Nuclei of Early-type Dwarf Galaxies: Are They Progenitors of Ultracompact Dwarf Galaxies?

Paudel, S., Lisker, T., Janz, J. 2010, ApJ, 724, L64

Park, Hong SooJournal Club – Galaxy Evolution, 2010. 11. 18.

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What is the origin of UCD (ultracompact dwarf galaxy) ?

The dE nuclei have younger stellar population ages than UCDs.The dE nuclei are on average more metal-rich than UCDs.

But, in the high-density cluster regions,the dE nuclei are as old and as metal-poor as UCDs. Virgo UCDs may have formed through the stripping of dE nuclei.

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Ultracompact dwarf galaxy (UCD) ?

Hilker et al. (1999, A&AS, 134, 75) : Two “new” members in Fornax cluster are very compact and have surface brightnesses comparable to GCs, however, their luminosities are in the range of dE nuclei.

Drinkwater et al. (2003, Nat, 423, 519) Phillipps et al. (2001, ApJ, 560, 201)

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Ultracompact dwarf galaxy (UCD) ?

Brighter and larger than GCs, and Fainter and smaller than dEs Reff <100pc, -10 >MV>-14

Evstigneeva et al. (2008, AJ, 136, 461) Phillipps et al. (2001)

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Ultracompact dwarf galaxy (UCD) ?

Colors of UCDs have proper range : 0.9<(V-I)0<1.3Fornax UCDs are redder on average than Virgo UCDs

Evstigneeva et al. (2008)

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Origin of UCD

(1) Intracluster GCsEvstigneeva et al. (2007, AJ, 133, 1722)

Old age(>8-10 Gyr)Metal poor(<-0.5dex) very luminous Intracluster GCsSuper-solar alpha-element

Mieske et al. (2002, A&A, 383, 823) Their magnitude distribution supports a smooth transition between the faint UCDs and the bright GCs.

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Origin of UCD

(2) Stripping of dE nuclei The remnant nuclei of dEs resemble UCDs in their structural

parameters.(Bekki et al. (2003, MN, 344, 399) numerical simulation result)

UCDs are the remnants of galaxies that have been significantly stripped in the cluster environment

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Advantage of this paper

Large sample of 34 nucleated dEs and 10 UCDs in the Virgo cluster: Previous studies used rather low numbers of objects

The extraction of nuclear spectra has been made with subtractingthe underlying galactic light, which can still contribute significantlyat the photometric center of the dEs

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Sample selection

Nucleated dEs (dE nuclei)

nucleus strength : <-1 difference between the nucleus magnitude(mr,nucleus

<21mag) and the host galaxy effective surface brightness(μr,eff,galaxy),※ μr,eff,galaxy : the brightness of a unit area of the galaxy

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Sample selection

Virgo UCDs : 10Evstigneeva2007(6)+Firth2009(3)+New(1)

Example)Virgo UCDs-14<MV<-10 mag0.9< (V-I)0 <1.3Starlike(<100 pc) Virgo cluster member : velocity range with 400<v<3000 km/s

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Photometry

dE nuclei and UCDs magnitude : SDSS DR5 dE nuclei magnitude elliptical model image (fit with 2”-2.5” radius) original – model circular aperture photometry (r=2”) UCDs magnitude circular aperture photometry (r=2”)

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Spectroscopy

Using VLT/FORS2

They removed most of the galactic light contamination from the nuclear spectrausing the galaxy spectrum extracted in a radial interval from 3” to 8”.

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Age, metallicity, and alpha-elements

Comparing Lick index measurements to the model (Thomas et al. 2003, MN, 339, 897)Using Chi2 minimization (Proctor & Sansom2002)With 9 indices for dE nuclei and 4 indices for UCDs

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Results

Figure 2Age, metallicity, and [α/Fe] abundance vs. local projected galaxy Density : dE nuclei (black) and UCDs (green)Local projected galaxy density :

circular projected area enclosing the 10th neighbor

Age break is seen at a projected density ≈ 40 sq. deg-1

lower/higher-density cluster regions

Age dE nuclei : age(low-density)=2.6 Gyr age(high-density)=11.6 GyrUCDs : age(high-density)=11.7 15Gyr In the high-density region,age, metallicity, and [α/Fe] between the UCDs and the dE nuclei are same.

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Results

Metallicity [Z/H]The dE nuclei also show a difference betweenthe low- and high-density regionsThe UCDs also have a fairly large scatterFrom 0.1 to -1.4

Alpha-elements [α/Fe]The dE nucleimore or less consistent with solar, a slight increase with densityThe UCDsMore clearly increase with density7 out of 10 have super-solar [α/Fe]

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Results

Figure 3The relation between the stellar population parameters and the r-band magnitudes: not see a relation of age and [α/Fe] with luminosity, both for nuclei and UCDs: the metallicity of both UCDs and dE nuclei tends to increase with increasing luminosity following the well-known metallicity–luminosity(mass) relation

Bimodal peaks of the age and metallicitydistributions of dE nuclei young metal-rich group

old metal-poor groupBrighter nuclei have metal-rich stellar populations consistent with solar values.The metal-poor nuclei (average of −1.0 dex), and most of them are less luminous than UCDs.

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Discussion

Because the stripping process can alter structural parameters(size, color, brightness) of the embedded nuclei,a direct comparison of structural parameters may notbe able to constrain the formation scenarios of UCDs properly.Age and metallicity of this paper can constrain the formation scenario of UCDs properly.

The age and metallicity distributions of UCDs and dE nucleiare similar in the high- density galaxy region. This supports the idea of the stripping of dEs inhigh-density environments to form the UCDs.

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Discussion

dE nuclei at low densities are younger than UCDs :Not only the orbital average should be located in the outskirts,but they should also not have passed right through the center,otherwise, the gas would have been stripped by ram pressure and tidal force.

We do not see young UCDs:dE destruction is only possible if the galaxy really goes through the center and experiences the strongest tidal forces. If we assume that the full destruction takes significantly more orbital time than the gas stripping, thennucleated dEs with an orbit leading through the center first lost their gas, halting any star formation, and then became destroyed.Therefore,We have no young UCDs.

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Discussion

The estimated ages and metallicities of UCDsnicely agree with the ages and metallicities of nuclei of dEs fromthe dense cluster regions. On the other hand, most of the old nuclei are fainter than the UCDs.(Figure 3) The UCD discoveries might suffer a selection effect: if a faint nucleus was stripped, it would now be automatically counted into the GC system.

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Summary

What is the origin of UCDs ?

In the high-density cluster regions,the dE nuclei are as old and as metal-poor as UCDs. Virgo UCDs may have formed through the stripping of dE nuclei.

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