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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.