  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 N_gamma (although for the FDD with a flatter slope) and
they become equal at N_gamma = 2*10^50 s^-1 where f_esc = 0.11. FDD allows
escape fractions of the same order also at lower 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 N_gamma ~ 9*10^50 s^-1, corresponding to a star
formation rate (SFR) of roughly 2 M_o yr^-1; at this ionising luminosity the
FDD is less transparent (f_esc ~ 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.
