evolution of galaxy groups michael balogh department of physics university of waterloo
TRANSCRIPT
Evolution of Galaxy groupsEvolution of Galaxy groups
Michael BaloghDepartment of PhysicsUniversity of Waterloo
Outline
1. Introduction2. Groups at low redshift (2PIGG)3. Groups at z=0.5 (CNOC2)4. Conclusions
Collaborators:– Richard Bower, Vince Eke (Durham)– Dave Wilman (Durham -> MPE)– Ray Carlberg (Toronto)– Gus Oemler, John Mulchaey (Carnegie)– Pasquale Mazzotta (Rome)
Galaxy clusters: reviewGalaxy clusters: review
• 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?
z=0 z=0.39 z=0.83
Cluster SFR evolution
• Based on sparsely-sampled [OII] spectroscopy
• Suggests fraction of star-forming galaxies evolves only weakly in clusters
• Different from colour evolution?
Clusters
Field
2dF
Nakata et al. (2005)
Postman, Lubin & Oke 2001van Dokkum et al. 2000
Fisher et al. 1998
Czoske et al. 2001
• “Butcher-Oemler effect” also seen in the general field
• Is the effect stronger in clusters?
0 0.3 1Redshift
MV < -20
High density
Low density
All galaxies
Redshift
Red
gal
axy
frac
tio
n
Evolution of the red sequence
Bell et al (2004)
• Conflicting results from photo-z surveys?
Nuijten et al. 2005
Red
gal
axy
frac
tio
n
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], H, 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
LDSS2-Magellan for deeper, more complete spectroscopy
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
Fra
ctio
n o
f g
ala
xies
wit
hou
t S
F
Distance from centre Local galaxy density
Wilman et al. (2004)
Morphologies: early results
• 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
Sp
iral
fract
ion
E/S
0 f
ract
ion
Groups
Groups
Groups
FieldS
pir
al
fract
ion
Vel. Dispersion (km/s)
The connection between star formation rate, morphology and environment
Like clusters, groups contain passive spirals: disk morphology but low star formation rates
FieldGroups
Elliptical
Early spiral Late spiral
S0
Distributions are corrected for differences in luminosity function between group and field
Evolution in groups
Wilman et al. (2004)
Fra
ctio
n o
f n
on
-SF
g
ala
xies
• 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
• Fraction of non-SF galaxies increases with redshift
• for both groups and field
• Insensitive to aperture effects
Fra
ctio
n o
f n
on
-SF
g
ala
xies
Groups
Group SFR evolutionF
ract
ion
of
non
-SF
g
ala
xies
Field
Group SFR evolution
Wilman et al. 2004
• shape of [OII] distribution evolves with redshift but does not depend on environment
• Result sensitive to aperture effects
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)
2PIGGz: X-ray observations
T=2-3.5 keV=550-700 km/s
9/18 groups in REFLEX survey
6/18 groups in RASS
Complete, “mass”-selected group sample
2PIGGz: near-infrared• Stellar masses from K-band data
– Obtained with ISPI (CTIO) for most groups observed with Chandra or XMM-Newton