Aims: Molecular emission lines originating in the nuclei of luminous
infra-red galaxies are used to determine the physical properties of the nuclear
ISM in these systems.
Methods: A large observational database of molecular emission lines is
compared with model predictions that include heating by UV and X-ray radiation,
mechanical heating, and the effects of cosmic rays.
Results: The observed line ratios and model predictions imply a separation of
the observedsystems into three groups: XDRs, UV-dominated high-density (n>=10^5
cm-3) PDRs, and lower-density (n=10^4.5 cm-3) PDRs that are dominated by
mechanical feedback.
Conclusions: The division of the two types of PDRs follows naturally from the
evolution of the star formation cycle of these sources, which evolves from
deeply embedded young stars, resulting in high-density (n>=10^5 cm-3) PDRs, to
a stage where the gas density has decreased (n=10^4.5 cm-3) and mechanical
feedback from supernova shocks dominates the heating budget.Comment: 4 pages, 3 figures, published as Letter to the Editor in A&A (see
http://www.aanda.org/articles/aa/abs/2008/34/aa10327-08/aa10327-08.html