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

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 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 L X Total HI content vs L X

Some initial HI Results NGC 1052 Group

New galaxy in the NGC 1052 Group ? No optical catalogue counterpart HI Mass 2x10 9 M o

The NGC 5044 Group: no intragroup HI gas

New galaxy in the NGC 5044 Group ? No optical catalogue counterpart HI Mass 6x10 8 M o 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 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 MNRAS

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

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 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 Are groups simply scaled down clusters ?

L X vs T relation