THE MIDINFRARED COLOR-LUMINOSITY RELATION AND THE LOCAL 12 MICRON LUMINOSITY FUNCTION

We have established a model to systematically estimate the contributio n 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 Sp ectrophotometer observation of 16 galaxies by Lu et al. in 1997. The m odel is used to estimate more properly the k-corrections for calculati ng the rest frame 12 and 25 mu m fluxes and luminosities of IRAS galax ies. We have studied the 12-25 mu m color-luminosity relation for a sa mple of galaxies selected at 25 mu m The color is found to correlate w ell 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 l uminosities are more sensitive to different populations of galaxies, w hile 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 diff erent populations. The luminous and ultraluminous infrared galaxies ha ve redder 12-25 mu m colors than those of the quasars. These relations provide powerful tools to differentiate different populations of gala xies. 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 Sp ace Infrared Telescope Facility. We have selected a sample of 668 gala xies 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 t he first time in the literature, effects of density variation in the l ocal universe are considered and corrected in the calculation of the 1 2 mu m luminosity function. It is also found that the 12 mu m selected sample is dominated by quasars and active galaxies, which therefore s trongly affect the 12 mu m luminosity function at high luminosities. T he ultraluminous infrared galaxies are relatively rare at 12 mu m comp ared to a 25 mu m sample.