Very Isolated Early-Type Galaxies

We use the Karachentseva Catalogue of Very Isolated Galaxies to investigate a candidate list of more than 100 very isolated early-type galaxies. Broadband imaging and low-resolution spectroscopy are available for a large fraction of these candidates and result in a sample of 102 very isolated early-type galaxies, including 65 elliptical (E) and 37 S0 galaxies. Many of these systems are quite luminous, and the resulting optical luminosity functions of the E and early-type (E+S0) galaxies show no statistical differences when compared to luminosity functions dominated by group and cluster galaxies. However, whereas S0 galaxies outnumber E galaxies 4:1 in the CfA survey, isolated E outnumber S0 galaxies by nearly 2:1. We conclude that very isolated elliptical galaxies show no evidence of a different formation and/or evolution process compared to those formed in groups or clusters, but that most S0 galaxies are formed by a mechanism (e.g., gas stripping) that occurs only in groups and rich clusters. Our luminosity function results for elliptical galaxies are consistent with very isolated elliptical galaxies being formed by merger events, in which no companions remain. Chandra observations were proposed specifically to test the merger hypothesis for isolated elliptical galaxies. However, this program has resulted in the observation of only one isolated early-type galaxy, the S0 KIG 284, which was not detected at a limit well below that expected for a remnant group of galaxies. Therefore, the hypothesis remains untested that very isolated elliptical galaxies are the remains of a compact group of galaxies that have completely merged.