Lyα blobs as an observational signature of cold accretion streams into galaxies

Recent hydrodynamic simulations of galaxy formation reveal streams of cold (T similar to 104 K) gas flowing into the centres of dark matter haloes as massive as 1012-13.5 M-circle dot at redshifts z similar to 1-3. In this paper, we show that if greater than or similar to 20 per cent of the gravitational binding energy of the gas is radiated away then the simulated cold flows are spatially extended Ly alpha sources with luminosities, Ly alpha linewidths and number densities that are comparable to those of observed Ly alpha blobs. Furthermore, the filamentary structure of the cold flows can explain the wide range of observed Ly alpha blob morphologies. Since the most massive haloes form in dense environments, the association of Ly alpha blobs with overdense regions arise naturally. We argue that Ly alpha blobs - even those which are clearly associated with starburst galaxies or quasars - provide direct observational support for the cold accretion mode of galaxies. We discuss various testable predictions of this association.