Evolutionary studies of low surface brightness dwarf galaxies.

摘要

The results of an investigation of the evolutionary status of fifteen HI rich, low surface brightness dwarf galaxies (LSBDGs) are presented. The target objects were identified via a systematic survey for extended HI envelopes around isolated low luminosity galaxies with the Arecibo telescope. The results of the Arecibo survey indicated that the majority of systems distinguished by high catalogued values of M{dollar}sb{lcub}H{rcub}{dollar}/L{dollar}sb{lcub}B{rcub}{dollar} had evidence of an extended HI distribution. Due to the low surface brightness nature of these systems, the catalogued magnitudes are a severe underestimate of their true luminosities ({dollar}sim{dollar}1.5{dollar}sp{lcub}m{rcub}{dollar}). However, even with the revised luminosities, the average M{dollar}sb{lcub}H{rcub}{dollar}/L{dollar}sb{lcub}B{rcub}{dollar} of the LSBDGs is a factor of 2 higher than typical for dwarf systems. The LSBDGs are rotation-dominated systems which are underluminous for their HI mass, suggesting that the star formation process has been inefficient.; The current star formation rates of the LSBDGs are low, with a median value of 0.023 M{dollar}sb{lcub}odot{rcub}{dollar} yr{dollar}sp{lcub}-1{rcub}{dollar}. Similar to other low luminosity systems, the gas depletion time scales are quite long, on the order of 10{dollar}sp{lcub}11{rcub}{dollar} yr. Further, the optical colors are consistent with an exponentially decreasing star formation rate over the last few Gyr. Thus, despite the presence of relatively unprocessed gas and low surface brightness, these systems are not "young." Rather, the star formation has been suppressed.; One key to the star formation history of the LSBDGs is that despite a moderate mass of HI, their global gas surface densities are a factor of 2 below the Toomre instability threshold. Only on local scales does the gas column density approach the instability limit. Peak column densities are on the order of 10{dollar}sp{lcub}21{rcub}{dollar} atoms cm{dollar}sp{lcub}-2{rcub}{dollar}, and, as expected, are correlated with sites of current star formation. Thus, the star formation process in the LSBDGs appears to be inhibited globally, but occurs locally.