Christopher J. Conselice

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Dwarf Galaxies in Clusters

• Christopher J. Conselice
• (Caltech)

Christopher J. Conselice (Nottingham)
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Why study dwarf galaxies in clusters ?
1. Most numerous type of galaxy, particularly
so in clusters - they are normal galaxies.
The ultimate galaxy formation/evolution
scenario must account for these objects. 2. In
hierarchical galaxy formation, low mass systems
are the first to form. Low-mass
galaxies may be the oldest galaxies in the
universe and we can learn how large
galaxies formed through merging by studying the
properties of these systems
Kauffmann White (1993)
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Dwarf ellipticals exist in clusters and
morphologically appear similar to Local Group
dwarf ellipticals
Discovered in 1950s by G. Reaves through a
Virgo cluster imaging survey

• Low-Mass Galaxies in the Perseus Cluster

There are however 5 - 10 as many low-mass cluster
galaxies per giant galaxy as there are in groups
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What do we know about cluster dwarf elliptical
galaxies

• Similar to Local Group dwarfs in terms of
  morphology and luminosity scaling relationships -
  e.g., surface brightness and magnitude

Conselice, Gallagher Wyse 2002
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Cluster dwarfs do not cluster around giants as
they do in groups
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Distribution of galaxy types in the Virgo Cluster
Dwarf Ellipticals
Giant ellipticals
Number densities vs. distance from cluster center
Conselice, Gallagher Wyse (2001)
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Velocity distribution of dEs is large, with
more substructure than giant cluster
galaxies
A sign of their formation mechanism?
Conselice et al. (2001)
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Cluster dEs deviate from the color-magnitude
relation
Clues for this deviation are old (e.g., Held
Mould 1994)

• Color-Magnitude diagram for Perseus galaxies with
  Local
• Group dEs plotted.

Both 'blue' and 'red dEs
Conselice, Gallagher Wyse (2003)
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Other studies have found similar trends
In the Fornax cluster (Rakos et al. 2001)
and Coma cluster (Poggianti et al. 2001)
Others (e.g., Adami et al. 2000) find similar
results
Low mass galaxies in clusters are not a
homogenous population, especially at M gt -15
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Cluster dEs likely have a mix of stellar
populations - some might be metal rich
Conselice et al. (2003)
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In general a mix of ages and metallicities are
needed to account for stellar populations of
LMCGs (e.g., Held Mould 1994 Rakos 2001)
Breakdown of color-magnitude relationship are
caused by a variety of ages and metallicities at
a given luminosity Very different from trend
found for early type cluster giants, but similar
to field galaxies
Rakos et al. (2001)
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Cluster dEs appear to have properties
fundamentally different from low-mass
galaxies in
groups Question What part of the cluster
environment produced this difference? Perhaps
cluster dEs do not form by the standard
supernova feedback mechanism (Dekel Silk
1986). Some Possibilities (all with interesting
implications) 1. Delayed formation dEs
are not old 2. Containment by the
intracluster medium 3. Removal of mass
from accreted galaxies 4. Very massive
dark halos
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Stellar Mass Functions FOR all galaxies to z1.4
The stellar mass function can be computed at
redshifts up to z1.4 Using large area fields 2
sq. degree, we find that the most massive
galaxies are already formed by z1.
Massive galaxies formed by z1, lower mass
systems form later
Bundy et al. (2006)
Bundy et al. (2006)
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Stellar mass functions divided into
different colors
Massive red galaxies are nearly fully formed by
z1
Lower mass galaxies end their star formation
later
Downsizing
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See no or little environmental dependence
Most of the stellar mass in dense and low
density environments is in place by
z1
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Do dwarfs in clusters form after giants?
Their globular clusters may tell us
Keck spectroscopic study of globular clusters
surrounding the dE VCC 1386 in the
Virgo cluster
Numbered objects are the GCs with spectroscopy
Conselice 2006, ApJ, 639, 120
VCC 1386 in the Virgo Cluster
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Balmer indicies
Comparing measured Balmer lines, Hß, H?, H?, to
Bruzual Charlot single burst models
gives ages 5 Gyr, or gt
12 Gyr
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Color vs. H? diagram
Similar comparison with (V-I) colors shows that
ages are gt 5 Gyr for these globular clusters
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Metallicity vs. H? diagram
Conclusions globular clusters in VCC 1386 have
an old age and a low metallicity with Age gt 5 Gyr
and Fe/H lt -1 at 3 sigma confidence
The body of VCC 1386 itself has a younger derived
age and a higher metallicity
Dwarf ellipticals in clusters have old stars, but
are not homogeneously old
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Can find dEs which are HI emitters - usually in
outer parts
Arecibo observations of dEs in Virgo, with HI
emitters as solid circles
Upper limits 5105 M(solar) in HI
Most emitters are in the outer part of the Virgo
cluster - evidence of infall and future stripping
of material? or gas for a future generation of
star formation?
Conselice, ONeil, Gallagher Wyse 2003
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Clusters can form by group mergers, but only by
accreting and stripping field galaxies of stellar
mass
Total Perseus luminosity function has a slope a
-1.4 After removing red dEs, the luminosity
function is more Similar to the field, with a
-1.25
Remember Cluster dEs cannot form from simple
mergers of groups of galaxies!
Conselice (2002)
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Hubble Space Telescope Survey of the Perseus
Cluster core
12 Orbits centered around the core of Perseus
(Conselice (PI), Held, DeRijke)
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Perseus Cluster HST survey (Conselice,
Gallagher Wyse 03 sample)
Real Dwarfs
Compact dwarfs
Background Spirals?
Ambiguous cases
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Real Dwarfs
Background Spirals?
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Compact dwarfs
Ambiguous cases
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65-80 of CGW dwarfs are cluster objects
Many other dwarfs are found in the HST imaging
that are not identified by CGW Many of these
are very smooth and symmetric
Morphological Evidence that dark matter
must exist in these systems given their extremely
low surface brightness
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Summary I. Cluster dwarf
elliptical galaxies have properties that are
fundamentally different from low-mass
galaxies in groups, including - spatial
extent - relative density compared with
giants - types of stellar populations II.
Some dEs in clusters have kinematic and stellar
populations which appear fundamentally
different from giant cluster galaxies
suggests a different formation scenario III.
Dwarf ellipticals in clusters have stellar
populations with a mix of ages and
metallicities at a given luminosity IV. These
properties can be explained if some low-mass
cluster galaxies formed both with the
cluster, while others are the remnants of
stripped galaxies accreted after the cluster
was in place and/or delayed formation