SIMBAD references

We have searched the Sloan Digital Sky Survey Data Release 12 for ghostly strong Lyα (DLA) systems. These systems, located at the redshift of the quasars, show strong absorption from low-ionization atomic species but reveal no H I Lyα absorption. Our search has, for the first time, resulted in a sample of 30 homogeneously selected ghostly absorbers with z_QSO > 2.0. Thirteen of the ghostly absorbers exhibit absorption from other H I Lyman series lines. The lack of Lyα absorption in these absorbers is consistent with them being dense and compact with projected sizes smaller than the broad-line region of the background quasar. Although uncertain, the estimated median H I column density of these absorbers is logN(H I) ∼ 21.0. We compare the properties of ghostly absorbers with those of eclipsing DLAs that are high-column-density absorbers, located within 1500 km s^–1 of the quasar emission redshift and showing strong Lyα emission in their DLA trough. We discover an apparent sequence in the observed properties of these DLAs, with ghostly absorbers showing wider H I kinematics, stronger absorptions from high-ionization species, C II and Si II excited states, and a higher level of dust extinction. Since we estimate that all these absorbers have similar metallicities, logZ/Z_☉∼-1.0, we conclude that ghostly absorbers are part of the same population as eclipsing DLAs, except that they are denser and located closer to the central active galactic nuclei.