Lyman Absorption at Low Redshifts and Hot Gas in Galactic Haloes

Motivated by recent observation of Lanzetta et al. that most luminous galaxies at low redshifts produce Ly absorption at impact parameter l < 160h 1kpc, we propose that these absorbers are clouds con ned 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 ( 109 yrs) in the galactic halo. The pressure and temperature of the gas at radius r 100 kpc are P = (10 100)cm 3K, T = 10(5:5 6:5)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 1037 40erg s 1. The plausibility for such gas to exist in current models of galaxy formation is discussed.