The Magellan uniform survey of damped Lyman α systems - II. Paucity of strong molecular hydrogen absorption

We present the first large, blind and uniformly selected survey for molecular hydrogen (H₂) in damped Lyman α systems (DLAs) with moderate-to-high resolution spectra. 86 DLAs were searched for absorption in the many Lyman and Werner H₂ transitions, with ≈79 per cent completeness for H₂ column densities above N(H₂) = 10^17.5 cm^-2 for an assumed Doppler broadening parameter b = 2 km s^-1. Only a single strong H₂ absorber was found - a system detected previously in Very Large Telescope/Ultraviolet and Visual Echelle Spectrograph spectra. Given our distribution of N(H₂) upper limits, this ∼1 per cent detection rate is smaller than expected from previous surveys at 99.8 per cent confidence. Assuming the N(H₂) distribution shape from previous surveys, our detection rate implies a covering factor of ∼1 per cent for N(H₂) ≥ 10^17.5 cm^-2 gas in DLAs (<6 per cent at 95 per cent confidence). We obtained new Magellan/Magellan Echellette (MagE) spectra for 53 DLAs; 8 km s^-1 resolution spectra were available for 27 DLAs. MagE's moderate resolution (≈71 km s^-1) yields weaker N(H₂) upper limits and makes them dependent on the assumed Doppler parameter. For example, half the (relevant) previous H₂ detections have N(H₂) ≥ 10^18.1 cm^-2, a factor of just 3 higher than our median upper limit. Nevertheless, several tests suggest our upper limits are accurate, and they would need to be increased by 1.8 dex to bring our detection rate within 95 per cent confidence of previous surveys.