Abstract
Quasi Stellar Objects (QSOs) whose spectral energy distributions (SEDs) are
reddened by dust either in their host galaxies or in intervening absorber
galaxies are to a large degree missed by optical color selection criteria like
the ones used by the SDSS. To overcome this bias against red QSOs, we employ a
combined optical and near-infrared color selection. In this paper, we present a
spectroscopic follow-up campaign of a sample of red candidate QSOs which were
selected from the SDSS and the UKIRT Infrared Deep Sky Survey (UKIDSS). The
spectroscopic data and SDSS/UKIDSS photometry are supplemented by photometry
from the Wide-field Infrared Survey Explorer (WISE). In our sample of 159
candidates, 154 (97 %) are confirmed to be QSOs. We use a statistical algorithm
to identify sightlines with plausible intervening absorption systems and
identify 9 such cases assuming dust in the absorber similar to Large Magellanic
Cloud sightlines. We find absorption systems towards 30 QSOs, two of which are
consistent with the best-fit absorber redshift from the statistical modelling.
Furthermore, we observe a broad range in SED properties of the QSOs as probed
by the rest-frame 2 $\mu$m flux. We find QSOs with a strong excess as well as
QSOs with a large deficit at rest-frame 2 $\mu$m relative to a QSO template.
Potential solutions to these discrepancies are discussed. Overall, our study
demonstrates the high efficiency of the optical/near-infrared selection of red
QSOs.
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