Evolutionary status of dwarf "transition" galaxies

We present deep B-band, R-band, and H alpha imaging of three dwarf galaxies: NGC 3377A NGC 4286, and IC 3475. Based on previous broadband imaging and H I studies, these mixed morphology galaxies have been proposed to be, respectively, a gas-rich low surface brightness Im dwarf, a nucleated dwarf that has lost most of its gas and is in transition from Im to dS0, N, and the prototypical example of a gas-poor "huge low surface brightness" early-type galaxy. From the combination of our broadband and H alpha imaging with the published information on the neutral gas content of these three galaxies, we find that (1) NGC 3377A is a dwarf spiral, similar to those found by Schombert and coworkers and Matthews & Gallagher; (2) both NGC 3377A and NGC 4286 have comparable amounts of ongoing star formation, as indicated by their H alpha emission, while IC 3475 has no detected H II regions to a very low limit; (3) the global star formation rates are at least a factor of 20 below those of 30 Doradus for NGC 3377A and NGC 4286; (4) while the amount of star formation is comparable, the distribution of star-forming regions is very different between NGC 3377A and NGC 4286, with Ha emission scattered over most of the optical face of NGC 3377A and all contained within the inner half of the optical disk of NGC 4286; (5) given their current star formation rates and gas contents, both NGC 3377A and NGC 4286 can continue to form stars for more than a Hubble time; (6) both NGC 3377A and NGC 4286 have integrated total B-R colors that are redder than the integrated total B-R color for IC 3475 and thus it is unlikely that either galaxy will ever evolve into an IC 3475 counterpart; and (7) IC 3475 is too blue to be a dE. We thus conclude that we have not identified potential precursors to galaxies such as IC 3475, and unless significant changes occur in the star formation rates, neither NGC 3377A nor NGC 4286 will evolve into a dwarf elliptical or dwarf spheroidal within a Hubble time. Furthermore, optical morphology, even when coupled with the knowledge of the neutral gas content of a dwarf galaxy, is not sufficient to determine its evolutionary phase. The evolutionary states of NGC 3377A and NGC 4286 are thus unclear and more complicated than might be inferred from either previous broadband imaging or H I content alone.