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Molecular fraction limits in damped Lyman-alpha absorption systems

S. J. Curran·M. T. Murphy·Y. M. Pihlstroem·J. K. Webb·A. D. Bolatto·G. C. Bower
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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-alpha absorption systems (DLAs). This brings the number of published systems searched from 18 to 30. In 17 cases we reach $3σ$ limits of $τ\leq0.1$, which is a significant improvement over the previous searches and more than sufficient to detect the 4 known redshifted millimetre absorbers ($τ\gapp1$). While 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 \MOLH-bands in the UV ($z_{\rm abs}\lapp1.8$). Using a model for the DLA metallicity evolution combined with assumed HCO$^+$/\MOLH ~and CO/\MOLH conversion ratios, we use the molecular column density limits to calculate plausible \MOLH ~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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S. J. Curran, M. T. Murphy, Y. M. Pihlstroem, J. K. Webb, A. D. Bolatto, G. C. Bower. 2004-04-27. Molecular fraction limits in damped Lyman-alpha absorption systems. https://doi.org/10.1111/j.1365-2966.2004.07945.x

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