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Summary: The paper discusses the search for CO and other molecules in emission and absorption in front of a sample of 30 targets, comprising 16 “associated” and 14 “intervening” HI 21-cm absorbers. The observations were done with the IRAM-30m telescope, simultaneously at 3mm and 2mm, exploring several lines of the CO ladder and HCO+, according to the redshift. Eight targets have been detected in emission, of which five are new. The derived molecular gas masses range from 10^9 to 7×10^11 M⊙2. The authors also report four new detections in absorption. Two of the associated CO absorption line detections at high redshift (z=1.211 and 1.275) resulted from the high spatial resolution follow-up with NOEMA. The disparity between the mm molecular and HI 21-cm absorption lines for these and another intervening system detected in HNC at z=1.275, is attributable to radio and mm sight lines tracing different media. The authors indicate that two distinct phases exist: one near galaxy centers with a larger CO-to-HI abundance ratio, and another with lower molecular abundance in the outer regions of the galaxy. The detection of absorption requires sufficient spatial resolution to overcome the dilution by emission, which will be an important criterion for mm follow-up of 21-cm absorbers from ongoing large-scale surveys.
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