A 22 GHz search for molecular absorption at z ∼ 3 with the upgraded ATCA

We report a λ > ∼ 1 cm search for rotational molecular absorption towards quasars, now possible with the upgraded Australia Telescope Compact Array (ATCA). The targets were PKS 0201+113, PKS 0336–017 and Q 0537–286, where known damped Lyman-alpha absorption systems (DLAs) could cause redshifted molecular absorption in the 12 mm band of the telescope. We place 3σ upper limits on any HCO 0 → 1 absorption features of < 30 mJy per 3.4 km s channel. The non-detections could be attributed to the inherent low metallicities in DLAs leading to generally low H2, and thus HCO, column densities. In general, the detection of molecular rotational transitions in DLAs could be further hindered by a lower than expected CO-to-H2 conversion ratio, whether due either to photoionization of carbon or its relative underabundance at high redshift.