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 f _ { esc } increases with ionisation rate \dot { { \cal N } } _ { \gamma } ( although for the FDD with a flatter slope ) and they become equal at \dot { { \cal N } } _ { \gamma } = 2 \times 10 ^ { 50 } s ^ { -1 } where f _ { esc } = 0.11 .
FDD allows escape fractions of the same order also at lower \dot { { \cal N } } _ { \gamma } , when Lyc photon escape is sharply suppressed by GDD .
Values of the escape fraction as high as 0.6 can be reached ( GDD ) for \dot { { \cal N } } _ { \gamma } \approx 9 \times 10 ^ { 50 } s ^ { -1 } , corresponding to a star formation rate ( SFR ) of roughly 2 M _ { \odot } yr ^ { -1 } ; at this ionising luminosity the FDD is less transparent ( f _ { esc } \approx 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 .