IRS Spectra of Two Ultraluminous Infrared Galaxies at z = 1.3

April 2006 • 2006ApJ...641..133D

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

Abstract • We present low-resolution (64<R<124) mid-infrared (8-38 μm) spectra of two z~1.3 ultraluminous infrared galaxies (ULIRGs) with L_8-1000μm~10¹³ L_solar. 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.

Links

• PREPRINT http://arxiv.org/abs/astro-ph/0512215
• NED https://ned.ipac.caltech.edu/uri/NED::InRefcode/2006ApJ...641..133D
• ELECTR https://doi.org/10.1086%2F500426
• SIMBAD http://simbad.u-strasbg.fr/simbo.pl?bibcode=2006ApJ...641..133D
• PDF https://stacks.iop.org/0004-637X/641/133/pdf
• DATA https://irsa.ipac.caltech.edu/bibdata/2006/D/2006ApJ...641..133D.html
• SPIRES http://inspirehep.net/search?p=find+eprint+astro-ph/0512215