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The important role of galaxy groups. Michael Balogh Department of Physics and Astronomy University of Waterloo. Galaxy Clusters. A standard picture to motivate environmental effects: Clusters are dominated by bright, red ellipticals. Massive galaxies. - PowerPoint PPT Presentation
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The important role of galaxy groups
Michael BaloghDepartment of Physics and Astronomy
University of Waterloo
Galaxy Clusters• A standard picture to motivate environmental
effects: Clusters are dominated by bright, red ellipticals
Massive galaxies• Not really
environmental effect: just result of biased galaxy formation Brightest galaxies
form early, rapidly SCUBA galaxies are
likely proto-ellipticals in clusters
Bower et al. 2006
Quenching
• Virial shocks become important above ~1012 Msun and after z~2
• But still require some mechanism to shut off star formation Must operate at ~1012
Msun to get CMD right AGN? Clumpy
accretion? Other things?
Dekel & Birnboim 2006
Low-mass galaxies• Galaxies with M~109 MSun are well below the
“threshold” mass.• But the fraction of blue galaxies depends
strongly on environment.
Baldry et al. (2006)Haines et al. 2007
Possible environmental effects
• Primordial? Dark matter mass accretion rate depends on environment
(Maulbetsch et al. 2006). Squelching of galaxies that form after reionization (Tully et al.
2002)• Enhanced merging, harassment• Ram-pressure (or tidal) stripping of cold gas• Strangulation: ram-pressure stripping, or shock heating, of
galaxy corona
• Most models currently include primordial plus merging effects, and a simplified strangulation prescription.
Direct evidence of environmental effects
Kenney et al. 2003Vollmer et al. 2004
• Ram-pressure stripping in Virgo
H for Virgo galaxyH for normal galaxy
• Truncated H disks in cluster spirals
• These galaxies have -20<MB<-18
Koopmann & Kenney 2004also: Vogt et al. 2004
• Passive Spirals• S0, dSph, UCDs• Wolf’s dusty spirals?
Peak in infall region?
Environmental quenching
• Simple galaxy formation models assume that gas accretion shuts off immediately when haloes merge
• Naturally leads to “downsizing” as low mass galaxies are incorporated into more massive haloes This environmental effect is a crucial ingredient!
Dekel & Birnboim 2006
Predictions•Isolated galaxies: only most massive galaxies are red.•Satellite galaxies at z=0 will be red, independent of halo mass, above ~1012 MSun.
•High red dwarf/giant ratio in groups and clusters•Little evolution in cluster and group colours to z=1
Isolated Galaxies
Incr
easi
ng s
tell
ar m
ass
• No low-mass, red galaxies, as predicted
Balogh, Salton et al. in prep.
Red sequence
threshold
Predictions• SFH of central galaxy depends on halo mass • SFH of satellite galaxies independent of
halo mass for M>1012MSun
Croton et al. 2006 models; from Weinmann et al. 2006
• Note: satellites are red even if they have low mass
• Fraction of late-type satellite galaxies independent of halo mass for M>1014 MSun Models do a
reasonable job at matching the most massive clusters (e.g.Diaferio 2001; Springel et al. 2001; Okamoto & Nagashima 2003)
Weinmann et al. 2006
Fra
ctio
n of
Lat
e-ty
pe g
alax
ies
Groups and clusters
The faint red galaxy problem!
• Models predict too many faint, red satellites in groups • This has a strong effect on the overall population, which
is dominated by groups
Weinmann et al. 2006
Red Galaxy luminosity function
• Dwarfs: -18.2>Mv>-20• Giants Mv<-20
Weinmann et al. haloes (SDSS)Courtesy: Sean McGee
De Lucia et al. (2007)
Red
Red Dwarf/Giant Ratio
• Faint red galaxies build up with time.
• Note: these “dwarfs” are MV~-19
VVDS: Zucca et al. 2006
COMBO-17: Bell et al. 2004
Driver et al. 2006Marzke & Da Costa 1997
PISCES: Tanaka et al. 2006
Field surveys
Red Dwarf/Giant Ratio
• Local clusters have more red dwarfs than the field.
• Clusters evolve strongly: faint red population appears at z<1 ?
