Discovery Of A Low-Redshift Damped Ly Alpha System In A Foreground Extended Disk Using A Starburst Galaxy Background Illuminator

We present the discovery of a low-redshift damped Ly alpha (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 rho = 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 10(20) 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 kpc(2). Assuming a uniform column density over the entire DLA system, we estimate its mass to be M-DLA >= 5.3 x 10(6) 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.