Emission features and source counts of galaxies in the mid-infrared

In this work we incorporate the newest Infrared Space Observatory (ISO) res ults on the mid-infrared spectral energy distributions (MIR SEDs) of galaxi es into models for the number counts and redshift distributions of MIR surv eys. A three-component model, with empirically determined MIR SED templates of (1) a cirrus/photodissociation region component, (2) a starburst compon ent, and (3) an active galactic nucleus (AGN) component, is developed for i nfrared (3-120 mu m) SEDs of galaxies. The model includes a complete IRAS 2 5 mu m selected sample of 1406 local galaxies (z less than or equal to 0.1) . Results based on these 1406 spectra show that the MIR emission features c ause significant effects on the redshift dependence of the K-corrections fo r fluxes in the WIRE 25 mu m band and ISOCAM 15 mu m band. This in turn wil l affect deep counts and redshift distributions in these two bands, as show n by the predictions of two evolution models [a luminosity evolution model with L proportional to (1 + z)(3) and a density evolution model with rho pr oportional to (1 + z)(4)]. Th, dips and bumps on curves of MIR number count s, caused by the emission features, should be useful indicators of the evol ution mode. The strong emission features at similar to 6-8 mu m will help t he detections of relatively high redshift (z similar to 2) galaxies in MIR surveys. On the other hand, determinations of the evolutionary rate based o n the slope of source counts and studies on the large-scale structures usin g the redshift distribution of MIR sources will have to treat the effects o f the MIR emission features carefully. We have also estimated a 15 mu m loc al luminosity function from the predicted 15 mu m fluxes of the 1406 galaxi es using the bivariate (15 mu m vs. 25 mu m luminosities) method. This lumi nosity function will improve our understanding of the ISOCAM 15 mu m survey s.