Virial shocks in galactic haloes?

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 with the infall rate, the postshock gas becomes unstable; it collapses inwards and cannot support the shock. We find for a monatomic gas that the shock is stable when the post-shock pressure and density obey eff (d ln P/dt)/(d ln /dt) > 10 7 . When expressed in terms of the pre-shock gas properties at radius r it reads as r (T )/u 3

• NA
  National Aeronautics and Space Administration
• IS
  Israel Science Foundation