Gravity-driven Lyα blobs from cold streams into galaxies: Cold streams as Lyα blobs

Abstract

We use high-resolution cosmological hydrodynamical AMR simulations to predict the characteristics of Lα emission from the cold gas streams that fed galaxies in massive haloes at high redshift.The Lα 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 UV background contributes only < 20% to the gas heating.The Lα emissivity is due primarily to electron-impact excitation cooling radiation in gas ∼ 2 × 10 4 K.We calculate the Lα emissivities assuming collisional ionisation equilibrium (CIE) at all gas temperatures.The simulated streams are self-shielded against the UV background, so photoionisation and recombination contribute negligibly to the Lα line formation.We produce theoretical maps of the Lα surface brightnesses, assuming that ∼ 85% of the Lα photons are directly observable.We do not consider transfer of the Lα radiation, nor do we include the possible effects of internal sources of photoionisation 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 ∼ 10 12-13 M ⊙ at z ∼ 3 emit as Lα blobs (LABs) with luminosities 10 43-44 erg s -1 .Most of the Lα comes from the extended (50-100 kpc) narrow, partly clumpy, inflowing, cold streams of (1 -5) × 10 4 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 × 10 -18 erg s -1 cm -2 arcsec -2 is ∼ 2 -100 arcsec 2 , consistent with observations.The linewidth is expected to range from 10 2 to more than 10 3 km s -1 with a large variance.The typical Lα surface brightness profile is ∝ 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 Lα 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.