2021ApJ...908..188F

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 x 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 (log(N_HI/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.