Virial shocks in galactic haloes? (original) (raw)

We investigate the conditions for the existence of an expanding virial shock in the gas falling within a spherical dark-matter halo. The shock relies on pressure support by the shock-heated gas behind it. When the radiative cooling is efficient compared to the infall rate the post-shock gas becomes unstable; it collapses inwards and cannot support the shock. We find for a monoatomic gas that the shock is stable when the post-shock pressure and density obey γ eff ≡ (d ln P/dt)/(d ln ρ/dt) > 10/7. When expressed in terms of the pre-shock gas properties at radius r it reads ρrΛ(T )/u 3 < .0126, where ρ is the gas density, u is the infall velocity and Λ(T ) is the cooling function, with the post-shock temperature T ∝ u 2 . This result is confirmed by hydrodynamical simulations, using an accurate spheri-symmetric Lagrangian code. When the stability analysis is applied in cosmology, we find that a virial shock does not develop in most haloes that form before z ∼ 2, and it never forms in haloes less massive than a few 10 11 M ⊙ . In such haloes, the infalling gas is not heated to the virial temperature until it hits the disc, thus avoiding the cooling-dominated quasi-static contraction phase. The direct collapse of the cold gas into the disc should have nontrivial effects on the star-formation rate and on outflows. The soft X-ray produced by the shock-heated gas in the disc is expected to ionize the dense disc environment, and the subsequent recombination would result in a high flux of L α emission. This may explain both the puzzling low flux of soft X-ray background and the L α emitters observed at high redshift.