Massive star formation in galaxies: radiative transfer models of the UV tomillimetre emission of starburst galaxies

We present illustrative models for the UV to millimetre emission of starbur st galaxies which are treated as an ensemble of optically thick giant molec ular clouds (GMCs) centrally illuminated by recently formed stars. The mode ls follow the evolution of the GMCs owing to the ionization-induced expansi on of the H II regions and the evolution of the stellar population within t he GMC according to the Bruzual & Charlot stellar population synthesis mode ls. The effect of transiently heated dust grains/PAHs on the radiative tran sfer, as well as multiple scattering, is taken into account. The expansion of the H II regions and the formation of a narrow neutral she ll naturally explain why the emission from PAHs dominates over that from ho t dust in the near- to mid-infrared, an emerging characteristic of the infr ared spectra of starburst galaxies. The models allow us to relate the observed properties of a galaxy to its ag e and star formation history. We find that exponentially decaying 10(7)-10( 8) yr old bursts can explain the IRAS colours of starburst galaxies. The mo dels are also shown to account satisfactorily for the multiwavelength data on the prototypical starburst galaxy M82 and NGC 6090, a starburst galaxy r ecently observed by ISO. In M82 we find evidence for two bursts separated b y 10(7) yr. In NGC 6090 we find that at least part of the far-infrared exce ss may be due to the age of the burst (6.4 x 10(7) yr). We also make predic tions about the evolution of the luminosity of starbursts at different wave lengths which indicate that far-infrared surveys may preferentially detect older starbursts than mid-infrared surveys.