Modelling ISO Galaxy Counts with Luminosity and Merger Rate Evolution

We model galaxy evolution in the 6.75μm and 15μm 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 infra-red. Our derived model is consistent with the local 12μm galaxy luminosity function (from IRAS), including the bright end if we place a small fraction (∼ 3.5 per cent at z = 0) of the model’s spiral galaxies in interacting pairs with mid-infra-red luminosities enhanced by major starbursts. Source counts from deep ISO surveys in the 6.75μm and 15μm passbands significantly exceed non-evolving predictions. We find the number counts, redshift distributions and the wide range of mid-infra-red to optical flux ratios of the detected galaxies to be reasonably consistent with our evolving model. Our models suggest that the steep number count of 6.75μm sources can be explained primarily by the evolving E/S0 galaxies. About one-third of 6.75μm sources do appear to be E/S0s or early-type spirals, consistent with this model, but starburst galaxies at 0.6 < z < 1.1 are more prominent in this passband than we expected. In the 15μm passband the main contributors appear to be evolving, star-forming spirals and starbursting galaxy mergers, as in our model. Visibly interacting galaxies form ∼ 31 per cent of the 15μm detections to 200μJy, and have high mid-infra-red/optical flux ratios consistent with our models for major starbursts with high dust extinction (up 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-infra-red luminosity function if the merger-triggered starbursts undergo significant luminosity evolution, of ∼ (1+ z) to z ∼ 1, in addition to the ∼ (1+ z) number evolution reflecting the increase with redshift in the fraction of merging/interacting galaxies (as estimated from optical surveys).