Lyman continuum escape from an inhomogeneous interstellar medium

We have studied the effects of gas density inhomogeneities on the escape of ionizing Lyman continuum (Lyc) photons from Milky Way type galaxies via 3D numerical simulations using the Monte Carlo radiative transfer code crash . To this aim a comparison between a smooth Gaussian distribution (GDD) and an inhomogeneous, fractal one (FDD) has been made with realistic assumptions for the ionizing stellar sources based on available data in the solar neighbourhood. In both cases the escape fraction f _esc increases with ionization rate  ˙ _γ   (although for the FDD with a flatter slope) and they become equal at  ˙ _γ =2×10⁵⁰ s^-1  where   f _esc=0.11  . FDD allows escape fractions of the same order also at lower  ˙ _γ ,  when Lyc photon escape is sharply suppressed by GDD. Values of the escape fraction as high as 0.6 can be reached (GDD) for  ˙ _γ ≈9×10⁵⁰ s^-1  , corresponding to a star formation rate (SFR) of roughly 2 M_⊙ yr⁻¹; at this ionizing luminosity the FDD is less transparent  ( f _esc≈0.28)  . If high-redshift galaxies have gas column densities similar to local ones, and are characterized by such high SFRs and by a predominantly smooth (i.e. turbulence-free) interstellar medium, our results suggest that they should considerably contribute to – and possibly dominate – the cosmic UV background.