Can galactic outflows explain the properties of Ly-alpha emitters?

Abstract

We study the properties of Ly-alpha emitters in a cosmological framework by computing the escape of Ly-alpha photons through galactic outflows. We combine the GALFORM semi-analytical model of galaxy formation with a Monte Carlo Ly-alpha radiative transfer code. The properties of Ly-alpha 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 characterise the differences in the Ly-alpha 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-alpha line profile of the Shell geometry is broader and more asymmetric, and the Ly-alpha 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-alpha emitters over the whole observed redshift range. The models are consistent with the observationally inferred Ly-alpha escape fractions, equivalent width distributions and with the shape of the Ly-alpha line from composite spectra. Interestingly, our predicted UV luminosity function of Ly-alpha emitters and the fraction of Ly-alpha emitters in Lyman-break galaxy samples at high redshift are in partial agreement with observations. Attenuation of the Ly-alpha line by the presence of a neutral intergalactic medium at high redshift could be responsible for this disagreement. We predict that Ly-alpha 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.

28 pages, 21 figures, submitted to MNRAS

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