We review observations of molecular absorption line systems at high redshift toward red quasars and gravitational lenses. The steeply falling power law column density distribution function of QSO absorption line systems implies that for every 1000 Ly α forest lines from absorbers with hydrogen column density, NH < 10 15 cm −2 there will be only one system with NH ≥ 10 22 cm −2 (Hu et al. 1995). Although rare, these extreme high column density systems provide the most direct and detailed probes of the dense, pre-star-forming interstellar medium (ISM) in galaxies at substantial look-back times. Unfortunately, such high column densities also lead to substantial optical dust extinction, e.g. for a normal dust-to-gas ratio, AV> 6 for NH> 10 22 cm −2; hence finding such absorbers using optical spectroscopy is problematic. One method used for finding these systems is to search for molecular and HI 21cm absorption toward flat spectrum radio sources with red (or absent) optical counterparts. Four high redshift molecular absorption line systems have been discovered this way, in two of which the absorption occurs in
To submit an update or takedown request for this paper, please submit an Update/Correction/Removal Request.