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In our current galaxy formation paradigm, high-redshift galaxies arepredominantly fuelled by accretion of cool, metal-poor gas from theintergalactic medium. Hydrodynamical simulations predict that this materialshould be observable in absorption against background sightlines within agalaxy's virial radius, as optically thick Lyman-limit systems (LLSs) with lowmetallicities. Here we report the discovery of exactly such a strong metal-poorabsorber at an impact parameter R_perp = 58 kpc from a star-forming galaxy at z= 2.44. Besides strong neutral hydrogen [N(HI) = 10^(19.50 +/- 0.16) cm^-2] wedetect neutral deuterium and oxygen, allowing a precise measurement of themetallicity: log10(Z / Zsolar) = -2.0 +/- 0.17, or (7-15) x 10^-3 solar.Furthermore, the narrow deuterium linewidth requires a cool temperature <20,000 K. Given the striking similarities between this system and thepredictions of simulations, we argue that it represents the direct detection ofa high redshift cold-accretion stream. The low-metallicity gas cloud is asingle component of an absorption system exhibiting a complex velocity,ionization, and enrichment structure. Two other components have metallicities >0.1 solar, ten times larger than the metal-poor component. We conclude that thephotoionized circumgalactic medium (CGM) of this galaxy is highlyinhomogeneous: the majority of the gas is in a cool, metal-poor andpredominantly neutral phase, but the majority of the metals are in ahighly-ionized phase exhibiting weak neutral hydrogen absorption but strongmetal absorption. If such inhomogeneity is common, then high-resolution spectraand detailed ionization modelling are critical to accurately appraise thedistribution of metals in the high-redshift CGM.

Metal-Poor, Cool Gas in the Circumgalactic Medium of a z = 2.4 Star-Forming Galaxy: Direct Evidence for Cold Accretion?