1. 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

2. 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?

3. 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…

4. The Present-Day Universe Uniform populations vs niggley details!

5. 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

6. 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

7. Clusters Today  The evidence for old stellar populations  Line Indices -> direct measure of age and metals  Bright ellipticals form a tight metalicity sequence  Greater diversity in the faint galaxy population  Greater diversity when you examine each galaxy in detail (eg Kuntschner’s talk – 50% show some sign of intermediate age star formation) Coma: Poggianti et al 2003

8. 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

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

10. Clusters in the past  Compare and contrast:  the Butcher-Oemler effect  versus  CMR evolution  FP evolution  “star forming” fraction Butcher & Oemler, 1984

11. Clusters in the past  Compare and contrast:  the Butcher-Oemler effect  versus  CMR evolution  FP evolution  “star forming” fraction (Ellis et al; Kodama et al; Gladders et al)

12. Clusters in the past  Compare and contrast:  the Butcher-Oemler effect  versus  CMR evolution  FP evolution  “star forming” fraction  Take care!  “progenitor bias” (van Dokkum et al; Jorgensen et al)

13. Clusters in the past  Compare and contrast:  the Butcher-Oemler effect  versus  CMR evolution  FP evolution  “star forming” fraction (Nakata et al 2004)

14. 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

15. Bright vs Faint galaxies  the cosmic down sizing hypothesis  the build-up of the CMR  Care is needed! De Lucia et al 2004; Kodama et al, 2004

16. Other Environments in the Past

17. Other environments in the past (Hopkins et al 2004; Bell 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 exist at z=1  groups vs field

18. 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)

19. M V < -20 High density Low density All galaxies Redshift Red galaxy fraction 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

20. 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)

21. Groups at z=0.4  20% success rate in targeted groups  295 group members in 26 groups  Typical group has 10 members.

22. 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

23. 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

24. 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=0.5 z=1 z=2

25. 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”

26. 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

27. 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

28. 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?

29. E+A galaxies  an important clue?  Evidence that galaxies are transformed  Gives us chance to identify the mechanism

30. SLOAN image Combined specrum strong A-star features weak OII Next Steps – Sneak Preview...

31. Red continuum OII H?H? Narrow-band Images Constructed from GMOS data cube \Maps old stars Maps continuing star formation Maps 1 Gyr old stellar population

32. H?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

33. The Star formation history of the universe " what is the impact of the growth of large scale structure? Redshift 0 0.30.5 5 10 Star Formation Rate (OII eqiuv. Width) 15 20 1.0 Total Star Formation rate Cluster Galaxies Group Galaxies

34. Colour-magnitude relation Baldry et al. 2003 (see also Hogg et al. 2003) Corrected for volume

35. Morphologies at z~0.75 split by environment from GEMS Low-density environment High-density environment Spheroid-dominatedDisk-dominated 30% of early-types blue 50% of early-types blue  Consistent with later addition of early-type population in lower density environments…