The local galaxy 8 μm luminosity function

A Spitzer Space Telescope survey in the NOAO Deep Wide Field in Bootes provides a complete, 8 mu m-selected sample of galaxies to a limiting (Vega) magnitude of 13.5. In the 6.88 deg(2) 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 mu m luminosity, stellar photospheres account for 19%, and AGN emission accounts for roughly 1%. A subsample of the 8 mu m-selected galaxies have blue, early-type colors, but even most of these have significant PAH emission. The luminosity functions for the total 8 mu m luminosity and for the dust emission alone are both well fit by Schechter functions. For the 8 mu m luminosity function, the characteristic luminosity is vL(v)*(8.0 mu m) = 1.8 x 10(10) L(circle dot), while for the dust emission alone it is 1.6 x 10(10) L(circle dot). The average 8 mu m luminosity density at z < 0.3 is 3.1 x 10(7) L(circle dot) Mpc(-3), and the average luminosity density from dust alone is 2.5 x 10(7) L(circle dot) Mpc(-3). This luminosity arises predominantly from galaxies with 8 mu m luminosities (vL(v)) between 2 x 10(9) and 2 x 10(10) L(circle dot), i.e., normal galaxies, not luminous or ultraluminous infrared galaxies (LIRGs/ULIRGs).