The bulk of the molecular gas in spiral galaxies is under the form of cold H2, that does not radiate and is only suspected through tracer molecules, such as CO. All tracers are biased, and in particular H2 could be highly underestimated in low metallicity regions. Our knowledge is reviewed of the H2 content of galaxies, according to their types, environment, or star-forming activities. The HI and CO components are generally well-mixed (spiral arms, vertical distribution), although their radial distributions are radically different, certainly due to radial abundance gradients. The hypothesis of H2 as dark matter is discussed, as well as the implications on galaxy dynamics, or the best perspectives for observational tests.

How to observe H2 in galaxies?

The bulk of molecular hydrogen in a galaxy is cold, around 10–20K, and therefore invisible. The first rotational level, accessible only through a quadrupolar transition, is more than 500 K above the fundamental. The presence of H2 is inferred essentially from the CO tracer. The carbon monoxide is the most abundant molecule after H2; its dipolemoment is small (0.1 Debye) and therefore CO is easily excited, the emission of CO(1–0) at 2.6mm (first level at 5.52K) is ubiquitous in the Galaxy.

The H2/CO conversion ratio

To calibrate the H2/CO ratio, the most direct and natural is to compare the UV absorption lines of CO and H2 along the same line of sight (Copernicus, e.g. Spitzer & Jenkins 1975; ORFEUS, cf Richter et al., this conference).