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 $ROSAT$ HRI data and deep three-color optical imaging of the group, I find that both the hot ($k$T $\sim$ 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 $ROSAT$ 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 $k$T $\sim 1$ 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$_X$-L$_B$ 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