Ghostly Damped Ly$\alpha$ Systems: Tracers of Gas Flows in the Close Vicinity of Quasars ?

Abstract

We have searched the Sloan Digital Sky Survey Data Release 12 for ghostly Damped Ly$\alpha$ (DLA) systems. These systems, located at the redshift of the quasars, show strong absorption from low-ionization atomic species but reveal no HI Ly$\alpha$ absorption. Our search has, for the first time, resulted in a sample of 30 homogeneously selected ghostly DLAs with $z_{\mathrm{QSO}}$$>$2.0. Thirteen of the ghostly DLAs exhibit absorption from other HI Lyman series lines. The lack of Ly$\alpha$ absorption in these absorbers is consistent with them being dense and compact with projected sizes smaller than the broad line region (BLR) of the background quasar. Although uncertain, the estimated median HI column density of these absorbers is log$N$(HI)$\sim$21.0. We compare the properties of ghostly DLAs with those of eclipsing DLAs that are high column density absorbers, located within 1500 km/s of the quasar emission redshift and showing strong Ly$\alpha$ emission in their DLA trough. We discover an apparent sequence in the observed properties of these DLAs with ghostly DLAs showing wider HI kinematics, stronger absorptions from high-ionization species, CII and SiII excited states, and higher level of dust extinction. Since we estimate that all these DLAs have similar metallicities, log$Z/Z_{\odot }$$\sim$$-$1.0, we conclude that ghostly DLAs are part of the same population as eclipsing DLAs, except that they are denser and located closer to the central active galactic nuclei (AGNs).

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