The photometric properties of isolated early-type galaxies

Isolated galaxies are important because they probe the lowest density regimes inhabited by galaxies. We define a sample of 36 nearby isolated early-type galaxies for further study. Our isolation criteria require them to have no comparable-mass neighbours within 2 B-band magnitudes, 0.67 Mpc in the plane of the sky and 700 km s(-1) in recession velocity. New widefield optical imaging of 10 isolated galaxies with the Anglo-Australian Telescope confirms their early-type morphology and relative isolation. We also present imaging of four galaxy groups as a control sample. The isolated galaxies are shown to be more gravitationally isolated than the group galaxies. We find that the isolated early-type galaxies have a mean effective colour of (B0R)(e) = 1.54 +/- 0.14, similar to their high-density counterparts. They reveal a similar colour-magnitude relation slope and small intrinsic scatter to cluster ellipticals. They also follow the Kormendy relation of surface brightness versus size for luminous cluster galaxies. Such properties suggest that the isolated galaxies formed at a similar epoch to cluster galaxies, such that the bulk of their stars are very old. However, our galaxy modelling reveals evidence for dust lanes, plumes, shells, boxy and disc isophotes in four out of nine galaxies. Thus at least some isolated galaxies have experienced a recent merger/accretion event, which may have induced a small burst of star formation. We derive luminosity functions for the isolated galaxies and find a faint slope of -1.2, which is similar to the 'universal' slope found in a wide variety of environments. We examine the number density distribution of galaxies in the field of the isolated galaxies. Only the very faintest dwarf galaxies (M-R greater than or similar to -15.5) appear to be associated with the isolated galaxies, whereas any intermediate-luminosity galaxies appear to lie in the background. Finally, we discuss possible formation scenarios for isolated early-type galaxies. Early epoch formation and a merger/accretion of galaxies are possible explanations. The collapse of a large, virialized group is an unlikely explanation, but that of a poor group remains viable.