The Ly-alpha forest around high-redshift galaxies

Motivated by the relative lack of neutral hydrogen around Lyman-break galaxies deduced from recent observations, we investigate the properties of the Lyα forest around high-redshift galaxies. The study is based on improved numerical SPH simulations implementing, in addition to standard processes, a new scheme for multiphase and outflow physics description. Although on large scales our simulations reproduce a number of statistical properties of the intergalactic medium (because of the small filling factor of shock-heated gas), they underpredict the Lyα optical depth decrease inside 1 Mpc h−1 of the galaxies by a factor of ≈15. We interpret this result as arising from the combined effect of infall occurring along the filaments, which prevents efficient halo gas clearing by the outflow, and the insufficient increase of (collisional) hydrogen ionization produced by the temperature increase inside the hot, outflow-carved bubble. Unless either feedback is not properly modelled in cosmological simulations or an observational selection bias is present, we speculate that local photoionization could be a viable explanation to solve the puzzle.