  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.

Astrophysics