Gravity-driven Lyα blobs from cold streams into galaxies

We use high-resolution cosmological hydrodynamical adaptive mesh refinement (AMR) simulations to predict the characteristics of Ly alpha emission from the cold gas streams that fed galaxies in massive haloes at high redshift. The Ly alpha luminosity in our simulations is powered by the release of gravitational energy as gas flows from the intergalactic medium into the halo potential wells. The ultraviolet UV background contributes only < 20 per cent to the gas heating. The Ly alpha emissivity is due primarily to electron-impact excitation cooling radiation in gas at similar to 2 x 104 K. We calculate the Ly alpha emissivities assuming collisional ionization equilibrium at all gas temperatures. The simulated streams are self-shielded against the UV background, so photoionization and recombination contribute negligibly to the Ly alpha line formation. We produce theoretical maps of the Ly alpha surface brightnesses, assuming that similar to 85 per cent of the Ly alpha photons are directly observable. We do not consider transfer of the Ly alpha radiation, nor do we include the possible effects of internal sources of photoionization such as star-forming regions. Dust absorption is expected to obscure a small fraction of the luminosity in the streams. We find that typical haloes of mass M-v similar to 1012-1013 M-circle dot at z similar to 3 emit as Ly alpha blobs (LABs) with luminosities 1043-1044 erg s-1. Most of the Ly alpha comes from the extended (50-100 kpc) narrow, partly clumpy, inflowing, cold streams of (1-5) x 104 K that feed the growing galaxies. The predicted LAB morphology is therefore irregular, with dense clumps and elongated extensions. The integrated area contained within surface brightness isophotes of 2 x 10-18 erg s-1 cm-2 arcsec-2 is similar to 2-100 arcsec2, consistent with observations. The linewidth is expected to range from 102 to more than 103 km s-1 with a large variance. The typical Ly alpha surface brightness profile is proportional to r-1.2 where r is the distance from the halo centre. Our simulated LABs are similar in luminosity, morphology and extent to the observed LABs, with distinct kinematic features. The predicted Ly alpha luminosity function is consistent with observations, and the predicted areas and linewidths roughly recover the observed scaling relations. This mechanism for producing LABs appears inevitable in many high-z galaxies, though it may work in parallel with other mechanisms. Some of the LABs may thus be regarded as direct detections of the cold streams that drove galaxy evolution at high z.