The unresolved hard X-ray background: the missing source population implied by the Chandra and XMM-Newton deep fields

By M. Worsley, A. C. Fabian, F. Bauer, D. Alexander, G. Hasinger, S. Mateos, H. Brunner, W. N. Brandt, D. P. Schneider. 2004

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We extend our earlier work on X-ray source stacking in the deep XMM-Newton observation of the Lockman Hole, to the 2-Ms Chandra Deep Field North (CDF-N) and the 1-Ms Chandra Deep Field South (CDF-S). The XMM-Newton work showed the resolved fraction of the X-ray background (XRB) to be ∼80-100 per cent at ≤ 2 keV but this decreased to only ∼50 per cent above ∼8 keV. The CDF-N and CDF-S probe deeper, and are able to fill in some of the missing fraction in the 4-6 keV range, but the resolved fraction in the 6-8 keV band remains only ∼60 per cent, confirming the trend seen with XMM-Newton. The missing XRB component has a spectral shape that is consistent with a population of highly obscured active galactic nuclei (AGN) at redshifts ∼0.5-1.5 and with absorption column densities of ∼10 23 -10 24 cm -2 .

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Citations in the corpus

A synthesis model for AGN evolution: supermassive black holes growth and feedback modes (2008, background): “Indeed, this class of objects dominates the counts in the X-ray energy band where almost the entire XRB radiation has been resolved into individual sources (Hasinger et al. 1998; Rosati et al. 2002; Alexander et al. 2003; Moretti et al. 2003; Worsley et al. 2005).”
Optical Spectroscopy and X-Ray Detections of a Sample of Quasars and Active Galactic Nuclei Selected in the Mid-Infrared from Two Spitzer Space Telescope Wide-Area Surveys (2006, background): “A large population of hidden AGN is also predicted from models of the X-ray background (e.g. Comastri et al. 1995; Worsley et al. 2005), though most models predict these 1Spitzer Science Center, Caltech, Mail Code 220-6, Pasadena, CA 91125; mlacy@ipac.caltech.edu,sajina@ipac.caltech.edu,…”
Spectral Energy Distributions of Hard X-Ray Selected Active Galactic Nuclei in the XMM-Newton Medium Deep Survey (2007, unknown): “The most recent background synthesis models predict that the missing population is made of sources at intermediate redshifts (z 1), with hard X-ray spectra ( 1:4) and moderate luminosities ( 1043 erg s 1; Worsley et al. 2005; Gilli et al. 2007).”
Swift BAT Survey of AGNs (2007, background): “It is now clear that much of the background at E > 8 keV is not produced by the sources detected in the 2Y8 keV band (Worsley et al. 2005), and is likely to come from a largely unobserved 1 NASA Goddard Space Flight Center, Astrophysics Science Division, Greenbelt, MD 20771.”
Multiwavelength Study of Massive Galaxies at z ~ 2. II. Widespread Compton-thick Active Galactic Nuclei and the Concurrent Growth of Black Holes and Bulges (2007, methodology): “…GOODS-N hereafter), and with XMM-Newton(e.g., Hasinger 2004 in the Lockman hole) resolved most of the X-ray background at energies 0.5–6 keV, but a substantial fraction of the higher energy background remains unresolved (Krivonos et al. 2005; Worsley et al. 2005; 2006; Hickox & Markevitch 2007).”
The incidence of obscuration in active galactic nuclei (2013, background): “This class of objects dominates the counts in the lower X-ray energy band, where almost the entire CXRB radiation has been resolved into individual sources (Worsley et al. 2005).”
Infrared Power-Law Galaxies in the Chandra Deep Field-South: Active Galactic Nuclei and Ultraluminous Infrared Galaxies (2006, methodology): “…galaxies at off-axis angles of less than 7A5 and lying farther away INFRARED POWER-LAW GALAXIES IN THE CDF-S 177No. 1, 2006 than 3 times the 90% encircled energy radii of other X-ray sources in the field, following the procedure of S. Immler et al. (2006, in preparation) and Worsley et al. (2005).”
