Yan, Lin and Sajina, Anna and Fadda, Dario and Choi, Phil and Armus, Lee and Helou, George and Teplitz, Harry and Frayer, David and Surace, Jason (2007) Spitzer Mid-Infrared Spectroscopy of Infrared Luminous Galaxies at z ~ 2. I. The Spectra. Astrophysical Journal, 658 (2). pp. 778-793. ISSN 0004-637X http://resolver.caltech.edu/CaltechAUTHORS:20091117-094832114
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We present the mid-infrared spectra obtained with the Spitzer IRS for 52 sources, selected as infrared luminous, z ≳ 1 candidates in the Extragalactic First Look Survey. The sample selection criteria are f_(24 μm) ≳ 0.9 mJy, νf_ν(24 μm)/νf_ν(8 μm) ≳ 3.16, and νf_ν(24 μm)/νf_ν(0.7 μm) ≳ 10. Of the 52 spectra, 47 (90%) produced measurable redshifts based solely on the mid-IR spectral features, with 35/47 (74%) at 1.5 ≲ z ≲ 3.2. Keck spectroscopy of a subsample (17/47) agrees with the mid-IR redshift measurements. The observed spectra fall into three categories: (1) 33% (17/52) have strong PAH emission and are probably powered by star formation with total IR luminosity roughly a factor of 5 higher than the local starburst ULIRGs; (2) 33% (17/52) have only deep silicate absorption at 9.8 μm, indicatiing deeply embedded dusty systems—these data alone cannot determine the energetic nature of the heating sources in these systems; and (3) the remaining 34% are mid-IR continuum-dominated systems with weak PAH emission and/or silicate absorption. This subsample is probably AGNs. We derived monochromatic, rest-frame 5.8 μm, continuum luminosities (νL_ν), ranging from 10^(10.3) to 10^(12.6) L_☉. Our spectra have mid-IR slope α_(5-15 μm) ≳ 2.1, much redder than the median value of 1.3 for the optically selected PG quasars. From the silicate absorption feature, we estimate that roughly two-thirds of the sample have optical depth τ_(9.8 μm) > 1. Their L_(1600 Å) and L_(IR) suggest that our sample is among the most luminous and most dust-enshrouded systems of its epoch. Our study has revealed a significant population of dust-enshrouded galaxies at z ~ 2, whose enormous energy output, comparable to that of quasars, is generated by AGNs, as well as starbursts. This IR-luminous population has very little overlap with submillimeter and UV-selected populations.
|Additional Information:||© 2007 American Astronomical Society. Print publication: Issue 2 (2007 April 1); received 2006 October 13; accepted for publication 2006 December 4. We would like to thank many helpful discussions with Henrik Spoon, Tom Soifer, and Guilaine Lagache. Support for this work was provided by NASA through an award issued by JPL/Caltech. A. Sajina acknowledges support through NASA grant 09865 from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract BAS5-26555.|
|Subject Keywords:||galaxies: evolution; galaxies: high-redshift; galaxies: starburst; infrared: galaxies|
|Official Citation:||Spitzer Mid-Infrared Spectroscopy of Infrared Luminous Galaxies at z ~ 2. I. The Spectra Lin Yan, Anna Sajina, Dario Fadda, Phil Choi, Lee Armus, George Helou, Harry Teplitz, David Frayer, and Jason Surace 2007 ApJ 658 778-793 doi: 10.1086/511516|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Jason Perez|
|Deposited On:||17 Nov 2009 23:42|
|Last Modified:||26 Dec 2012 11:33|
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