Publication: Mid-infrared spectroscopy of lensed galaxies at 1 < z < 3: The nature of sources near the MIPS confusion limit
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2008-03-01
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American Astronomical Society
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We present Spitzer IRS mid-infrared spectra for 15 gravitationally lensed, 24 μm-selected galaxies, and combine the results with four additional very faint galaxies with IRS spectra in the literature. The median intrinsic 24 μm flux density of the sample is 130 μJy, enabling a systematic survey of the spectral properties of the very faint 24 μm 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 μm-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 SMM J163554.2+661225 agrees remarkably well with low-redshift, lower luminosity templates. We compare the rest-frame 8 μm 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 μm measurements, in the sense that at z ~ 2, a given observed frame 24 μm luminosity corresponds to a lower bolometric luminosity than would be inferred from low-redshift templates of similar luminosity at the corresponding rest wavelength.
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© 2008 The American Astronomical Society. All rights reserved.
We thank J. Donley for computing X-ray flux upper limits for the nondetections, and D. Sand for making available his reduction of the WFPC2 images. We thank D. Lutz and H. Teplitz for sending electronic versions of their published spectra. This work is based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. Support for this work was provided in part by NASA through contract 1255094 issued to the University of Arizona by JPL/Caltech. J. Rigby was supported in part by NASA through the Spitzer Space Telescope Fellowship Program. This work has made use of archival data from the Chandra X-Ray Observatory, the Hubble Space Telescope, and the Spitzer Space Telescope. This research has made use of NASA’s Astrophysics Data System Bibliographic Services and the NASA/ IPAC Extragalactic Database.
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