1997A&A...328...48W

A detailed study of the absorbing molecular clouds towards the radio source PKS 1413+135, at a redshift z=0.247, is reported. Physical conditions (density, temperature, filling factors) are derived for the molecular gas, in the frame work of two models: a homogeneous multicomponent model with equal filling factors, and a two-phase medium consisting of dense clumps embedded in a more diffuse component. It is shown that our absorption data are consistent with the presence of a diffuse gas component, dominating the observed opacity, and a dense component, accounting for most of the mass. We also show that without knowledge of the small scale structure of the absorbing molecular gas, we can only derive lower limits to the column density. Given the very narrow absorption spectrum, the size of the overall absorbing cloud along the line of sight must be quite small, of the order of 1pc. The variability of the absorption spectrum has been studied over a time range of more than 2 years. We find that the opacity ratio between two absorbing components in CO has varied by a factor 2.3±0.3. The variations are interpreted as a change of the line of sight due to structural changes in the background source. We discuss what can be derived from molecular absorption line observations concerning the invariance of physical constants, such as the mass of the molecules. We discuss molecular line data from redshifts 0.27-0.89, corresponding to look-back times of 30% to 60% of the age of the universe.