Galaxies With Spectral Energy Distribution Peaking Near 60microns. II. Optical Broadband Properties

In this paper (the second in a series), we present results of optical multicolor broadband CCD imaging of IRAS galaxies with spectral energy distributions peaking near 60 microns. As roughly two-thirds of the galaxies are previously unidentified in optical catalogues, this paper also serves as an optical catalogue for 60 micron peakers. All the 60 micron peakers are strong emission-line galaxies (Vader et al.) [AJ (1993), in press]. Our most striking result is that spiral structure is noticeably lacking among the 60 micron peakers, in view of the fact that IRAS galaxies and optically selected samples of emission-line galaxies are typically spirals. The majority of galaxies in our far-infrared (FIR) selected sample have a peculiar morphology (tidal tails, plumes, rings, shells, etc.) and the remaining galaxies have an amorphous appearance. This is reflected in a proposed morphological classification scheme consisting of four classes, ranging from amorphous to peculiar. Morphology class correlates well with optical colors and magnitudes; spectroscopic type; and FIR spectral indices. We interpret these correlations in terms of two scenarios, in which an interaction/merger is a key element that accounts for the observed morphology of the galaxies and the nuclear activity producing the "warm" FIR colors. Scenario A attributes the different optical morphologies of 60 micron peakers to differences in the precursor galaxies before the interaction, while scenario B attributes the morphological classification scheme to an evolutionary sequence that links galaxy morphology to nuclear activity. Both scenarios can explain the data equally well. The wide range in luminosity of our sample indicates that the short-lived phase of nuclear activity and 60 micron emission are not restricted to high luminosity galaxies.