A glance at the host galaxy of high-redshift quasars using strong damped Lyman-alpha systems as coronagraphs

TL;DR

This study uses strong damped Lyman-alpha systems as natural coronagraphs to detect and analyze narrow Ly extbackslash alpha\ emission from high-redshift quasar host galaxies, revealing insights into their properties and environment.

Contribution

It presents a large statistical sample of DLAs acting as natural coronagraphs, enabling the first systematic study of quasar host galaxy Ly extbackslash alpha\ emission at high redshift.

Findings

25 extbackslash% of DLAs show narrow Ly extbackslash alpha\ emission.

Emission luminosity suggests ionizing radiation from the AGN dominates.

Most DLAs are associated with neighboring galaxies, not the host itself.

Abstract

We searched quasar spectra from the SDSS-III Baryon Oscillation Spectroscopic Survey (BOSS) for the rare occurrences where a strong damped Lyman-alpha absorber (DLA) blocks the Broad Line Region emission from the quasar and acts as a natural coronagraph to reveal narrow Ly\alpha\ emission from the host galaxy. We define a statistical sample of 31 DLAs in Data Release 9 (DR9) with log N(HI) > 21.3 cm^-2 located at less than 1500 km s^-1 from the quasar redshift. In 25% (8) of these DLAs, a strong narrow Ly\alpha\ emission line is observed with flux ~25 x 10^-17 erg s^-1 cm^-2 on average. For DLAs without this feature in their troughs, the average 3-\sigma\ upper limit is < 0.8 x 10^-17 erg s^-1 cm^-2. Our statistical sample is nearly 2.5 times larger than the anticipated number of intervening DLAs in DR9 within 1500 km s^-1 of the quasar redshift. We also define a sample of 26 DLAs from DR9 and DR10 with narrow Ly\alpha\ emission detected and no limit on the HI column density to better characterize properties of the host galaxy emission. Analyzing the statistical sample, we do not find substantial differences in the kinematics, metals, or reddening for the two populations with and without emission detected. The highly symmetric narrow Ly\alpha\ emission line profile centered in the HI trough indicates that the emitting region is separate from the absorber. The luminosity of the narrow Ly\alpha\ emission peaks is intermediate between that of Lyman-alpha emitters and radio galaxies, implying that the Ly\alpha\ emission is predominantly due to ionizing radiation from the AGN. Galaxies neighboring the quasar host are likely responsible for the majority (> 75%) of these DLAs, with only a minority (< 25%) arising from HI clouds located in the AGN host galaxy.