A robust method for investigating galactic evolution in the submillimetre waveband: II the submillimetre background and source counts

This is the second of two papers describing a model of galactic evolution in the submillimetre waveband. The model incorporates a self-consistent treatment of the evolution of dust and stars, is normalized to the submillimetre properties of galaxies in the local universe, and can be used to make predictions for both disk and elliptical galaxies and for `closed-box', `inflow', and `outflow' models of galactic evolution. In Paper I we investigated whether it is possible to explain the extreme dust masses of high-redshift quasars and radio galaxies by galactic evolution. In this paper we use the model to make predictions of the submillimetre background and source counts. All our disk-galaxy models exceed at short wavelengths the submillimetre background recently measured by Puget et al. (1996). We also find that it is possible to fit the background over the entire wavelength range with a elliptical model but not with a disk model. We make source count predictions at 190 $\mu$m for the ISOPHOT instrument on ISO and at 850 $\mu$m for SCUBA. We show that the shape of the 850 $\mu$ m source counts depends almost entirely on the mass spectrum of the radiating objects. Finally, we consider the limitations of the models. We find that one of the biggest uncertainties in the model is our lack of information about the submillimetre properties of nearby galaxies, in particular the lack of a direct measurement of the submillimetre luminosity function.