  The effect of a newly born star cluster inside a giant molecular cloud (GMC)
is to produce a hot bubble and a thin, dense shell of interstellar gas and dust
swept up by the H II expansion, strong stellar winds, and repeated supernova
explosions. Lying at the inner side of the shell is the photodissociation
region (PDR), the origin of much of the far-infrared/sub-millimeter/millimeter
(FIR/sub-mm/mm) radiation from the interstellar medium (ISM). We present a
model for the expanding shell at different stages of its expansion which
predict mm/sub-mm and far-IR emission line intensities from a series of key
molecular and atomic constituents in the shell. The kinematic properties of the
swept-up shell predicted by our model are in very good agreement with the
measurements of the supershell detected in the nearby starburst galaxy M 82. We
compare the modeling results with the ratio-ratio plots of the FIR/sub-mm/mm
line emission in the central 1.0 kpc region to investigate the mechanism of
star forming activity in M 82. Our model has yielded appropriate gas densities,
temperatures, and structure scales compared to those measured in M 82, and the
total H2 content is compatible with the observations. This implies that the
neutral ISM of the central star-forming region is a product of fragments of the
evolving shells.
