼ 3 Gyr.

In both strangulation scenarios, we therefore assume a metal-licity increase by more than a factor of two intenrich ∼ 3 Gyr, asjustified by the Pégase2 closed-box models, which leads to fi-nal metallicity values of Zend = 8.9 (black and blue tracks inFig. 10). These Zend values of the model starvation paths lie inthe region of the mass-metallicity diagram populated by highermetallicity SDSS objects (note that the metallicity of the redsymbols in panel b in Fig. 10 are in agreement with the localSDSS MZR). This indicates that starvation is a plausible sce-nario for the evolution of the metallicities ofz ∼ 0.4 infallinggalaxies to higher metallicities after 3 Gyrs, consistent with theupper, higher metallicity part, of thez ∼ 0.06 MZR. Addition-ally, the Zend values of the model starvation paths are also inagreement with the O/Hs of several accreted cluster galaxies atz ∼ 0.4 (red filled circles in Fig. 10). Therefore,z ∼ 0.4 accretedcluster members with higher metallicities are likely to have beeninfalling into the cluster at earlier times (z ∼ 0.8), and to havesuffered strangulation in the time span of about 3 Gyrs betweenz ∼ 0.8 andz ∼ 0.4.

6. Summary

This study of environmental effects on the Z(M,SFR) rela-tion is based on VIMOS spectroscopy of cluster galaxies in theMAC0416 cluster, supplemented by a zCOSMOS field galaxysample at similar redshiftsz ∼ 0.4. The main results can be sum-marized as follows:

1. The star-forming cluster galaxies atz ∼ 0.4 are not domi-nated by AGNs, and they follow the star-forming abundance se-quence in the BPT-diagram (Fig. 2).



2. We provide and verify the reliability of new calibrationsof O/H for galaxies following this star-forming abundance se-quence (Fig. 3). They are based on the line ratios of only twoELs close in wavelength ([OIII]/Hβ or [NII] /Hα), which havethe advantage that extinction corrections are negligible.

3. Cluster and field galaxies atz ∼ 0.4 show on aver-age similar MZR and mass−sSFR relations for stellar masses9.2 < log(M/M⊙) < 10.2 (Figs. 4 and 6), in accordance withfindings of numerical studies and local observations. On theother hand, we find that bulge-dominated (smooth) and compactcluster galaxies have lower sSFRs and higher O/Hs compared totheir disk-dominated and peculiar counterparts (Figs. 5 and 7),in agreement with an FMR scenario combined with the effect ofdominant bulges lowering SFRs.

4. We find several indications for a strangulation scenario forgalaxies with log(M/M⊙) < 10.2 infalling into a cluster:

i) First, based on the location of galaxies in the projectedphase-space diagram, a higher fraction of “accreted” galaxies(63%) are metal-rich compared to the fraction of 28% of metal-rich “infalling” galaxies (Fig. 8).

ii) Second, while the measured O/Hs of field and infallingcluster galaxies are in quite good agreement with the FMR, ac-creted galaxies have a distribution shifted to higher metallicitiesthan predicted by the Li13 models (Fig. 9). This trend is intensi-fied for low mass (log(M/M⊙) < 9.2) accreted cluster galaxies,which have 2− 3 times higher metallicities than predicted bythe models with inflowing pristine gas, indicating that a strangu-lation scenario in which gas infall has stopped enhancing theirmetallicities is a plausible mechanism.

iii) Third, the paths of model galaxies in the MZR and in the∆sSFR− ∆O/H diagrams show that strangulation is needed, ad-ditional to a constant or exponentially declining SFR, to explainthe higher metallicities of accreted cluster galaxies (Fig. 10).

The main conclusion is that a strangulation scenario is plau-sible to explain the metallicities of “infalling” and “accreted”log(M/M⊙) < 10.2 cluster galaxies in MAC0416. While theZ(M,SFR) measurements of most field and infalling clustergalaxies are in agreement with the FMR assuming a primor-dial gas inflow, we find higher than from the FMR expectedmetallicity values in accreted cluster galaxies, especially at lowermasses, indicating that the inflow of pristine gas is being stoppedin these galaxies as a result of cluster environmental effects.

Acknowledgements. This publication is supported by the Austrian Science Fund(FWF).

