A Long Stream of Metal-poor Cool Gas around a Massive Starburst Galaxy at z = 2.67

Abstract

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_⊙ yr^-1 and a molecular gas reservoir of 1.3 × 10¹¹ M_⊙. We characterize its CGM with two background QSOs at impact parameters of 93 kpc and 176 kpc. We detect strong H I and metal-line absorption near the redshift of the SMG toward both QSOs, each consisting of three main subsystems spanning over 1500 km s^-1. 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 H I column densities ( ${\rm{log}}({N}_{{\rm{H}}{\rm\small{I}}}/{{\rm{cm}}}^{-2})=20.2,18.6$ ), a low metallicity ([M/H] ≈ -2.0), and nearly the same radial velocity (δv ∼ -300 km s^-1). While the H I column densities match previous results on the massive halos hosting QSOs, the metallicity is lower by more than an order of magnitude, making it an outlier in the line width-metallicity relation of damped Lyα absorbers. The large physical extent, the velocity coherence, the high surface densities, 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_⊙ yr^-1 from three such streams. At this rate, it takes only a gigayear to acquire the molecular gas reservoir of the central starburst.