Overluminous CO in the central regions of spiral galaxies

Even though the interstellar medium (ISM) in a galaxy plays a critical role in its evolution, it is difficult to quantitatively measure the ISM content of a galaxy when the gas is primarily molecular. In this paper I present a method for quantitatively deriving the dust mass in a galaxy using optical and near-infrared images. These images are used as inputs to a radiative transfer model to generate a dust optical depth map. By simulating observational errors, I show that up to an optical depth of A(V) = 8 the model is not affected by saturation effects. The method is then applied to the central region (< 1.5 kpc) of four nearby spiral galaxies and is used to calibrate the conversion factor between CO luminosity and H-2 mass. The resulting conversion factor shows that the CO is overluminous by 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-infrared colors an I-K color map is a good qualitative representation of the dust morphology of a galaxy.