Molecular QSO Absorption Line Systems

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 there will be only one system with NH ≥ 10 cm (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; 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 a gravitational lens (0218+357 at z = 0.685 and 1830−211 at z = 0.886; Wiklind & Combes 1995, 1996a). In the other two systems (1413+135 at z = 0.247 and 1504+357 at z = 0.673) the absorption takes place in the host galaxy of an AGN (Wiklind & Combes 1994, 1996b). So far, 15 different molecules have been identified in these redshifted systems, plus many of their isotopomers, including complex, multi-atom molecules such as the cyclic species C3H2 (Combes & Wiklind 1999; Menten et al. 1999). The lines can be relatively broad, as for 1504+377 with a FWHM = 100 km s, or extremely narrow, as for 1413+135 (FWHM = 1 km s). As opposed to emission studies, observations of redshifted absorption provide the ability to probe very narrow pencil beams through the intervening galaxy and, thus, given a sufficiently strong background source, to determine the physical and chemical conditions in single molecular clouds by observations of rare molecular species. The best studied of the high-z molecular absorption line sources in terms of high resolution imaging is that associated with the gravitational lens system 1830−211 (Wiklind & Combes 1996a). At its redshift of 0.89, many of the ground level rotational transitions of molecules commonly found in galactic molecular clouds redshift into the observing bands of the VLA and the VLBA (Carilli et