Lyman alpha absorption at low redshifts and hot gas in galactic haloes

Abstract

Motivated by recent observation of Lanzetta et al. that most luminous galaxies at low redshifts produce $\lya$ absorption at impact parameter $l\lsim 160 \kpch$, we propose that these absorbers are clouds confined by the pressure of ambient hot gas in galactic haloes. We determine the properties of this hot gas and of the absorption systems on the basis of observational and theoretical constraints. The absorbing clouds need to be replenished on about one orbital time ($\sim 10^9$ yrs) in the galactic halo. The pressure and temperature of the gas at radius $r\sim 100\kpc$ are $P=(10-100){\rm cm^{-3} K}$, $T=10^{(5.5-6.5)}{\rm K}$. The model requires that about 10 per cent of the gas in low-redshift galactic haloes is in the hot phase. Such gas in galactic haloes emits x-ray with bolometric luminosity of the order $10^{37-40}{\rm erg\,s^{-1}}$. The plausibility for such gas to exist in current models of galaxy formation is discussed.