A Long Stream of Metal-Poor Cool Gas around a Massive Starburst Galaxy
  at z = 2.67
  We present the first detailed dissection of the circumgalactic medium (CGM)
of massive starburst galaxies at z > 2. Our target is a submillimeter galaxy
(SMG) at z = 2.674 that has a star formation rate of 1200 $M_\odot$/yr and a
molecular gas reservoir of $1.3\times10^{11} M_\odot$. We characterize its CGM
with two background QSOs at impact parameters of 93 kpc and 176 kpc. We detect
strong HI and metal-line absorption near the redshift of the SMG towards both
QSOs, each consisting of three main subsystems spanning over 1500 km/s. The
absorbers show remarkable kinematic and metallicity coherence across a
separation of 86 kpc. In particular, the cool gas in the CGM of the SMG
exhibits high HI column densities ($\log N_{\rm HI}/{\rm cm}^{-2} = 20.2,
18.6$), low metallicities ([M/H] $\approx$ -2.0), and similar radial velocities
($\approx$ -300 km/s). While the HI column densities match previous results on
the CGM around QSOs at z > 2, the metallicities are lower by more than an order
of magnitude, making it an outlier in the line width$-$metallicity relation of
damped Ly$\alpha$ absorbers. The large physical extent, the velocity coherence,
the high surface density, and the low metallicity are all consistent with the
cool, inflowing, and near-pristine gas streams predicted to penetrate hot
massive halos at z > 1.5. We estimate a total gas accretion rate of ~100
$M_\odot$/yr from three such streams, which falls short of the star formation
rate but is consistent with simulations. At this rate, it takes about a
gigayear to acquire the molecular gas reservoir of the central starburst.
