Molecular fraction limits in damped Lyman α absorption systems

We have used the Green Bank Telescope (GBT) and Berkeley-Illinois-Maryland Association (BIMA) array to search for redshifted millimetre absorption in a sample of damped Lyman α absorption systems (DLAs). This increases the number of published systems searched from 18 to 30. In 17 cases we reach 3σ limits of τ<= 0.1, which is a significant improvement over the previous searches and more than sufficient to detect the four known redshifted millimetre absorbers (τ>~ 1). Although the CO rotational (millimetre) column density limits obtained are weaker than the electronic (optical) limits, they may provide useful limits below the atmospheric cut-off for the Lyman and Werner H₂ bands in the UV (z_abs<~ 1.8). Using a model for the DLA metallicity evolution combined with assumed HCO⁺/H₂ and CO/H₂ conversion ratios, we use the molecular column density limits to calculate plausible H₂ molecular fraction limits. Finally, we use these results to discuss the feasibility of detecting rotational CO transitions in DLAs with the next generation of large radio telescopes.