Redshifted Neutral Hydrogen 21cm Absorption toward Red Quasars

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Abstract
We have searched for redshifted neutral hydrogen 21cm absorption toward sources from the Stickel et al. `red quasar' sub-sample. Five of these red quasars have been searched for redshifted HI 21cm absorption to optical depth levels of a few percent, and four show strong absorption. This 80% success rate for the red quasars compares to the much lower success rate of only 11% for detecting HI 21cm absorption associated with optically selected Mg II absorption line systems. The large neutral hydrogen column densities seen toward the red quasars provide circumstantial evidence supporting the dust reddening hypothesis, as opposed to an intrinsically red spectrum for the AGN emission mechanism. The data on the red quasar sub-sample support the models of Fall and Pei for dust obscuration by damped Ly alpha absorption line systems and suggest that: (i) there may be a significant, but not dominant, population of quasars missing from optically selected samples due to dust obscuration, perhaps as high as 20% at the POSS limit for an optical sample with a redshift distribution similar to the 1 Jy, flat spectrum quasar sample, and (ii) optically selected samples may miss about half the high column density quasar absorption line systems. The redshifted HI 21cm absorption line detections are toward the sources: 0108+388 at z = 0.6685, 0500+019 at z = 0.5846, and 1504+377 at z = 0.6733. No absorption is seen toward 2149+056 at z = 0.740 at a level below that seen for the three detections. In some systems the absorbing gas is in the vicinity of the AGN, either circumnuclear material or material in the general ISM of the AGN's host galaxy, and in other systems the absorption is by gas associated with galaxies cosmologically distributed along the line of sight to the quasar.

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