Dwarf Elliptical Galaxies in the Coma Cluster from the HST Treasury Survey or Clusters in Dwarfs in

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Dwarf Elliptical Galaxies in the Coma Cluster
from the HST Treasury SurveyorClusters in
Dwarfs in Clusters

• Henry Ferguson
• STScI
• August 28, 2008

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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 dEs in clusters form earlier? Survive
  longer?
• Why do there appear to be more dEs per ES0 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 GCs? UCDs?

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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 dEs should have same
  sizes, ages, etc as satellite dEs.

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The HST Coma Cluster Treasury Program
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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 EA galaxies
• Selection of samples on surface brightness and
  morphological criteria for spectroscopy.

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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

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Observed before ACS failed 25 fields (275
sq. arcmin)
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Focus on nuclei

• Resolution 50 parsecs at Coma
• Comparable to the best ground-based resolution
  for Virgo
• Cluster dE are relatively easy to distinguish
  from background galaxies by morphology alone
• Its 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

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Properties of dE nuclei

• Nuclei represent lt10 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

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Universal CMO relation?
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Nuclei are slightly redder than the surrounding
galaxy
Grant et al. 2005
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Fornax
Virgo
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Cote et al. 2006
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Lisker et al. 2006
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Virgo Cluster dE shapes (SDSS)
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Coma Treasury Bright dE,N
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Bright dE
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Fainter dE,N
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Fainter dE
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Coma Treasury -- Bright dE,N
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Bright dE
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Fainter dE,N
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Fainter dE
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Nuclei tend to be in brighter dE (selection?)
dE,N
dE
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Spatial segregation
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Spatial segregation
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Strongly nucleated
Weakly nucleated
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Throwing out weakly nucleated dEs
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Throwing out weakly nucleated dEs
K-S test probability lt 3 (bright dE vs dE,N)
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Colors (preliminary!)
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Colors of bright dE
K-S test probability 5
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Conclusions

• Central nuclear star clusters are common in
  Coma-cluster dE
• Trends for Virgo and Fornax appear to hold in
  Coma
• Brighter dEs 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