The Optical and Near-Infrared Morphologies of Isolated Early-Type Galaxies

To study early-type galaxies in their simplest environments, we have constructed a well-defined sample of 30 isolated galaxies. The sample contains all early-type galaxies listed in the Third Reference Catalogue of Bright Galaxies (RC3) with no other cataloged galaxy with a known redshift lying within a projected radius of 1 h Mpc and ±1000 km s -1 (where we use the recession velocities in the RC3). We have obtained optical and near-infrared images of 23 of the galaxies and of a comparison sample of 13 early-type galaxies in X-ray–detected poor groups of galaxies. We have applied the techniques of unsharp-masking, galaxy model division, and color maps to search for morphological features that might provide clues to the evolution of these galaxies. Evidence for dust features is found in approximately 75% of both the isolated and group galaxies (17 of 22 and 9 of 12, respectively). However, shells or tidal features are much more prevalent in our isolated sample than in our group sample (9 of 22 = 41% vs. 1 of 12 = 8%, respectively). The isolation and colors of these shell galaxies make it unlikely that tidal interactions or asymmetric star formation are the causes of such features. One model that is not ruled out is that mergers produce the shells. If shells and dust are both merger signatures, the absence of shells in group elliptical galaxies implies that shells (1) form more easily, (2) are younger, and/or (3) are longer lived in isolated environments.