Halpha3: an Halpha imaging survey of HI selected galaxies from ALFALFA.
  II. The star formation properties of galaxies in the Virgo cluster and
  surroundings
  We present the analysis of Halpha3, an Halpha imaging survey of 409 galaxies
selected from the HI Arecibo ALFALFA Survey in the Local Supercluster,
including the Virgo cluster. We explore the relations between the stellar mass,
the HI mass and the current, massive SFR of nearby galaxies in the Virgo
cluster and we compare them with those of isolated galaxies in the Local
Supercluster, disentangling the role of the environment in shaping the star
formation properties of galaxies at the present cosmological epoch. We
investigate the relationships between atomic neutral gas and newly formed stars
in different environments, across many morphological types, and over a wide
range of stellar masses adopting an updated calibration of the HI deficiency
parameter. Studying the mean properties of late-type galaxies in the Local
Supercluster, we find that galaxies in increasing local galaxy density
conditions (or decreasing projected angular separation from M87) show a
significant decrease in the HI content and in the mean specific star formation
rate, along with a progressive reddening of their stellar populations. The
gradual quenching of the star formation occurs outside-in, consistently with
the predictions of the ram pressure model. Once considered as a whole, the
Virgo cluster is effective in removing neutral hydrogen from galaxies, and this
perturbation is strong enough to appreciably reduce the SFR of its entire
galaxy population. An estimate of the present infall rate of 300-400 galaxies
per Gyr in the Virgo cluster is obtained from the number of existing HI-rich
late-type systems, assuming 200-300 Myr as the time scale for HI ablation. If
the infall process has been acting at constant rate this would imply that the
Virgo cluster has formed approximately 2 Gyr ago, consistently with the idea
that Virgo is in a young state of dynamical evolution.
