Sharing the baryons : Stars and gas in local volume galaxies

Abstract

The baryonic content of a galaxy is divided into two distinct, albeit related, components; the stellar component and the gaseous component. Although many individual galaxies have had their stars and gas studied in great detail, overall the relation between these two baryonic components is poorly understood. This thesis presents a study of the stellar and gas properties of galaxies in the Local Volume (D < 10 Mpc) and investigates the relation between the atomic hydrogen and stellar components of a galaxy. We present H-band (1.65um) surface photometry of 57 galaxies drawn from the Local Sphere of Influence (LSI) with distances of less than 10 Mpc from the Milky Way. The images with a typical surface brightness limit 4 mag fainter than 2MASS (24.5 mag/arcsec{u0302}2 < SB_lim < 26 mag/arcsec{u0302}2 ) have been obtained with IRIS2 on the 3.9 m Anglo-Australian Telescope. A total of 22 galaxies that remained previously undetected in the near-IR and potentially could have been genuinely young galaxies were found to have an old stellar population with a star density 1-2 magnitudes below the 2MASS detection threshold. The cleaned near-IR images reveal the morphology and extent of many of the galaxies for the first time. For all program galaxies, we derive radial luminosity profiles, ellipticities, and position angles, together with global parameters such as total magnitude, mean effective surface brightness and half-light radius. Our results show that 2MASS underestimates the total magnitude of galaxies with effective surface brightness between 18-21 mag/arcsec{u0302}2 by up to 2.5 mag. The Sersic parameters best describing the observed surface brightness profiles are also presented. Adopting accurate galaxy distances and a H-band mass-to-light ratio of 1.0 +/- 0.4, the LSI galaxies are found to cover a stellar mass range of 5.6 < log_10 M < 11.1. The results are discussed along with previously obtained optical data. Our sample of low luminosity galaxies is found to follow closely the optical-infrared B versus H luminosity relation defined by brighter galaxies with a slope of 1.14 +/- 0.02 and scatter of 0.3 magnitudes. Finally we analyse the luminosity - surface brightness relation to determine an empirical mass-to-light ratio of 0.78 +/- 0.08 for late-type galaxies in the H-band. The atomic hydrogen distribution and kinematics for Local Volume galaxies have been obtained as part of the 'Local Volume \HI\ Survey' (LVHIS). We use the new, high resolution, HI line data obtained with the Australia Telescope Compact Array for a kinematical analysis of 12 nearby galaxies. For six galaxies in our sample we present the only resolved HI imaging available to date which reveals the atomic hydrogen distribution for the first time. Detailed kinematic information is presented as global HI line profiles and rotation curves. The rotation parameters obtained from the HI line profile are shown to be an accurate measure of the true rotation derived via rotation curve analysis. The kinematics of the 12 galaxies are analysed in detail and the classical and baryonic Tully-Fisher relations determined. Near-IR and 21cm data for 1394 nearby galaxies are analysed to investigate the relation between the atomic hydrogen and stellar components of a galaxy. We show that there is an upper limit for the HI mass in a galaxy, a quantity that correlates with its stellar mass. We examine the relation between the stellar and baryonic mass and find that the minimum stellar mass fraction for galaxies increases systematically to unity as their baryonic mass goes from 108 to 1012 solar masses. This result suggests that there is a physical limit to the maximum baryonic mass a galaxy has at the current epoch with giant cluster ellipticals being the most extreme systems. A similar mass limit has been proposed on theoretical grounds by Rees & Ostriker (1977) and Silk (1977).