Cirrus models for local and high-z SCUBA galaxies

We present a model for the ultraviolet to submillimetre emission from stars embedded in the general interstellar dust in galaxies (the `infrared cirrus' component). Such emission is characterized by relatively low optical depths of dust and by cool (<30 K) dust temperatures. The model incorporates the stellar population synthesis model of Bruzual & Charlot and the dust model of Siebenmorgen & Krügel which includes the effects of small grains/polycyclic aromatic hydrocarbons. We apply the model to fit the optical to submillimetre spectral energy distributions (SEDs) of nearby galaxies, which are dominated by cirrus emission, and we find that our simple model is quite adequate to explain the observed SEDs.

We also, more controversially, apply this cirrus model to the SEDs of high redshift sources detected in blank-field submillimetre surveys with the Submillimetre Common User Bolometer Array (SCUBA). Surprisingly, an excellent fit is found for many of these sources, with typical values for the optical depth A_V and the surface brightness of the stellar radiation field ψ being only a factor of 2-3 higher than for nearby galaxies. This increase is not unreasonable given the expected evolution of dust optical depth in currently favoured star-formation history models.

We conclude that the tendency to interpret the high-z SCUBA galaxies as very highly obscured starbursts may be premature and that these galaxies may be more closely linked to optically selected high redshift galaxies than previously assumed.