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 fueled 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 sub([perpendicular]) = 58 kpc from a star-forming galaxy at z = 2.44. Besides strong neutral hydrogen (N sub(H0) = 10 super(19.50+ or -0.16) cm super(-2)) we detect neutral deuterium and oxygen, allowing a precise measurement of the metallicity: log sub(10)(Z/Z sub([middot in circle])) = -2.0+ or -0.17, or (7-15) x 10 super(-3) solar. Furthermore, the narrow deuterium linewidth requires a cool temperature 0.1 solar, 10 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 modeling are critical to accurately appraise the distribution of metals in the high-redshift CGM.