The PalermoSwift-BAT hard X-ray catalogue: II. Results after 39 months of sky survey (2009, background): “…of sources gives the larger contribution: while the CXB at energies lower than 10 keV has been almost entirely resolved (80–90%, Moretti et al. 2003; Worsley et al. 2005, 2006; Brandt & Hasinger 2005), only a∼1.5% of the CXB at higher energies can be associated with resolved sources (Ajello et al.…”
Spitzer Observations of Massive, Red Galaxies at High Redshift (2005, background): “In addition, Worsley et al. (2005) conclude that as much as 40% of the hard X-ray background stems from heavily obscured AGNs at zk 0:5.”
Constraints on sterile neutrinos as dark matter candidates from the diffuse X-ray background (2005, background): “Therefore, in what follows we will assume that such a situation does not occur (in particular, this does not occur in models attributing the existing XRB shape to AGN emissions: Worsley et al. 2005; Treister & Urry 2005).”
THREE-YEAR SWIFT–BAT SURVEY OF ACTIVE GALACTIC NUCLEI: RECONCILING THEORY AND OBSERVATIONS? (2010, background): “…density, these sources contribute an ∼10%–25% fraction of the CXB emission (Gilli et al. 2007; Treister et al. 2009, respectively) at 30 keV, but at the same time are expected to be fairly numerous representing up to ∼30% of the entire AGN population (Risaliti et al. 1999; Worsley et al. 2005).”
Spitzer Power-Law Active Galactic Nucleus Candidates in the Chandra Deep Field-North (2007, background): “…now resolved 70-90% of the cosmic X-ray background (CXRB) at 2-8 keV into discreet sources (Mushotzky et al. 2000; Giacconi et al. 2002; Alexander et al. 2003; Bauer et al. 2004; Worsley et al. 2004, 2005), detecting the majority of the Xray unobscured AGN population in the deep X-ray fields.”
A NEW DIAGNOSTIC OF ACTIVE GALACTIC NUCLEI: REVEALING HIGHLY ABSORBED SYSTEMS AT REDSHIFT >0.3 (2011, methodology): “For the X-ray stacking we adopt a similar approach to Worsley et al. (2005), who stacked sources and calculated the significance of the stacked result using 10,000 Monte Carlo trials.”
THE NUCLEAR SPECTROSCOPIC TELESCOPE ARRAY (NuSTAR) HIGH-ENERGY X-RAY MISSION (2013, background): “These surveys have resolved 70– 90% of the 0.5–10 keV XRB (e.g., Worsley et al. 2005; Hickox & Markevitch 2006; Xue et al. 2012), revealing obscured and unobscured AGNs out to z ∼ 5–6 (e.g., Tozzi et al. 2006; Brusa et al. 2009; see Brandt & Alexander 2010 for a recent review).”
The synthesis of the cosmic X-ray background in the Chandra and XMM-Newton era (2006, background): “The integrated emission from the considered populations reproduces the entire resolved XRB flux (e.g. Worsley et al. 2005) up to 5–6 keV, while in the 6–10 keV range it is slightly above it.”
The evolution of massive black hole seeds (2007, background): “The integrated emission from these Xray quasars generates the cosmic X-ray background (XRB), and its spectrum suggests that most black-hole growth is obscured in optical wavelengths (Fabian & Iwasawa 1999; Mushotzky et al. 2000; Hasinger et al. 2001; Barger et al. 2003, 2005; Worsley et al. 2005).”
Chandra and Spitzer Unveil Heavily Obscured Quasars in the Chandra/SWIRE Survey (2006, background): “Less than 30% is resolved above 10 keV, where the bulk of the CXRB energy density is produced (Worsley et al. 2004, 2005).”
Cosmic X-Ray Background and Earth Albedo Spectra with Swift BAT (2008, background): “However, the fraction of CXB emission resolved into AGNs declines with energy being <50% above 6 keV (Worsley et al. 2005).”