Astrophysical studies of redshifted spectral lines provide a powerful probe of the contents of the distant (and therefore earlier) Universe. We have embarked upon a survey for neutral hydrogen (HI, at a wavelength of 21-cm) and the hydroxyl molecule (OH at 18-cm) in intervening absorbers along the lines-of-sight towards radio quasars, some of the most distant objects in the Universe. Absorption systems at optical wavelengths are numerous since these are easily found by pointing a telescope at an optically bright quasar, giving in a single observation a spectrum which is essentially a 'bar-code' of all the systems which absorb or emit radiation at all redshifts between us and the quasar (Fig. 1).

Radio telescopes cover much smaller relative bandwidths than their optical counterparts and so this method cannot be used to discover absorption systems in the radio band. Atomic (HI) lines are usually found by observing at a frequency determined from the optical spectrum. However, molecular absorption (OH) in the radio band has never been found by this method. Why is this? We believe a major factor is the amount of dust present in the absorbing system. Dust extinguishes blue and ultra-violet light, leaving the light redder (think of a sunset) and fainter, and protects molecular gas from the ultra-violet light that would break the molecules into their constituent atoms. A strong correspondence is then expected between the amount of dust and the amount of molecular gas in absorbing systems. This is seen in Fig. 2, which shows the molecular fraction against the degree of reddening, which is due to dust.

Although opaque to the shorter optical wavelengths, radio waves travel though dust unhindered. So by selecting quasars which are very strong radio emitters, but optically dim, there's a fair chance that there's a dark molecular cloud lying somewhere in the billions of light-years between us and the radio source, which is blocking, or reddening, most of the light. Since the redshifts cannot be determined through optical observations, we are undertaking spectral-scans towards dim radio sources with the world's largest steerable radio telescope, at Green Bank in the USA, in the hope of finding the molecules protected by this intervening obscuration.

Fig. 1: Absorption of optical and radio lines along the line-of-sight to a distant quasar. Figure definitely not to scale.
Fig. 2: • Left: The fraction of the gas in the molecular state versus the 'redness' for the few known distant molecular absorbers. • Right: Detail showing the ratio of the hydroxyl/hydrogen absorption strength against colour for the four absorbers detected at microwave frequencies.

Steve Curran, Matthew Whiting and John Webb