Spitzer spectra of a 10 mJy galaxy sample and the star formation rate in the local universe

A complete flux-limited sample of 50 galaxies is presented having f(v)(24 mu m) > 10 mJy, chosen from a survey with MIPS on Spitzer of 8.2 deg(2) in the NOAO Deep Wide-Field Survey (NDWFS) region in Bootes. Spectra obtained with the low-resolution modules of IRS on Spitzer are described for 36 galaxies in this sample; 25 show strong PAH emission features characteristic of starbursts, and 11 show silicate absorption or emission, emission lines, or featureless spectra characteristic of AGNs. Infrared or optical spectral classifications are available for 48 of the entire sample of 50; 33 galaxies are classified as starbursts and 15 as AGNs. [There are an additional 19 Galactic stars with f(v)(24 mu m) > 10 mJy in the survey area.] Using a relation between 7: 7 mu m PAH luminosity and SFR derived from previous IRS observations of starbursts, the SFR per unit volume of the local universe (SFRD) is determined from the complete sample and is found to be 0: 008 M-circle dot yr(-1) Mpc(-3). This provides an extinction-free measurement of SFRD independent of optical properties and provides a parameter that can be used for direct comparison to high-redshift starbursts being discovered with Spitzer. Individual sources in the sample have SFRs from 0: 14 to 160 M-circle dot yr(-1). The derived value for the local SFRD is about half that of the local SFRD deduced from bolometric luminosities of the IRAS 60 mu m Bright Galaxy Sample, with the deficiency being at lower luminosities and arising primarily from the small number of low-luminosity sources in the 10 mJy sample. The agreement for higher luminosities confirms the validity of using the 7: 7 mu m PAH feature as a measure of SFRD in the high-redshift universe, where this is often the only indicator available for faint sources.