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| - IV. The current star formation in nearby clusters of galaxies
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| - H α surface photometry of galaxies in the Virgo cluster
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| - H α+[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 ~ $60 \%$ 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 ( Θ). The expected decrease of the star formation rate (SFR), as traced by the H α EW, with decreasing Θ 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 information 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 a higher rate than their giant counterparts which experienced most of their star formation activity at earlier cosmological epochs. The gas-deficient objects, primarily members of the Virgo cluster, have a 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 of 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 ( $\mu_{\rm o}=$ 31.77) than the distance of their gas-deficient counterparts ( $\mu_{\rm o}=$ 30.85), suggesting that the gas healthy objects belong to a cloud projected onto the cluster center, but in fact lying a few Mpc behind Virgo, thus unaffected by the dense IGM of the cluster.
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