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 $\rm \rho=36~kpc$ from the
star forming galaxy, SDSS J111324.08+293051.2 ($z=0.17077$). We measure an HI
column density of $N($HI$)\rm =3.47\times10^{20}~cm^{-2}$ along with multiple
low-ionization species such as NI, NII, SiII, CII, and SiIII. We also make an
estimate of the covering fraction to be 0.883, giving us a limiting size of the
DLA to be $A_{DLA}\rm \geq3.3~kpc^2$. Assuming a uniform column density over
the entire DLA system, we estimate its mass to be $M_{DLA}\geq5.3\times
10^6~M_\odot$. 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 HI column
density, and the gas kinematics, we believe this DLA is tracing the warm
neutral gas in the HI 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.
