The relationship between gas content and star formation rate in spiral   galaxies. Comparing the local field with the Virgo cluster
  Despite many studies of the star formation in spiral galaxies, a complete and coherent understanding of the physical processes that regulate the birth of stars has not yet been achieved, nor a unanimous consent was reached, despite the many attempts, on the effects of the environment on the star formation in galaxies member of rich clusters. We focus on the local and global Schmidt law and we investigate how cluster galaxies have their star formation activity perturbed. We collect multifrequency imaging for a sample of spiral galaxies, member of the Virgo cluster and of the local field; we compute the surface density profiles for the young and for the bulk of the stellar components, for the molecular and for the atomic gas. Our analysis shows that the bulk of the star formation correlates with the molecular gas, but the atomic gas is important or even crucial in supporting the star formation activity in the outer part of the disks. Moreover, we show that cluster members which suffer from a moderate HI removal have their molecular component and their SFR quenched, while highly perturbed galaxies show an additional truncation in their star forming disks. Our results are consistent with a model in which the atomic hydrogen is the fundamental fuel for the star formation, either directly or indirectly through the molecular phase; therefore galaxies whose HI reservoirs have been depleted suffer from starvation or even from truncation of their star formation activity. 
M. Fumagalli (1 and 2), G. Gavazzi (1) ((1) Universita di   Milano-Bicocca, (2) University of California, Santa Cruz)