A ghostly damped Ly$\alpha$ system revealed by metal absorption lines
  We report the discovery of the first 'ghostly' damped Ly$\alpha$ absorption
system (DLA), which is identified by the presence of absorption from strong
low-ion species at $z_{\rm abs}=1.70465$ along the line of sight to the quasar
SDSSJ113341.29$-$005740.0 with $z_{\rm em}=1.70441$. No Ly$\alpha$ absorption
trough is seen associated with these absorptions because the DLA trough is
filled with the leaked emission from the broad emission line region of the
quasar. By modeling the quasar spectrum and analyzing the metal lines, we
derive log$N$(HI)(cm$^{-2}$)$\sim$21.0 $\pm$ 0.3. The DLA cloud is small ($\le$
0.32 pc) thus not covering entirely the broad line region and is located at
$\ge$ 39 pc from the central active galactic nucleus (AGN). Although the DLA is
slightly redshifted relative to the quasar, its metallicity
([S/H]=$-$0.41$\pm$0.30) is intermediate between what is expected from
infalling and outflowing gas. It could be possible that the DLA is part of some
infalling material accreting onto the quasar host galaxy through filaments, and
that its metallicity is raised by mixing with the enriched outflowing gas
emanating from the central AGN. Current DLA surveys miss these 'ghostly' DLAs,
and it would be important to quantify the statistics of this population by
searching the SDSS database using metal absorption templates.
