REDSHIFTED NEUTRAL HYDROGEN-21 CENTIMETER ABSORPTION TOWARD RED QUASARS

We have searched for redshifted neutral hydrogen 21 cm absorption towa rd sources from the Stickel et al. ''red quasar'' subsample. The red q uasar subsample is taken from the 1 Jy sample of flat-spectrum radio s ources and is comprised of the 15 sources that are undetected on the P OSS. Five of these red quasars have been searched for redshifted H I 2 1 cm absorption to optical depth levels of a few percent, and four sho w strong absorption, with neutral hydrogen column densities between 4 and 80 x 10(18) x (T-s/f) cm(-2). This 80 % success rate for the red q uasars compares to the much lower success rate of only 11 % for detect ing H I 21 cm absorption associated with optically selected Mg II abso rption line systems. The large neutral hydrogen column densities seen toward the Stickel et al. red quasars provide circumstantial evidence supporting the dust-reddening hypothesis, as opposed to an intrinsical ly red spectrum for the AGN emission mechanism. The lower limits to re st-frame values of A, are between two and seven, leading to lower limi ts to the spin temperatures for the neutral hydrogen between 50 K and 1000 K, assuming a Galactic dust-to-gas ratio. We consider the questio n of biases in optically selected samples of quasars caused by dust ob scuration. Overall, the data on the red quasar subsample support the m odels of Fall & Pei for dust obscuration by damped Ly alpha absorption line systems and suggest that (1) there may be a significant, but not dominant, population of quasars missing from optically selected sampl es because of dust obscuration, perhaps as many as 20 % at the POSS li mit for an optical sample with a redshift distribution similar to the 1 Jy flat-spectrum quasar sample, and (2) optically selected samples m ay miss about one-half the high column density quasar absorption line systems. The redshifted H I 21 cm absorption line detections presented herein 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 towar d 2149 + 056 at z = 0.740 at a level below that seen for the three det ections, although there is some uncertainty in this case as to the exp ected line redshift. In some systems, the absorbing gas is in the vici nity of the AGN, as either circumnuclear material or material in the g eneral ISM of the AGN's host galaxy, as is probably the case for 0108 + 388 and 1504 + 377, and in other systems the absorption is by gas as sociated with galaxies cosmologically distributed along the line of si ght to the quasar, as may be the case for 0500 + 019. The Westerbork S ynthesis Radio Telescope spectrum of 1504 + 377 confirms the lack of H I 21 cm absorption associated with the narrow molecular absorption li ne system at z = 0.67150.