Redshifted Neutral Hydrogen 21 Centimeter Absorption toward Red Quasars

Abstract

We have searched for redshifted neutral hydrogen 21 cm absorption toward sources from the Stickel et al. "red quasar" subsample. The red quasar subsample is taken from the 1 Jy sample of flat-spectrum radio sources and is comprised of the 15 sources that are undetected on the POSS. Five of these red quasars have been searched for redshifted H I 21 cm absorption to optical depth levels of a few percent, and four show strong absorption, with neutral hydrogen column densities between 4 and 80 × 10¹⁸ × (T_s/f) cm^-2. This 80% success rate for the red quasars compares to the much lower success rate of only 11% for detecting H I 21 cm absorption associated with optically selected Mg II absorption 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 intrinsically red spectrum for the AGN emission mechanism. The lower limits to rest-frame values of A_V are between two and seven, leading to lower limits to the spin temperatures for the neutral hydrogen between 50 K and 1000 K, assuming a Galactic dust-to-gas ratio. We consider the question of biases in optically selected samples of quasars caused by dust obscuration. Overall, the data on the red quasar subsample support the models of Fall & Pei for dust obscuration by damped Lyα absorption line systems and suggest that (1) there may be a significant, but not dominant, population of quasars missing from optically selected samples because of dust obscuration, perhaps as many as 20% at the POSS limit for an optical sample with a redshift distribution similar to the 1 Jy flat-spectrum quasar sample, and (2) optically selected samples may 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 toward 2149+056 at z = 0.740 at a level below that seen for the three detections, although there is some uncertainty in this case as to the expected line redshift. In some systems, the absorbing gas is in the vicinity of the AGN, as either circumnuclear material or material in the general 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 associated with galaxies cosmologically distributed along the line of sight to the quasar, as may be the case for 0500+019. The Westerbork Synthesis Radio Telescope spectrum of 1504+377 confirms the lack of H I 21 cm absorption associated with the narrow molecular absorption line system at z = 0.67150.