An Upper Limit of the Redshifted CO (J = 1–0) Absorption Line toward AO 0235+164

Abstract

A search was made for a redshifted CO absorption line (J= 1–0) toward the BL Lac object AO 0235+164, for which 21-cm and Lya absorption lines have been detected at the redshift of z=0.524. Observations were made with the 45-m telescope at the Nobeyama Radio Observatory and the upper limit of the optical thickness of ~0.05 was obtained. The ratio of the upper limit of column density of CO molecules to that of the neutral hydrogen atoms is estimated to be about 3 × 10–7, an order of magnitude less than the typical galactic value.