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http://arxiv.org/abs/1511.02522




Fig. 1. Color-luminosity (left) and color-mass (right) diagrams for MAC0416 cluster galaxies atz ∼ 0.4 with spectroscopic redshifts. Cyandata points are the spectroscopic confirmed MAC0416 blue cluster members, representative of the star-forming blue cloud galaxies, while the 76galaxies used for this metallicity study (Zgals sample) are shown in blue. The red lines and numbers indicatethe fraction ofZgals objects (bluesymbols) with respect to the parent sample (cyan points). Weachieve a uniform coverage at intermediate luminosities/masses. Lowest/highestluminosity/mass bins are under-represented by our sample.

luminosity (R magnitude) stellar mass color B − R

bins f aint → → bright low → → high blue → → red

members 21% 37% 32% 10% 15% 41% 29% 16% 13% 47% 25% 15%

Zgals 12% 39% 41% 8% 11% 39% 42% 8% 11% 55% 24% 11%

f raction 16% 30% 36% 22% 20% 27% 42% 14% 22% 33% 27% 20%

Table 1. Demonstration of the coverage of our sample. First row: Percentage of spectroscopic members of MAC0416 representativefor the bluecloud in the given bins in luminosity, mass and color; Secondrow: selectedZgals objects for this metallicity study in the given bins in luminosity,mass and color; Third row: the fractions ofZgals objects in different luminosity/mass/color bins with respect to the parent sample of blue clustermembers (numbers shown in Fig. 1).



Fig. 2. BPT (Baldwin et al. 1981) diagnostic diagram to disentanglestar formation-dominated galaxies from AGNs. The left diagram showsthe field galaxies atz ∼ 0.4, and the right panel theZgals cluster members of MAC0416 (cluster galaxies of similar high masses as the fieldcomparison sample are depicted in blue). Both the field and cluster galaxies atz ∼ 0.4 lie under and to the left of the theoretical (solid) curve ofKewley et al. (2001) and of the empirical (dashed) curve of Kauffmann et al. (2003), which separate star-foming galaxies (below/left of the curves)from AGNs (above/right of the curves), indicating that in all of them the dominant source of ionization in the gas is recent star formation.Mostgalaxies lie on the star-forming metallicity sequence withrough boundaries indicated by the thick dashed lines.

Fig. 3. The difference between the O/Hs calculated using the O3N2 calibration, and the O/Hs calculated using Eq. 1 (left panel) and Eq. 2 (rightpanel) forZgals MAC0416 galaxies which lie on the star-forming abundance sequence (Fig. 2). Given typical O/H uncertainties of 0.1-0.2 dex,the agreement between the O3N2 calibration and the Eq. 1 and Eq. 2 calibrations is good, especially for the Eq. 1 calibration.



Fig. 4. Left panel: The mass−sSFR relation for MAC0416 cluster galaxies (blue symbols) and zCOSMOS field galaxies (green) atz ∼ 0.4. Theoblique solid thick black line shows the MS atz ∼ 0.4 and its dispersion (indicated by the gray area), and the dashed red line depicts the localSDSS MS, using Eq. 1 in Peng et al. (2010) forz ∼ 0.4 andz ∼ 0.06, respectively. Right panel: the median values of the mass−sSFR relations atz ∼ 0.4 are shown as blue (cluster) and green (field) solid lines andbig symbols, while the corresponding dashed oblique lines enclose 68% of thedata. The dashed thick light green line shows the completeness limit of ourZgals cluster sample, SFR∼ 0.3 M⊙/yr.

Fig. 5. The offsets of cluster galaxies with different morphologies from the MS atz ∼ 0.4. Blue squares are disks, red circles smooth, blackcompact, and green irregulars. The red solid lines show the MS atz ∼ 0.4 predicted by Peng et al. (2010). The shaded gray area is the 1σ scatterof the respective morphological category, while the dashedblack solid line is the median value.



