The morphology and stellar populations of the dwarf amorphous galaxies NGC 216 and NGC 2915

Abstract

The definitions of the amorphous and blue compact dwarf (BCD) classes of galaxies are very similar. One key difference is that BCDs are often selected for their apparent compactness (i.e. a small angular size), which selects against nearby objects, whereas amorphous galaxies must be extended. We present initial results of a project to determine the population distribution in dwarf (M_B < −18) amorphous galaxies (dAgs) and determine which dAgs can be classified as BCDs. We have used the 3.9m Anglo-Australian Telescope to obtain deep B and R CCD images of two dAgs: NGC 216 (M_B = −17.3) and NGC 2915 (M_B = −14.1). The morphology of NGC 216 is that of a dusty late-type edge-on disk galaxy, with a peculiar one-sided bar. It would not be classified as a BCD if seen face-on. However, NGC 2915 does have all the properties of a BCD, and can be classified so. It has numerous condensations near its center. Many of these are likely to be individual stars. We derive a distance to NGC 2915 of 5 Mpc if the brightest blue non-extended objects are blue supergiants, and if there is little internal extinction. Similar condensations are seen in the dAgs NGC 1705 (Meurer, et al., 1989. Astrophys. Space Sci., 156: 141) and NGC 5253 (Caldwell and Phillips, 1989. Astrophys. J., 338: 789) which are also likely to be BCDs.