The impact of the IGM on high-redshift Lyα emission lines

Abstract

We calculate the impact of the intergalactic medium (IGM) on the observed Lyα lines emitted by galaxies in an ionized IGM at. Our model accounts for gas clumping in the IGM and for the fact that high-redshift galaxies reside in overdense regions, which causes the velocity field of the IGM to depart from the Hubble flow. The observed shape of the Lyα line varies widely, with dependence on the intrinsic width and systemic velocity of the line, a galaxies star formation rate and the local extragalactic UV background. For large star formation rates and levels of the UV background, absorption in the IGM does not result in a Lyα line that is asymmetric as is common among known high-redshift Lyα emitters (LAEs). For models in which the lines do show the observed strong asymmetry, the IGM typically transmits only 10-30 per cent of the Lyα flux. The increase in the ionizing background that accompanied the completion of reionization barely increased the IGM transmission, which suggests that LAEs of comparable luminosity should not appear to be significantly dimmer prior to overlap. In this light, we briefly discuss the potential of LAEs as a probe into the epoch of reionization.