1. Evolution of Galaxy groups Michael Balogh Department of Physics University of Waterloo

2. Outline 1.Introduction 2.Groups at low redshift (2PIGG) 3.Groups at z=0.5 (CNOC2) 4.Conclusions Collaborators: –Richard Bower, Vince Eke (Durham) –Dave Wilman (Durham -> MPE) –Ray Carlberg (Toronto) –Gus Oemler, John Mulchaey (Carnegie) –Pasquale Mazzotta (Rome)

3. Galaxy clusters: review Galaxy clusters are dominated by passively evolving galaxies with high formation redshifts How does the evolution compare with the general field? Nature or nurture: clusters are built from groups. How do groups evolve? z=0z=0.39 z=0.83

4. Butcher-Oemler Effect Andreon, Lobo & Iovino 2004 Concentrated clusters at high redshift may have more blue galaxies than concentrated clusters at low redshift But blue fraction depends strongly on luminosity and radius so care needs to be taken to evaluate blue fraction at same luminosity limit, and within same (appropriate) radius. Blue fraction

5. Cluster SFR evolution Based on sparsely- sampled [OII] spectroscopy Suggests fraction of star- forming galaxies evolves only weakly in clusters Different from colour evolution? Clusters Field 2dF Nakata et al. (2005) Postman, Lubin & Oke 2001 van Dokkum et al. 2000 Fisher et al. 1998 Czoske et al. 2001 “Butcher-Oemler effect” also seen in the general field Is the effect stronger in clusters? 0 0.3 1 Redshift

6. M V < -20 High density Low density All galaxies Redshift Red galaxy fraction Evolution of the red sequence Bell et al (2004) Conflicting results from photo-z surveys? Nuijten et al. 2005 Red galaxy fraction

7. Tying star formation to structure growth Groups Clusters

8. Groups Make up ~60% of local population; abundance evolves strongly with redshift Much harder to do because contrast with background is lower. Individual groups have few members. Things to find out: –What is efficiency of galaxy formation in groups? Need stellar mass, gas mass, dynamical mass –What is star formation rate? [OII], H , UV –What is morphological composition? S0 galaxies? Irregulars? –How does all this evolve?

9. Evolution in groups z~0.05: 2dFGRS (Eke et al. 2004) –Based on friends-of-friends linking algorithm –calibrated with simulations. Reproduces mean characteristics (e.g. velocity dispersion) of parent dark matter haloes z~0.45: CNOC2 (Carlberg et al. 2001) –selected from redshift survey, 0.3<z<0.55 –~30 nights of dedicated time with LDSS2- Magellan for deeper, more complete spectroscopy

10. CNOC2 groups

11. Low redshift groups R>2R 200 2dFGRS: Lewis et al. 2003 SDSS: Gomez et al. 2004 Field Clusters Relation between star formation rate and local density holds in the infall regions of clusters (i.e. in groups)

12. CNOC2 groups: z~0.45 ~30 nights of dedicated Magellan time 295 spectroscopic members in 26 groups (r~23) Single-orbit ACS images for all 26 groups Fraction of galaxies without SF Distance from centreLocal galaxy density Wilman et al. (2004)

13. Morphologies Spiral-dominated group  =270 km/s E/S0-dominated group  =226 km/s

14. Morphologies: early results There are fewer spiral galaxies in groups than in the field, at the same redshift. No evidence for more disturbance/irregularities in group galaxies Field Spiral fraction E/S0 fraction Groups Field Spiral fraction Vel. Dispersion (km/s)

15. The connection between star formation rate, morphology and environment Like clusters, groups contain passive spirals: disk morphology but low star formation rates Field Groups Elliptical Early spiral Late spiral S0 Distributions are corrected for differences in luminosity function between group and field

16. Evolution

17. Evolution in groups Wilman et al. (2004) Fraction of non-SF galaxies Use [OII] equivalent width to find fraction of galaxies without significant star formation most galaxies in groups at z~0.4 have significant star formation – in contrast with local groups

18. Wilman et al. 2004 Fraction of non-SF galaxies increases with redshift for both groups and field Insensitive to aperture effects Fraction of non-SF galaxies Groups Group SFR evolution Fraction of non-SF galaxies Field

19. Group SFR evolution Wilman et al. 2004 shape of [OII] distribution evolves with redshift but does not depend on environment Result sensitive to aperture effects

20. Future Work

21. Better SFRs: GALEX 9 orbits awarded in Cycle 1: 3 orbits in each of 3 CNOC2 patches with deep Magellan spectroscoopy Only 1.5 orbits obtained so far Preliminary match with CNOC2 spectroscopy shows we detect most group members in the near-UV (rest frame far-UV)

22. GALEX d

23. 2PIGGz: X-ray observations T=2-3.5 keV  =550-700 km/s 9/18 groups in REFLEX survey 6/18 groups in RASS Complete, “mass”-selected group sample

24. 2PIGGz: near-infrared Stellar masses from K-band data –Obtained with ISPI (CTIO) for most groups observed with Chandra or XMM-Newton

25. Baryon content of nearby groups 24 arcmin

26. Conclusions More star formation in groups at z=0.5 than at z=0 On average, groups at 0<z<0.5 have less star formation and fewer spiral galaxies than the field. Passive spiral galaxies are a key component of groups at z=0.5 Much more to come…