The mid-infrared color-luminosity relation and the local 12 micron luminosity function

We have established a model to systematically estimate the contribution of the mid-IR emission features between 3 and 11.6 mu m to the IRAS in-band fluxes, using the results of Infrared Space Observatory PHT Spectrophotometer observation of 16 galaxies by Lu et al. in 1997. The model is used to estimate more properly the k-corrections for calculating the rest frame 12 and 25 mu m fluxes and luminosities of IRAS galaxies. We have studied the 12-25 mu m color-luminosity relation for a sample of galaxies selected at 25 mu m The color is found to correlate well with the 25 mu m luminosity, the mid-IR luminosity, and the ratio of far-infrared and blue luminosities. The relations with the mid-IR luminosities are more sensitive to different populations of galaxies, while a single relation of the 12-25 mu m color versus the ratio of the far-infrared and blue luminosities applies equally well to these different populations. The luminous and ultraluminous infrared galaxies have redder 12-25 mu m colors than those of the quasars. These relations provide powerful tools to differentiate different populations of galaxies. The local luminosity function at 12 mu m provides the basis for interpreting the results of deep mid-IR surveys planned or in progress with infrared Space Observatory, Wide-Field Infrared Explorer, and Space Infrared Telescope Facility. We have selected a sample of 668 galaxies from the IRAS Faint Source Survey flux density limited at 200 mJy at 12 mu m. A 12 mu m local luminosity function is derived and, for the first time in the literature, effects of density variation in the local universe are considered and corrected in the calculation of the 12 mu m luminosity function. It is also found that the 12 mu m selected sample is dominated by quasars and active galaxies, which therefore strongly affect the 12 mu m luminosity function at high luminosities. The ultraluminous infrared galaxies are relatively rare at 12 mu m compared to a 25 mu m sample.