Comparison of dust‐to‐gas ratios in luminous, ultraluminous, and hyperluminous infrared galaxies

AbstractThe 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 (SED) using composite models which account for the photoionizing radiation from H II 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 the mid‐IR dust emission. Clouds with shock velocities of 500 km s–1 prevail in hyperluminous infrared galaxies. An AGN is found in nearly all of the ultraluminous infrared galaxies and in half of the luminous infrared galaxies of the sample. High IR luminosities depend on dust‐to‐gas ratios as high as ∼0.1 by mass, however most hyperluminous IR galaxies show dustto‐gas ratios much lower than those calculated for the luminous and ultraluminous IR galaxies. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)