Molecular gas at intermediate redshifts

Abstract

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.