Star Formation at the Extreme Ends of the Hubble Sequence

Joseph Helmboldt

I present the results of two different studies aimed at exploring star formation within galaxies at the two extreme ends of the Hubble sequence and how they may contribute to our knowledge of galaxy evolution and star formation in general. The first of these two projects uses broad-band (B and R) and narrow-band (H-alpha) imaging of 69 galaxies drawn from a single-dish HI survey (the HI Parkes All Sky Survey) to examine star formation in a galaxy sample that is not biased against late-type, low surface brightness galaxies as optically selected samples typically are. The results imply that while the surface brightness bias inherent to flux limited optical catalogs may cause these catalogs to miss more than 10% of the total galaxy number density, they will only miss about 3-4% of the total star formation rate density and a negligable amount of the total luminosity density. While they may not contribute significantly to the overall amount of star formation in the local universe, the lower surface brightness galaxies in this sample have HII region and diffuse ionized gas (DIG) properties that imply that they may provide interesting examples of star formation occuring under extreme conditions. The relative amounts of H-alpha emission from the DIG components and the shapes of the HII region luminosity functions for these galaxies are consistent with them having sporadic star formation histories and forming some fraction of their stars outside of star clusters.

The second project uses a sample of ~37,000 early-type galaxies culled from the Sloan Digital Sky Survey (SDSS) to identify a subsample of actively star forming early-type galaxies with the aim of establishing them as the progenitors of post-starburst E+A galaxies which may be tied to galaxy morphology evolution and galaxy merger activity. The results show that star forming early-type galaxies have properties (namely luminosity, stellar mass, and velocity dispersion) that are very similar to those of E+A galaxies and that both classes of galaxies preferentially reside in relatively low density environments (i.e. outside of clusters). Within the SDSS sample, the star forming early-type galaxies are about an order of magnitude more numerous than E+A galaxies. With the help of population synthesis models, it was found that this discrepancy in number density can be resolved by taking into account the facts that not all star forming galaxies will go through large enough bursts to be classified as E+A galaxies, that the typical time scale for the star forming phase is longer than that for the E+A phase, and that a significant fraction of galaxies that should be classified as E+A galaxies will not be because they have emission lines produced by AGN. Therefore, it appears that star forming early-type galaxies (or at least some fraction of them) are the progenitors of E+A galaxies. The model results also imply that on average, the relative increase in stellar mass caused by the starbursts that likely produce E+A galaxies is less than 3%. Narrow- and broad-band imaging of a small subset of star forming early galaxies revealed that the star formation is centralized and that while the starbursts may only increase the total stellar mass by ~3%, they are capable of increasing the bulge fraction by about 10%.