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

TL;DR

This study conducted millimetre-wave absorption searches in high-redshift reddened quasars but found no molecular absorption, suggesting that such detections are rarer than previously expected under certain assumptions.

Contribution

It provides the first systematic millimetre-band absorption survey in high-redshift reddened quasars and analyzes the factors affecting detectability of molecular absorption.

Findings

No absorption detected in eight quasars with detected continuum flux.

Only 12 out of 40 objects are potentially detectable under assumed conditions.

Detection likelihood decreases with increasing excitation temperature.

Abstract

We have undertaken a search for millimetre-wave band absorption (through the CO and HCO+ rotational transitions) in the host galaxies of reddened radio sources (z = 0.405-1.802). 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 ultra-violet luminosities of the sources, we find that, even if these assumptions are valid, only twelve of the forty objects (mainly from this work) are potentially detectable. This is assuming an excitation temperature of 10 K at z=0, with the number decreasing with increasing temperatures (to zero detectable at 100 K).