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 μm luminosity functions (LFs) and the corresponding star-forming LFs at z < 0.3 derived from the 5MUSES sample. Spectroscopic redshifts have been obtained for ~98% of the objects and the median redshift is ~0.12. The 5-35 μm 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 μm, and compare with the total 15 and 24 μm 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 μm and 78% at 24 μm, while it goes up 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 ~ 0.7 and confirm that evolution on both luminosity and density is required to explain the difference in the LFs at different redshifts.