Explaining the Observed Lyman Alpha `Blobs' as Cold Streams of Gas in the Halos of Galaxies

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Abstract
Over the past decade, mysterious blobs of Lyman alpha (Lya) emission by hydrogen have been discovered in the vicinity of galaxies at early cosmic times. Although a number of possible models were proposed to explain these blobs, none appeared to be consistent with all available data. In a seemingly unrelated frontier, the latest computer simulations of galaxy formation reveal streams of cold (T~1e4 K) gas feeding the cores of dark matter halos as massive as 1e12-1e13 Msun. Here we show that the simulated cold flows are spatially extended Lya sources with luminosities, Lya line widths, and abundances that are similar to those of observed Lya blobs. The predicted filamentary structure of cold flows explains the wide range of observed Lya blob morphologies. The most luminous cold flows are associated with massive halos, which preferentially reside in dense environments, in agreement with observations. We suggest that Lya blobs -- even those that are clearly associated with starburst galaxies or active galactic nuclei -- provide the first direct observational evidence for the cold accretion mode of galaxies.

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