  The dust-to-gas ratios in three different samples of luminous, ultraluminous,
and hyperluminous infrared galaxies are calculated by modelling their radio to
soft X-ray spectral energy distributions using composite models which account
for the photoionizing radiation from HII regions, starbursts, or AGNs, and for
shocks. The models are limited to a set which broadly reproduces the mid-IR
fine structure line ratios of local, IR bright, starburst galaxies. The results
show that two types of clouds contribute to the IR emission. Those
characterized by low shock velocities and low preshock densities explain the
far-IR dust emission, while those with higher velocities and densities
contribute to mid-IR dust emission. An AGN is found in nearly all of the
ultraluminous IR galaxies and in half of the luminous IR galaxies of the
sample. High IR luminosities depend on dust-to-gas ratios of about 0.1 by mass,
however, most hyperluminous IR galaxies show dust-to-gas ratios much lower than
those calculated for the luminous and ultraluminous IR galaxies.
