GMC Formation in M33: the Role of Pressure

We present a complete map of the Giant Molecular Clouds in M33 and show that they lie exclusively on filaments of HI. The GMCs are localized on the extended HI filaments, and HI filaments extend well beyond the radius where the GMCs are found, suggesting that the H_2 forms from the HI and not vice-versa. We propose that the fraction of neutral gas that is molecular at a particular radius is determined primarily by the ambient hydrostatic pressure at that radius. This leads to a prediction that the transition radius, the radius where the mean surface density of HI and H_2 are equal, occurs at a constant value of the stellar surface density. We show that for a sample of 26 galaxies, the stellar surface density at the transition radius is constant to 40%.