Metal-Poor, Cool Gas in the Circumgalactic Medium of a z = 2.4
  Star-Forming Galaxy: Direct Evidence for Cold Accretion?
  In our current galaxy formation paradigm, high-redshift galaxies are
predominantly fuelled by accretion of cool, metal-poor gas from the
intergalactic medium. Hydrodynamical simulations predict that this material
should be observable in absorption against background sightlines within a
galaxy's virial radius, as optically thick Lyman-limit systems (LLSs) with low
metallicities. Here we report the discovery of exactly such a strong metal-poor
absorber 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] we
detect neutral deuterium and oxygen, allowing a precise measurement of the
metallicity: 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 the
predictions of simulations, we argue that it represents the direct detection of
a high redshift cold-accretion stream. The low-metallicity gas cloud is a
single 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 the
photoionized circumgalactic medium (CGM) of this galaxy is highly
inhomogeneous: the majority of the gas is in a cool, metal-poor and
predominantly neutral phase, but the majority of the metals are in a
highly-ionized phase exhibiting weak neutral hydrogen absorption but strong
metal absorption. If such inhomogeneity is common, then high-resolution spectra
and detailed ionization modelling are critical to accurately appraise the
distribution of metals in the high-redshift CGM.
