Transcript 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.

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.
2.
3.
“traditional” projected distance to 5th nearest neighbour
3-dimensional density on 1 and 5 Mpc scales
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)
Ha distribution
• Ha 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 starforming 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: Ha in z~0.3 clusters
(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
Ha luminosity
Couch et al. (2001)
Balogh et al. (2002)
Correlation with density
• The fraction of
star-forming
galaxies varies
strongly with
density
2dFGRS
• Correlation at all
densities; still a
flattening near the
critical value
Isolated Galaxies
All galaxies
Bright 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.
Large scale structure
• Little dependence
on cluster velocity
dispersion
• SFR depends
mostly on galaxy
density, not
embedding halo
mass.
Comparison with models
GALFORM model
Cole et al. (2000)
Observations
Comparison with models
GALFORM model
Cole et al. (2000)
Observations
Slow decay models (strangulation) do not work
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
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
(u-r)0
Baldry et al. (2003)
Isolated Galaxies
• Fraction of SF galaxies in
lowest density
environments is not much
larger than the average
Average value
in full sample
2dFGRS
– So strong evolution in
global average cannot be
due only to a change in
densities
Large scale structure
● s > 600 km/s
● 200 < s < 400
• Measured 3-d
density on 1.1 and
5.5 Mpc scales
• groups are wellseparated in this
plane, by velocity
dispersion
Large scale structure
r5.5 (Mpc-3)
0.050
0.010
0.005
• Emission-line
fraction appears to
depend on 1 Mpc
scales and on 5.5
Mpc scales.
Increasing fraction of Ha emitters