Lyα Radiation from Collapsing Protogalaxies. I. Characteristics of the Emergent Spectrum

We present Monte Carlo calculations of Lyα radiative transfer through optically thick, spherically symmetric, collapsing gas clouds. These represent simplified models of protogalaxies in the process of their assembly. Such galaxies produce Lyα flux over an extended solid angle, either from a spatially extended Lyα emissivity, or from scattering effects, or both. We present a detailed study of the effect of the gas distribution and kinematics and of the Lyα emissivity profile on the emergent spectrum and surface brightness distribution. The emergent Lyα spectrum is typically double peaked and asymmetric. In practice, however, we find energy transfer from the infalling gas to the Lyα photons to significantly enhance the blue peak and to render the red peak, in most cases, undetectable. The resulting effective blueshift, combined with scattering in the intergalactic medium, renders extended Lyα emission from collapsing protogalaxies difficult to detect beyond redshift z>~4. We find a strong wavelength dependence of the slope of the surface brightness distribution (with preferential flattening at the red side of the line) to be a robust indication that Lyα photons are being generated (rather than just scattered) in a spatially extended, collapsing region around the galaxy. For self-ionized clouds whose effective Lyα optical depth is <~10³, infall and outflow models can produce nearly identical spectra and surface brightness distributions and are practically indistinguishable. The presence of cosmic abundance of deuterium may produce a detectable dip in the spectra of systems with moderate hydrogen column densities, N_H=10¹⁸-10²⁰ cm^-2. Finally, we present a new analytic solution for the emerging Lyα spectrum in the limiting case of a static uniform sphere, extending previous solutions for slabs.