Molecular gas at intermediate redshifts

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) and 1830-211 (z \~ 0.886; Chengalur et al. 1999), 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 1996) persists out to z \~ 1. It has been suggested (Liszt & Lucas 1999) 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.