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Absorption lines at high redshift in front of quasars are rare in the mmdomain. Only five associated and five intervening systems have been reported inthe literature. These bring very useful information complementary to emissionlines, for instance, to distinguish between inflows and outflows. They are alsogood candidates to study the variations of the fundamental constants. We reporthere the search for molecules in emission and absorption in front of a sampleof 30 targets, comprising 16 associated and 14 intervening HI 21-cm absorbers.The observations have been done with the IRAM-30m telescope, simultaneously at3mm and 2mm, exploring CO ladder and HCO+ lines. Eight targets have beendetected in emission, of which five are new. Their molecular gas masses rangefrom 10^9 to 7 10^11 Mo. We also report four new detections in absorption. Twoof the associated CO absorption line detections at high redshift (z=1.211 and1.275) resulted from the high spatial resolution follow-up with NOEMA. Thedisparity between the mm molecular and HI 21-cm absorption lines for these andanother intervening system detected in HNC at z = 1.275, is attributable toradio and mm sight lines tracing different media. Comparing HI and H2 in the 14known high redshift molecular absorbers, associated HI absorption lines arebroad, with multiple components and the molecular absorption corresponds to thebroader and weaker 21-cm absorption component. This indicates two distinctphases: one near galaxy centers with a larger CO-to-HI abundance ratio, andanother with lower molecular abundance in the outer regions of the galaxy. Thecomparison of interferometric and single dish observations shows that thedetection of absorption requires sufficient spatial resolution to overcome thedilution by emission, and will be an important criterion for mm follow-up of21-cm absorbers from ongoing large-scale surveys.

Cold molecules in HI 21cm absorbers across redshifts 0.1-4