The influence of environment The influence of environment on galaxy populationson galaxy populations
Michael BaloghUniversity of Waterloo, Canada
Outline
• Low redshift– Simple trends encompass most of
what we know of as environmental influences
• Models: what works and what doesn’t
• Redshift evolution• The future: what’s next?
The influence of The influence of environment environment on galaxy on galaxy populationspopulations
Populations• Current star formation
rate• Recent star formation • Stellar mass (average
SFR)• Morphology (of stars,
neutral gas, ionized gas)
• AGN • Gas content
Environment• Mass of dark
matter halo• Position within
halo• Local density• Large-scale
density
The The influenceinfluence of environment of environment on galaxy populationson galaxy populations
• Nature vs. nurture?• Entangled in current models
– Gas accretion, merger, and feedback history scale with halo mass.
– No longer the right question?• A better question: what physics
operates in haloes of a given mass, at a given epoch?– Today’s population is the result of
different environments at different epochs: cannot try to isolate one mechanism as responsible for the observed trends.
The local Universe
Colour-magnitude distribution
• Nearby galaxies seem to fall into two surprisingly well-defined, smoothly varying distributions.
• Colour, luminosity, concentration, star formation rate
Blanton et al. 2004
Colour-magnitude distribution
• Colour distribution in 0.5 mag bins can be fit with two Gaussians
• Mean and dispersion of each distribution depends strongly on luminosity
• Dispersion includes variation in dust, metallicity, SF history, and photometric errors
• At bright magnitudes, significant fraction of “blue” population “contaminates” red: c.f. talk by Wolf.
(u-r)
Bright
Faint
Baldry et al. 2003
• Fraction of red galaxies depends strongly on density. This is the primary influence of environment on the colour distribution.
• Mean colours depend weakly on environment: transitions between two populations must be rapid (or rare at the present day)
Balogh et al. 2004
• Fraction of red galaxies depends strongly on density. This is the primary influence of environment on the colour distribution.
• Mean colours depend weakly on environment: transitions between two populations must be rapid (or rare at the present day)
• Trend is not completely absent for fainter galaxies; but never dominant
Balogh et al. 2004
The star-forming population
• Rines et al. 2005: H distribution in virial, infall and field regions nearly identical.
• Carter et al. (2001) – 3150 nearby galaxies
• H for SF galaxies does not depend on environment
– Triggering of SF occurs on small spatial scales
•Hard to explain with simple, slow-decay models (e.g. Balogh et al. 2000)
Halo mass dependence
• Environment: halo mass– Use luminosity
as tracer of mass. Compare with theoretical mass function
• At fixed mass the late-fraction depends weakly on luminosity
• Late-type fraction depends most strongly on halo mass
Weinmann et al. 2005
[-21,-22][-22,-23]
[-20,-21][-19,-
20][-18,-19]
R luminosity
Halo mass dependence
colo
ur
SF
Rco
nce
ntr
ati
on
• Average properties of galaxies in either peak is independent of halo mass– But depends
on luminosity
[-21,-22][-22,-23]
[-20,-21][-19,-
20][-18,-19]
R luminosity
Weinmann et al. 2005
Local effects?
• Still a (weak) trend with radius in haloes of fixed mass
• Dependence on luminosity (surprisingly?) weak
Weinmann et al. 2005
1014<M<1015
1013<M<1014
Conformity• Properties of “satellite” galaxies appear to be
connected with properties of “central” (actually brightest) galaxy
Weinmann et al. 2005
Similar to effect seen in 2PIGG groups? See Vince Eke’s talk.
Definition of central?
Implications
• Simple dependence of “late-type” fraction on environment characterizes much of observed trends (e.g. SFR-density, morphology-density, colour-density etc.).
• Interpretation?1. Two modes of formation. Within each
peak is variance due to dust, metallicity (second-order effects).
2. Transitions: Where do S0, E+A fit in? 3. Burst vs. continuous SFR (Kauffmann et
al. 2005)
Signs of Nurture: Virgo spirals
Kenney et al. 2003Vollmer et al. 2004
• Ram-pressure stripping in Virgo
H for Virgo galaxy
H for normal galaxy
• Truncated H disks in clusters
Koopmann & Kenney 2004also: Vogt et al. 2004
Signs of Nurture: morphology and SFR
• Passive Spirals• E+A galaxies?• S0, dSph, UCDs• Wolf’s dusty
spirals? Peak in infall region?
