1. Henry Ferguson STScI August 28, 2008
Dwarf Elliptical Galaxies in the Coma Cluster from the HST Treasury Survey or Clusters in Dwarfs in Clusters
Henry Ferguson
STScI
August 28, 2008

2. Open Questions Are dE just E galaxies in smaller halos?
Or are they stripped irregulars/spirals?
Do dE properties depend on environment?
Are dE and dSph the same beast?
Did dE’s in clusters form earlier? Survive longer?
Why do there appear to be more dE’s per E+S0 in richer groups than in poorer ones?
What are dE nuclei?
How do they relate to black holes in more massive galaxies?
Are they the progenitors of some GC’s? UCDs?

3. Cluster dwarfs and the missing satellites
No missing satellite problem for Virgo
Whatever solutions work for the local-group must not break Virgo
True dwarfs probably vastly outnumber remnants of stripped giant galaxies
Cluster dwarfs may survive longer
Some dwarfs that would otherwise merge with the central galaxy in their halo are tidally liberated when that halo merges into a larger halo
Not obvious that cluster dE’s should have same sizes, ages, etc as satellite dE’s.

4. The HST Coma Cluster Treasury Program

5. Survey objectives Luminosity Function to MV ~ -9.
Morphologies for a wide range of luminosity and environment.
Colors and color gradients in a wide range of luminosity and environment.
Bright and faint ends of the global scaling laws.
C-M diagrams for globular clusters.
Detection of Ultra-Compact Dwarfs (UCDs)
Morphology of “E+A” galaxies
Selection of samples on surface brightness and morphological criteria for spectroscopy.

6. Cycle 15 survey F475W and F814W 82 fields planned (900 sq. arcmin)
42 fields in central mosaic
40 centered on known members in the outskirts
Supporting observations: Hectospec DEIMOS Subaru VLA …

7. Observed before ACS failed:
25 fields (275 sq. arcmin)

8. Focus on nuclei… Resolution ~50 parsecs at Coma Covers 0.2 Mpc2
Comparable to the best ground-based resolution for Virgo
Cluster dE are relatively easy to distinguish from background galaxies by morphology alone
It’s possible to see and measure the nuclei
Covers 0.2 Mpc2
(Compared to 0.02 Mpc2 for the HST Virgo Cluster survey)
~500 eyeball-classified members

9. Properties of dE nuclei
Nuclei represent <10% of the total light (typically ~ 2%)
Nuclear magnitude increases with galaxy magnitude
Generally brighter than globular clusters
Radii
r1/2 in Virgo range from 60 to 2 pc (median 4 pc)
Colors
Color-luminosity relation
On average redder than surrounding galaxy, with large scatter
Some blue ones
Velocity Dispersions
~10-50 km/s in the brighter ones

10. Universal “CMO” relation?

11. Nuclei are slightly redder than the surrounding galaxy
Grant et al. 2005

12. Virgo
Fornax

13. Cote et al.
2006

14. Lisker et al. 2006

15. Virgo Cluster dE shapes (SDSS)

16. Coma Treasury Bright dE,N

17. Bright dE

18. Fainter dE,N

19. Fainter dE

20. Fainter dE,N

21. Fainter dE

22. Nuclei tend to be in brighter dE (selection?)
dE,N dE

23. Spatial segregation

24. Spatial segregation

25. Strongly nucleated
Weakly nucleated

26. Throwing out weakly nucleated dE’s…

27. Throwing out weakly nucleated dE’s…
K-S test probability < 3% (bright dE vs dE,N)

28. Colors (preliminary!)

29. Colors of bright dE
K-S test probability 5%

30. Conclusions
Central nuclear star clusters are common in Coma-cluster dE
Trends for Virgo and Fornax appear to hold in Coma:
Brighter dE’s tend to have nuclei
dE,N population is strongly concentrated toward the cluster center
Bright dE population is less strongly concentrated
Hint of a color difference at fixed mag for bright dE?
dE with no nuclei are a younger infalling population
dE,N and most of the faint dE are an old, relaxed population
survivors of early formation of the cluster