The Evolution of Groups and Clusters

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. 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

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

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

10. 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)

11. 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)

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

13. 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

14. 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

15. Other Environments in the Past

16. 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 Other environments in the past (Hopkins et al 2004; Bell 2004)

17. 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)

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 Red galaxy fraction High density All galaxies Low density MV < -20 Redshift (Bell et al 2004)

19. “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 • 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.

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

21. Groups at z~0.4 Comparison with star forming fraction in the 2df-GRS Fraction of passive galaxies Lowredshift • 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? inter-mediateredshift Wilman et al 2004

22. 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

23. The Growth of Clusters z=0.5 • 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=2

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? “clusters” “groups” “galaxies” z=.4 z=.1 From z=0.1 to 0, average cluster accretes 10%, of its mass: 40% is “groups” 20% is “galaxies”

25. 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 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”)

26. 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

27. 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?

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

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

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

31. Narrow-band Images OII velocity A-star population has no discernable velocity structure, but OII has 100 km/s (p-p) rotation about minor axis H? \Maps old stars Maps continuing star formation OII equivalent width

32. The Star formation history of the universe • what is the impact of the growth of large scale structure? 20 Total Star Formation rate 15 Star Formation Rate (OII eqiuv. Width) 10 Group Galaxies 5 Cluster Galaxies Redshift 0 0.3 0.5 1.0

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