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

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 Lyemission 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 1 500 km s −1 from the quasar redshift. In 25% (8) of these DLAs, a strong narrow Ly� emission line is observed with flux ∼25×10 −17 erg s −1 cm −2 on average. For DLAs without this feature in their troughs, the average 3-� upper limit is 75%) of these DLAs, with only a minority (< 25%) arising from Hi clouds located in the AGN host galaxy.