Extragalactic H2 and its variable relation to CO
  A steep metallicity dependence of the CO-to-H2 conversion factor X of the form log X = 12.2 - 2.5 log[O]/[H], based on a sample of 20 (extra)galactic determinations, is presented and discussed. The apparently less steep dependence found from millimeter interferometer observations is explained within the context of photon-dominated regions. 
F.P. Israel (Sterrewacht Leiden)