Lyα radiation from collapsing protogalaxies.: II.: Observational evidence for gas infall

We model the spectra and surface brightness distributions for the Ly alpha radiation expected from protogalaxies that are caught in the early stages of their assembly. We use the results of a companion paper to characterize the radiation emerging from spherically collapsing gas clouds. We then modify these spectra to incorporate the effect of subsequent resonant scattering in the IGM. Using these models, we interpret a number of recent observations of extended Ly alpha blobs (LABs) at high redshift. We suggest, based on the angular size, energetics, relatively shallow surface brightness profiles, and double-peaked spectra, that several of these LABs may be associated with collapsing protogalaxies. We suggest two follow-up observations to diagnose the presence of gas infall. High-S/N spectra of LABs should reveal a preferential flattening of the surface brightness profile at the red side of the line. Complementary imaging of the blobs at redshifted H alpha wavelengths should reveal the intrinsic Ly alpha emissivity and allow its separation from radiative transfer effects. We show that Ly alpha scattering by infalling gas can reproduce the observed spectrum of the Steidel et al. LAB2 as accurately as a recently proposed outflow model. Finally, we find similar evidence for infall in the spectra of pointlike Ly alpha emitters. The presence of scattering by the infalling gas implies that the intrinsic Ly alpha luminosities and derived quantities, such as the star formation rate, in these objects may have been underestimated by about an order of magnitude.