Overluminous CO in the Central Regions of Spiral Galaxies

Abstract

Even though the interstellar medium (ISM) in a galaxy plays a criticalrole in its evolution, it is difficult to quantitatively measurethe ISM content of a galaxy when the gas is primarily molecular.In this paper I present a method for quantitatively deriving thedust mass in a galaxy using optical and near-infrared images. Theseimages are used as inputs to a radiative transfer model to generatea dust optical depth map. By simulating observational errors, I showthat up to an optical depth of A(V) = 8 the model is not affectedby saturation effects. The method is then applied to the centralregion (< 1.5 kpc) of four nearby spiral galaxies and is used tocalibrate the conversion factor between CO luminosity and H-2 mass.The resulting conversion factor shows that the CO is overluminousby a factor of at least 5-20 compared to the CO in the solar neighborhood.I also show that in the absence of a full set of optical and near-infraredcolors an I-K color map is a good qualitative representation of thedust morphology of a galaxy.