Lyα RADIATIVE TRANSFER WITH DUST: ESCAPE FRACTIONS FROM SIMULATED HIGH-REDSHIFT GALAXIES

Abstract

The Lyman alpha emission line is an essential diagnostic tool for probing\ngalaxy formation and evolution. Not only is it commonly the strongest\nobservable line from high-redshift galaxies but from its shape detailed\ninformation about its host galaxy can be revealed. However, due to the\nscattering nature of Lya photons increasing their path length in a non-trivial\nway, if dust is present in the galaxy the line may be severely suppressed and\nits shape altered. In order to interpret observations correctly, it is thus of\ncrucial significance to know how much of the emitted light actually escapes the\ngalaxy.\n In the present work, using a combination of high-resolution cosmological\nhydro-simulations and an adaptively refinable Monte Carlo Lya radiative\ntransfer code including an advanced model of dust, the escape fractions f_esc\nof Lya radiation from high-redshift (z = 3.6) galaxies are calculated. In\naddition to the average escape fraction, the variation of f_esc in different\ndirections and from different parts of the galaxies is investigated, as well as\nthe effect on the emergent spectrum.\n Escape fractions from a sample of simulated galaxies of representative\nphysical properties are found to decrease for increasing galaxy virial mass\nM_vir, from f_esc approaching unity for M_vir ~ 10^9 M_sun to f_esc less than\n10% for M_vir ~ 10^12 M_sun. In spite of the dust being almost grey, it is\nfound that the emergent spectrum is affected non-uniformly, with the escape\nfraction of photons close to the line center being much higher than of those in\nthe wings, thus effectively narrowing the Lya line.\n