OPTICAL OBSERVATIONS OF NGC-2915 - A NEARBY BLUE COMPACT DWARF GALAXY

This paper presents B and R band CCD images and medium resolution spec troscopy of NGC 2915, a relatively isolated BCD (blue compact dwarf) g alaxy at a distance of approximately 5 Mpc. NGC 2915 contains two stel lar populations: a high surface brightness blue core population and a red diffuse population. The core population contains all of the H II, and numerous embedded objects. It is the locus of current high mass st ar formation. The brightest embedded objects are likely to be young io nizing clusters, while many of the fainter objects are likely to be in dividual supergiant stars with masses up to approximately 25 M., or bl ends of a few such stars. Curious aligned structures on the SE side of the galaxy are seen and their nature discussed. The spectrum of the c ore is dominated by bright narrow emission lines like that of a high e xcitation and low metallicity (less than half solar) H II region. The continuum is flat, with Balmer and Ca ii features seen in absorption. The velocity of the Ca II features suggest contamination by galactic i nterstellar absorption. There is a significant velocity gradient in th e spectra, probably indicative of rotation. Outside of its core, NGC 2 915 resembles a dE (dwarf elliptical) galaxy, in that it has an expone ntial surface brightness profile, is red [(B - R)0 = 1.65], and has a low extrapolated central surface brightness [B(0)c = 22.44]. NGC 2915' s properties are compared with other BCDs, concentrating on two morpho logically similar BCDs that are near enough to resolve into stars: NGC 1705 and NGC 5253. It is noted that the presence of winds in BCDs inv alidates closed box chemical evolution models and the remaining constr aints on star formation duration are relatively weak. Some BCDs, inclu ding NGC 2915, may be able to maintain their present star formation ra te for Gyr time scales. This suggests that the overall evolution of th ese BCDs may be much slower than the approxmately10 Myr burst time sca les commonly quoted. However, shortly after the formation of a massive (10(6) M.) cluster a BCD will have all the properties of strong starb urst galaxy.