Lyα Radiation from Collapsing Protogalaxies. II. Observational Evidence for Gas Infall

We model the spectra and surface brightness distributions for the Lyman alpha(Lya) radiation expected from protogalaxies that are caught in the early stagesof their assembly. We use the results of a companion paper to characterize theradiation emerging from spherically collapsing gas clouds. We then modify theintrinsic spectra to incorporate the effect of subsequent resonant scatteringin the intergalactic medium (IGM). Using these models, we interpret a number ofrecent observations of extended Lya blobs (LABs) at high redshift. We suggest,based on the angular size, energetics, as well as the relatively shallowsurface brightness profiles, and double-peaked spectra, that several of theseLABs may be associated with collapsing protogalaxies. We suggest two follow-upobservations to diagnose the presence of gas infall. High S/N spectra of LABsshould reveal a preferential flattening of the surface brightness profile atthe red side of the line. Complementary imaging of the blobs at redshiftedBalmer alpha wavelengths should reveal the intrinsic Lya emissivity and allowits separation from radiative transfer effects. We show that Lya scattering byinfalling gas can reproduce the observed spectrum of Steidel et al's LAB2 asaccurately as a recently proposed outflow model. Finally, we find similarevidence for infall in the spectra of point-like Lyman alpha emitters. Thepresence of scattering by the infalling gas implies that the intrinsic Lyaluminosities, and derived quantities, such as the star-formation rate, in theseobjects may have been underestimated by about an order of magnitude.Comment: Accepted for Publication in ApJ, 11 emulateapj pages with 6 figures, together with a companion pape