Download The evolution of galaxy groups

Evolution of Galaxy groups
Michael Balogh
Department of Physics
University of Waterloo
Outline
1.
2.
3.
4.
Introduction
Groups at low redshift (2PIGG)
Groups at z=0.5 (CNOC2)
Conclusions
Collaborators:
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–
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Richard Bower, Vince Eke (Durham)
Dave Wilman (Durham -> MPE)
Ray Carlberg (Toronto)
Gus Oemler, John Mulchaey (Carnegie)
Pasquale Mazzotta (Rome)
Galaxy clusters: review
z=0
z=0.39
z=0.83
• Galaxy clusters are dominated by passively evolving galaxies with high formation
redshifts
• How does the evolution compare with the general field?
• Nature or nurture: clusters are built from groups. How do groups evolve?
Cluster SFR evolution
• “Butcher-Oemler effect” also seen in the general field
• Is the effect stronger in clusters?
Field
Nakata et al. (2005)
2dF
Postman, Lubin & Oke 2001
van Dokkum et al. 2000
Fisher et al. 1998
Clusters
Czoske et al. 2001
0
0.3
Redshift
1
• Based on sparselysampled [OII]
spectroscopy
• Suggests fraction of starforming galaxies evolves
only weakly in clusters
• Different from colour
evolution?
Evolution of the red
sequence
Red galaxy fraction
Red galaxy fraction
• Conflicting results from photo-z
surveys?
High density
All galaxies
MV < -20
Low density
Redshift
Nuijten et al. 2005
Bell et al (2004)
Groups
• Make up ~60% of local population;
abundance evolves strongly with
redshift
• Much harder to do because contrast
with background is lower. Individual
groups have few members.
• Things to find out:
– What is efficiency of galaxy formation in
groups? Need stellar mass, gas mass,
dynamical mass
– What is star formation rate? [OII], Ha,
UV
– What is morphological composition? S0
galaxies? Irregulars?
– How does all this evolve?
Evolution in groups
z~0.05: 2dFGRS (Eke et al. 2004)
– Based on friends-of-friends linking algorithm
– calibrated with simulations. Reproduces mean
characteristics (e.g. velocity dispersion) of parent
dark matter haloes
z~0.45: CNOC2 (Carlberg et al. 2001)
– selected from redshift survey, 0.3<z<0.55
– ~30 nights of dedicated time with LDSS2Magellan for deeper, more complete
spectroscopy
CNOC2 groups
CNOC2 groups: z~0.45
• ~30 nights of dedicated Magellan time
• 295 spectroscopic members in 26 groups (r~23)
• Single-orbit ACS images for all 26 groups
Fraction of galaxies without SF
Wilman et al. (2004)
Distance from centre
Local galaxy density
Morphologies
E/S0-dominated
group
s=226 km/s
Spiral-dominated
group
s=270 km/s
E/S0 fraction
Morphologies: early results
Groups
• There are fewer spiral galaxies in
groups than in the field, at the same
redshift.
• No evidence for more
disturbance/irregularities in group
galaxies
Field
Spiral fraction
Spiral fraction
Field
Groups
Groups
Vel. Dispersion (km/s)
The connection between star
formation rate, morphology
and environment
Field
Groups
Distributions are corrected for differences in
luminosity function between group and field
S0
Elliptical
Early spiral
Late spiral
Like clusters, groups contain passive spirals: disk
morphology but low star formation rates
Evolution
Fraction of non-SF galaxies
Evolution in groups
• Use [OII] equivalent width to
find fraction of galaxies
without significant star
formation
• most galaxies in groups
at z~0.4 have significant
star formation – in
contrast with local
groups
Wilman et al. (2004)
Group SFR evolution
Groups
Fraction of non-SF galaxies
• Fraction of non-SF galaxies
increases with redshift
• for both groups and field
• Insensitive to aperture effects
Fraction of non-SF galaxies
Field
Wilman et al. 2004
Group SFR evolution
• shape of [OII]
distribution evolves with
redshift but does not
depend on environment
• Result sensitive to
aperture effects
Wilman et al. 2004
Future Work
Better SFRs: GALEX
• 9 orbits awarded in Cycle 1: 3
orbits in each of 3 CNOC2
patches with deep Magellan
spectroscoopy
• Only 1.5 orbits obtained so
far
• Preliminary match with
CNOC2 spectroscopy shows
we detect most group
members in the near-UV (rest
frame far-UV)
GALEX
• d
2PIGGz: X-ray observations
9/18 groups in REFLEX survey
6/18 groups in RASS
Complete, “mass”-selected
group sample
T=2-3.5 keV
s=550-700 km/s
2PIGGz: near-infrared
• Stellar masses from K-band data
– Obtained with ISPI (CTIO) for most groups observed
with Chandra or XMM-Newton
Baryon content of nearby
groups
Conclusions
• More star formation in groups at z=0.5 than
at z=0
• On average, groups at 0<z<0.5 have less star
formation and fewer spiral galaxies than the
field.
• Passive spiral galaxies are a key component
of groups at z=0.5
• Much more to come…