MULTIFREQUENCY WINDOWS ON SPIRAL GALAXIES .4. GAS CONTENT AND STAR-FORMATION IN THE VIRGO CLUSTER

We present a multifrequency statistical analysis of spiral galaxies be longing to the Virgo cluster using photometrical data available in the literature or taken by us. The analysis shows that in spiral galaxies the global star formation activity is tightly coupled with the atomic gas content, while it is almost independent of the molecular gas rese rvoir (as determined from the CO lines). In spiral discs the surface b rightness of stars emitting in the visible and in the infrared H band is correlated with the H-2 column density. These observational evidenc es are interpreted as a result of using the CO emission of spiral gala xies to infer their global molecular gas content. This technique, in f act, is valid only in solar-neighbourhood environments, with abundant molecular gas and star formation, while it leads to an underestimate o f the H-2 content in those regions of spiral discs which have abundant cold molecular gas and low mass-star formation activity. In high-mass star formation regions, instead, the intense far-UV radiation field p roduced by young O-B stars partially photodissociates the diffuse mole cular gas, weakening the expected relation between star formation and molecular gas content. In the Virgo spiral galaxies the molecular gas content as traced by CO is not influenced by the cluster environment. Even galaxies showing strong signs of interaction with the intergalact ic medium, as determined from their perturbed HI morphology, have a no rmal molecular gas content.