Emission Features and Source Counts of Galaxies in the Mid‐Infrared
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Abstract
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 (3120 km) SEDs of galaxies. The model includes a complete IRAS 25 km selected sample of 1406 local galaxies (z 0.1). Results based on these 1406 spectra show that the MIR emission features cause signicant e ects on the redshift dependence of the K-corrections for uxes in the WIRE 25 km band and ISOCAM 15 km band. This in turn will a ect deep counts and redshift distributions in these two bands, as shown by the predictions of two evolution models [a luminosity evolution model with L P (1 ] z)3 and a density evolution model with o P (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 D68 km will help the detections of relatively high redshift (z D 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 e ects of the MIR emission features carefully. We have also estimated a 15 km local luminosity function from the predicted 15 km uxes of the 1406 galaxies using the bivariate (15 km vs. 25 km luminosities) method. This luminosity function will improve our understanding of the ISOCAM 15 km surveys.