The Local Galaxy 8 μm Luminosity Function

A Spitzer Space Telescope survey in the NOAO Deep Wide Field in Boötes provides a complete, 8 μm-selected sample of galaxies to a limiting (Vega) magnitude of 13.5. In the 6.88 deg² field sampled, 79% of the 4867 galaxies have spectroscopic redshifts, allowing an accurate determination of the local (z < 0.3) galaxy luminosity function. Stellar and dust emission can be separated on the basis of observed galaxy colors. Dust emission (mostly PAH) accounts for 80% of the 8 μm luminosity, stellar photospheres account for 19%, and AGN emission accounts for roughly 1%. A subsample of the 8 μm-selected galaxies have blue, early-type colors, but even most of these have significant PAH emission. The luminosity functions for the total 8 μm luminosity and for the dust emission alone are both well fit by Schechter functions. For the 8 μm luminosity function, the characteristic luminosity is νL(8.0 μm) = 1.8 × 10¹⁰ L_☉, while for the dust emission alone it is 1.6 × 10¹⁰ L_☉. The average 8 μm luminosity density at z < 0.3 is 3.1 × 10⁷ L_☉ Mpc^-3, and the average luminosity density from dust alone is 2.5 × 10⁷ L_☉ Mpc^-3. This luminosity arises predominantly from galaxies with 8 μm luminosities (νL_ν) between 2 × 10⁹ and 2 × 10¹⁰ L_☉, i.e., normal galaxies, not luminous or ultraluminous infrared galaxies (LIRGs/ULIRGs).