Emission Features and Source Counts of Galaxies in the Mid-Infrared

In this work we incorporate the newest Infrared Space Observatory (ISO) results on the mid-infrared spectral energy distributions (MIR SEDs) of galaxies into models for the number counts and redshift distributions of MIR surveys. A three-component model, with empirically determined MIR SED templates of (1) a cirrus/photodissociation region component, (2) a starburst component, and (3) an active galactic nucleus (AGN) component, is developed for infrared (3-120 μm) SEDs of galaxies. The model includes a complete IRAS 25 μm selected sample of 1406 local galaxies (z ≤ 0.1). Results based on these 1406 spectra show that the MIR emission features cause significant effects on the redshift dependence of the K-corrections for fluxes in the WIRE 25 μm band and ISOCAM 15 μm band. This in turn will affect deep counts and redshift distributions in these two bands, as shown by the predictions of two evolution models [a luminosity evolution model with L ∝ (1 + z)³ and a density evolution model with ρ ∝ (1 + z)⁴]. The dips and bumps on curves of MIR number counts, caused by the emission features, should be useful indicators of the evolution mode. The strong emission features at ~6-8 μm will help the detections of relatively high redshift (z ~ 2) galaxies in MIR surveys. On the other hand, determinations of the evolutionary rate based on the slope of source counts and studies on the large-scale structures using the redshift distribution of MIR sources will have to treat the effects of the MIR emission features carefully. We have also estimated a 15 μm local luminosity function from the predicted 15 μm fluxes of the 1406 galaxies using the bivariate (15 μm vs. 25 μm luminosities) method. This luminosity function will improve our understanding of the ISOCAM 15 μm surveys.