MASS STRIPPING OF LYMAN-ALPHA CLOUDS BY RAM PRESSURE UNDER THE MINIHALO MODEL

As a model for the Lyalpha forest we examine intergalactic clouds conf ined by the gravity of cold dark matter (CDM), so-called minihalos. Fi rst, we study the interaction of minihalos with supersonic gas flows t o consider the evolution of gas clouds via ram pressure using two-dime nsional hydrodynamical calculations. Such a supersonic wind will occur when a minihalo moves with supersonic velocity relative to the surrou nding intergalactic matter or when a blast wave or detonation wave, wh ich is caused by the activity of quasars, overtakes the minihalo. We d etermine the conditions for cloud destruction and obtain the time scal e for gas ablation. We find that the blast or detonation wave ejected from a quasar with energy larger than 10(63) ergs almost completely bl ows out the gas from the dark halo out to a radius of a few megaparsec s from the source quasar. Second, we explore the case when minihalos m ake a cluster, which is naturally expected in the CDM cosmogony. Such a cluster will evolve to a group of minihalos or protogalaxies within which each minihalo moves with the virial velocity of the group and fe els the ram pressure of surrounding intergalactic medium. Taking the m ass and radius of a galaxy for that of a cluster of minihalos, we expe ct that the gas of a given minihalo would be completely stripped in th e virial stage. As a result, the net clustering of Lyalpha clouds will be suppressed, in agreement with observations of Lyalpha forests.