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IRS Spectra of Two Ultraluminous Infrared Galaxies at z = 1.3
The Astrophysical Journal (2006)
  • V. Desai, California Institute of Technology
  • L. Armus, California Institute of Technology
  • B. T. Soifer, California Institute of Technology
  • D. W. Weedman, Cornell University
  • Sarah J.U. Higdon, Cornell University
  • C. Bian, California Institute of Technology
  • C. Borys, California Institute of Technology
  • H. W. W. Spoon, Cornell University
  • V. Charmandaris, Cornell University
  • K. Brand, National Optical Astronomy Observatory
  • M. J. I. Brown, Princeton University
  • A. Dey, National Optical Astronomy Observatory
  • J. L. Higdon, Cornell University
  • J. R. Houck, Cornell University
  • B. T. Jannuzi, National Optical Astronomy Observatory
  • E. Le Floc'h, University of Arizona
  • M.L.N. Ashby, Harvard-Smithsonian Center for Astrophysics
  • H. A. Smith, Harvard-Smithsonian Center for Astrophysics
We present low-resolution (64 < R < 124) mid-infrared (8–38 m) spectra of two z  1:3 ultraluminous infrared galaxies (ULIRGs) with L8 1000 m  1013 L. The spectra were taken with the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope. Both objects were discovered in the NOAO Deep Wide-Field Survey (NDWFS) Bootes field. MIPS J142824.0+352619 is a bright 160 m source with a large infrared-to-optical flux density ratio. Previous authors provided evidence for a foreground lens and estimated an amplification of P10, although this factor is currently poorly constrained. The 6.2, 7.7, 11.3, and 12.8 m polycyclic aromatic hydrocarbon (PAH) emission bands in its IRS spectrum indicate a redshift of z  1:3. The large equivalent width of the 6.2 m PAH feature indicates that at least 50% of the mid-infrared energy is generated in a starburst, an interpretation supported by a large ½Ne ii/½Ne iii ratio and a low upper limit on the X-ray luminosity. SST24 J142827.19+354127.71 has the brightest 24 m flux (10.55 mJy) among optically faint (R > 20) galaxies in the NDWFS. Its mid-infrared spectrum lacks emission features, but the broad 9.7 m silicate absorption band places this source at z  1:3. Optical spectroscopy confirms a redshift of z ¼ 1:293  0:001. Given this redshift, SST24 J142827.19+354127.71 has among the largest rest-frame 5 m luminosities known. The similarity of its SED to those of known AGN-dominated ULIRGs and its lack of either PAH features or large amounts of cool dust indicate that the mid-infrared emission is dominated by an AGN rather than a starburst. 
  • Galaxies,
  • Active,
  • Evolution,
  • Formation,
  • Starburst,
  • Infrared
Publication Date
April 10, 2006
Publisher Statement
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Citation Information
V. Desai, L. Armus, B. T. Soifer, D. W. Weedman, et al.. "IRS Spectra of Two Ultraluminous Infrared Galaxies at z = 1.3" The Astrophysical Journal Vol. 641 Iss. 1 (2006) p. 133 - 139
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