The Astronomical Journal, 127:1336-1343, 2004. Paper Website abstract bibtex
We use the Karachentseva Catalogue of Very Isolated Galaxies to \ninvestigate a candidate list of more than 100 very isolated early-type \ngalaxies. Broadband imaging and low-resolution spectroscopy are \navailable for a large fraction of these candidates and result in a \nsample of 102 very isolated early-type galaxies, including 65 elliptical \n(E) and 37 S0 galaxies. Many of these systems are quite luminous, and \nthe resulting optical luminosity functions of the E and early-type \n(E+S0) galaxies show no statistical differences when compared to \nluminosity functions dominated by group and cluster galaxies. However, \nwhereas S0 galaxies outnumber E galaxies 4:1 in the CfA survey, isolated \nE outnumber S0 galaxies by nearly 2:1. We conclude that very isolated \nelliptical galaxies show no evidence of a different formation and/or \nevolution process compared to those formed in groups or clusters, but \nthat most S0 galaxies are formed by a mechanism (e.g., gas stripping) \nthat occurs only in groups and rich clusters. Our luminosity function \nresults for elliptical galaxies are consistent with very isolated \nelliptical galaxies being formed by merger events, in which no \ncompanions remain. Chandra observations were proposed specifically to \ntest the merger hypothesis for isolated elliptical galaxies. However, \nthis program has resulted in the observation of only one isolated \nearly-type galaxy, the S0 KIG 284, which was not detected at a limit \nwell below that expected for a remnant group of galaxies. Therefore, the \nhypothesis remains untested that very isolated elliptical galaxies are \nthe remains of a compact group of galaxies that have completely merged.