CARBON-MONOXIDE EMISSION AS A PRECISE TRACER OF MOLECULAR GAS IN THE ANDROMEDA GALAXY

Stars are known to form in clouds of cold molecular hydrogen, which ar e relatively poorly understood despite being one of the main component s of the interstellar medium. The problem is that H-2 is invisible in the cold interstellar medium, so its distribution and motion must be i nferred from observations of minor constituents of the clouds, such as carbon monoxide and dust. Most of our present knowledge comes from ob servations of CO emission, but there is much debate on whether this is an effective tracer of H-2: it might miss a large fraction of the mol ecular gas(1). It is difficult to address this question on the basis o f observations within the Milky Way alone, whose edge-on orientation m akes it hard to discern the distant cloud structures. We have therefor e surveyed the CO emission of the molecular clouds of M31 (the Androme da galaxy), the nearest spiral galaxy to the Milky Way, and investigat ed the extent to which it follows the extinction of starlight by dust. We find a remarkably tight association between the CO emission and th e dust, from which we conclude that CO does indeed trace all of the mo lecular gas.