Molecular transitions as probes of the physical conditions of
  extragalactic environments
  Ab initio grids of time dependent chemical models, varying in gas density,
temperature, cosmic ray ionization rate, and radiation field, are used as input
to RADEX calculations. Tables of abundances, column densities, theoretical line
intensities, and line ratios for some of the most used dense gas tracers are
provided. The degree of correlation as well as degeneracy inherent in molecular
ratios is discussed. Comparisons of the theoretical intensities with example
observations are also provided. We find that, within the parameters space
explored, chemical abundances can be constrained by a well defined set of gas
density-gas temperature-cosmic ray ionization rate for the species we
investigate here. However, line intensities, as well as, more importantly, line
ratios, from different chemical models can be very similar leading to a clear
degeneracy. We also find that the gas subjected to a galactic cosmic ray
ionization rate will not necessarily have reached steady state by 1 Myr. The
species most affected by time dependency effects are HCN and CS, both high
density tracers. We use our method to fit an example set of data from two
galaxies. We find that (i) molecular line ratios can be easily matched even
with erroneous individual line intensities; (ii) no set of species can be
matched by a one-component ISM; (iii) a species may be a good tracer of an
energetic process but only under specific density and temperature conditions.
We show that by taking into consideration the chemistry behind each species and
the individual line intensities, many degeneracies that arise by just using
molecular line ratios can be avoided. Finally we show that using a species or a
ratio as a tracer of an individual energetic process (e.g. cosmic rays, UV)
ought to be done with caution.
