Galaxy groups Michael Balogh Department of Physics and Astronomy University of Waterloo.
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Transcript of Galaxy groups Michael Balogh Department of Physics and Astronomy University of Waterloo.
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Galaxy groups
Michael BaloghDepartment of Physics and Astronomy
University of Waterloo
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Outline
1. Where do groups fit in the hierarchy?
2. Group selection methods3. Properties of galaxies in groups4. Theoretical challenges
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What is a group?
• ~few L* galaxies
• Mhalo~1012-5x1013 (<500 km/s)
• Physically associated – but not necessarily virialized
• At higher masses, galaxy population seems to be weakly dependent on halo mass
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Buildup of structure
• Group abundance evolves strongly
• Fraction of galaxies in groups (N>6) increases by about a factor 3 since z=1
Knobel et al. (2009)
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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)
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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
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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.
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Slow truncation
• Without preprocessing: not only would groups be field-like, but clusters would show much more scatter
Fra
ctio
n of
pro
cess
ed g
alax
ies
Halo massMcGee et al. (2009)
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Slow truncation
• And z evolution would be rapid
• Ellingson et al. (2001) used this argument to support long (T~3Gyr) timescales from CNOC clusters
Fra
ctio
n of
pro
cess
ed g
alax
ies
Halo massMcGee et al. (2009)
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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
Halo mass McGee et al. (2009)
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Group Selection Methods
• Redshift surveys• Xray• Photometric surveys
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Redshift surveys
• 2dFGRS/SDSS >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
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X-ray selection: low-z
• ROSAT able to detect nearby systems with ~100 km/s or greater Zabludoff & Mulchaey (1998) Osmond & Ponman (2004) Rasmussen et al. (2008)
Mulchaey & Zabludoff (1998)
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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)
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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)
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Photometric selection
• McConnachie et al. (2008) use SDSS to detect 7400 compact groups, photometrically.
• Attempt to correct for contamination using simulations
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Photometric selection
• RCS: not effective in the group regime
• Completeness trusted down to ~300 km/s.
Gilbank et al. (2007)
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Group properties
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SDSS groups
• Weak correlation with halo mass for clusters
• Evidence for larger blue fractions in groups
Bamford et al. (2009)
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• Low-mass satellite galaxies show dependence on halo mass on group scales
Kimm et al. 2009
Groups and clusters
Also Weinmann et al. 2006, Pasquali et al. 2009
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Properties of X-ray groups• Spiral fraction in X-ray
groups correlates with , 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)
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Groups at z=0.5
• At fixed stellar mass, groups have fewer blue galaxies than the field
Balogh et al. (2009)
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Groups at z=0.5
Balogh et al. (2009)
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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)
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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)
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Theoretical challenges
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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)
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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 low-mass galaxies (e.g. Haines, Rasmussen)… why so quick in GALFORM?
Balogh et al. (2009)
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Conclusions
• 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?