MNRAS, submitted

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 Mr<-20.6 (SDSS); Mb<-19.5 (2dFGRS)

3 measures of environment:1. “traditional” projected distance to 5th

nearest neighbour2. 3-dimensional density on 1 and 5 Mpc scales3. 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 tovirial 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 galaxiesBright 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 ObservationsCole 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 BJ-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 environmentD. 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 (

Mp

c-3)

0.050

0.010

0.005 Increasing fraction of Hemitters