Most of the H2 in our Galaxy resides in the cold interiors of molecular clouds. The most reliable way to trace the H2 content of a molecular cloud is, in principle, to measure the distribution of dust through it. In this contribution we present a new observational approach that uses infrared dust extinction of starlight to construct high resolution maps of the distribution of dust (H2) inside molecular clouds over unprecedented ranges of cloud depth: 1 < Av < 40 magnitudes. We also present a comparison of our results with conventional molecular-line column density tracer C18O and conclude that for cloud depths of Av > 10 magnitudes this species is a very poor tracer of H2.

Introduction

Molecular clouds are the reservoirs of H2 in the Galaxy. They contain about half of the mass of the Interstellar Medium and hence an important fraction of the mass of the Galaxy. By far the most important characteristic of molecular clouds is that they are the nurseries out of which stars like our Sun were born. This creation process not only determines the origins of stars and planetary systems in our Galaxy but also regulates the structure and evolution of galaxies on the large scale. To understand star and planet formation is to understand how cold H2 clouds evolve.