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The Evolution of Groups and Clusters
Richard Bower, ICC, Durham
With thanks to the collaborators that have shaped my views
Mike Balogh, Dave Wilman, Taddy Kodama, Ivan Baldry, Bob Nichol, John Mulchaey, Gus Oemler
And people that gave me viewfoils
Mike Balogh, Roger Davies, Eric Bell
Outline Focus on clusters and the star formation history of their galaxies… Clusters and Groups in the local universe
The evidence for old stellar populations Bright vs faint galaxies
Clusters in the distant universe
Evolution in stellar populations and star formation rates Comparison with field galaxy evolution Evolution of the stellar mass function
Other environments in the past
The properties of galaxies in distant groups Can we understand what we see?
The Spirit of this Talk
The interaction of galaxies with their environment is complicated The growth of the universe if complicated Star formation is complicated
…our job is comprehend the elegant simplicity of the universe…
The Present-Day Universe
Uniform populations vs niggley details!
Clusters Today
The evidence for uniform old stellar populations
The colour magnitude relation Small scatter (between clusters
and within)
Interesting aside: is the CMR really flat? (Bernardi et al)
The fundamental Plane -> galaxy M/L (ciotti & Renzini 1993)
Line Indices -> direct measure of age and metals
Lopez-Cruz et al 2004
Clusters Today
The evidence for old stellar populations
The colour magnitude relation
The fundamental Plane -> galaxy M/L
Line Indices -> direct measure of age and metals
Bright ellipticals form a tight metalicity sequence
Greater diversity in the faint and S0 galaxy population
Fornax: Kuntschner & Davies 1998
Different Environments - Today groups (Balogh's talk): in lower
density environments
Fraction of star forming galaxies suppressed in dense environments – but it’s a continuous trend
Local density is more important than halo mass
Luminosity is more important than environment
isolated galaxies
Even isolated regions contain “passive” galaxies
Balogh et al. 2004
Evolution – What were these environments like in the past?
Passive evolution vs niggley details?
Clusters in the past
Compare and contrast: the Butcher-Oemler effect
versus CMR evolution FP evolution “star forming” fraction
Butcher & Oemler, 1984
Clusters in the past
Compare and contrast: the Butcher-Oemler effect
versus CMR evolution FP evolution “star forming” fraction
(Ell
is e
t al;
Kod
ama
et a
l; G
ladd
ers
et a
l)
Clusters in the past
Compare and contrast: the Butcher-Oemler effect
versus CMR evolution FP evolution “star forming” fraction
Take care! “progenitor bias”
(van
Dok
kum
et a
l; J
orge
nsen
et a
l)
Clusters in the past
Compare and contrast: the Butcher-Oemler effect
versus CMR evolution FP evolution “star forming” fraction
(Nak
ata
et a
l 200
4)
Star formation history vs stellar mass assembly
In cluster cores, both star formation and mass assembly seem to have happened a long time ago.
Toft et al 2004
Bright vs Faint galaxies
the cosmic down sizing hypothesis
the build-up of the CMR
Care is needed!
De
Luc
ia e
t al 2
004;
Kod
ama
et a
l, 20
04
Other Environments in the Past
Other environments in the past
(Hopkins et al 2004; Bell 2004)
the field The cosmic star formation
rateRapid increase over z=0 to 1
abundance of starsModest decrease – little
evolution in the mass fn.
But …even in the field, many “passive” galaxies exist at z=1
groups vs field
Other environments in the past
the field The cosmic star formation
rate Rapid increase over z=0 to
1
abundance of stars Modest decrease – little
evolution in the mass fn.
But …even in the field, many “passive” galaxies exit at z=1
groups vs field(Galzebrook et al, 2004; Bell 2004)
MMVV < -20 < -20
High densityHigh density
Low densityLow density
All galaxiesAll galaxies
RedshiftRedshift
Re
d g
ala
xy fr
act
ion
Re
d g
ala
xy fr
act
ion
Other environments in the past
(Bell et al 2004)
the field The cosmic star formation
rate Rapid increase over z=0 to
1
abundance of stars Modest decrease – little
evolution in the mass fn.
