Isolated Elliptical Galaxies

One of the outstanding questions in galaxy evolution is how mergers transform galaxies. Studies of interacting galaxies in the field and clusters at high redshifts suggest that mergers are a common feature of galaxy evolution. Numerical simulations demonstrate that many kinds of mergers eventually produce a galaxy with a mass profile similar to that inferred from the optical luminosity profile of ellipticals. However, studies of ellipticals have been limited mostly to the densest environments, clusters and groups of galaxies, where they are most often found. Because there are many possible mechanisms besides mergers which might influence a galaxy's evolution in these environments, the interpretation of the data is complicated, and, consequently, we do not know whether many ellipticals are evolved merger products.

To study elliptical galaxies in the simplest of environments, Ann Zabludoff (University of Arizona) and I have compiled a unique and well-defined sample of 30 isolated ellipticals with z < 0.03. This sample is the first of its kind and made possible only because of the recent electronic availability of huge, typed databases of galaxy redshifts. The sample galaxies are bright and do not lie within a projected radius of 1 Mpc from any other galaxy in the RC3. Because surveys like the RC3 are notoriously incomplete, we confirm the isolation of these galaxies by inspection of sky survey images and comparison with group and cluster catalogs drawn from magnitude-limited redshift surveys. These criteria ensure that these ellipticals lie in the most rarefied environments of the nearby universe.

For each galaxy in our sample, we have obtained deep optical and near-infrared images and long-slit spectroscopy (along the major and minor axes). Analyses of the stellar compositions, kinematics, and morphologies of these galaxies provide strong evidence for recent mergers: seven of these ellipticals are AGN, several have a substantial young A-type stellar component signifying recent star-formation, and approximately 30% have luminous shells on the scales of tens-of-kpc.

Although the optical data are very suggestive, they do not allow us to distinguish between the merging of several large galaxies and the accretion of smaller satellites, since all of the observed morphological features can be explained by either scenario. However, X-ray observations may allow a clean separation of the two possibilities. This is because the cooling time for the hot gas in groups is much longer than the Hubble time. Therefore, while the galaxies in the group may have had enough time to merge and form a single galaxy, the intragroup medium will be essentially unchanged. Thus, we would expect isolated elliptical galaxies to have large extended X-ray halos, if they are the merged product of a group. Furthermore, the temperature and luminosity of this halo should be similar to that observed in groups today. Recent observations of the NGC 1132 galaxy with ASCA are consistent with the merged group scenario. This galaxy is surrounded by a large group-like X-ray halo with properties similar to those of X-ray groups.

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