driving downsizing with groups of galaxies michael balogh department of physics and astronomy...
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Driving Downsizing with groups of galaxies
Michael BaloghDepartment of Physics and Astronomy
University of Waterloo
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or: the faint red galaxy problem
CollaboratorsDavid Gilbank, Sean McGee, Robbie Henderson
(Waterloo)Dave Wilman, Daniel Pierini (MPE, Garching)
Richard Bower, Simon Morris (Durham)John Mulchaey, Gus Oemler (Carnegie)
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Outline
I. Review: Galaxy formation models
II. Evolution of faint red galaxiesIII. Galaxy groups at z=0.4IV. Revisiting starvation
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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
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The halo modelThe halo model
Radiativecooling
Hot baryons
Dark matter
~106 K for galaxies, hence invisible
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>95% of baryons are dark
The inefficiency of star formationThe inefficiency of star formationstars= 0.0014 ± 0.00013
stars/ baryon =0.03
(Balogh et al. 2001; Cole et al. 2002)
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Galaxy Luminosity Galaxy Luminosity FunctionFunction
Benson et al. 2003
Nu
mb
er
den
sity
of g
ala
xie
s
Luminosity
Theory
Data
stars/ baryon =0.03
(Balogh et al. 2001; Cole et al. 2002)
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Stellar mass• Blue galaxies are absent above ~3x1010 MSun
• Star formation today occurs in low-mass galaxies
Kauffmann et al. (2003)
Baldry et al. (2004) 11 10 9 8 log M*
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Gas Accretion• Halo mass scale
constant with time, ~2x1011 MSun.
• Separates “hot” and “cold” accretion (e.g. White & Frenk 1991)
• AGN feedback helps eliminate bright blue galaxies (Springel et al. 2005; Croton et al. 2006; Bower et al. 2006)
Dekel & Birnboim 2006
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Galaxy Clusters• A standard picture to motivate environmental
effects: Clusters are dominated by bright, red ellipticals
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Low-mass galaxies
• Galaxies with M~109 MSun are well below the “threshold” mass.
• But the fraction of red galaxies STILL depends strongly on environment.
Baldry et al. (2006)
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Strangulation/Starvation
• 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
Kenney et al. 2003Vollmer et al. 2004
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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):
• Low stellar-mass, red galaxies are predicted to be in groups, above the critical mass limit
GyrM
Mt
Sun
3.0
10*
102.2
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Satellite galaxies at z=0• 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?
Weinmann et al. 2006
Model predictions
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Part II: Evolution of faint red
galaxies
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Red Galaxy luminosity function
• Faint red galaxies have appeared recently in clusters
De Lucia et al. (2007)
• Dwarfs: -18.2>Mv>-20• Giants Mv<-20
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• Faint red galaxies have built up in clusters since z~1
Cluster data from:Gilbank et al. (2007)Stott et al. (2007)Hansen et al. (2007)Barkhouse et al. (2007)Andreon (2007)Tanaka et al. (2005)De Lucia et al. (2004)
Observed galaxy clusters
Gilbank & Balogh (2008) Redshift
Red
Dw
arfs
/Gia
nts
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• Faint red galaxies are less common in the field – but also increasing with time (more rapidly?)
Field data from:Bell et al. (2003, 2004)Driver et al. (2006)Scarlata et al. (2007)Brown et al. (2007)Zucca et al. (2006)Baldry et al. (2004)
Observed galaxy clusters
Observed field galaxies
Gilbank & Balogh (2008) Redshift
Red
Dw
arfs
/Gia
nts
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Observed galaxy clusters
Observed field galaxies
Bower et al. (2006) model predictions
Gilbank & Balogh (2008) Redshift
Red
Dw
arfs
/Gia
nts
• Models predict a large fraction of faint, red galaxies at all redshifts, even in the field
• Due to the red satellite galaxies in small groups
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Gilbank & Balogh (2008) Redshift
Red
Dw
arfs
/Gia
nts
• The evolution in the field can be explained if faint, red galaxies are produced only in groups with masses greater than 1012.5 MSun.
