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Session 81 - Dwarf Galaxies.
Display session, Friday, January 09
An outstanding question in galaxy evolution is whether there is an evolutionary connection between the various morphological classes of dwarf galaxies. In particular, it has been suggested that dIm galaxies may evolve into dE/dS0 galaxies after they lose their gas, either through supernovae-driven winds during episodes of intense star formation, or through stripping processes induced by galaxy-galaxy interactions. Previous studies, which have concentrated mainly on the broad optical morphological differences between dwarf galaxies and to some extent their neutral gas content, have not firmly established a connection between dIms and dEs. Additional information, namely the ionized gas associated with ongoing star formation, may elucidate the situation.
We present deep B-band, R-band, and H\alpha imaging of 3 dwarf galaxies: NGC 3377A, NGC 4286, and IC 3475. Based on previous broadband imaging and \ionH1 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, and the prototypical example of a gas-poor ``huge low surface brightness'' early-type galaxy (Sandage amp; Hoffman 1989, Sandage amp; Binggeli 1984). From the combination of our broadband and H\alpha imaging, we find that, unlike IC 3475, NGC 3377A and NGC 4286 both contain several regions of active star formation. Furthermore, NGC 3377A shows no evidence of a nuclear concentration in H\alpha seen in NGC 4286 (the bright nuclear knot is likely a foreground star), and has a high \ionH1 mass-to-light ratio, while IC 3475 and NGC 4286 both have low values of M_HI/L. The evolutionary state of NGC 3377A is thus unclear and more complicated than might be inferred from either previous broad-band imaging or \ionH1 content alone.
We will present evolutionary indicators such as color maps and star formation rates for these galaxies and discuss their gas content and morphological relationships in detail.
Program listing for Friday