2002A&A...381L..73K

We present Giant Metrewave Radio Telescope (GMRT) observations of OH absorption in B3 1504+377 (z∼0.673) and PKS 1413+135 (z∼0.247). OH has now been detected in absorption towards four intermediate redshift systems, viz. the lensing galaxies towards B 0218+357 (z∼0.685; Kanekar et al., 2001, in prep.) and 1830-211 (z∼0.886; Chengalur et al., 1999A&A...343L..79C), in addition to the two systems listed above. All four systems also give rise to well studied millimetre wavelength molecular line absorption from a host of molecules, including HCO⁺. Comparing our OH data with these millimetre line transitions, we find that the linear correlation between N_OH and N_HCO+^_ found in molecular clouds in the Milky Way (Liszt & Lucas, 1996A&A...314..917L) persists out to z∼1. It has been suggested (Liszt & Lucas, 1999, ASP Conf. Ser., 156, 188) that OH is a good tracer of H₂, with N_H2/N_OH≃10⁷ under a variety of physical conditions. We use this relationship to estimate N_H2 in these absorbers. The estimated N_H2 is >10²² in all four cases and substantially different from estimates based on CO observations.