Redshifted Neutral Hydrogen 21cm Absorption toward Red Quasars

We have searched for redshifted neutral hydrogen 21cm absorption toward sources from the Stickel et al. (1996a) ‘red quasar’ sub-sample. The red quasar sub-sample is taken from the 1 Jy sample of flat spectrum radio sources, and is comprised of the 15 sources which are undetected on the POSS. 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, with neutral hydrogen column densities between 4 and 80×10×(s f ) cm. 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 (Briggs and Wolfe 1983, Lane et al. 1997). 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 AV are between 2 and 7, 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 due to dust obscuration. Overall, the data on the red quasar sub-sample support the models of Fall and Pei (1993) for dust obscuration by damped Lyα 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.