Evolving Starburst Modeling of FIR/sub-mm/mm Line Emission. II.
  Application to M 82
  We present starburst models for far-infrared/sub-millimeter/millimeter
(FIR/sub-mm/mm) line emission of molecular and atomic gas in an evolving
starburst region, which is treated as an ensemble of non-interacting hot
bubbles which drive spherical shells of swept-up gas into a surrounding uniform
gas medium. These bubbles and shells are driven by stellar winds and supernovae
within massive star clusters formed during an instantaneous starburst. The
underlying stellar radiation from the evolving clusters affects the properties
and structure of photodissociation regions (PDRs) in the shells, and hence the
spectral energy distributions (SEDs) of the molecular and atomic line emission
from these swept-up shells and the associated parent giant molecular clouds
(GMCs) contains a signature of the stage of evolution of the starburst. The
physical and chemical properties of the shells and their structure are computed
using a a simple well known similarity solution for the shell expansion, a
stellar population synthesis code, and a time-dependent PDR chemistry model.
The SEDs for several molecular and atomic lines ($^{12}$CO and its isotope
$^{13}$CO, HCN, HCO$^+$, C, O, and C$^+$) are computed using a non-local
thermodynamic equilibrium (non-LTE) line radiative transfer model. By comparing
our models with the available observed data of nearby infrared bright galaxies,
especially M 82, we constrain the models and in the case of M 82, we provide
estimates for the ages (5 - 6 Myr, 10 Myr) of recent starburst activity. We
also derive a total H$_2$ gas mass of $\sim$ 2 - 3.4 $\times$ 10$^8$
M$_{\odot}$ for the observed regions of the central 1 kpc starburst disk of M
82.
