Modelling ISO galaxy counts with luminosity and merger rate evolution

We model galaxy evolution in the 6.75- and 15-mu m passbands of the ISO satellite, by combining models of galaxy evolution at optical wavelengths (which are consistent with the optical galaxy counts) with observed spectral energy distributions in the infrared. Our model is consistent with the local 12-mu m galaxy luminosity function (from IRAS), including the bright end if we place a small fraction (similar to 3.5 per cent at z = 0) of the model's spiral galaxies in interacting pairs with mid-infrared luminosities enhanced by major starbursts. Source counts from deep ISO surveys in the 6.75- and 15-mu m passbands significantly exceed nonevolving predictions. We find the number counts, redshift distributions and the wide range of mid-infrared/optical flux ratios of the detected galaxies to be reasonably consistent with our evolving model. Our models suggest that the steep number count of 6.75-mu m sources can be explained primarily by the evolving E/S0 galaxies. About one-third of 6.75-mu m sources do appear to be E/S0s or early-type spirals, but starburst galaxies at 0.6 < z < 1.1 are more prominent in this passband than we expected. In the 15-mu m passband the main contributors appear to be evolving, star-forming spirals and starbursting galaxy mergers, as in our model. Visibly interacting galaxies form similar to 31 per cent of the 15-mu m detections to 200 mu JY, and have high mid-infrared/optical flux ratios consistent with our models for major starbursts with high dust extinction (up to similar to 1.8 mag in the rest-frame blue band). The numbers and high mean redshift (0.68 +/- 0.08) of merging/interacting galaxies detected in these ISO surveys are only consistent with the local mid-infrared luminosity function if the merger-triggered starbursts undergo significant luminosity evolution, of similar to(1+z)(2) to z similar to 1, in addition to the similar to(1 + z)(2) number evolution reflecting the increase with redshift in the fraction of merging/interacting galaxies (as estimated from optical surveys).