But …even in the field, many “passive” galaxies exit at z=1
groups vs field
Groups at z=0.4
follow-up observations with Magellan to gain higher completeness and depth
Aim of comparing star formation rates in groups at z~0.4 and locally
Also infrared data from WHT; HST ACS imaging being analysed now.
“LDSS-2 Distant Group Survey”: Based on the CNOC2 redshift survey aimed at z~0.5. Group selection and inital look at properties described in Carlberg et al. (2001)
Groups at z=0.4 20% success
rate in targeted groups
295 group members in 26 groups
Typical group has 10 members.
Groups at z~0.4
Wilman et al 2004
Fraction of passive galaxies
inter-mediate redshift
Low redshift
Evidence for evolution in galaxy groups. Groups were a much more active environment in the past – but is this because:
•groups are more recently assembled?•the galaxies forming the groups are more active?
Comparison with star forming fraction in the 2df-GRS
So what does it all mean?
To make sense of it all we need to know how to connect together different environments over a
range of redshift
The Growth of Clusters
cluster formation history comparing local/past
clusters Most massive progenitor? Mass distribution of
progenitors? Are clusters built from the
infall of groups? What else do we want to
know?
z=0.5z=1
z=2
The Growth of Clusters
cluster formation history comparing local/past
clusters Most massive progenitor? Mass distribution of
progenitors? Are clusters built from the
infall of groups? What else do we want to
know?
z=.1
z=.4
From z=0.1 to 0, average cluster accretes 10%, of its mass:
40% is “groups”
20% is “galaxies”
“galaxies”
“groups”“clusters”
Summary: some things we’ve learned
In Clusters
Uniform populations indicate old ages
…but not in faint galaxies or if you look in detail
At higher redshift
Evolution of the CMR and FP suggests high formation redshifts
Mass function is non-evolving too …but
the blue fractions evolve in clusters (but not the star forming fraction)
You can see the build up of the CMR (cosmic “downsizing”)
In groups and the field
A continuous transition in the fraction of passive galaxies
Even isolated galaxies can be “red and dead”, particularly if bright
…transformation is not a cluster specific phenomenon
…it must act quickly At higher redshift
Star forming/red galaxies are a smaller fraction of the population
This holds for groups, not just “field” galaxies
…the evolution is not just a result of the lower abundance of groups
Galaxy Transformation
do we need transformation? ("nature" vs "nurture")
internally or externally driven? gas consumption vs stripping/triggering
Mechanisms - which ones are still viable? Ram pressure Strangulation Gravitational interactions
Star formation history is not morphology!
star formation rate and morphology are not the same thing!
does morphological transformation take longer?
Is it the same mechanism?
E+A galaxies
an important clue? Evidence that galaxies are transformed Gives us chance to identify the mechanism
SLOAN image
Combined specrumstrong A-star featuresweak OII
Next Steps – Sneak Preview...
Red continuum
OII H?
Narrow-band ImagesConstructed from GMOS data cube
\Maps old stars Maps continuing star formation
Maps 1 Gyr old stellar population
H?
Narrow-band Images
\Maps old stars Maps continuing star formation
OII velocity
OII equivalent width
A-star population has no discernable velocity structure, but OII has 100 km/s (p-p) rotation about minor axis
The Star formation history of the universe
what is the impact of the growth of large scale structure?
Redshift0 0.3 0.5
5
10
Sta
r F
orm
atio
n R
ate
(OII
eqi
uv.
Wid
th)
15
20
1.0
Total Star
Formati
on rate
Cluster Galaxies
Group Galaxies
Colour-magnitude relation
Baldry et al. 2003
(see also Hogg et al. 2003)
Corrected for volume