The Mid-infrared Luminosity Function at z < 0.3 from 5MUSES: Understanding the Star Formation/Active Galactic Nucleus Balance from a Spectroscopic View

We present rest-frame 15 and 24 mu m luminosity functions (LFs) and the corresponding star-forming LFs at z < 0.3 derived from the 5MUSES sample. Spectroscopic redshifts have been obtained for similar to 98% of the objects and the median redshift is similar to 0.12. The 5-35 mu m Infrared Spectrograph spectra allow us to estimate accurately the luminosities and build the LFs. Using a combination of starburst and quasar templates, we quantify the star formation (SF) and active galactic nucleus (AGN) contributions in the mid-IR spectral energy distribution. We then compute the SF LFs at 15 and 24 mu m, and compare with the total 15 and 24 mu m LFs. When we remove the contribution of AGNs, the bright end of the LF exhibits a strong decline, consistent with the exponential cutoff of a Schechter function. Integrating the differential LF, we find that the fractional contribution by SF to the energy density is 58% at 15 mu m and 78% at 24 mu m, while it goes up to similar to 86% when we extrapolate our mid-IR results to the total IR luminosity density. We confirm that the AGNs play more important roles energetically at high luminosities. Finally, we compare our results with work at z similar to 0.7 and confirm that evolution on both luminosity and density is required to explain the difference in the LFs at different redshifts.