8 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]
Could a Local Group X-Ray Halo Affect the X-Ray and Microwave Backgrounds?
Suto et al. (1996, astro-ph/9602061 and ApJ, 461, L33) have suggested that an
X-ray halo in the Local Group might explain both the observed low-energy excess
in the X-ray background and the quadrupole anisotropy in the cosmic microwave
background. Recent observations of poor groups of galaxies by the ROSAT PSPC
set reasonable limits on how extensive and dense such a halo could be. The poor
groups most similar to the Local Group do not have a detectable halo, and the
upper limits of these observations suggest that any Local Group halo would be
nearly two orders of magnitude too tenuous to produce the effects Suto et al.
(1996) discuss. In particular, the Sunyaev-Zel'dovich effect cannot contribute
significantly to the quadrupole anisotropy measured by COBE.Comment: 8 pages, to appear in The Astrophysical Journal Letters, paper can
also be obtained at http://hea-www.harvard.edu/~pildis/smio.htm
Gas-Rich Dwarf Galaxies from the PSS-II --- II. Optical Properties
We describe the optical properties of a sample of 101 gas-rich field dwarf
galaxies found on PSS-II (Second Palomar Sky Survey) plates, most newly
discovered as part of a survey to investigate the clustering properties of
dwarf galaxies relative to giants. These galaxies have low surface brightnesses
and are relatively distant, with recession velocities ranging up to 10,000
km/s. They have bluer V-I colors (median value of 0.75) than either actively
star-forming giant galaxies or low metallicity globular clusters, implying that
these dwarfs have both low metallicities and little past star formation. These
galaxies are also extremely gas rich, with a median HI mass to V luminosity
ratio of approximately 2 in solar units. We divide the sample into two groups:
true dwarfs with diameters (at 25 I mag arcsec^-2) less than 7.5 kpc and
Magellanic dwarfs with diameters greater than that value. The true dwarfs have
greater HI mass to V luminosity ratios and slightly bluer V-I colors than the
Magellanic dwarfs. Overall, the optical properties of our sample of dwarf
galaxies point towards their being quiescent objects that have undergone little
star formation over the age of the universe. They are not faded objects, but
instead may be going through one of their first periods of weak star formation.Comment: 27 pages, to appear in 20 May 1997 ApJ, paper also available at
http://www.astro.nwu.edu/astro/pildis/dwarfphot.html and
http://zebu.uoregon.edu/~js/dwarf.htm