Transcript driving galaxy evolution since z=1

Galaxy groups
Driving galaxy evolution since z=1
Michael Balogh
Department of Physics and Astronomy
University of Waterloo
Outline
1. What we know about galaxy
formation
a) The local Universe
b) Evolution since z<1
2. GEEC: Groups at 0.3<z<0.5
3. Model development
• We observe
starlight – which
results from the
condensation of
baryonic matter
Matter and Energy
• Baryons make
up less than
5% of the
matter and
energy in the
Universe
Spergel et. al 2003, 2006
The halo model
• The growth of
dark matter
structure is now
well understood
• Galaxy formation
history is tightly
coupled to dark
matter halo mass
www.nbody.net
The halo model
Hot baryons
~106 K for
galaxies, hence
invisible
Radiative
cooling
Dark matter
The cooling catastrophe
Cooling occurs primarily through
bremsstrahlung radiation, so
tcool  T1/2r-1
The typical density of haloes is higher
at early times: r  (1+z)3
Thus, gas cools very efficiently in
small haloes at high redshift.
The inefficiency of star formation
Wstars= 0.0014 ± 0.00013
Wstars/ Wbaryon =0.03
 >95% of baryons are dark
Why so few stars?
Simulation: dark matter
in the Local Group
Dark matter
Stars
• Overcooling leads to the
formation of hundreds more
small galaxies than are
observed.
Stellar mass
• Blue galaxies are
absent above
~3x1010 MSun
• Star formation
today occurs in
low-mass
galaxies
From GALEX & SDSS data
Salim et al. 2007
Stellar mass
SFR Density
• Most star
formation today
occurs in
M=10.5 galaxies.
Why?
Gilbank et al. 2009
Low-mass galaxies
• Low masses:
photoionization
and supernovae
reduce SFR
Massive galaxies
• Halo mass scale
constant with time,
~2x1011 MSun.
• Separates “hot” and
“cold” accretion (e.g. White
& Frenk 1991)
Dekel & Birnboim 2006
Massive galaxies
• Outflows from
massive black hole
accretion can
provide up to
6x1054 J
• AGN feedback helps
eliminate bright blue
galaxies (Springel et al. 2005;
Croton et al. 2006; Bower et al.
2006)
Galaxy Clusters
• Clusters are characterised by
bright, red ellipticals
The role of environment: halo
mass
• equally strong
dependence on
halo mass and
stellar mass
• Even low-mass
galaxies in
clusters are
mostly passive
Kimm et al. 2009 (SDSS data)
Evolution
Evolution I
• Gradual reduction in SFR at all
masses, among “active” population
“Star-forming”
galaxies in the
AEGIS survey
Noeske et al. 2007
Evolution II
z=0
• Plus growth of
“red-and-dead”
galaxies, starting
with the most
massive
zCOSMOS: Pozzetti et al. 2009
0.75<z<1
0.1<z<0.35
Group evolution in zCOSMOS
z=0
Fpassive
Groups
Field
• Evolution is
more
advanced in
clusters
• Restricted
to massive
galaxies
Iovino et al. 2009
GEEC
Group Environment Evolution
Collaboration
Michael Balogh, Sean McGee (Waterloo)
Richard Bower (Durham)
John Mulchaey, Gus Oemler (Carnegie)
Dave Wilman, Jen Connelly, Alexis Finoguenov (MPE)
Laura Parker, Annie Hou (McMaster)
Groups at 0.3<z<0.5
~200 groups
between z~0.1 and
z~0.55
Millennium Simulation
All haloes
•
•
•
•
•
•
Selected from CNOC2 survey
26 groups 0.3<z<0.55
followed-up at Magellan
IRAC and MIPS
3 Orbit GALEX
Deep Chandra/XMM
HST ACS (1 orbit in F775W)
for 20 groups
GEEC Groups
0.3<z<0.5
McGee et al. 2007
GEEC: GALEX data
SED fits
Star-forming group galaxies
• SSFR-M
correlation
independent of
environment
GEEC
GEEC
McGee et al. in prep
Groups at z=0.5
• There are more
galaxies in
groups without
any star
formation
• Note that most
of the lowestmass galaxies
are still actively
forming stars
GEEC
GEEC
zCOSMOS
limit
McGee et al. in prep
SFR evolution
• Low-redshift
comparison
sample from
SDSS
• Field in good
agreement with
Noeske et al.
• Group
environments
identical to
field.
McGee et al. in prep
Rapid evolution in groups
• Groups have been diverging from the field since
z=0.4
McGee et al. in prep
Group evolution
• Field galaxies evolve slowly: SSFR
decreases steadily with time.
• In addition to this, star formation is
shut off in group galaxies,
accelerating their evolution.
Models
Strangulation/Starvation
Kenney et al. 2003
Vollmer et al. 2004
• Gas around satellite galaxies
may be shock-heated, tidally- or
ram-pressure stripped
• Stripping the cold, dense gas in
the disk requires high velocities
and ICM densities
• The hot halo can perhaps be
stripped more easily (Larson, Tinsley
& Caldwell 1980)
Kawata & Mulchaey
2007
Environment: models
• Standard assumption is that satellite
galaxies instantly lose their entire hot
halo.
 SFR then declines on a typical timescale
(Balogh, Navarro & Morris 2000):
 M* 
t  2.2 10

