May
2005
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2005AJ....129.2074F
Authors
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Franceschini, Alberto
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Manners, James
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Polletta, Maria del Carmen
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Lonsdale, Carol
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Gonzalez-Solares, Eduardo
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Surace, Jason
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Shupe, Dave
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Fang, Fan
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Xu, C. Kevin
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Farrah, Duncan
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Berta, Stefano
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Rodighiero, Giulia
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Perez-Fournon, Ismael
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Hatziminaoglou, Evanthia
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Smith, Harding E.
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Siana, Brian
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Rowan-Robinson, Michael
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Nandra, Kirpal
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Babbedge, Tom
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Vaccari, Mattia
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Oliver, Seb
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Wilkes, Belinda
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Owen, Frazer
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Padgett, Deborah
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Frayer, Dave
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Jarrett, Tom
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Masci, Frank
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Stacey, Gordon
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Almaini, Omar
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McMahon, Richard
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Johnson, Olivia
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Lawrence, Andrew
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Willott, Chris
Abstract
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We exploit deep combined observations with Spitzer and Chandra of the Spitzer Wide-Area Infrared Extragalactic Survey (SWIRE) in the ELAIS N1 region to investigate the nature of the faint X-ray and IR sources in common, to identify active galactic nucleus (AGN)/starburst diagnostics, and to study the sources of the X-ray and IR cosmic backgrounds (XRB and CIRB). In the 17'×17' area of the Chandra ACIS-I image there are approximately 3400 SWIRE near-IR sources with 4 σ detections in at least two Infrared Array Camera (IRAC) bands and 988 sources detected at 24 μm with the Multiband Imaging Photometer (MIPS) brighter than S24~=0.1 mJy. Of these, 102 IRAC and 59 MIPS sources have Chandra counterparts, out of a total of 122 X-ray sources present in the area with S0.5-8keV>10-15 ergs cm-2 s-1. We have constructed spectral energy distributions (SEDs) for each source using data from the four IRAC wavebands, Chandra fluxes in the hard (2-8 keV) and soft (0.5-2 keV) X-rays, and optical follow-up data in the wavebands U, g', r', i', Z, and H. We fit a number of spectral templates to the SEDs at optical and IR wavelengths to determine photometric redshifts and spectral categories and also make use of diagnostics based on the X-ray luminosities, hardness ratios, X-ray to IR spectral slopes, and optical morphologies. Although we have spectroscopic redshifts for only a minority of the Chandra sources (10 type 1 QSOs or Seyfert sources and three galaxies), the available SEDs constrain the redshifts for most of the sample sources, which turn out to be typically at 0.5<z<2. We find that 39% of the Chandra sources are dominated by type 1 AGN emission (QSOs or Seyfert 1), 23% display optical/IR spectra typical of type 2 AGNs, while the remaining 38% show starburst-like or even normal galaxy spectra (including five passively evolving early-type galaxies). Since we prove that all these galaxies are dominated by AGN emission in X-rays (considering their large 0.5-8 keV rest-frame X-ray luminosities and their high X-ray to IR flux ratios), this brings the fraction of type 1 AGNs to 80% of the type 2 AGNs; even assuming that all the Chandra sources undetected by Spitzer are type 2 AGNs, the type 1 fraction would exceed 1/3 of the total population. Our analysis of the mid-IR MIPS 24 μm-selected sources, making up ~50% of the CIRB, shows that the fraction of those dominated by an AGN (either type 1 or type 2) is relatively constant with the IR flux and around 10%-15%. Our combined IR and hard X-ray observations allow us to verify that the dust covering fraction in type 1 AGNs is widely distributed between ~10% and 100%. A significant fraction, from 15% to 30% or more, of the sources of the XRB are hosted in galaxies whose optical/IR spectra are dominated by starburst (or normal galaxy) emission and for which only the hard X-ray spectra reveal the presence of a moderately luminous hidden AGN.
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