IV. The current star formation in nearby clusters of galaxies

Halpha+[NII] imaging observations of 369 late-type galaxies in the Virgo cluster and in the Coma/A1367 supercluster are analyzed. They constitute an optically selected sample (m_p10^9yrs) stars. Put together, the young and the old stellar indicators give the ratio of currently formed stars over the stars formed in the past, or "birthrate" parameter b. We also determine the "global gas content" combining HI with CO observations. We define the "gas deficiency" parameter as the logarithmic difference between the gas content of isolated galaxies of a given Hubble type and the measured gas content.For the isolated objects we find that b decreases with increasing NIR luminosity. The gas-deficient objects, primarily members to the Virgo cluster, have their birthrate significantly lower than the isolated objects with normal gas content and of similar NIR luminosity. This indicates that the current star formation is regulated by the gaseous content of spirals.Whatever mechanism (most plausibly ram-pressure stripping) is responsible for the pattern of gas deficiency observed in spiral galaxies members to rich clusters, it also produces the observed quenching of the current star formation