8 Summary and conclusion

Using spectro-photometric data available in the literature for a small sample of nearby galaxies we have analysed the relationships between the X = N(H₂)/I(CO) conversion factor and various parameters characterizing the physical properties of the ISM. The behaviour of the nearby galaxies, whose value of X has been measured by independent techniques, is compared to that of a sample of 266 late-type galaxies with available multifrequency data. For these 266 galaxies X is estimated by assuming a metallicity-dependent dust to gas ratio, where the dust masses have been determined using ISOPHOT and IRAS 100-200 $\mu$m data. Both samples show an anticorrelation (correlation) between X and the metallicity (or the UV radiation field), with higher values of X in metal-poor, star forming galaxies. These observational results can be explained if the diffuse carbon monoxide in the outskirts of the giant molecular clouds is dissociated by UV photons at a rate higher than the molecular hydrogen which, given its higher density, is more efficiently self-shielded. The ratio between N(H₂) and I(CO) is thus expected to increase when the UV radiation field increases, as in star forming regions or in regions where the extinction, therefore the dust content and the metallicity, are low.

Given the strong relationship between metallicity, star formation activity and luminosity, the sample of nearby galaxies is used to calibrate a luminosity-dependent X conversion factor. Using this new calibration, we re-analyse the molecular gas properties of galaxies spanning a large range in morphological type and luminosity. Low-mass, dwarf galaxies have higher molecular gas masses (per unit galaxy mass) than early-type, massive spirals. The molecular gas fraction in clouds or complexes is $\sim$15% of the total HI reservoir for all late-type galaxies.

Galaxies strongly interacting with the cluster environment have, on average, a molecular gas content comparable to isolated, unperturbed objects.

The star formation rate of late-type galaxies is strongly related to their molecular gas content. This relationship is valid for galaxies spanning a large range in luminosity.

The efficiency in transforming gas into stars is roughly constant in galaxies of different type and luminosity and belonging to different environments.

Acknowledgements

We want to thank the 12 m telescope operators for their invaluable help during the remote observations. We thank J. Kenney for providing us with some unpublished CO data, and C. Hoopes and N. Devereux for the H$\alpha +[$NII]EW of NGC 891 and M 33. We thank C. Bonfanti for providing us with some metallicity measurements, V. Buat, A. Contursi and J. M. Deharveng for interesting discussions, and S. Zibetti for providing us with fitting routines. We thank the anonymous referee for comments and suggestions which helped improving the quality of the manuscript. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center, funded by the National Aeronautics and Space Administration and the National Science Foundation.