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

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

Spiral-dominated group=270 km/s

E/S0-dominated group=226 km/s

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

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

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

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

Baryon content of nearby groups

24 arcmin

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…