Mid-infrared spectroscopy of lensed galaxies at 1 < z < 3:: The nature of sources near the MIPS confusion limit

We present Spitzer IRS mid-infrared spectra for 15 gravitationally lensed, 24 mu m-selected galaxies, and combine the results with four additional very faint galaxies with IRS spectra in the literature. The median intrinsic 24 mu m flux density of the sample is 130 mu Jy, enabling a systematic survey of the spectral properties of the very faint 24 mu m sources that dominate the number counts of Spitzer cosmological surveys. Six of the 19 galaxy spectra (32%) show the strong mid-IR continuua expected of AGNs; X-ray detections confirm the presence of AGNs in three of these cases, and reveal AGNs in two other galaxies. These results suggest that nuclear accretion may contribute more flux to faint 24 mu m-selected samples than previously assumed. Almost all the spectra show some aromatic (PAH) emission features; the measured aromatic flux ratios do not show evolution from z = 0. In particular, the high signal-to-noise mid-IR spectrum of SMMJ163554.2+ 661225 agrees remarkably well with low-redshift,lower luminosity templates. We compare the rest-frame 8 mu m and total infrared luminosities of star-forming galaxies, and find that the behavior of this ratio with total IR luminosity has evolved modestly from z = 2 to z = 0. Since the high aromatic-to-continuum flux ratios in these galaxies rule out a dominant contribution by AGNs, this finding implies systematic evolution in the structure and/or metallicity of infrared sources with redshift. It also has implications for the estimates of star-forming rates inferred from 24 mu m measurements, in the sense that at z similar to 2, a given observed frame 24 mu m luminosity corresponds to a lower bolometric luminosity than would be inferred from low-redshift templates of similar luminosity at the corresponding rest wavelength.