Fig. 6. Left panel: MZR for 76 MAC0416 cluster members (Zgals sample, blue symbols) and 93 field galaxies (green symbols) at z ∼ 0.4.Polynomial fits to the data points (blue and green thick lines) show that the MZRs for cluster and field galaxies are similarat intermediatemasses Mmed, where theZgals sample has a uniform coverage with respect to the parent sample (see Fig. 1). Right panel: the median values ofMZR at z ∼ 0.4 are shown in blue (cluster) and green (field), with the corresponding dashed oblique lines enclosing 68% of the data. In bothpanels we extrapolated the MZR of the SDSS sample to lower stellar masses by assuming that the slope of the low-mass MZR stays constant forlog(M/M⊙) < 9.2.

Fig. 7. Median offsets to the local relation forZgals MAC0416 galaxies, divided by their morphologies. We find different behaviors forsmooth/compact versus disk/peculiar types with respect to the local relation. While forthe latter we measure O/H abundances lower than thelocal relation, those galaxies with more prominent bulges and compact galaxies are more enhanced in gas metallicities.



Fig. 8. Phase-space diagram based onall MAC0416 spectroscopic members (black open circles). Our study concentrates on the objects withmetallicities measured (Zgals sample). Restricting ourselves to intermediate masses 9.2 < log(M/M⊙) < 10.2 we depict high metallicity (consis-tent with the local MZR) objects as red symbols and galaxies with lower metallicities (below the local MZR) as blue symbols. The dashed verticalline indicates R200 ∼ 1.8 Mpc of the cluster (as reported in B15), the dashed magenta line indicates the caustic determined by B15. The diagramenables us to divide the cluster galaxies in “infalling” (above and to the right of the magenta line, star symbols) and “accreted” objects (filledcircles).



Fig. 9. Difference between the measured O/Hs of z ∼ 0.4 Zgals cluster galaxies (panel c) and zCOSMOS field objects (panel d) and the expectedO/Hs from the formulations of Li13 for different infall metallicitiesZ0 relative to the yieldy, Z0/y = 0.1 (enriched inflow, filled circles) andZ0/y = 0 (pristine inflow, open circles). In addition, in panel c) the position of cluster galaxies in the phase-space diagram (see Fig. 8) is indicatedin red (blue) for accreted (infalling) galaxies. The upper histograms (panels a and b) for Mmed objects show that the infalling cluster members (bluehistogram) and field galaxies (panel b) have in general lowermetallicities, in quite good agreement with the FMR, unlikeaccreted galaxies (redhistogram) which have in general higher metallicities thanpredicted by the FMR.



Fig. 10. Panel a: Offsets∆O/H to the local MZR vs.∆sSFR to the MS atz ∼ 0.4 for cluster galaxies with masses 9.2 < log(M/M⊙) < 10.2 (Mmed).Filled circles (stars) symbols indicate accreted (infalling) galaxies based on their position in the phase-space diagram (Fig. 8). Galaxies with O/Hsconsistent with the local MZR are shown in red, other objectsin blue. The MS mean values atz ∼ 0.8, z ∼ 0.4 andz ∼ 0.06, based on the equationof the evolution of the sSFR from Peng et al. (2010), are indicated by dashed, solid and dotted magenta lines. The dashed-dotted red line indicatesthe median local MZR of our matched SDSS sample (see also Fig.6). Four tracks of a model galaxy start with 1.2 M⊙/yr and log(M/M⊙) = 9.5.The model galaxy continues forming stars for 3 Gyrs with a constant SFR (cyan track) or exponentially declining SFR with aτ = 3 Gyrs (greentrack), and the corresponding tracks are shown for a strangulation scenario with constant SFR (blue track) and exponentially declining SFR (blacktrack). Panel b: MZR for Mmed cluster galaxies with data symbols and paths of model galaxies color-coded like in panel a. The magenta lines showthe Li13 expectations of the Z(M,SFR) for galaxies on the MS at different redshifts, with dashed, solid and dotted lines corresponding to differentz like the magenta lines in the panel a. Panel c: The mass−sSFR relation for cluster galaxies color-coded in red for O/Hs consistent with the localMZR. Dashed and dotted lines show the median and dispersion of the mass−sSFR relation according to Peng et al. (2010). The green filled circleswith a black cross in panels b and c indicate the starting point of the toy models described in Sect. 5.2: log(Mstart/M⊙) = 9.5, SFRstart = 1.2M⊙/yrand Zstart= 8.55.


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