Physical Properties of Low Mass Galaxies in the Local Volume

Abstract

The motivation of this research came from a desire to accurately trace the underlying stellar distributions of low mass irregular galaxies in the Local Volume and utilize the derived properties in various avenues of research. To this effect, deep H-band surface photometry and analysis of 40 Local Volume galaxies was performed, a sample primarily composed of dwarf irregulars in the Cen A group, obtained using the IRIS2 detector at the 3.9m AngloAustralian Telescope. The imagery probes to a surface brightness of ∼25 mag arcsec−2 , reaching a 40 times lower stellar density than the Two Micron All Sky Survey (2MASS) which had insufficient image depth to study dwarf galaxies in the NIR. This research has developed extremely careful and rigorous cleaning techniques to remove contaminating sources, with the surface photometry on the 33 detected galaxies deriving the observed total magnitude, effective surface brightness and best fitting Sérsic parameters. For selected galaxies, image quality and surface photometry comparisons to 2MASS and VISTA Hemispheric Survey (VHS) are made demonstrating that deep targeted surveys are still the most reliable means of obtaining accurate surface photometry. From analysis of the NIR imagery it was suspected that the structural properties of irregular galaxies were similar. I tested this suspicion through structure-luminosity relationships for the low mass irregulars in the sample and dwarf ellipticals and spheroidal galaxies obtained from other catalogues. I demonstrate that a significant fraction of the Local Volume dwarf irregular population have underlying structural properties similar to both Local Volume and Virgo Cluster dwarf ellipticals. Linear regressions to structure-luminosity relationships for the Local Volume galaxies and Virgo Cluster dwarf ellipticals show significant differences in both slope and scatter around the established trend lines, suggesting that environment might regulate the structural scaling relationships of dwarf galaxies in comparison to their more isolated counterparts. High quality resolved H i observations are only possible for a handful of dwarf galaxies in the Local Volume. Future H i surveys are unlikely to improve the current situation. I therefore explored a method for estimating the surface density of the atomic gas from global H i parameters, which are conversely widely available. I perform empirical tests using galaxies with resolved H i maps, and find that the approximation presented in this study produces values for the surface density of atomic hydrogen within typically 0.5 dex of the true value. I apply this method to a sample of 147 galaxies drawn from modern NIR stellar photometric surveys. I confirm a strict correlation between the atomic gas surface density and the star formation rate surface density. Comparison of these data to available models shows that a model in which the thermal pressure balances the vertical gravitational field captures better the shape of the ΣSFR-Σgas relationship. Show more