Surveys of several hundred galaxies show a systematic trend in the fraction of molecular gas to neutral gas for galaxies along the `Hubble sequence' (see Fig. 6). This relationship is not fully understood although possible answers include the effect of the large-scale gravitational field in the formation or disruption of molecular clouds. For example, the stronger gravitational field in the bulge-dominated (early) galaxies may encourage fragmentation of the gas as a first step to forming dense clouds through collapse. It appears that there is also an enhanced fraction of molecules to neutral atoms in merging galaxies and cluster galaxies. In the latter case (e.g. Virgo), it is thought that the diffuse hydrogen has been swept away by the intracluster gas as the galaxy moves through the cluster.
Figure 6. The ratio of molecular to atomic hydrogen for different `Hubble types' of spiral galaxies. The histograms are presented in order of declining bulge to disk ratios, with the S0/Sa galaxies having the largest bulges. The increasing fraction of dense molecular clouds towards earlier Hubble types may reflect a higher cloud formation rate in the presence of a stronger gravitational field. (Courtesy of J.S. Young, University of Massachussetts.)