The Evolution of Groups and Clusters " Richard Bower, ICC, Durham " With thanks to the collaborators...

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




(Ell

is e

t al;

Kod

ama

et a

l; G

ladd

ers

et a

l)




Take care! “progenitor bias”

(van

Dok

kum

et a

l; J

orge

nsen

et a

l)




(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


(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



rate Rapid increase over z=0 to

1

abundance of stars Modest decrease – little


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


(Bell et al 2004)


rate Rapid increase over z=0 to

1

abundance of stars Modest decrease – little


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

The Evolution of Groups and Clusters " Richard Bower, ICC, Durham " With thanks to the collaborators...

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