AVERAGE INFRARED GALAXY SPECTRA FROM SPITZER FLUX-LIMITED SAMPLES

The mid-infrared spectroscopic analysis of a flux-limited sample of galaxies with f(nu)(24 mu m) > 10 mJy is presented. Sources observed are taken from the Spitzer First Look Survey (FLS) catalog and from the NOAO Deep Wide-Field Survey region in Bootes (NDWFS). The spectroscopic sample includes 60 of the 100 sources in these combined catalogs having f(nu)(24 mu m) > 10 mJy. New spectra from the Spitzer Infrared Spectrograph are presented for 25 FLS sources and for 11 Bootes active galactic nuclei (AGNs); these are combined with 24 Bootes starburst galaxies previously published to determine the distribution of mid-infrared spectral characteristics for the total 10 mJy sample. Sources have 0.01 < z < 2.4 and 41.8 < log nu L(nu)(15 mu m) < 46.2 (ergs s(-1)). Average spectra are determined as a function of luminosity; lower luminosity sources (log nu L(nu)(15 mu m) < 44.0) are dominated by polycyclic aromatic hydrocarbon (PAH) features and higher luminosity sources (log nu L(nu)(15 mu m) > 44.0) are dominated by silicate absorption or emission. We find that a rest-frame equivalent width of 0.4 mu m for the 6.2 mu m PAH emission feature provides a well-defined division between lower luminosity, "pure" starbursts and higher luminosity AGN or composite sources. Using the average spectra, fluxes f(nu)(24 mu m) which would be observed with the Spitzer MIPS are predicted as a function of redshift for sources with luminosities that correspond to the average spectra. AGNs identical to those in this 10 mJy sample could be seen to z = 3 with f(nu)(24 mu m) > 1 mJy, but starbursts fall to f(nu)(24 mu m) < 1 mJy by z similar to 0.5. This indicates that substantial luminosity evolution of starbursts is required to explain the numerous starbursts found in other IRS results having f(nu)(24 mu m) similar to 1 mJy and z similar to 2.