1994A&A...286L...9W

We report the first detection of CO absorption lines in an intermediate redshift galaxy, at z=0.24671. The backgroung continuum source is the BL Lac object PKS 1413+135, which is likely to be situated in the nucleus of the spiral galaxy hosting the absorbing clouds. At least three CO(J=0->1) components were detected. The lines are very narrow, down to 0.6km/s. The filling factor of the absorbing clouds is large (≳80%). The extent of the continuum source is unknown. VLBI cm data suggests a size of several parsecs, whereas rapid variability suggests a size of ∼10³ AU. Since the filling factor and optical depth cannot be determined independently, we can only derive a lower limit to the column density of CO. In the case of a diffuse component completely covering the background source, the N(CO) column density is 1x10^16 cm^–2, but the true values could be much higher. The CO absorption lines change in depth on time scale of a few weeks. We attribute these variations to the superluminal motions of the radio jet in PKS 1413+135 or to proper motion of very small-scale molecular clumps