2009ApJ...693..370W

The mid-infrared spectroscopic analysis of a flux-limited sample of galaxies with f_ν(24 µm) > 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_ν(24 µm) > 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 νL_ν(15 µm) < 46.2 (erg/s). Average spectra are determined as a function of luminosity; lower luminosity sources (log νL_ν(15 µm) < 44.0) are dominated by polycyclic aromatic hydrocarbon (PAH) features and higher luminosity sources (log νL_ν(15 µm) > 44.0) are dominated by silicate absorption or emission. We find that a rest-frame equivalent width of 0.4 µm for the 6.2 µm 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_ν(24 µm) 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_ν(24 µm) > 1 mJy, but starbursts fall to f_ν(24 µm) < 1 mJy by z∼ 0.5. This indicates that substantial luminosity evolution of starbursts is required to explain the numerous starbursts found in other IRS results having f_ν(24 µm) ∼ 1 mJy and z∼ 2.