Redshifted Neutral Hydrogen 21 Centimeter Absorption toward Red Quasars

We have searched for redshifted neutral hydrogen 21cm absorption towardsources from the Stickel et al. `red quasar' sub-sample. Five of these redquasars have been searched for redshifted HI 21cm absorption to optical depthlevels of a few percent, and four show strong absorption. This 80% success ratefor the red quasars compares to the much lower success rate of only 11% fordetecting HI 21cm absorption associated with optically selected Mg IIabsorption line systems. The large neutral hydrogen column densities seentoward the red quasars provide circumstantial evidence supporting the dustreddening hypothesis, as opposed to an intrinsically red spectrum for the AGNemission mechanism. The data on the red quasar sub-sample support the models ofFall and Pei for dust obscuration by damped Ly alpha absorption line systemsand suggest that: (i) there may be a significant, but not dominant, populationof 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 redshiftdistribution similar to the 1 Jy, flat spectrum quasar sample, and (ii)optically selected samples may miss about half the high column density quasarabsorption line systems. The redshifted HI 21cm absorption line detections aretoward the sources: 0108+388 at z = 0.6685, 0500+019 at z = 0.5846, and1504+377 at z = 0.6733. No absorption is seen toward 2149+056 at z = 0.740 at alevel below that seen for the three detections. In some systems the absorbinggas is in the vicinity of the AGN, either circumnuclear material or material inthe general ISM of the AGN's host galaxy, and in other systems the absorptionis by gas associated with galaxies cosmologically distributed along the line ofsight to the quasar