The energetic environment and the dense interstellar medium in ULIRGs

TL;DR

This study models the spectral energy distributions of 30 ULIRGs to determine key physical parameters, revealing the dominance of starbursts over AGNs and the evolution of star formation efficiency.

Contribution

It introduces a comprehensive modeling approach combining starburst and AGN components to derive physical parameters and refine the HCN-to-molecular gas conversion factor.

Findings

Most ULIRGs are starburst-dominated energetically.

The HCN-to-molecular gas conversion factor is half of previous estimates.

Star formation efficiency decreases with starburst age.

Abstract

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.