Galaxy evolution in the infrared:: comparison of a hierarchical galaxy formation model with Spitzer data

We present predictions for the evolution of the galaxy luminosity function, number counts and redshift distributions in the infrared (IR) based on the Lambda CDM cosmological model. We use the combined GALFORM semi-analytical galaxy formation model and GRASIL spectrophotometric code to compute galaxy spectral energy distributions including the reprocessing of radiation by dust. The model, which is the same as that given by Baugh et al., assumes two different initial mass functions (IMFs): a normal solar neighbourhood IMF for quiescent star formation in discs, and a very top-heavy IMF in starbursts triggered by galaxy mergers. We have shown previously that the top-heavy IMF seems to be necessary to explain the number counts of faint submillimetre galaxies. We compare the model with observational data from the Spitzer Space Telescope, with the model parameters fixed at values chosen before Spitzer data became available. We find that the model matches the observed evolution in the IR remarkably well over the whole range of wavelengths probed by Spitzer. In particular, the Spitzer data show that there is strong evolution in the mid-IR galaxy luminosity function over the redshift range z similar to 0-2, and this is reproduced by our model without requiring any adjustment of parameters. On the other hand, a model with a normal IMF in starbursts predicts far too little evolution in the mid-IR luminosity function, and is therefore excluded.