Transcript Document

Galaxy groups
Michael Balogh
Department of Physics and Astronomy
University of Waterloo
Outline
1. Where do groups fit in the
hierarchy?
2. Group selection methods
3. Properties of galaxies in groups
4. Theoretical challenges
What is a group?
• ~few L* galaxies
• Mhalo~1012-5x1013 (s<500 km/s)
 At higher masses, galaxy population
seems to be weakly dependent on halo
mass
• Physically associated – but not
necessarily virialized
Buildup of structure
•
Most galaxies today
are in groups
•
Abundance evolves
strongly
Fraction of galaxies
in groups (N>6)
increases by about
a factor 3 since z=1
•
Knobel et al. (2009)
Satellites/Centrals
• Nice idea
• Which one is the
central galaxy?
• Not surprisingly:
data show little
difference
between
correlations of
satellite/central
galaxies
Satellites/Centrals
• Nice idea
• Which one is the
central galaxy?
Satellites/Centrals
• Nice idea
• Which one is the
central galaxy?
• Not surprisingly:
data show little
difference
between
correlations of
satellite/central
galaxies
Weinmann et al. (2006)
Halo mass
Cluster growth via groups
• Clusters grow via:
 Major mergers
between clusters
 Accretion of groups
 Accretion of isolated
galaxies
• Low-mass clusters
may accrete much
of their mass
directly from the
field
Berrier et al. (2008)
Cluster growth via groups
• M=1014.2
clusters accrete
35% of galaxies
via groups
• For Coma-like
clusters, fraction
is 50%.
McGee et al. (2009), using Font et al. (2008) model
Pre-processing
• Importance of groups also depends
on how long these galaxies reside in
group environment. And main
progenitor was itself a group at
some point.
 Use “processed galaxies” as tracer of
accretion histories.
 Assume galaxies “transform” T Gyr
after first accretion into a halo >M.
• Without
preprocessing: not
only would groups
be field-like, but
clusters would show
much more scatter
McGee et al. (2009)
Fraction of processed galaxies
Slow truncation
Halo mass
• And z evolution would
be rapid
McGee et al. (2009)
Fraction of processed galaxies
Slow truncation
Halo mass
Group preprocessing
• Slow timescale, low mass
threshold predicts:
 Tight red sequence at z=0
 Weak dependence on halo
mass
 Moderate evolution:
negligible red fraction by
z=1.5 (also: Ellingson et al.
2001)
McGee et al. (2009)
Group Selection Methods
• Redshift surveys
• X-ray
• Photometric surveys
Redshift surveys
• 2dFGRS/SDSS/6dF
 >4500 sq degrees
 >5000 groups with z<0.1
• CNOC2
 1.5 sq degrees
 200 groups 0.2<z<0.55
 Extensive follow-up of
~30 groups
• zCOSMOS
 1.7 sq degree
 800 groups 0.1<z<1
• DEEP II
 1 sq degree
 899 groups with 2 or
more members
 0.7<z<1.4
X-ray selection: low-z
• ROSAT able to detect nearby
systems with s~100 km/s or
greater



Zabludoff & Mulchaey (1998)
Osmond & Ponman (2004)
Rasmussen et al. (2008)
Mulchaey & Zabludoff (1998)
X-ray selection: higher z
• XMM-LSS (~10 ks)

Willis et al. (2005)
• Mulchaey et al.
(2007); Jeltema et al.
(2007, 2008)

Nine X-ray groups
at 0.2<z<0.6, from
ROSAT DCS
• These probe low-mass
cluster regime, but
not true groups
Mulchaey et al. (2006)
X-ray selection: higher z
• CNOC2 fields: Chandra and XMM data – combined depth
equivalent to 469 ksec (Chandra)
• c.f. ~160 ks in COSMOS
z=0.4
See also Knobel et
al. (2009)
Finoguenov et al. (in prep)
Photometric selection
• McConnachie et al.
(2008) use SDSS to
detect 7400
compact groups,
photometrically.
• Attempt to correct
for contamination
using simulations
Photometric selection
• RCS: not
effective in the
group regime
• Completeness
trusted down
to s~300
km/s.
Gilbank et al. (2007)
Group properties
SDSS groups
• Weak correlation
with halo mass
for clusters
• Evidence for
larger blue
fractions in
groups
Bamford et al. (2009)
Groups and clusters
•
“For
satellite
galaxies, a
nearly equally
strong
dependence on
halo mass and
stellar mass is
seen.”
Also Weinmann et al. 2006,
Pasquali et al. 2009
Kimm et al. 2009
Properties of X-ray groups
• Spiral fraction in X-ray
groups correlates with
s, Tx
 X-ray bright groups tend
to be spiral-poor (e.g.
Brough et al. 2006)
 Significant scatter in
early fraction (Mulchaey
& Zabludoff 1998)
• HI deficiency
independent of X-ray
properties in compact
groups (Rasmussen et
al. 2008)
Osmond & Ponman (2004)
Groups at z=0.5
• At fixed
stellar mass,
groups have
fewer blue
galaxies than
the field
Balogh et al. (2009)
Groups at z=0.5
• At fixed stellar mass, groups have fewer
blue galaxies than the field
Balogh et al. (2009)
Groups and clusters at z=0.5
• Galaxies show a
halo-mass
dependence:
 Red fractions of
groups
intermediate
between cluster
and field
environments
Balogh et al. (2009)
Low-sfr galaxies
• Mounting evidence that there may be a transition
population of dust-reddened, low-sfr galaxies found in
intermediate environments

STAGES supercluster: Wolf et al. (2008); Gallazzi et al.
(2008)
• SDSS: Skibba et al. (2008); Bamford et al. (2008)
• Virgo: Crowl & Kenney (2008); Hughes et al. (2009)
• HCGs: Johnson et al. (2007); Gallagher et al. (2008)
Theoretical challenges
Rapid strangulation
• Compare z=0.5 group
galaxy colour
distribution with
models

Narrow range of NIR
luminosity
• Simple models
overpredict the red
fraction (but actually
do a pretty good job)
• The blue galaxies are
near the group halo –
but not actually
subhaloes
Balogh et al. (2009)
Slow strangulation
• Models which slow the
rate of transformation
 Destroys distinct
bimodality
• Maybe only a fraction
of group galaxies
should be affected;
orbit-dependent?
• Puzzle: strangulation
should be slow for lowmass galaxies (e.g.
Haines, Rasmussen)…
why so quick in
GALFORM?
Balogh et al. (2009)
Halo mass is King(?)
• Galaxy properties at z=0
depend almost equally on
stellar mass and halo mass
• SFR/SN/AGN/gas accretion
rate are the king’s advisors


Theorists have worked hard
to make models that work,
assuming they correlate
primarily with halo mass.
Does success in matching
the luminosity function
under this assumption mean
we know what’s going on?
• Or have we just given the emperor a new set of clothes?
Conclusions/Future Directions
• Robust samples of groups at 0<z<1 now
routinely available
 All require good mock catalogues to account
for contamination, selection effects
• Need more precise measures of SFH
 Dust-obscured star formation
 SF on long vs short timescales
• Need to find source of scatter in group
properties
 Lx-M residuals? Concentration? Dynamics?
Associated large-scale structure?