Lyα radiation from collapsing protogalaxies.: I.: Characteristics of the emergent spectrum

We present Monte Carlo calculations of Ly alpha 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 Ly alpha flux over an extended solid angle, either from a spatially extended Ly alpha emissivity, or from scattering effects, or both. We present a detailed study of the effect of the gas distribution and kinematics and of the Ly alpha emissivity profile on the emergent spectrum and surface brightness distribution. The emergent Ly alpha spectrum is typically double peaked and asymmetric. In practice, however, we find energy transfer from the infalling gas to the Ly alpha 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 Ly alpha emission from collapsing protogalaxies difficult to detect beyond redshift z greater than or similar to 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 Ly alpha photons are being generated (rather than just scattered) in a spatially extended, collapsing region around the galaxy. For self-ionized clouds whose effective Ly alpha optical depth is less than or similar to 10(3), 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) = 10(18)-10(20) cm(-2). Finally, we present a new analytic solution for the emerging Ly alpha spectrum in the limiting case of a static uniform sphere, extending previous solutions for slabs.