Passive Spiral Formation from Halo Gas Starvation: Gradual Transformation into S0s

Recent spectroscopic and high resolution $HST$-imaging observations haverevealed significant numbers of ``passive'' spiral galaxies in distantclusters, with all the morphological hallmarks of a spiral galaxy (inparticular, spiral arm structure), but with weak or absent star formation.Exactly how such spiral galaxies formed and whether they are the progenitors ofpresent-day S0 galaxies is unclear. Based on analytic arguments and numericalsimulations of the hydrodynamical evolution of a spiral galaxy's halo gas(which is a likely candidate for the source of gas replenishment for starformation in spirals), we show that the origin of passive spirals may well beassociated with halo gas stripping. Such stripping results mainly from thehydrodynamical interaction between the halo gas and the hot intracluster gas.Our numerical simulations demonstrate that even if a spiral orbits a clusterwith a pericenter distance $\sim$ 3 times larger than the cluster core radius,$\sim$ 80 % of the halo gas is stripped within a few Gyr and, accordingly,cannot be accreted by the spiral. Furthermore, our study demonstrates that thisdramatic decline in the gaseous infall rate leads to a steady increase in the$Q$ parameter for the disk, with the spiral arm structure, although persisting,becoming less pronounced as the star formation rate gradually decreases. Theseresults suggest that passive spirals formed in this way, gradually evolve intored cluster S0s.Comment: 13 pages 4 figures (fig.1 = jpg format), accepted by Ap