Modelling Mechanical Heating in Star-Forming Galaxies: CO and 13CO Line
  Ratios as Sensitive Probes
  We apply photo-dissociation region (PDR) molecular line emission models, that
have varying degrees of enhanced mechanical heating rates, to the gaseous
component of simulations of star-forming galaxies taken from the literature.
Snapshots of these simulations are used to produce line emission maps for the
rotational transitions of the CO molecule and its 13CO isotope up to J = 4-3.
  We consider two galaxy models: a small disk galaxy of solar metallicity and a
lighter dwarf galaxy with 0.2 \zsun metallicity. Elevated excitation
temperatures for CO(1 - 0) correlate positively with mechanical feedback, that
is enhanced towards the central region of both model galaxies. The emission
maps of these model galaxies are used to compute line ratios of CO and 13CO
transitions. These line ratios are used as diagnostics where we attempt to
match them These line ratios are used as diagnostics where we attempt to match
them to mechanically heated single component (i.e. uniform density, Far-UV
flux, visual extinction and velocity gradient) equilibrium PDR models.
  We find that PDRs ignoring mechanical feedback in the heating budget
over-estimate the gas density by a factor of 100 and the far-UV flux by factors
of ~10 - 1000. In contrast, PDRs that take mechanical feedback into account are
able to fit all the line ratios for the central < 2 kpc of the fiducial disk
galaxy quite well. The mean mechanical heating rate per H atom that we recover
from the line ratio fits of this region varies between $10^{-27}$ --
$10^{-26}$~erg s$^{-1}$.
