57 research outputs found
The Destruction of a Compact Group of Galaxies
The very high apparent galaxy densities in Hickson compact groups (HCGs)
should produce copious amounts of galaxy-galaxy interaction, if these groups
are bound dynamical units. I examine the evidence for such interactions in HCG
94, a group of seven galaxies with an envelope of diffuse optical light and an
extremely high X-ray luminosity. Using HRI data and deep three-color
optical imaging of the group, I find that both the hot (T 4 keV) gas
and the diffuse optical light trace the same elongated potential well, which is
offset from the galaxy distribution. The luminosity and colors of the diffuse
optical light imply that the group galaxies are being ripped apart to form a
cD-type remnant galaxy. The similarity in shape of the X-ray and optical
contours may mean that this destruction process may require several billion
years.Comment: AASTeX, 9 pages--to be published in ApJ, 20 December 1995--tar'ed and
compressed postscript figures and postscript text available at
ftp://astro.lsa.umich.edu/pub/get/pildi
Deep Optical Observations of Compact Groups of Galaxies
Compact groups of galaxies appear to be extremely dense, making them likely
sites of intense galaxy interaction, while their small populations make them
relatively simple to analyze. In order to search for optical interaction
tracers such as diffuse light and galaxy tidal features in Hickson compact
groups (HCGs), we carried out deep photometry in three filters on a sample of
HCGs with observations. Using a modeling procedure to subtract the
light of bright early-type galaxies, we found shell systems and extended
envelopes around many, but not all, of those galaxies. Only one group in our
sample, HCG 94, has diffuse light in the group potential (with a luminosity of
7 L); the other groups do not contain more than 1/3 L in diffuse light.
With the exception of HCG 94 (which is the most X-ray--luminous HCG), we found
no correlation between the presence of shells or other tidal features and the
X-ray luminosity of a group. Better predictors of detectable group X-ray
emission are a low spiral fraction and belonging to a larger galaxy
condensation---neither of which are correlated with optical disturbances in the
group galaxies. Two elliptical galaxies that are extremely optically luminous
but X-ray--faint are found to have shells and very complex color structures.
This is likely due to recent infall of gas-rich material into the galaxies,
which would produce both the disruption of stellar orbits and a significant
amount of star formation.Comment: 24 pages, to appear in October 1995 Astronomical Journal; postscript
text and figures (low resolution scans, tar'ed and compressed) available at
ftp://astro.lsa.umich.edu/pub/get/pildis
Properties of Simulated Compact Groups of Galaxies
We analyze compact groups of galaxies appearing in a galaxy formation
simulation dominated by cold dark matter (omega_0=1, omega_{baryon}=0.1). The
simulation uses an N-body code to model the behavior of the non-baryonic matter
and smoothed particle hydrodynamics to model the baryons. One run includes gas
dynamics alone, and the other incorporates star formation as well. Groups
identified as physically compact at z=0 form originally along filaments and
become compact in the final ~20% of the simulation; they contain
x-ray--luminous diffuse gas well before becoming compact. The component masses,
baryon fractions, and gas-to-galaxy mass fractions of the simulated groups are
roughly similar somewhat more gas-rich and have x-ray temperatures a factor of
3 lower than those seen in HCGs. These discrepancies may be alleviated by
adding the effects of energy input into the diffuse group gas by star formation
and supernovae.Comment: to appear in AJ in August 1996; 32 pages using AASTeX 4.0 macros;
paper with full-resolution figures available at
http://hea-www.harvard.edu/~pildis/sims.htm
ROSAT Observations of Compact Groups of Galaxies
We have systematically analyzed a sample of 13 new and archival ROSAT PSPC
observations of compact groups of galaxies: 12 Hickson Compact Groups plus the
NCG 2300 group. We find that approximately two-thirds of the groups have
extended X-ray emission and, in four of these, the emission is resolved into
diffuse emission from gas at a temperature of T keV in the group
potential. All but one of the groups with extended emission have a spiral
fraction of less than 50\%. The baryon fraction of groups with diffuse emission
is 5--19\%, similar to the values in clusters of galaxies. However, with a
single exception (HCG 62), the gas-to-stellar-mass ratio in our groups has a
median value near 5\%, somewhat greater than the values for individual
early-type galaxies and two orders of magnitude lower than in clusters of
galaxies. The X-ray luminosities of individual group galaxies are comparable to
those of similar field galaxies, although the L-L relation for
early-type galaxies may be flatter in compact groups than in the field.Comment: 27 pages in AASTeX format, figures available from
[email protected]
Early versus late type galaxies in compact groups
We find a strong correlation between the effective radius of the largest
early-type galaxies in compact groups of galaxies and the velocity dispersion
of the groups. The lack of a similar correlation for late type galaxies is
supportive of the so called second generation merging scenario which predicts
that ellipticals should dominate the internal dynamics of the groups, while
late-type galaxies are mainly recent interlopers which are still in an early
stage of interaction with the group potential.Comment: Astron. Nachr., IN PRES
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