High Redshift HI 21cm Absorption toward Red Quasars
  We have searched for redshifted absorption in the 21 cm line of neutral hydrogen toward `red quasars', which are extragalactic radio sources with a steep spectral drop between optical and infrared wavelengths. The success rate for detecting HI 21cm absorption toward a representative sample of such sources is 80%. This compares to the much lower success rate of 11% for detecting HI 21cm absorption associated with optically selected Mg II absorption line systems. The large neutral hydrogen column densities seen toward red quasars supports the hypothesis that these sources are reddened by dust, as opposed to having an intrinsically red spectrum due to the AGN emission mechanism. The lower limits to the spin temperatures for the neutral hydrogen are 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. 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. 
C.L. Carilli (NRAO), Karl M. Menten (MPIfR), C.P. Moore (Kapteyn)