The mid-IR Luminosity Function at z<0.3 from 5MUSES: Understanding the Star-formation/AGN Balance from a Spectroscopic View

TL;DR

This study derives mid-infrared luminosity functions at z<0.3 from the 5MUSES sample, quantifying star formation and AGN contributions, and compares these with higher redshift results to understand galaxy evolution.

Contribution

First detailed mid-IR luminosity functions at z<0.3 using spectroscopic data and spectral decomposition to separate star formation and AGN contributions.

Findings

Star formation contributes 58% at 15 um and 78% at 24 um to IR energy density.

Bright-end luminosity decline follows Schechter function after removing AGN.

Evolution in luminosity and density explains differences with z~0.7 results.

Abstract

We present rest-frame 15 and 24 um luminosity functions and the corresponding star-forming luminosity functions 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 um IRS spectra allow us to estimate accurately the luminosities and build the luminosity functions. Using a combination of starburst and quasar templates, we quantify the star-formation and AGN contributions in the mid-IR SED. We then compute the star-formation luminosity functions at 15 um and 24 um, and compare with the total 15 um and 24 um luminosity functions. When we remove the contribution of AGN, the bright end of the luminosity function exhibits a strong decline, consistent with the exponential cutoff of a Schechter function. Integrating the differential luminosity function, we find that the fractional contribution by star formation to the energy density is 58% at 15 um and 78% at 24 um, while it goes up to ~86% when we extrapolate our mid-IR results to the total IR luminosity density. We confirm that the active galactic nuclei 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.