MNRAS, submitted

Galaxy evolution Evolution in global properties reasonably well established What drives this evolution? How does it depend on environment? Steidel et al. (1999)

Cluster Galaxies Evolution clearly depends on environments – clusters are extreme examples But even more common environments (groups) show differential evolution

Theoretical expectations (?) Isolated galaxies have (invisible) halo of hot gas that can cool and replenish the disk –allows star formation to continue for longer Cluster galaxies lose this gas, so their SFR declines more quickly. Also cluster galaxies form earlier. –therefore SFRs should be lower in clusters no ram-pressure stripping, harassment needed to achieve reasonable match to observed clusters (Diaferio et al. 2001; Okamoto et al. 2003).

First steps: nearby clusters Analysis of 2dFGRS –reduced SFRs at low densities –relation holds outside clusters –“critical density”? Gradient is consistent with the strangulation hypothesis (Balogh et al. 2000) Lewis et al. (2002)

New study based on combination of SDSS and 2dF galaxy redshift surveys Volume-limited sample of 24,968 galaxies at  0.05<z<0.1  M r <-20.6 (SDSS); M b <-19.5 (2dFGRS) 3 measures of environment: 1.“traditional” projected distance to 5 th nearest neighbour 2.3-dimensional density on 1 and 5 Mpc scales 3.velocity dispersion of embedding cluster or group  catalogues of Nichol, Miller et al. and Eke et al.

Group catalogues 2dFGRS (Eke et al.) –Based on friends-of-friends linking algorithm –calibrated with simulations. Reproduces mean characteristics (e.g. velocity dispersion) of parent dark matter haloes –is highly complete, at expense of having unphysical contamination, esp. at low masses –selected subsample with at least 10 members above our luminosity limit. SDSS (Nichol, Miller et al.) –Search for clustering in spatial and colour space; also calibrated with simulations –Selected subsample with Gaussian velocity dispersions –is a highly pure sample, at expense of being incomplete

2dF groups SDSS groups circle size is proportional to virial radius (vel. dispersion)

H  distribution H  distribution is distinctly bimodal: SFR is not continuous –also seen in colours: (Baldry et al. 2003; Strateva et al. 2001) galaxies do not have arbitrarily low SFR So mean/median do not necessarily trace a change in SFR

The star-forming population Amongst the star- forming population, there is no trend in mean SFR with density! Hard to explain with simple, slow-decay models (e.g. Balogh et al. 2000)

Recalling: H  in z~0.3 clusters Balogh et al. (2002) Couch et al. (2001) (Field) Number of emission lines galaxies is low in all clusters However, shape of luminosity function similar to field: –consistent with shift in normalisation; not in H  luminosity

Correlation with density The fraction of star-forming galaxies varies strongly with density Correlation at all densities; still a flattening near the critical value 2dFGRS

Isolated Galaxies Selection of isolated galaxies: –non-group members, with low densities on 1 and 5.5 Mpc scales ~30% of isolated galaxies show negligible SF –challenge for models? –environment must not be only driver of evolution. All galaxies Bright galaxies

Large scale structure Little dependence on cluster velocity dispersion SFR depends mostly on galaxy density, not embedding halo mass.

Comparison with models GALFORM model Observations Cole et al. (2000)

Comparison with models GALFORM model Observations Slow decay models (strangulation) do not work Cole et al. (2000)

Redshift evolution? Comparison of 2dFGRS with CNOC2 groups/field [OII] distributions for rest B J -limited samples Average [OII] for SF galaxies does not appear to depend on redshift or environment Fraction of [OII] emitters depends on both redshift and environment D. Wilman et al.

Conclusions Any environment-induced change to galaxy SFR must be rapid, and occur in low-density environments Galaxy-galaxy interactions are the most likely cause of observed segregation: only environment directly observed to influence galaxy evolution Models of galactic cooling flows must be incomplete

Bimodality SDSS colours show two distinct populations Red population may be the result of major mergers at high redshift, followed by passive evolution Baldry et al. (2003) (u-r) 0

Isolated Galaxies Fraction of SF galaxies in lowest density environments is not much larger than the average –So strong evolution in global average cannot be due only to a change in densities Average value in full sample 2dFGRS

Large scale structure Measured 3-d density on 1.1 and 5.5 Mpc scales groups are well- separated in this plane, by velocity dispersion ●  > 600 km/s ● 200 <  < 400

Large scale structure Emission-line fraction appears to depend on 1 Mpc scales and on 5.5 Mpc scales.  5.5 (Mpc -3 ) Increasing fraction of H  emitters