Can galactic outflows explain the properties of Ly α emitters?

Abstract

We study the properties of Ly emitters in a cosmological framework by computing the escape of Ly photons through galactic outflows. We combine the GALFORM semi-analytical model of galaxy formation with a Monte Carlo Ly radiative transfer code. The properties of Ly emitters at 0 < z < 7 are predicted using two outflow geometries: a shell of neutral gas and a wind ejecting material, both expanding at constant velocity. We characterize the differences in the Ly line profiles predicted by the two outflow geometries in terms of their width, asymmetry and shift from the line centre for a set of outflows with different hydrogen column densities, expansion velocities and metallicities. In general, the Ly line profile of the Shell geometry is broader and more asymmetric, and the Ly escape fraction is lower than with the Wind geometry for the same set of parameters. In order to implement the outflow geometries in the semi-analytical model GALFORM, a number of free parameters in the outflow model are set by matching the luminosity function of Ly emitters over the whole observed redshift range. The resulting neutral hydrogen column densities of the outflows for observed Ly emitters are predicted to be in the range 10 18 -10 23 cm -2 . The models are consistent with the observationally inferred Ly escape fractions, and with the shape of the Ly line from composite spectra. Interestingly, our predicted UV luminosity function of Ly emitters and the fraction of Ly emitters in Lyman-break galaxy samples at high redshift are in partial agreement with observations. We predict that Ly emitters constitute a subset of the galaxy population with lower metallicities, lower instantaneous star formation rates and larger sizes than the overall population at the same UV luminosity.