Lu et al. 2007
De Lucia et al. 2007
Stott et al. 2007
Tanaka et al. 2007
VVDS: Zucca et al. 2006
COMBO-17: Bell et al. 2004
Driver et al. 2006Marzke & Da Costa 1997
PISCES: Tanaka et al. 2006
Clusters
Gilbank et al. 2007
Red Dwarf/Giant Ratio
• Models get local clusters right!
Bower et al. 2006 (clusters)
Lu et al. 2007
De Lucia et al. 2007
Stott et al. 2007
Tanaka et al. 2007
VVDS: Zucca et al. 2006
COMBO-17: Bell et al. 2004
Driver et al. 2006Marzke & Da Costa 1997
PISCES: Tanaka et al. 2006
Gilbank et al. 2007
Red Dwarf/Giant Ratio
• Oops. Too many faint red galaxies in the field!
Lu et al. 2007
De Lucia et al. 2007
Stott et al. 2007
Tanaka et al. 2007
VVDS: Zucca et al. 2006
COMBO-17: Bell et al. 2004
Driver et al. 2006Marzke & Da Costa 1997
PISCES: Tanaka et al. 2006
Bower et al. 2006 (field)
Gilbank et al. 2007
Red Dwarf/Giant Ratio
• Models predict little evolution
• Strangulation is too effective in small groups, which are dominant at z<1
Models
Clusters
Field
Two solutions to the faint red galaxy problem
1. Mass threshold? 1013-1014 MSun works well for clusters
(Balogh, Navarro & Morris 2000; Poggianti et al. 2006)
Would improve match to the field Harassment, ram pressure stripping
should be stronger in these systems.
2. Timescale? Maybe it’s still strangulation, operating in
smaller haloes, but more slowly?
The importance of galaxy groups
• Masses 1013-1014 MSun
If there is a “threshold mass”, this is probably it.
• Common today and evolve strongly with redshift
• Should be close to the action, in time, even if the threshold mass is 1012 MSun. In rich clusters it seems we are observing most
galaxies long after their SF has shut down.
CNOC2:Groups at z~0.4• ~200 groups
between z~0.1 and z~0.55
• 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
“CNOC2” GroupsZ=0.5
Millennium Simulation
All haloes
McGee et al. 2007
Evolution in Groups
• SFH of galaxies in groups are similar to the field, and evolve with it
Wilman et al. 2005
Groups @ z=0.5
• Active fraction weakly depressed relative to field
• No evidence for dramatic effects.
• Models predict much lower fractions
Balogh et al. 2006
Bower et al. model groups
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
Two solutions to the faint red galaxy problem
1. Mass threshold? 1013-1014 MSun works well for clusters
(Balogh, Navarro & Morris 2000; Poggianti et al. 2006)
Would improve match to the field Harassment, ram pressure stripping
should be stronger in these systems.
2. Timescale? Maybe it’s still strangulation, operating in
smaller haloes, but more slowly?
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
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
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
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.
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.
Extra slides
Formation of massive galaxies in clusters
• At z>1 brightest galaxies are blue, clustered. Formation of
giant ellipticals Can be
understood in terms of cooling time, feedback, and biased galaxy formation.
DEEP2 (Cooper et al. 2006;2007)
Rich Clusters and bright galaxies
• Models give a pretty good match to rich clusters predict too few active galaxies in clusters, brighter
than MR=-20.7 But not a huge effect
SDSS clusters: Finn et al. 2007 Bower et al. 2006 models
Threshold mass• Difficult to match observations if all SF
shuts off above ~1012 MSun
Poggianti et al. 2006
SDSS clusters
1012 MSun threshold
Threshold mass• Difficult to match observations if all SF
shuts off above ~1012 MSun
Poggianti et al. 2006
SDSS clusters
1014 MSun threshold
Groups: Model predictions
• Bower et al. (2006) models predict groups should be more like clusters
• Too large a difference compared with the field
Group m
ean
Field
M/L ratios
• Compare total luminosity within R200 to dynamical mass (M200).
• Triangles include correction for passive evolution and dark halo mass growth. zavg ≈ 0.4
z ≈ 0 (Ramella et al. 2004)
Balogh et al. 2006
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