On the absence of molecular absorption in high-redshift millimetre-band searches

We have undertaken a search for millimetre-waveband absorption (through the CO and HCO+ rotational transitions) in the host galaxies of reddened radio sources (z= 0.405& 8211;1.802). Despite the colour selection (optical–near-infrared colours of V−K≳ 5 in all but one source), no absorption was found in any of the eight quasars for which the background continuum flux was detected. On the basis of the previous (mostly intervening) H2 and OH detections, the limits reached here and in some previous surveys should be deep enough to detect molecular absorption according to their V−K colours. However, our survey makes the assumption that the reddening is associated with dust close to the emission redshift of the quasar and that the narrow millimetre component of this emission is intercepted by the compact molecular cores. By using the known millimetre absorbers to define the colour depth and comparing this with the ultraviolet luminosities of the sources, we find that, even if these assumptions are valid, only 12 of the 40 objects (mainly from this work) are potentially detectable. This is assuming an excitation temperature of Tx= 10 K at z= 0, with the number decreasing with increasing temperatures (to zero detectable at Tx≳ 100 K).