Galaxy Ecology
The role of galaxy environment in determining
the star formation history of the universe
Michael Balogh
ICC, University of Durham
Plus: Richard Bower, Ian Smail, Simon Morris, Vince Eke (Durham)
Ian Lewis and the 2df team
Bob Nichol, Percy Gomez, Chris Miller, Tomo Goto (CMU)
Ann Zabludoff (Arizona)
John Mulchaey, Gus Oemler (Carnegie)
Ray Carlberg (Toronto)
Warrick Couch (UNSW)
Galaxy Ecology
The role of galaxy environment in determining
the star formation history of the universe
Michael Balogh
ICC, University of Durham
Motivation: cosmological context of observations
Clusters at intermediate redshift
The local universe: 2dF galaxy redshift survey, and Sloan digital
sky survey
Low mass clusters and groups at intermediate redshift
Why Does Star Formation Stop?
Steidel et al. 1999
SFR ~ (1+z)1.7
(Wilson, Cowie et al. 2002)
A) Internal? i.e. gas consumption and “normal” aging
B) External? Hierarchical build-up of structure inhibits star formation
A Press-Schechter plot showing the growth of the mass
structure of the universe
LCDM cosmology
Growth of structure still
rapid
But how should curves be
normalised?
Groups
Clusters
Renormalised relative
to 1011 Msol
Galaxy clusters: the end of star
formation?
30
AC114 (z=0.31)
200
(Couch et al. 2001)
-100
0
Does star formation take
place in clusters at z>0 ?
-200
-300
DRA
100
Butcher-Oemler effect?
-200
-100
0
100
200
Abell 2390 (z~0.23)
3.6 arcmin
R image from
CNOC survey
(Yee et al. 1996)
Ha in Abell 2390
Balogh & Morris 2000
3.6 arcmin
Nod & Shuffle: LDSS++ (AAT)
band-limiting filter +
microslit = ~800
galaxies per 7’ field
Ha in Rich Clusters at z~0.3
LDSS++ with nod
and shuffle sky
subtraction, on
AAT
(Field)
Couch et al. 2001
ApJ 549, 820
Balogh et al. 2002
MNRAS, 335, 110
Fine for clusters – but what about groups?
Groups
Clusters
Mechanisms

Ram-pressure stripping
– Needs dense ICM and high velocities - clusters

Collisions / harassment
– Groups are preferred place!

"Strangulation"
– Removal of the gas halo
– Similar to ram-pressure stripping but much
easier!
Ram pressure - simulations
from Quilis et al. 2000
Gunn & Gott, 1972
Examples of galaxy
collisions
in the real universe and in a
simulation (Moore et al
1995)
Strangulation ?
Use numerical model of
infall to estimate timescale
for disruption of SFR
Radial gradients in CNOC
clusters suggest t ~2 Gyr
(Balogh, Navarro & Morris 2000)
Strangulation - removal of the gas halo
Could occur in
groups
First suggested by Larson, Tinsley & Caldwell, 1984
Where do environmental
effects become
important?
The Local Universe

2dFGRS (Lewis et al. 2002, MNRAS 334, 673)
– Ha in 11000 galaxies within 20 Mpc of 17
clusters, down to MB=-19
– SFR-density, SFR-radius relations in clusters
with s>400 km/s

SDSS (Gomez et al. 2002, ApJ in press)
– volume-limited sample of 8600 galaxies from
the EDR, MR<-20.5
– SFR-density relation independent of proximity
to a cluster
Galaxy Transformation in the 2DF survey
A1620
Rvir
Data for 17 Abell-like clusters
Covers velocity dispersions
500 km/s - 1100 km/s
Region out to > 20 Rvir extracted from
the survey
Major advantages:
● Star formation rate measured from
Ha
● Complete redshift information - no
need to subtract background!
● Compare with surrounding field
directly
1 degree
(data extracted over ~7 deg field)
SFR-Environment Relation in the
2dFGRS
Field
SFR-Radius Relation
Field
Field
Lewis et al. 2002
MNRAS 334, 673
SFR-Environment Relation in the
2dFGRS
Field
SFR-Density Relation
Field
Field
Lewis et al. 2002
MNRAS 334, 673
SFR-Environment Relation in the
2dFGRS
Field
SFR-Density Relation
c.f. Morphology-Density Relation
R>2 Rvirial
Field
Field
Lewis et al. 2002
MNRAS 334, 673
SFR-Environment relation in the
SDSS
Star Formation Rate (Mo/yr)
Gomez et al. (2002)
Field 75th percentile
75th percentile
Median
Galaxy Surface Density (Mpc-2)
Field median
What You Must Remember!
Galaxy Transformation does not occur in clusters!
=> ram-pressure stripping not important
It occurs in groups
... these might then end up in clusters.
Groups at z=0.2-0.5

Low-Lx Clusters at z=0.25
– Factor ~10 less massive than CNOC clusters
– HST imaging, extensive ground-based
spectroscopy