SunM5.1210
SunM1310
SunM1210
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Red dwarf/giant ratio
• Models are far too efficient at quenching star formation in satellite (group) galaxies
• Galaxy groups at z=0.5 are critical for detailed study of transforming galaxies
Redshift
Red
Dw
arfs
/Gia
nts
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Part III: Galaxy groups at z=0.4
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Groups at z~0.4• ~200 groups between
z~0.1 and z~0.55, selected from the CNOC2 survey (Carlberg et al. 2001)
• Follow-up at Magellan• 26 groups targeted between
z =0.3 and z=0.55
• Observations of 20 groups for 1 orbit each in F775W filter with HST ACS camera
• 3 Orbit GALEX data• IRAC and MIPS data• XMM, Chandra
“CNOC2” GroupsZ=0.5
Millennium Simulation
All haloes
McGee et al. 2007
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Star formation in groups
• At all stellar masses, star-forming galaxies are found less frequently in groups
Bower et al. model groups
Balogh et al. 2006
Fra
ctio
n w
ith
[OII
] em
issi
on li
nes
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Passive galaxies
• Spitzer IRAC colours are an excellent tracer of low-levels of activity
Wilman et al. 2007
SpiralsE/S0
[8m]-[3.6m] colour
rest [m]
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Star formation in groups• Dusty and/or low-levels of star formation in massive
galaxies Break occurs at ~1011 MSun. Group galaxies still show less activity than field galaxies
of the same mass.
10.5 11 11.5log10 Mstellar/MSun
Infr
ared
Act
ive
frac
tion
Opt
ical
ly A
ctiv
e fr
actio
n
Wilman et al. 2007 Balogh et al. 2006
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Group morphologies
Fra
ctio
n of
dis
k ga
laxi
es
Fra
ctio
n of
dis
k ga
laxi
es
McGee et al. 2007
• 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
CNOC2MGC
Allen et al. 2006
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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
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GALEX• Starvation model seems a good fit to
the passive spirals in CNOC2 groups
McGee et al. in prep
CNOC2 groupsRed: passive spirals
Black: normal spiralsGreen: passive spirals
Blue: normal spirals
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Summary: z=0.4 groups
• There is evidence galaxies are being quenched in groups, but the effect is not dramatic
• We are embarking on a full multiwavelength analysis from FUV to MIR to constrain the star formation histories of group members
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Part IV: Revisiting starvation models
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Slow strangulation
• How quickly do galaxies lose their gas?• Consider analytic and numerical
(GADGET-2) models of “hot” gas+DM haloes merging with groups or clusters, on cosmologically sensible orbits.
McCarthy et al. 2007
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Hot stripping in a uniform medium
• Instantaneous stripping: a fixed fraction of gas will be removed
McCarthy et al. 2007
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Hot stripping in a uniform medium
• Instantaneous stripping: a fixed fraction of gas will be removed
• In reality there is a delay of ~1 Gyr which we model linearly:
McCarthy et al. 2007
Dark matter
Gas
Analytic prediction
sc
tM
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Hot stripping in clusters
• Onset of stripping is delayed• =2, =2/3 works well for a
variety of orbits, mass ratios.• Takes ~2 Gyr to remove half
the gas mass Still plenty of hot fuel left The amount of gas left depends
on orbit, mass ratio etc., but the time delay of at least 1-2 Gyr is fairly robust
• Through starvation alone, low-mass satellite galaxies could potentially continue star formation for a significant fraction of a Hubble time.
McCarthy et al. 2007
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Observational evidence
• Sun et al. (2007) detect hot coronae around galaxies in clusters Reduced luminosity compared with isolated galaxies, but
still significant.
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Summary
• There are environmental influences on galaxy formation after z=1
• Probably dominant in massive groups, not clusters.
• Current modeling of environmental effects is wrong and this has consequences for predictions of the general field (which is dominated by groups) Simple strangulation models may still work
well, if the instantaneous assumption is dropped.
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Extra slides
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Cosmic Time
• buildup of mass on the red-sequence occurs with the most massive galaxies first
• decrease in the “quenching” stellar mass with redshift
Cimatti et al. (2006)
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Universal relation
• Red fraction appears to depend on a simple linear combination of stellar mass and density
• Reflects the fact that stellar mass and density are correlated
Baldry et al. (astro-ph/0607648 )
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Evolution in Groups
• SFH of galaxies in groups are similar to the field, and evolve with it
Wilman et al. 2005
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Groups - morphology
• Use Gim2D to measure the fraction of light in the bulge (B/T)
• Low-z data from the MGC (Driver et al.)
• Models do well here. Merger history
OK. SFH needs work.
McGee et al. 2007
Black: dataRed: models