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Several hundreds of new high-sensitivity 21 cm and optical observations of spiral galaxies in the northern hemisphere are combined with previously published data in order to investigate the probable cause of gas deficiency. The normalcy of HI content for a galaxy of a given morphological type is defined from the observed mean hybrid neutral hydrogen surface brightness of $\sim 2000$ galaxies of types Sa or later located in low galactic density environments.
With respect to the comparison sample, spirals located in cluster cores usually show severe depletion of their interstellar HI. We confirm previous findings of the strong correlation of gas deficiency in clusters with both clustercentric distance and local density. We also demonstrate that HI deficiency of spirals inside cluster cores is not linked to the possible existence of clumps and, hence, that it is unlikely to have been established by the initial conditions existing at the epoch of galaxy formation. Correlations between integral properties of clusters seem to favor gas removal mechanisms depending on the gas density of the intracluster medium in front of those depending on its temperature. Special attention has been paid to reveal any dependence of HI deficiency on the velocity of galaxies with respect to the cluster where they are immersed. Indications are found that the velocity dispersion of gas deficient spirals tend to be, in any case, higher than that of non-deficient ones. These results are compatible with a scenario where gas deficient spirals are currently infalling into the cluster cores for the first time, the gas removal process being both efficient and swift.
J.M.S. acknowledges the support of a United States-Spanish Joint Commitee for Cultural \& Education Cooperation Fellowship No. II-D 91090.
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