CNOC2 groups at z=0.45
– Spectroscopy with LDSS-2 on Magellan 6.5-m
– Goal is complete group membership to M*+1
Low Lx Clusters at z~0.25
Cl0841
z=0.24
s=390
Cl0849
z=0.23
s=750
Cl1701
z=0.24
s=590
Cl1702
z=0.22
s=370
Cl0818
z=0.27
s=630
Cl0819
z=0.23
s=340
Cl1309
z=0.29
s=640
Cl1444
z=0.29
s=500
Lx ~ 1043 - 1044 ergs/s, ~ 10 X less massive than CNOC
Morphologies in Low-Lx Clusters
at z~0.25
Bulge/Total Fractions from GIM2D (Simard et al. 2002)
Intermediate
Disks
B/T<0.4
Bulges
B/T>0.6
Balogh et al. 2002, ApJ 566, 123
Low-Lx Disk Fractions
(from Medium Deep
Survey)
High-Lx Clusters
from the lensing
sample of
Smith et al. 2001
Balogh et al. 2002, ApJ 566, 123
Morphology-density relation
at z~0.25
Balogh et al. 2002
ApJ 566, 123
Bulge Creation or Disk
Destruction?
Balogh et al. 2002,
ApJ 566, 123
Disk Luminosity
Functions (at fixed
density)
Bulge Creation or Disk
Destruction?
Balogh et al. 2002,
ApJ 566, 123
Bulge Luminosity
Functions (at fixed
density)
Star Formation in Low-Lx
Clusters
Spectroscopy for 172 cluster
members Mr< -19 (h=1)
Balogh et al.
1997
SFR from [OII] emission line
Balogh et al. (2002)
MNRAS, in press
astro-ph/0207360
Disks Without Star Formation
Cl 1309 id=83
z=0.2934
[OII]
B/T = 0.39
Wo (OII)=-2.64.0
Wo (Hd)=3.8 2.1
3” HST Image
Disks Without Star Formation
[OII]
Cl 1444 id=78
z=0.2899
B/T = 0.42
Wo (OII)=3.5 2.7
Wo (Hd)=4.9 1.3
3” HST Image
Disks Without Star Formation
Cl 0818 id=58
z=0.2667
[OII]
B/T = 0.19
Wo (OII)=-9.6 7.8
Wo (Ha)=22.1 11.6
Wo (Hd)=2.0 3.6
3” HST Image
Ha
Disks Without Star Formation
Cl 0841 id=20
z=0.2372
[OII]
B/T = 0.42
Wo (OII)=-0.2 1.2
Wo (Ha)=-1.4 0.6
Wo (Hd)=0.0 0.6
3” HST Image
Ha
Low-Lx Clusters: Summary
 Bulge
formation may be more efficient in
more massive clusters

but star formation in disks is suppressed
in all clusters
The CNOC2 groups project

The CNOC2 redshift survey was aimed at measuring
correlation strengths and star formation rates in the
z=0.5 universe.

Group selection and inital look at properties described
in Carlberg et al 2001

Durham involvement: follow-up observations with
Magellan to gain higher completeness confirming
complete samples of group members using LDSS-2.

Overall aim of comparing star formation rates in
groups at z=0.5 and locally (Mulchaey & Zabludoff
etc, 2dfgrs coming soon!)
Bower, Mulchaey, Oemler, Carlberg et al - in prep.
CNOC2 Groups at z~0.45
Deep spectroscopy with LDSS-2
on Magellan 1
Infrared (Ks) images from INGRID
Combined with CNOC2 multicolour photometry and spectroscopy, we can
determine group structure, dynamics, stellar mass, and star formation history.
LDSS2 on Magellan
[OII]
[OII]
CNOC2 Groups at z~0.45
Preliminary results
based on only 12
CNOC2 groups
Have observed >30
groups to date
Balogh et al.
1997
Putting it all together…
20
CNOC2 Groups
15
?
CNOC1 Field
10
SDSS Field
Low-Lx Clusters
5
SDSS
Clusters
Mean EW [OII] (Angstroms)
Wilson et al. 2002
0
CNOC1 Clusters
0.3
Redshift
0.5
?
1.0
Local Groups in the 2dFGRS
Based on friends-of-friends
catalogue (V. Eke)
Mean SFR appears to be
suppressed in all galaxy
associations at z=0!
So where is star formation
going on??
The Environmental-Madau plot
20
CNOC2 Groups
15
?
CNOC1 Field
10
SDSS Field
2dF Clusters
Mean EW [OII] (Angstroms)
Wilson et al. 2002
5
0
?
Low-Lx Clusters
CNOC1 Clusters
0.3
Redshift
0.5
?
1.0
Summary

2dFGRS and SDSS: SFR-density relation shows
critical density at 1 Mpc-2
 SFR suppressed in all dense regions, in
structures more massive than groups
 Lack of strong evolution in clusters + abundance
of structure above the critical threshold suggests
environmental processes are important to global
evolution.
BUT: Strong evolution in group SFRs?