Are Pressure-Confined Clouds in Galactic Halo Possible for a Model of Ly$\alpha$ Cloud?

Abstract

(ABRIDGED)The Ly$\alpha$ clouds at low redshifts (z < 1.7)observed by HST showed us two unexpected features: Lanzetta et.al.establised the association between Ly$\alpha$ clouds and galaxies, and Ulmer pointed out the strong clustering of Ly$\alpha$ clouds. Motivated by these observations together with another observation which reports the detection of metal in the Ly$\alpha$ clouds at high redshift universe, we propose the two-component protogalaxy modelas a model for the Ly$\alpha$ cloud based upon the previous work. In our model,the Ly$\alpha$ clouds are supposed to be stable cold clouds confined by the pressure of ambient hot gas in galactic halo. We determine the properties of these cold clouds and hot gas on the basis of theoretical and observational constraints. Especially, we take into account the stability of a cold cloud in the galactic halo in addition to the general stability conditions in a two-component medium, and compare the derived quantities of Ly$\alpha$ clouds both cases in galactic halo and in intergalactic medium between high and low redshifts. We conclude that the condition that a cloud is stable against both evaporation and tidal destruction by a hot galactic halo is very restrictive. As a result, in the most noteworthy case, at z${\sim}$0.5,it is concluded that a pressure-confined, stable spherical Ly$\alpha$ cloud with typical neutral hydrogen column density $N_{HI}=10^{14}cm^{-2}$ cannot survive in the galactic halo, although much higher column density clouds of $N_{HI}= 10^{17}cm^{-2}$ can do there.