Lyman Continuum Escape from Inhomogeneous ISM

We have studied the effects of gas density inhomogeneities on the escape of ionising Lyman continuum (Lyc) photons from Milky Way-type galaxies via 3D numerical simulations using the Monte Carlo radiative transfer code CRASH (Ciardi et al. 2001). 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 ionising stellar sources based on available data in the solar neighborhood. In both cases the escape fraction fesc increases with ionisation rate Ṅγ (although for the FDD with a flatter slope) and they become equal at Ṅγ = 2 × 10 50 s where fesc = 0.11. FDD allows escape fractions of the same order also at lower Ṅγ , when Lyc photon escape is sharply suppressed by GDD. Values of the escape fraction as high as 0.6 can be reached (GDD) for Ṅγ ≈ 9× 10 50 s, corresponding to a star formation rate (SFR) of roughly 2 M⊙ yr ; at this ionising luminosity the FDD is less transparent (fesc ≈ 0.28). If high redshift galaxies have gas column densities similar to local ones, 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.