IRS spectra of two ultraluminous infrared galaxies at z = 1.3

V. Desai, L. Armus, B. T. Soifer, D. W. Weedman, S. Higdon, C. Bian, C. Borys, H. W.W. Spoon, V. Charmandaris, K. Brand, M. J.I. Brown, A. Dey, J. Higdon, J. Houck, B. T. Jannuzi, E. Le Floc'h, M. L.N. Ashby, H. A. Smith

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24 Scopus citations


We present low-resolution (64 < R < 124) mid-infrared (8-38 μm) spectra of two z ≈ 1.3 ultraluminous infrared galaxies (ULIRGs) with L 8-1000 μm ≈ 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 μm source with a large infrared-to-optical flux density ratio. Previous authors provided evidence for a foreground lens and estimated an amplification of ≲10, although this factor is currently poorly constrained. The 6.2, 7.7, 11.3, and 12.8 μm 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 μm PAH feature indicates that at least 50% of the mid-infrared energy is generated in a starburst, an interpretation supported by a large [Ne II]/[Ne III] ratio and a low upper limit on the X-ray luminosity. SST24 J142827.19+354127.71 has the brightest 24 μm 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 μm silicate absorption band places this source at z ≈ 1.3. Optical spectroscopy confirms a redshift of z = 1.293 ± 0.001. Given this redshift, SST24 J142827.19+354127.71 has among the largest rest-frame 5 μm 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.

Original languageEnglish (US)
Pages (from-to)133-139
Number of pages7
JournalAstrophysical Journal
Issue number1 I
StatePublished - Apr 10 2006


  • Galaxies: active
  • Galaxies: evolution
  • Galaxies: formation
  • Galaxies: starburst infrared: galaxies

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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