A ghostly damped Ly$\alpha$ system revealed by metal absorption lines

TL;DR

This paper reports the discovery of a unique 'ghostly' damped Lyα system identified through metal absorption lines, which is missed by traditional surveys due to its filled Lyα trough, revealing a new class of absorbers near quasars.

Contribution

The study introduces the first detection of a 'ghostly' DLA, demonstrating a new method to identify such systems via metal lines and highlighting their potential significance in galaxy evolution.

Findings

The 'ghostly' DLA has a high neutral hydrogen column density (~10^21 cm^-2).

It is located within 39 parsecs of the quasar, partially covering the broad line region.

Metallicity indicates enrichment possibly from outflows or infalling gas.

Abstract

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.