| Abstract
| - ABSTRACT. 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 (zabs≲ 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.
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