Galaxy Groups

Duncan A. Forbes

Centre for Astrophysics & Supercomputing, Swinburne University

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 ?

Specs

Loose Compact Cluster

Number of galaxies ~20 ~5 ~100

Density over field 20x 10^6x 10^6x

Velocity dispersion ~150 ~150 ~700

Hot gas temp. <1keV <1keV >1keV

Which environment is most conducive to ongoing mergers today (ie z~0) ?

Galaxy MergersThe rule of thumb for mergers to occur is:

vel. disp of group / galaxy internal vel. disp. < 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

=> smaller galaxies have a longer merger timescale.

Richstone & Malumuth 1983 ApJ 268 30

Hickson Compact Groups

Hickson 1982 ApJ 255 382

Optical selection based on

richness: >= 4 galaxies within 3 magnitudes of the brightest galaxy

isolation: no galaxies within 3x group radius

compactness: surface brightness < 26 mag/sq arc

=> ~100 HCGs (~90% are real).

HCGs shouldn’t exist !

Tmerger < < Hubble time

=> HCGs should have merged into a single larger galaxy by z=0

Why haven’t they ?

Athanassoula etal. 1997 MNRAS 286 825

Loose Groups

Using a friends-of-friends algorithm Garcia (1993, A&AS, 100, 47) on a database of 6,392 galaxies to B < 14.0 and Vres < 5,500 km/s derived an all-sky catalog of 485 groups of at least 3 galaxies.

Garcia (1995, A&A, 297, 56) defined ~120 compact groups from the 1993 group catalog. Compact groups are often found at the centres of larger loose groups.

X-ray Properties of Galaxy GroupsHot gas in groups may be the dominant baryon component in the Universe.

The gas has a temperature of about 10^6 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.

Mulchaey 2000 ARAA 38 289

Are groups simply scaled down clusters ?

LX vs T relation

Survey of Nearby Galaxy GroupsDespite their ubiquity, groups are poorly studied relative to clusters...

=> 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

Optical Imaging of Galaxy Groups

Colour-magnitude relation

Galaxy luminosity function

Giant to dwarf galaxy ratio

Globular cluster systems

Galaxy Luminosity Function

Hunsberger etal. 1998 ApJ, 505, 536

Shape of HCG galaxy LF correlates with X-ray luminosity. Note lack of moderate sized galaxies.

Loose group galaxy LF is largely unconstrained.

X-ray Gas in Galaxy Groups

Early type galaxy fraction vs LX

HI Gas in Galaxy Groups

NGC 1052 Group

Tidal HI gas ?

Spectral follow-up

Terlevich & Forbes 2001, MNRAS, submitted

Measure redshifts

Age-date galaxies via spectral line indices

Fossils

Fossils: massive isolated elliptical galaxies with group-like X-ray halos.

Are fossils merged compact groups ?

Jones, Ponman & Forbes 2000 MNRAS 312 319

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.

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.

Concluding Remarks

IAU Conference on “Small Galaxy Groups” in 1999, ASP press., Valtonen and Flynn editors.

Groups provide the link between clusters and the field but the physical processes in galaxy groups are poorly understood.

Groups contain 3-30 large galaxies, an unknown number of dwarf galaxies and an intragroup medium of hot gas.

Most galaxies in the Universe are found in Groups. The Group environment is most conducive to mergers.