ULTRA-DEEP MID-INFRARED SPECTROSCOPY OF LUMINOUS INFRARED GALAXIES AT z{approx} 1 AND z {approx} 2

Journal Article · Tue Aug 10 00:00:00 EDT 2010 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/719/1/425· OSTI ID:21455013

Fadda, Dario; Lin, Yan; Frayer, David T; Helou, George ^[1]; Lagache, Guilaine; Marcillac, Delphine ^[2]; Sajina, Anna ^[3]; Lutz, Dieter ^[4]; Wuyts, Stijn ^[5]; Le Floc'h, Emeric ^[6]; Caputi, Karina ^[7]; Spoon, Henrik W. W. ^[8]; Veilleux, Sylvain ^[9]; Blain, Andrew ^[10]

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1. IPAC, California Institute of Technology, Pasadena, CA 91125 (United States)
2. Institut d'Astrophysique Spatiale, University Paris-Sud 11 and CNRS (UMR 8617), F-91405 Orsay (France)
3. Haverford College, Haverford, PA 19041 (United States)
4. Max-Planck-Institut fuer Extraterrestrische Physik, 85741 Garching (Germany)
5. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States)
6. CEA Saclay, Service d'Astrophysique, 91191 Gif-sur-Yvette (France)
7. SUPA Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ (United Kingdom)
8. Department of Astronomy, Cornell University, 219 Space Sciences Building, Ithaca, NY 14853 (United States)
9. Department of Astronomy, University of Maryland, College Park, MD 20742 (United States)
10. Astronomy Department, California Institute of Technology, Pasadena, CA 91125 (United States)

We present ultra-deep mid-infrared spectra of 48 infrared-luminous galaxies in the GOODS-south field obtained with the Infrared Spectrograph on the Spitzer Space Telescope. These galaxies are selected among faint infrared sources (0.14-0.5 mJy at 24 {mu}m) in two redshift bins (0.76-1.05 and 1.75-2.4) to sample the major contributors to the cosmic infrared background at the most active epochs. We estimate redshifts for 92% of the sample using polycyclic aromatic (PAH) and Si absorption features obtaining, in particular, eight new redshifts difficult to measure from ground-based observations. Only a few of these galaxies (5% at z {approx} 1 and 12% at z {approx} 2) have their total infrared luminosity dominated by emission from active galactic nuclei (AGNs). The averaged mid-IR spectrum of the z {approx} 1 luminous infrared galaxies (LIRGs) is a very good match to the averaged spectrum of local starbursts. The averaged spectrum of the z {approx} 2 ultra-luminous infrared galaxies (ULIRGs), because of a deeper Si absorption, is better fitted by the averaged spectrum of H II-like local ULIRGs. Combining this sample with other published data, we find that 6.2 {mu}m PAH equivalent widths (EW) reach a plateau of {approx} 1 {mu}m for L {sub 24{mu}m} {approx}< 10{sup 11} L{sub sun}. At higher luminosities, EW{sub 6.2{mu}m} anti-correlates with L{sub 24{mu}m}. Intriguingly, high-z ULIRGs and sub-millimeter galaxies (SMGs) lie above the local EW{sub 6.2{mu}m}-L{sub 24{mu}m} relationship suggesting that, at a given luminosity, high-z ULIRGs have AGN contributions to their dust emission lower than those of local counterparts. A quantitative analysis of their morphology shows that most of the luminous IR galaxies have morphologies similar to those of IR-quiet galaxies at the same redshift. All z {approx} 2 ULIRGs of our sample are IR-excess BzK galaxies and most of them have L{sub FIR}/L{sub 1600A} ratios higher than those of starburst galaxies at a given UV slope. The 'IR excess' is mostly due to strong 7.7 {mu}m PAH emission and underestimation of UV dust extinction. On the basis of the AGN-powered L {sub 6{mu}m} continuum measured directly from the mid-IR spectra, we estimate an average intrinsic X-ray AGN luminosity of L{sub 2-10keV} = (0.1{+-} 0.6)x 10{sup 43} erg s{sup -1}, a value substantially lower than the prediction by Daddi et al.

• OSTI ID: 21455013
• Journal Information: Astrophysical Journal, Vol. 719, Issue 1; Other Information: DOI: 10.1088/0004-637X/719/1/425; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTROPHYSICS
COSMOLOGY AND ASTRONOMY
ABSORPTION SPECTROSCOPY
DUSTS
EMISSION
GALACTIC EVOLUTION
GALAXIES
GALAXY NUCLEI
INFRARED SPECTRA
LUMINOSITY
POLYCYCLIC AROMATIC HYDROCARBONS
RED SHIFT
TELESCOPES
AROMATICS
EVOLUTION
HYDROCARBONS
OPTICAL PROPERTIES
ORGANIC COMPOUNDS
PHYSICAL PROPERTIES
SPECTRA
SPECTROSCOPY