Cirrus models for local and high z SCUBA galaxies

Abstract

We present a model for the UV to submillimeter 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 & Krugel which includes the effects of small grains/PAHs. We apply the model to fit the optical to submillimeter spectral energy distributions (SEDs) of nearby galaxies which are dominated by cirrus emission and 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 submillimeter surveys with 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 psi being only a factor 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.