The energetic environment and the dense interstellar medium in ULIRGs
  We fit the near-infrared to radio spectral energy distributions of a sample
of 30 luminous and ultra-luminous infrared galaxies with models that include
both starburst and AGN components. The aim of the work was to determine
important physical parameters for this kind of objects such as the optical
depth towards the luminosity source, the star formation rate, the star
formation efficiency and the AGN fraction. We found that although about half of
our sample have best-fit models that include an AGN component, only 30 % have
an AGN which accounts for more than 10 % of the infrared luminosity whereas all
have an energetically dominant starburst. Our models also determine the mass of
dense molecular gas. Assuming that this mass is that traced by the HCN
molecule, we reproduce the observed linear relation between HCN luminosity and
infrared luminosity found by Gao and Solomon (2004). However, our derived
conversion factor between HCN luminosity and the mass of dense molecular gas is
a factor of 2 smaller than that assumed by these authors. Finally, we find that
the star formation efficiency falls as the starburst ages.
