Galaxies with anomalously high abundances of molecular hydrogen

A sample of 66 galaxies from the catalog of Bettoni et al. (CISM) with anomalously high molecular-to-atomic hydrogen mass ratios (Mmol/MHI > 2) is analyzed. The sample galaxies do not differ systematically from the other galaxies in the catalog with the same morphological types, in terms of their photometric parameters, rotational velocities, dust contents, or the integrated masses of gas (for galaxies with the same linear sizes and disk angular momenta). This suggests that the overabundances of H2 are due to the molecularization of HI. Galaxies with bars and active nuclei are found more frequently among galaxies with Mmol estimates in CISM. In a small fraction of cases, high Mmol/MHI ratios are due to overestimation of Mmol due to overstimating of the conversion factor for the translation of CO-line intensities into the number of H2 molecules along the line of sight. It is argued that the molecularization of the bulk of the gas mass could be due to the concentration of gas in the inner regions of the galactic disks and the resulting high gas pressures and relative low star-formation efficiencies, as is indeed observed in galaxies with high Mmol/MHI ratios.