10
M
Sun 

0.3
Gyr
• Low stellar-mass, red
galaxies are predicted to
be in groups
Satellite galaxies at z=0
Star-forming fraction
• Most faint, satellite galaxies are blue
• Models too efficient at shutting off gas
supply?


Too rapid? Too complete?
Or should this mechanism only apply to
massive haloes?
Model predictions
Weinmann et al. (2006); see also Gilbank & Balogh (2008)
Rapid strangulation
• Compare GEEC group
galaxy colour
distribution with models
• 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, hot stripping
• Idealised simulations
• Takes ~2 Gyr to remove
half the gas mass
 Still plenty of hot fuel left
• Through starvation alone,
low-mass satellite galaxies
could potentially continue
star formation for a
significant fraction of a
Hubble time.
McCarthy et al. 2007
Observational evidence
• Sun et al. (2007) detect hot coronae around galaxies in
clusters

Reduced luminosity compared with isolated galaxies, but
still significant.
Slow strangulation
• Models which slow
the rate of
transformation
 Destroys distinct
bimodality
 Is the problem with
the strangulation –
or with the normal
feedback cycles?
Balogh et al. (2009)
Conclusions/Future Directions
• Groups accelerate the termination of star
formation at z<1
 For reasons that are still not understood.
• GEEC2: Sample of 20 groups at z=1
selected from zCOSMOS
 X-ray detected from very deep
Chandra/XMM images
 Gemini spectroscopy proposed to return
~15-20 members per group
 HST data
Extra slides
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
•
z=0
z=0.5
z=0.8
Knobel et al. (2009)
Cluster growth via groups
• Clusters grow via:
 Major mergers
between clusters
 Accretion of groups
 Accretion of isolated
galaxies
• Scatter in cluster
properties can be a
good tracer of group
preprocessing (Balogh
et al. 2009)
McGee et al. (2009)
Group morphologies
• Only a small difference in galaxy morphology at z=0.4
This evolves strongly to z=0
Suggest morphological transformation may lag behind star
formation quenching
CNOC2
Fraction of disk galaxies
Fraction of disk galaxies


McGee et al. 2007
MGC
Allen et al. 2006
Passive spirals
• Moran et al. (2007) analyse GALEX colours
of passive spirals in two rich clusters at
z=0.5
• “starved” spirals appear to be found in
infalling groups
Timescales
• Starvation model seems a good fit to
the passive spirals in GEEC
Red: passive spirals
Black: normal spirals
GEEC groups
McGee et al. in prep