Group Evolution Multi-wavelength Survey (GEMS)

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Group Evolution Multi-wavelength Survey
(GEMS)

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

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Collaborators
David Barnes, Swinburne Trevor Ponman,
Birmingham Carole Mundell, Liverpool John
Moores Paul Goudfrooij, STScI
http//astronomy.swin.edu.au/staff/dforbes/groups.
html
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Cluster-centric view of galaxy formation
Ellipticals formed at z gt 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)
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GEMS Group Evolution Multi-wavelength Survey
Despite their ubiquity, groups are poorly studied
relative to clusters... GEMS gt 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
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Sample Selection
From an initial group catalogue of 4320 groups we
selected those groups observed with long ROSAT
exposures in a distance annulus 1000 lt V lt 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.
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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.
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Optical Imaging of Galaxy Groups
Colour-magnitude relation Galaxy luminosity
function Giant to dwarf galaxy ratio Globular
cluster systems
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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
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X-ray Gas in Galaxy Groups
Early type galaxy fraction vs LX Total HI content
vs LX
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Some initial HI Results
NGC 1052 Group
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New galaxy in the NGC 1052 Group ?
No optical catalogue counterpart HI Mass 2x109 Mo
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The NGC 5044 Group no intragroup HI gas
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New galaxy in the NGC 5044 Group ?
No optical catalogue counterpart HI Mass 6x108 Mo
Object lies outside of the group virial radius.
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The IC 1459 Group intragroup HI gas
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The NGC 3557 Group infalling galaxy ?
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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.

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Concluding Remarks
Most galaxies in the Universe are found in
Groups. The Group environment is most conducive
to mergers.
Groups provide the link between clusters and the
field but the physical processes in galaxy groups
are poorly understood.
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.
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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
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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
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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.
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Fossils
Fossils massive isolated elliptical galaxies
with group-like X-ray halos. Are fossils merged
compact groups ?
Jones, Ponman Forbes 2000 MNRAS 312 319
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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
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LX vs T relation