• e.g. Christlein & Zabludoff (2005)– Residual [OII] after subtracting
expectation for given B/T, D4000 and Mstar.
• SFR gradient is not entirely:– Consequence of MDR– Consequence of change in mass
function– Effect of initial conditions
AGN• AGN fraction independent of density
– Surprising?
Carter et al. (2001)
Miller et al. (2003)
Models
Semi-analytic approach
• Trace merger histories with N-body simulations (cannot use Press-Schechter because you need to know where the galaxies are)
• More massive haloes form earlier: longer merger history.– There is also a larger-scale bias: haloes of a
given mass form earlier in denser environments (Sheth & Tormen 2004; Abbas & Sheth 2005; Harker et al. 2005)
• Make simple assumptions about gas accretion (e.g. no accretion onto satellites) and feedback (supernova, AGN)
General trends: successes
Springel et al. 2001: morphology-density relation
Okamoto & Nagashima (2003)SFR-radius
Diaferio et al. (2001)colour-radius
0.0 0.5 1.0 1.5 2.0R/R200
Bimodality?•Springel et al. 2001; Diaferio et al. 2001
–Bimodality in field not clear
–All cluster galaxies are red
Okamoto & Nagashima 2003
–SFR is suppressed in all galaxies: blue peak is distorted
-24 -22 -20 -18 -16
MV-logh-24 -22 -20 -18 -16
MV-logh
Cole et al. 2000Supernova feedback prescription does not produce bimodal colour distribution at faint magnitudes. Spirals
Ellipticals
All
cluster
DataModel
SPH simulationsKeres et al. (2005): SPH simulations
reproduce trend of decreasing SFR with increasing density (see also Berlind et al. 2004).
Confirm this is due to reduced accretion of hot gas
But colour-distribution of galaxies doesn’t look quite right…
SFR
Hot accretion
Cold accretion
SPH
Observed
Improving the colour distribution
• Springel, Di Matteo & Hernquist (2005)– Including black hole
feedback terminates star formation more quickly. Leads to rapid reddening of merger remnants
• Sijacki & Springel 2005• AGN feedback removes
young population in cD galaxiesNo feedback
Magorrian-AGN
BH accretion rate
Improving the colour distribution
• Croton et al. (2005)• Radio-feedback most
efficient in large groups.• Proportional to Mgas×MBH
Cooli
ng
rate
(M
sun
/yr)
Models: summary
• When feedback parameters are tuned to reproduce the field luminosity function and colour distribution, what will we find as a function of environment?– General trends will be reproduced.
But will it be for the right reasons?– Any differences in detail: will they
signify “nurture” processes? Or just that feedback parameters need further tuning?
Back to observations: Evolution
Evolution: clusters(briefly)
• Morphology-density relation (see talks by Postman, Dressler)– Fewer S0 in z=1 clusters, but non-zero – Little evolution in MDR z=1 to z=0.5– Suggests high-z MDR is primordial, with
z<0.5 environment-driven evolution
• SFR and colour gradients– Radial gradients steeper in the past
(Ellingson et al. 2001; Kodama & Bower 2001)
– Can be related to truncation of star formation in an infalling field population
Clusters• Tanaka et al. 2005 (see poster)
– tight CMR in place in clusters to z=0.8– Faint end of CMR in groups formed z~0.5– No CMR in field at z=0.8– Also De Lucia (2004): faint end of red sequence disappears at
z>0.5
Clusters
Field
2dF
Nakata et al. 2005
Postman, Lubin & Oke 2001van Dokkum et al. 2000
Fisher et al. 1998
Czoske et al. 2001
Cluster galaxy evolution
• Supported by observed evolution in [OII]-emission fraction (Nakata et al. 2005)– Field evolves much more
strongly than clusters (for bright galaxies)
Evolution: photo-z surveys
• Similar rate of increase in red fraction in the field and clusters – average field red sequence
galaxy came into the sample later
All galaxies
CFHTLS: Nuijten et al. (2005)
Re
d g
ala
xy fr
act
ion
0
0.2
0
.4
0.6
0
.8
1.0
0.2
0
.4
0.6
0
.8
MV < -20
High density
Low density
RedshiftR
ed
ga
laxy
fra
ctio
n
COMBO-17: E. Bell et al.
