Galaxy Groups

1
Galaxy Groups

• Duncan A. Forbes
• Centre for Astrophysics Supercomputing,
  Swinburne University

2
Galaxy Environments

• Clusters party animals that love being in a
  crowd
• Groups social butterflies that like to mingle
• Field hermits that prefer isolation

In which environment do we find most galaxies
? How do we define the different environments ?
3
Specs
Loose Compact Cluster
Number of galaxies 20 5 100 Density over
field 20x 106x 106x Velocity
dispersion 150 150 700 Hot gas
temp. lt1keV lt1keV gt1keV
Which environment is most conducive to ongoing
mergers today (ie z0) ?
4
Galaxy Mergers
The rule of thumb for mergers to occur is vel.
disp of group / galaxy internal vel. disp. lt 2.
Dynamical friction (deceleration of a galaxy
moving through a background medium of masses) is
a key process in determining the merger
timescale. Tdyn (mass of galaxy)-1 (background
density)-1 gt smaller galaxies have a longer
merger timescale.
Richstone Malumuth 1983 ApJ 268 30
5
Hickson Compact Groups
Optical selection based on richness gt 4
galaxies within 3 magnitudes of the brightest
galaxy isolation no galaxies within 3x group
radius compactness surface brightness lt 26
mag/sq arc gt 100 HCGs (90 are real).
Hickson 1982 ApJ 255 382
6
HCGs shouldnt exist !
Tmerger lt lt Hubble time gt HCGs should have
merged into a single larger galaxy by z0 Why
havent they ?
Athanassoula etal. 1997 MNRAS 286 825
7
Loose Groups
Using a friends-of-friends algorithm Garcia
(1993, AAS, 100, 47) on a database of 6,392
galaxies to B lt 14.0 and Vres lt 5,500 km/s
derived an all-sky catalog of 485 groups of at
least 3 galaxies.
Garcia (1995, AA, 297, 56) defined 120 compact
groups from the 1993 group catalog. Compact
groups are often found at the centres of larger
loose groups.
8
X-ray Properties of Galaxy Groups
Hot gas in groups may be the dominant baryon
component in the Universe. The gas has a
temperature of about 106 K or 1keV and radiates
(cools) via thermal bremmstrahlung. Loose and
compact groups have similar (identical) X-ray
properties. The X-ray luminosity of individual
group galaxies appears to be the same as for the
field galaxies.
Are groups simply scaled down clusters ?
Mulchaey 2000 ARAA 38 289
9
LX vs T relation
10
Survey of Nearby Galaxy Groups
Despite their ubiquity, groups are poorly studied
relative to clusters... gt Multiwavelength study
of 35 nearby galaxy groups. Aim to understand
how the group environment affects the galaxies
and how groups evolve. Selection on the basis of
their X-ray luminosity (a rough measure of the
group dynamical state). Data X-ray, optical,
IR, and HI
11
Optical Imaging of Galaxy Groups
Colour-magnitude relation Galaxy luminosity
function Giant to dwarf galaxy ratio Globular
cluster systems
12
Galaxy Luminosity Function
Shape of HCG galaxy LF correlates with X-ray
luminosity. Note lack of moderate sized
galaxies. Loose group galaxy LF is largely
unconstrained.
Hunsberger etal. 1998 ApJ, 505, 536
13
X-ray Gas in Galaxy Groups
Early type galaxy fraction vs LX
14
HI Gas in Galaxy Groups
NGC 1052 Group
Tidal HI gas ?
15
Spectral follow-up
Measure redshifts Age-date galaxies via
spectral line indices
Terlevich Forbes 2001, MNRAS, submitted
16
Fossils
Fossils massive isolated elliptical galaxies
with group-like X-ray halos. Are fossils merged
compact groups ?
Jones, Ponman Forbes 2000 MNRAS 312 319
17
Group Evolution

• Compact groups do merge but they are replenished
  from the surrounding loose group.
• Loose groups continually collapse to form new
  compact groups.
• Compact groups have common dark matter halos
  which suppress merging.

18
Groups make cDs
It has been suggested (Merritt 1984, ApJ, 276,
26) that cD galaxies at the centre of clusters
formed from group ellipticals. The group,
containing a large elliptical, falls into the
cluster at early epochs, accreting galaxies and
growing its cD envelope by shredding nearby
galaxies.
19
Concluding Remarks
Most galaxies in the Universe are found in
Groups. The Group environment is most conducive
to mergers.
Groups contain 3-30 large galaxies, an unknown
number of dwarf galaxies and an intragroup medium
of hot gas.
Groups provide the link between clusters and the
field but the physical processes in galaxy groups
are poorly understood.
IAU Conference on Small Galaxy Groups in 1999,
ASP press., Valtonen and Flynn editors.