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

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

Halo White Dwarfs and the Hot Intergalactic Medium

We present a schematic model for the formation of baryonic galactic halos and hot gas in the Local Group and the intergalactic medium. We follow the dynamics, chemical evolution, heat flow and gas flows of a hierarchy of scales, including: protogalactic clouds, galactic halos, and the Local Group itself. Within this hierarchy, the Galaxy is built via mergers of protogalactic fragments. We find that early bursts of star formation lead to a large population of remnants (mostly white dwarfs), which would reside presently in the halo and contribute to the dark component observed in the microlensing experiments. The hot, metal-rich gas from early starbursts and merging evaporates from the clouds and is eventually incorporated into the intergalactic medium. The model thus suggests that most microlensing objects could be white dwarfs (m \sim 0.5 \msol), which comprise a significant fraction of the halo mass. Furthermore, the Local Group could have a component of metal-rich hot gas similar to, although less than, that observed in larger clusters. We discuss the known constraints on such a scenario and show that all local observations can be satisfied with present data in this model. The best-fit model has a halo that is 40% baryonic, with an upper limit of 77%.Comment: 15 pages, LaTex, uses aas2pp4.sty, 7 postscript figures. Substantially revised and enlarged to a full-length article. Somewhat different quantitative results, but qualitative conclusions unchange

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