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