Massive Star Formation in Galaxies: Radiative transfer models of the UV to mm emission of starburst galaxies

Abstract

We present illustrative models for the UV to millimeter emission of starburst galaxies which are treated as an ensemble of optically thick giant molecular clouds (GMCs) centrally illuminated by recently formed stars. The models follow the evolution of the GMCs due to the ionization-induced expansion of the HII regions and the evolution of the stellar population within the GMC according to the Bruzual & Charlot stellar population synthesis models. The effect of transiently heated dust grains/PAHs to the radiative transfer, as well as multiple scattering, is taken into account. The expansion of the HII regions and the formation of a narrow neutral shell naturally explains why the emission from PAHs dominates over that from hot dust in the near to mid-IR, an emerging characteristic of the infrared spectra of starburst galaxies. The models allow us to relate the observed properties of a galaxy to its age and star formation history. We find that exponentially decaying 10^7-10^8 yrs old bursts can explain the IRAS colours of starburst galaxies. The models are also shown to account satisfactorily for the multiwavelength data on the prototypical starburst galaxy M82 and NGC6090, a starburst galaxy recently observed by ISO. In M82 we find evidence for two bursts separated by 10^7yrs. In NGC6090 we find that at least part of the far-IR excess may be due to the age of the burst (6.4 x 10^7yrs). We also make predictions about the evolution of the luminosity of starbursts at different wavelengths which indicate that far-IR surveys may preferentially detect older starbursts than mid-IR surveys.