Chemical abundances and star-formation histories of low-mass dwarf galaxies

The study of the nature and evolution of the lowest-mass galaxies is important to our overall understanding of the formation and evolution of galaxies over time. Low-mass systems are a key to many lingering issues concerning galaxy formation including the apparent lack of very low-mass dwarf galaxies in the local universe. Additionally, low-mass dwarf galaxies are very metal-poor and thus provide possible analogs to processes of star-formation and evolution in the early universe. Low-mass galaxies are very low-luminosity and thus observing the lowest-mass systems is a diffcult task. Until recently, nearly all studies of low-mass galaxies were preformed with optically selected samples. These samples have an unavoidable bias toward higher surface brightness systems and it is unclear whether they can accurately characterize the dwarf galaxy population. In this work, we present studies performed on samples of gas-rich low-mass dwarf irregular galaxies chosen from the catalogs of two blind HI-surveys, the Arecibo Dual Beam Survey and Arecibo Legacy Fast ALFA Survey. These catalogs are free of optical biases and thus may provide a more comprehensive view of the gas-rich dwarf galaxy population. We find that optical properties of our samples are broadly consistent with other samples of dwarf irregulars, however our sample tends toward extremely low surface brightness in some cases. A small number of our galaxies have properties which indicate they may be nearly undetectable in typical optical surveys if in a more face-on orientation. The star-formation rates for our sample are roughly consistent with expectations for low surface brightness dwarf irregulars and distinctly lower than that seen in blue compact dwarf galaxies. We perform a spectral analysis of both samples and discover two new extremely-metal deficient (XMD) galaxies; this is particularly important as XMD galaxies have been largely elusive despite significant effort to find them. Our spectral analysis provides evidence that HI-selected samples may probe a chemically distinct population and that the luminosity-metallicity relationship may not be universal for low-mass systems. Understanding the deviations from this trend may help reveal the underlying physical mechanisms that cause the relationship to exist.