1. 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 Soo Journal Club – Galaxy Evolution, 2010. 11. 18. 1

2. 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. 2

3. 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) 3

4. Ultracompact dwarf galaxy (UCD) ? Brighter and larger than GCs, and Fainter and smaller than dEs R eff M V >-14 Evstigneeva et al. (2008, AJ, 136, 461)Phillipps et al. (2001) 4

5. Ultracompact dwarf galaxy (UCD) ? Colors of UCDs have proper range : 0.9<(V-I) 0 <1.3 Fornax UCDs are redder on average than Virgo UCDs Evstigneeva et al. (2008) 5

6. Ultracompact dwarf galaxy (UCD) ? UCD contain Dark matter ? ==> Yes : galactic origin Drinkwater 2003 6

7. Origin of UCD (1) Intracluster GCs Evstigneeva et al. (2007, AJ, 133, 1722) Old age(>8-10 Gyr) Metal poor(<-0.5dex)  very luminous Intracluster GCs Super-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. 7

8. 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 8

9. 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 subtracting the underlying galactic light, which can still contribute significantly at the photometric center of the dEs 9

10. Sample selection Nucleated dEs (dE nuclei) nucleus strength : <-1 difference between the nucleus magnitude(m r,nucleus <21mag) and the host galaxy effective surface brightness(μ r,eff,galaxy ), ※ μ r,eff,galaxy : the brightness of a unit area of the galaxy 10

11. Sample selection Virgo UCDs : 10 Evstigneeva2007(6)+Firth2009(3)+New(1) Example) Virgo UCDs -14<M V <-10 mag 0.9< (V-I) 0 <1.3 Starlike(<100 pc) Virgo cluster member : velocity range with 400<v<3000 km/s 11

12. 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 ” ) 12

13. Spectroscopy Using VLT/FORS2 They removed most of the galactic light contamination from the nuclear spectra using the galaxy spectrum extracted in a radial interval from 3 ” to 8 ”. 13

14. Age, metallicity, and alpha-elements Comparing Lick index measurements to the model (Thomas et al. 2003, MN, 339, 897) Using Chi 2 minimization (Proctor & Sansom2002) With 9 indices for dE nuclei and 4 indices for UCDs 14

15. Results Figure 2 Age, 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 Gyr UCDs : 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. 15

16. Results Metallicity [Z/H] The dE nuclei also show a difference between the low- and high-density regions The UCDs also have a fairly large scatter From 0.1 to -1.4 Alpha-elements [α/Fe] The dE nuclei more or less consistent with solar, a slight increase with density The UCDs More clearly increase with density 7 out of 10 have super-solar [α/Fe] 16

17. Results Figure 3 The 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 metallicity distributions of dE nuclei  young metal-rich group old metal-poor group Brighter 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. 17

18. Discussion Because the stripping process can alter structural parameters(size, color, brightness) of the embedded nuclei, a direct comparison of structural parameters may not be 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 nuclei are similar in the high- density galaxy region.  This supports the idea of the stripping of dEs in high-density environments to form the UCDs. 18

19. 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, then nucleated 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. 19

20. Discussion The estimated ages and metallicities of UCDs nicely agree with the ages and metallicities of nuclei of dEs from the 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. 20

21. 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. 21