Luminosity, density and redshift dependence of red
fraction
RCS z>0: Yee et al. (2005)SDSS z=0: Balogh et al. (2004)
Luminosity, density and redshift dependence of
colour
RCS z>0: Yee et al. (2005)SDSS z=0: Balogh et al. (2004)
Luminosity, density and redshift dependence of
colour• Little evolution in red peak colour
RCS z>0: Yee et al. (2005)SDSS z=0: Balogh et al. (2004)
Luminosity, density and redshift dependence of
colour• Little evolution in red peak colour• Colours of bright blue galaxies evolve strongly
RCS z>0: Yee et al. (2005)SDSS z=0: Balogh et al. (2004)
Galaxy groups at z=0.4
• Selected from CNOC2 survey• >30 nights Magellan spectroscopy
(better completeness, depth)• ACS image of ~30 groups• GALEX data rolling in slowly• Spitzer (IRAC and shallow MIPS)
data from GTO programs• Collaborators: Dave Wilman (MPE),
Richard Bower (Durham), Gus Oemler, John Mulchaey (Carnegie), Ray Carlberg (Toronto)
Groups at z=0.4: Morphologies
Spiral-dominated group=270 km/s
E/S0-dominated group=226 km/s
Morphologies: early results
• There are fewer spiral galaxies in groups than in the field, at the same redshift.
• No evidence for more disturbance/irregularities in group galaxies
Field
Sp
iral
fract
ion
E/S
0 f
ract
ion
Groups
Groups
Groups
FieldS
pir
al
fract
ion
Vel. Dispersion (km/s)
The connection between star formation rate, morphology and environment
Like clusters, groups contain passive spirals: disk morphology but low star formation rates
FieldGroups
Elliptical
Early spiral Late spiral
S0
Distributions are corrected for differences in luminosity function between group and field
Stellar mass-SFR
• Stellar masses from archival Spitzer (IRAC) data
• Significant star formation seen in more massive galaxies than locally: downsizing?
• No significant difference between group and field for this subsample.
Rosati? z=1
SDSS (Kauffmann et al.)
Evolution in groups
Wilman et al. (2004)
Fra
ctio
n o
f n
on
-SF
g
ala
xies
• Use [OII] equivalent width to find fraction of galaxies without significant star formation
• most galaxies in groups at z~0.4 have significant star formation – in contrast with local groups
• cf. Gonzalez talk: supergroup
Wilman et al. 2004
• Fraction of non-SF galaxies increases with redshift
• for both groups and field
• Insensitive to aperture effects
• Evolution cannot be account for by passive-evolution models. Require truncation of star formation (both groups and field)
Fra
ctio
n o
f n
on
-SF
g
ala
xies
Groups
Group SFR evolutionF
ract
ion
of
non
-SF
g
ala
xies
Field
Clusters
Field
2dF
Nakata et al. 2005
Postman, Lubin & Oke 2001van Dokkum et al. 2000
Fisher et al. 1998
Czoske et al. 2001
Group Evolution
Groups: Wilman et al. (2005)
High redshift• Spectroscopic survey: ~100 redshifts 1.48<z<2.89• Overdense region has more massive, older galaxies• Consistent with expectations for earlier formation time
(1600 Myr vs 800 Myr)
Steidel et al. (2005)
High redshift
• UV-selected LBG survey
• No environmental dependence of SFR
• Can be consistent: cluster galaxies get head start, but instantaneous SFR the same
• Even at z=0 it seems star-forming galaxies have a distribution independent of environment
Bouché & Lowenthal (2005)
The future
• Theory: still has a lot of catching up to do– Thus we are in discovery mode rather
than testing mode• Observations:
– Dust-obscured SF (Spitzer, Herschel)– AGN/SF connection at z>0– Lower luminosities– Spatial dependence of SFR (i.e. IFU
spectroscopy)– Transitional galaxies
Top Related