Local Luminosity Function at 15$\mu m$ and Galaxy Evolution Seen by ISOCAM 15$\mu m$ Surveys

Abstract

A local luminosity function at 15$\mu m$ is derived using the bivariate (15$\mu m$ vs. 60$\mu m$ luminosity) method, based on the newly published ISOCAM LW3-band (15$\mu m$) survey of the very deep IRAS 60$\mu m$ sample in the north ecliptic pole region (NEPR). New IRAS 60$\mu m$ fluxes are obtained using the SCANPI/SUPERSCANPI software at the new ISOCAM positions of the sources in the sample. It is found to be in excellent agreement with the 15$\mu m$ local luminosity function published by Xu et al (1998), which is derived from the {\it predicted} 15$\mu m$ luminosities of a sample of IRAS 25$\mu m$ selected galaxies. Model predictions of number counts and redshift distributions based on the local luminosity function and assumptions of its evolution with the redshift are calculated and compared with the data of ISOCAM 15$\mu m$ surveys. Strong luminosity evolution on the order of $L\propto (1+z)^{4.5}$ is suggested in these comparisons, while pure density evolution can be ruled out with high confidence. The sharp peak at about 0.4mJy in the Euclidean normalized differential counts at 15$\mu m$ can be explained by the effects of MIR broadband emission features, eliminating the need for any hypothesis for a 'new population'. It is found that the contribution from the population represented by ISOCAM 15$\mu m$ sources can account for the entire IR/submm background, leaving little room for any missing 'new population' which can be significant energy sources of the IR/submm sky.