The properties of submm galaxies in hierarchical models

We use the combined galform semi-analytical model of galaxy formation and grasil spectrophotometric code to investigate the properties of galaxies selected via their submillimetre (submm) emission. The fiducial model we use has previously been shown to fit the properties of local Ultra-Luminous Infrared Galaxies, as well as the number counts of faint submm galaxies. Here, we test the model in more detail by comparing the SEDs and stellar, dynamical, gas and halo masses of submm galaxies against observational data. We precisely mimic the submm and radio selection function of the observations and show that the predicted far-infrared properties of model galaxies with S850 > 5 mJy and S1.4 > 30 μJy are in good agreement with observations. Although the dust emission model does not assume a single dust temperature, the far-infrared SEDs are well described by single component modified blackbody spectrum with characteristic temperature 32 ± 5 K, in good agreement with observations. We also find evidence that the observations may have uncovered evolution in the far-infrared–radio relation in ULIRGs out to z∼ 2. We show that the predicted redshift distribution of submm galaxies provides a reasonable fit to the observational data with a median redshift z= 2.0. The radio-selected subset of submm galaxies are predicted to make up approximately 75 per cent of the population and peak at z= 1.7, in a reasonable agreement with the observed radio detected fraction and redshift distribution. However, the predicted K band and mid-infrared (3–8 μm) flux densities of the submm galaxies [and Lyman-Break Galaxies (LBGs)] are up to a factor of 10× fainter than observed. We show that including the stellar thermally pulsating asymptotic giant branch phase in the stellar population models does not make up for this deficit. This discrepancy may indicate that the stellar masses of the submm galaxies in the model are too low: M★∼ 1010M⊙, while observations suggest more massive systems, M★≳ 1011M⊙. However, if the predicted K- and 3–8-μm extinctions in the model could be dramatically reduced, then this would reduce, but not eliminate, this discrepancy. Finally, we discuss the potential modifications to the models which may improve the fit to the observational data, as well as the new observational tests which will be made possible with the arrival of new facilities, such as Submillimetre Common-User Bolometer Array2.