Galaxy Groups Michael Balogh University of Durham

z=0: Richard Bower, Vince Eke (Durham) Warrick Couch, Ian Lewis and the 2dF team Bob Nichol, Chris Miller (CMU) z=0.5: Dave Wilman, Richard Bower (Durham) John Mulchaey, Gus Oemler (Carnegie) Ray Carlberg (Toronto) z=1: Fumiaki Nakata, Mark Sullivan, Dave Gilbank, Ian Smail, Richard Bower (Durham) Taddy Kodama, Ichi Tanaka (Tokyo) Galaxy Groups

Number evolution of Groups Groups Clusters

Z=0 : The 2dFGRS and SDSS

Z=0 : The SDSS and 2dFGRS Combined volume-limited sample from both surveys. Final catalogue of ~15000 galaxies M r <-20.6, 0.05<z<0.1 Group catalogues from V. Eke (2dFgrs) and C. Miller (SDSS Early data release) Measure star formation rates from H  emission

Group Finder 2dFgrs Based on a friends-of- friends algorithm (Eke et al., in prep) Effective at finding small groups with  « 500 km/s SDSS Different algorithm, includes colour information (Miller et al. in prep). Effective at finding more massive groups

Star formation in groups Preliminary results: Average H  equivalent width within R vir is lower than the global average, for  >200 km/s Mean SFR is suppressed in all bound galaxy associations at z=0 Balogh et al. in prep Field

SFR-Density Relation in the 2dFGRS Lewis, Balogh et al MNRAS 334, 673 Field Lewis et al. (2002) identified a critical density of ~1 Mpc -2, where environmental effects become important This relation is independent of group velocity dispersion

X-ray properties of 2dF groups Mass-selected sample of 21 groups from 2dFgrs catalogue,  >500 km/s XMM proposal to observe 9 of the lowest X-ray luminosity groups, for which good data does not already exist P. Mazzotta, PI

Z=0.5 : The CNOC2 survey

Groups at Z=0.5 Durham involvement: follow- up observations with Magellan to gain higher completeness and depth Also infrared data from WHT Based on the CNOC2 redshift survey. Group selection and inital look at properties described in Carlberg et al. (2001) Wilman, Balogh, Bower Mulchaey, Oemler Carlberg

CNOC2 groups at z~0.5 Wilman et al. in prep Velocity dispersions range from ~200 km/s to ~650 km/s

CNOC2 groups at z~0.5 Wilman et al. in prep. Distance from centre (Mpc) Mean EW [OII] (Å) Preliminary result for 7 groups Within ~1 Mpc of group centre, galaxy SFR is low relative to surrounding field Compare with z=0 data to establish evolution of the “critial” density

Z > 1 : The Future

Groups at z > 1 Deep multicolour (VRi′z′JK s ) images of Lynx and Q (z=1.2). Proposals to observe high redshift radio galaxies and radio-loud quasars: known to reside in dense environments – IRIS2 narrow band H  and [OIII] at z=2.3 – GMOS/FORS2 narrow band filter + grism H  and [OII] spectroscopy at z=1.4, 1.47, 2.3

Lynx clusters: z=1.2 Subaru VRi’z’ INGRID JK s Identified 7 groups around the clusters from photometric redshifts. GMOS spectroscopy pending X (arcmin) Y (arcmin) Nakata et al. (2002)

Groups around the Lynx clusters CL1 CL2 GR3 Group at z=1.27, ~3 Mpc from main cluster CMR is present, perhaps with more scatter and fewer bright galaxies Nakata et al. in prep

Groups around the Lynx clusters M * clus ~ M * group ~ Nakata et al. in prep

Steidel et al The goal… What does the “Madau” plot look like for galaxy groups? This will distinguish evolution driven by large scale structure from evolution driven by local physics