H \alpha + [ NII ] imaging observations of 369 late-type ( spiral ) galaxies in the Virgo cluster and in the Coma/A1367 supercluster are analyzed , covering 3 rich nearby clusters ( A1367 , Coma and Virgo ) and nearly isolated galaxies in the Great-Wall .
They constitute an optically selected sample ( m _ { p } < 16.0 ) observed with \sim 60 \% completeness .
These observations provide us with the current ( T < 10 ^ { 7 } yrs ) star formation properties of galaxies that we study as a function of the clustercentric projected distances ( \Theta ) .
The expected decrease of the star formation rate ( SFR ) , as traced by the H \alpha E.W. , with decreasing \Theta is found only when galaxies brighter than M _ { p } \sim - 19.5 are considered .
Fainter objects show no or reverse trends .
We also include in our analysis Near Infrared data , providing us with informations on the old ( T > 10 ^ { 9 } yrs ) 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 .
For the considered galaxies 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 .
In other words less massive galaxies are currently forming stars at higher rate than their giant counterparts which experienced most of their star formation activity at earlier cosmological epochs .
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 .
A significant fraction of gas ” healthy ” ( i.e .
with a gas deficiency parameter less than 0.4 ) and currently star forming galaxies is unexpectedly found projected near the center of the Virgo cluster .
Their average Tully-Fisher distance is found approximately one magnitude further away ( \mu _ { o } = 31.77 ) than the distance of their gas-deficient counterparts ( \mu _ { o } = 30.85 ) , suggesting that the gas healthy objects belong to a cloud projected onto the cluster center , but in fact lying few Mpc behind Virgo , thus unaffected by the dense IGM of the cluster .