On the scarcity of redshifted OH and millimetre-band molecular absorption

ABSTRACT Despite much searching, redshifted decimetre- and millimetre-band absorption by molecular gas remains very rare, limited to just six systems at zabs ≳ 0.05. Detection of these transitions can yield precise diagnostics of the conditions of the star-forming gas in the earlier Universe, the hydroxyl (OH) radical being of particular interest as in the λ = 18 cm ground state there are four different transitions located close to neutral hydrogen 21- cm and thus detectable with the Square Kilometre Array and its pathfinders. The four transitions of OH have very different dependences on the fundamental constants, thus having much potential in testing for any evolution in these over large look-back times. By collating the photometry in a uniform manner, we confirm our previous hypothesis that the normalized OH absorption strength is correlated with the optical–near-infrared red colour of the sightline. Applying this to the published searches, we find that all, but one (J0414+054), have simply not been searched sufficiently deeply. We suggest that this is due to the standard selection of sources with reliable optical redshifts introducing a bias against those with enough dust with which to shield the molecular gas. For the single source searched to sufficient depth, we have reason to suspect that the high degree of reddening arises from another system along the sightline, thus not being inconsistent with our hypothesis. We also show that the same optical redshift bias can account for the scarcity of millimetre-band absorption.