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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