The Mid-Infrared Luminosity Function of Galaxies in the European Large Area Infrared Space Observatory Survey Southern Fields

We present the first determination of the 15 m luminosity function of galaxies from the European Large Area ISO Survey (ELAIS) southern fields. We have adopted a new criterion to separate the quiescent, nonevolving and the starburst, evolving populations based on the ratio of mid-infrared to optical luminosity. Strong evolution is suggested by our data for the starburst galaxy population, while normal spiral galaxies are consistent with no evolution. The starburst population must evolve both in luminosity and in density with rates of the order L(z) / (1þ z) and (z) / (1þ z), respectively, up to z 1. The evolutionary parameters of our model have been tested by comparing the model predictions with other observables, like source counts at all flux density levels (from 0.1 to 300 mJy) and redshift distributions and luminosity functions at high z (0:7 < z < 1:0 from Hubble Deep Field North [HDF-N] data). The agreement between our model predictions and the observed data is remarkably good. We use our data to estimate the star formation density of the universe up to z 1⁄4 0:4, and we use the luminosity function model to predict the trend of the star formation history up to z 1⁄4 1. Subject headings: galaxies: evolution — galaxies: luminosity function, mass function — galaxies: spiral — galaxies: starburst — infrared: galaxies