Stability of the Protogalactic Clouds: I. Field Length in the Adiabatic Models

Abstract

All gasdynamical models for the evolution of gaseous content of galaxies assume that cooling from the hot, virialized phase to the cold phase occured through some sort of thermal instability. Subsequent formation of colder clouds embedded in the hot, rarefied medium is a well-known process appearing in many astrophysical circumstances and environments. The characteristics of the condensed clouds depend on the relevant timescales for cloud formation and disruption due to either collisions or one of the operating instabilities. In this paper, the importance of the Kelvin-Helmholtz instability is investigated for the clouds forming in huge gaseous haloes of $L_\ast$ galaxies. Recent treatment of this problem by Kamaya (1997) is extended and a more realistic cooling function employed. Results show that the Kelvin-Helmholtz instability proceeds effectively on the same timescale whether we account for self-gravity or not. This has multiple significance, since these objects may have been seen as high-column density absorption line systems against the background QSOs, and probably represent the progenitors of the present-day globular clusters.