Anisotropic Lyman-alpha Emission
  As a result of resonant scatterings off hydrogen atoms, Lyman-alpha (Lya)
emission from star-forming galaxies provides a probe of the (hardly isotropic)
neutral gas environment around them. We study the effect of the environmental
anisotropy on the observed Lya emission by performing radiative transfer
calculations for models of neutral hydrogen clouds with prescriptions of
spatial and kinematic anisotropies. The environmental anisotropy leads to
corresponding anisotropy in the Lya flux and spectral properties and induces
correlations among them. The Lya flux (or observed luminosity) depends on the
viewing angle and shows an approximate correlation with the initial Lya optical
depth in the viewing direction relative to those in all other directions. The
distribution of Lya flux from a set of randomly oriented clouds is skewed to
high values, providing a natural contribution to the Lya equivalent width (EW)
distribution seen in observation. A narrower EW distribution is found at a
larger peak offset of the Lya line, similar to the trend suggested in
observation. The peak offset appears to correlate with the line shape (full
width at half maximum and asymmetry), pointing to a possibility of using Lya
line features alone to determine the systemic redshifts of galaxies. The study
suggests that anisotropies in the spatial and kinematic distributions of
neutral hydrogen can be an important ingredient in shaping the observed
properties of Lya emission from star-forming galaxies. We discuss the
implications of using Lya emission to probe the circumgalactic and
intergalactic environments of galaxies.
