Lyman-alpha emission as a probe of galaxy environments

As a result of resonant scatterings off hydrogen atoms, Lya emission from star-forming galaxies provides a potential probe of the neutral gas environment around them. In order to determine the utility of Lya emission as a probe of gas environments we study the effects of 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 a correlation with Lya optical depth along the line of sight of the viewing direction relative to the optical depths 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 EW distribution seen in observations. A narrower EW distribution is found at larger peak offset of the line, similar to a trend suggested in recent observation. The peak offset appears to correlate with the line shape (a product of the width and the asymmetry of the line) suggesting the possibility of using the Lya line features alone to determine systemic redshift 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 and introduce an in-progress follow-up study using realistic gas environments from a cosmological galaxy formation simulation.