Dust-to-gas ratios in the starburst regions of luminous infrared galaxies

We investigate the properties of dust and dust-to-gas ratios in various starburst regions of luminous infrared galaxies (LIGs). We refer to the sample of seven LIGs recently observed in the mid-infrared by Soifer et al. using the Keck Telescope with spatial resolution approaching the diffraction limit. These seven objects are among the closest LIGs and have been classified as starburst galaxies from optical spectroscopy. Our goal consists in modelling the continuum spectral energy distribution of each galaxy, particularly in the infrared range. Models are further constrained by observed emission-line ratios in the optical range. Multicloud models consistently account for the coupled effects of shock, photoionization by hot stars and diffuse secondary radiation from the shock-heated gas. Emission from clouds in the neighbourhood of evolved starbursts and with high shock velocities (∼500 km s⁻¹) explains both the bremsstrahlung and reradiation from dust in the mid-infrared. Clouds with lower velocity (100 km s⁻¹), which correspond to younger starbursts, also contribute to both line and continuum spectra. Both low- and high-velocity clouds are thus present in nearly all the sample galaxies. For all the galaxies, an old stellar population is revealed by blackbody emission in the optical/near-infrared range. Dust-to-gas ratios vary in different regions of individual galaxies.