The Evolution of Groups and ClustersRichard Bower, ICC, Durham

With thanks to the collaborators that have shaped my viewsMike Balogh, Dave Wilman, Taddy Kodama, Ivan Baldry, Bob Nichol, John Mulchaey, Gus OemlerAnd people that gave me viewfoilsMike Balogh, Roger Davies, Eric Bell

OutlineFocus on clusters and the star formation history of their galaxiesClusters and Groups in the local universeThe evidence for old stellar populationsBright vs faint galaxiesClusters in the distant universeEvolution in stellar populations and star formation ratesComparison with field galaxy evolutionEvolution of the stellar mass functionOther environments in the pastThe properties of galaxies in distant groupsCan we understand what we see?

The Spirit of this TalkThe interaction of galaxies with their environment is complicatedThe growth of the universe if complicatedStar formation is complicated

our job is comprehend the elegant simplicity of the universe

The Present-Day UniverseUniform populations vs niggley details!

Clusters TodayThe evidence for uniform old stellar populations

The colour magnitude relationSmall 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 metalsLopez-Cruz et al 2004

Clusters TodayThe evidence for old stellar populations

The colour magnitude relationThe fundamental Plane -> galaxy M/LLine Indices -> direct measure of age and metalsBright ellipticals form a tight metalicity sequenceGreater diversity in the faint and S0 galaxy populationFornax: Kuntschner & Davies 1998

Clusters TodayThe evidence for old stellar populations

Line Indices -> direct measure of age and metalsBright ellipticals form a tight metalicity sequenceGreater diversity in the faint galaxy populationGreater diversity when you examine each galaxy in detail (eg Kuntschners talk 50% show some sign of intermediate age star formation)Coma: Poggianti et al 2003

Different Environments - Todaygroups (Balogh's talk): in lower density environmentsFraction of star forming galaxies suppressed in dense environments but its a continuous trendLocal density is more important than halo mass Luminosity is more important than environmentisolated galaxiesEven isolated regions contain passive galaxiesBalogh et al. 2004

Evolution What were these environments like in the past?Passive evolution vs niggley details?

Clusters in the pastCompare and contrast:the Butcher-Oemler effect versusCMR evolutionFP evolutionstar forming fractionButcher & Oemler, 1984

Clusters in the pastCompare and contrast:the Butcher-Oemler effect versusCMR evolutionFP evolutionstar forming fraction(Ellis et al; Kodama et al; Gladders et al)

Clusters in the pastCompare and contrast:the Butcher-Oemler effect versusCMR evolutionFP evolutionstar forming fractionTake care!progenitor bias(van Dokkum et al; Jorgensen et al)

Clusters in the pastCompare and contrast:the Butcher-Oemler effect versusCMR evolutionFP evolutionstar forming fraction(Nakata et al 2004)

Star formation history vs stellar mass assemblyIn cluster cores, both star formation and mass assembly seem to have happened a long time ago.Toft et al 2004

Bright vs Faint galaxiesthe cosmic down sizing hypothesis the build-up of the CMRCare is needed! De Lucia et al 2004; Kodama et al, 2004

Other Environments in the Past

Other environments in the pastthe field The cosmic star formation rateRapid increase over z=0 to 1abundance of starsModest decrease little evolution in the mass fn.But even in the field, many passive galaxies exist at z=1groups vs field(Hopkins et al 2004; Bell 2004)

Other environments in the pastthe field The cosmic star formation rateRapid increase over z=0 to 1abundance of starsModest decrease little evolution in the mass fn.But even in the field, many passive galaxies exit at z=1groups vs field(Galzebrook et al, 2004; Bell 2004)

Other environments in the pastthe field The cosmic star formation rateRapid increase over z=0 to 1abundance of starsModest decrease little evolution in the mass fn.But even in the field, many passive galaxies exit at z=1groups vs fieldMV < -20 High densityLow densityAll galaxiesRedshiftRed galaxy fraction(Bell et al 2004)

Groups at z=0.4follow-up observations with Magellan to gain higher completeness and depthAim of comparing star formation rates in groups at z~0.4 and locallyAlso infrared data from WHT; HST ACS imaging being analysed now.

Groups at z=0.420% success rate in targeted groups295 group members in 26 groups Typical group has 10 members.

Groups at z~0.4Wilman et al 2004Fraction of passive galaxiesinter-mediate redshiftLow redshiftEvidence 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 Clusterscluster formation historycomparing local/past clustersMost massive progenitor?Mass distribution of progenitors?Are clusters built from the infall of groups?What else do we want to know?z=0.5z=1z=2

The Growth of Clusterscluster formation historycomparing local/past clustersMost massive progenitor?Mass distribution of progenitors?Are clusters built from the infall of groups?What else do we want to know?z=.1z=.4From z=0.1 to 0, average cluster accretes 10%, of its mass:40% is groups20% is galaxies

galaxiesgroupsclusters

Summary: some things weve learned In ClustersUniform populations indicate old agesbut not in faint galaxies or if you look in detailAt higher redshiftEvolution of the CMR and FP suggests high formation redshiftsMass function is non-evolving toobut 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 fieldA continuous transition in the fraction of passive galaxiesEven isolated galaxies can be red and dead, particularly if brighttransformation is not a cluster specific phenomenonit must act quicklyAt higher redshiftStar forming/red galaxies are a smaller fraction of the populationThis holds for groups, not just field galaxiesthe evolution is not just a result of the lower abundance of groups

Galaxy Transformationdo we need transformation? ("nature" vs "nurture")internally or externally driven?gas consumption vs stripping/triggeringMechanisms - which ones are still viable?Ram pressureStrangulationGravitational 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 galaxiesan important clue? Evidence that galaxies are transformedGives us chance to identify the mechanism

Next Steps Sneak Preview...SLOAN imageCombined specrumstrong A-star featuresweak OII

Narrow-band ImagesRed continuumOIIH?Constructed from GMOS data cube\Maps old starsMaps continuing star formationMaps 1 Gyr old stellar population

Narrow-band ImagesH?\Maps old starsMaps continuing star formationOII velocityOII equivalent widthA-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 universewhat is the impact of the growth of large scale structure?

Redshift00.30.5510Star Formation Rate (OII eqiuv. Width)15201.0Total Star Formation rateCluster GalaxiesGroup Galaxies

Colour-magnitude relationBaldry et al. 2003(see also Hogg et al. 2003)Corrected for volume

Morphologies at z~0.75 split by environment from GEMSLow-density environmentHigh-density environmentSpheroid-dominatedDisk-dominated30% of early-types blue50% of early-types blueConsistent with later addition of early-type population in lowerdensity environments

We dont have to look at galaxy populations now. Look at higher redshift and join the environmentsProjgenitor bias: select Es or if colours are bimodal and transition rapid the results can be misleadingImportant not just star formation historyClusters are very special; What I want to do is to contrast what we see in clusters with what we see in lower density environmentsHidden slide! But shows what is being discussed.