2021ApJ...907..103D

We present the discovery of a low-redshift damped Lyα (DLA) system in the spectrum of background starburst galaxy SDSS J111323.88+293039.3 (z = 0.17514). The DLA is at an impact parameter of ρ = 36 kpc from the star-forming galaxy, SDSS J111324.08+293051.2 (z = 0.17077). We measure an H I column density of N(H I) = 3.47 x 10²⁰ cm^–2 along with multiple low-ionization species such as N I, N II, Si II, C II, and Si III. We also make an estimate of the covering fraction to be 0.883, giving us a limiting size of the DLA to be A_DLA >= 3.3 kpc². Assuming a uniform column density over the entire DLA system, we estimate its mass to be M_DLA >= 5.3 x 10⁶ M_☉. The extended illuminator and the low redshift of this DLA give us the unique opportunity to characterize its nature and the connection to its host galaxy. We measure a velocity offset of +131 km s^–1 from the systemic velocity of the host for the DLA. This velocity is -84 km s^–1 from the projected rotation velocity of the host galaxy as measured using a newly constructed rotation curve. Based on the size of the host galaxy, the H I column density, and the gas kinematics, we believe this DLA is tracing the warm neutral gas in the H I disk of the foreground galaxy. Our detection adds to a small set of low-redshift DLAs that have confirmed host galaxies, and is the first to be found using an extended background source.