Group Evolution Multi-wavelength Survey (GEMS)

Duncan A. Forbes

Centre for Astrophysics & Supercomputing, Swinburne University

Collaborators

David Barnes, Swinburne

Trevor Ponman, Birmingham

Carole Mundell, Liverpool John Moores

Paul Goudfrooij, STScI

http://astronomy.swin.edu.au/staff/dforbes/groups.html

Cluster-centric view of galaxy formation

“Ellipticals formed at z > 3”

Colour-Magnitude Relation and Fundamental Plane studies constrain the formation and evolution of Cluster ellipticals.

Very little is known about the formation epoch of Group or Field ellipticals. And yet...

Most galaxies are found in Groups

Groups are where we expect most mergers (a key process in elliptical galaxy formation)

GEMS: Group Evolution Multi-wavelength Survey

Despite their ubiquity, groups are poorly studied relative to clusters...

GEMS => First multiwavelength study of 30 nearby galaxy (loose and compact) groups.

Aim: to understand how the group environment affects the galaxies and how groups evolve.

Data: X-ray, optical, and HI

Sample Selection

From an initial group catalogue of 4320 groups we selected those groups observed with long ROSAT exposures in a distance annulus 1000 < V < 3000 km/s (to match the ROSAT field-of-view).

The remaining 30 groups cover a range of RA and Dec. and X-ray luminosities.

X-ray luminosity is a rough measure of the dynamical state of the group and hence gives us an important physical selection criterion.

Data Collection Progress Report

X-ray: 30 groups with archive ROSAT data, ~20 with Chandra or XMM data approved.

Optical: ~20 groups with wide field-of-view multi-filter imaging on hand. AAT run in Feb 2002 approved.

HI: 6 groups mapped with Parkes multibeam, further Parkes and ATCA followup requested.

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

Total HI content vs LX

Some initial HI Results

NGC 1052 Group

New galaxy in the NGC 1052 Group ?

No optical catalogue counterpart

HI Mass 2x109 Mo

The NGC 5044 Group: no intragroup HI gas

New galaxy in the NGC 5044 Group ?

No optical catalogue counterpart

HI Mass 6x108 Mo

Object lies outside of the group virial radius.

The IC 1459 Group: intragroup HI gas

The NGC 3557 Group: infalling galaxy ?

Loose vs Compact Groups

• Compact groups merge but are continually replenished from a surrounding loose group.

• Loose groups continually collapse to form new compact groups.

• Compact groups have common dark matter halos which suppress merging. No relation to loose groups.

Concluding Remarks

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

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

GEMS aims to understand the processes operating within groups and the evolution of groups as a system.

This willl be achieved using multiwavelength observations of a carefully selected sample of 30 nearby galaxy groups.

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.

Fossils

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

Are fossils merged compact groups ?

Jones, Ponman & Forbes 2000 MNRAS 312 319

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