Studies Of Unusually Gas-Rich Galaxies In Alfalfa At Both Ends Of The Mass Spectrum

I present two major investigations enabled by the sensitivity and volume of the ALFALFA survey: one of dwarf early-type dwarfs (ETDs) in the Virgo Cluster, and the other of the high Hi mass, gas-rich HIghMass sample. The Virgo Cluster contains hundreds of ETDs, which are mostly red, non-starforming, and relatively gas-free galaxies. According to standard evolutionary scenarios, most such galaxies are the result of the stripping of faint late-type dwarfs which fall onto the cluster. These galaxies have their gas removed, and eventually cease forming new stars. However, ALFALFA has detected 6 ETDs which are both as gas-rich as a typical dwarf irregular or blue compact dwarf but which have the star formation properties of a typical 'red and dead' ETD. I investigate possible explanations for this population of galaxies: that they are infalling unstripped galaxies, that they are dwarfs with 'bursty' star formation between phases of star formation, that their gas is recycled, and that their gas is newly reaccreted. I argue that the properties of these ETDs are most consistent with gas reaccretion. Observationally, it is observed that galaxies with higher stellar masses are the most efficient at forming stars from their gas, and thus tend to have lower gas fractions (MHI /M* ). This is in agreement with both the semi-analytic theory of hierarchical clustering and the 'down-sizing' found observationally. However, in ALFALFA we have identified a sample of 34 galaxies which have both high stellar masses and high gas fractions for their stellar masses, which we call HIghMass. I present an in-depth study of two of these galaxies, UGC 9037 and UGC 12506, using high-resolution Hi observations obtained at the Very Large Array. UGC 9037 has very high Hi surface densities and strong non-circular motions, and appears to be a galaxy in transition, and will go through a period of heightened star formation in the near future. UGC 12506 has very low surface densities for its Hi mass, and is stable kinematically, making it a prime candidate for a galaxy which has intrinsically high dark matter halo spin. I also present preliminary results for the combined Hi and H2 for three additional HIghMass galaxies, but no firm conclusions are drawn