The Chandra Deep Field North Survey. VI. The Nature of the Optically Faint X-Ray Source Population

By D. Alexander, W. N. Brandt, A. Hornschemeier, G. Garmire, D. P. Schneider, F. E. Bauer, R. Griffiths. 2001

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We provide constraints on the nature of the optically faint (I ≥ 24) X-ray source population from a 1 Ms Chandra exposure of a 8.′4 × 8.′4 region within the Hawaii flanking-field area containing the Hubble Deep Field North region. We detect 47 (2400 deg-2) optically faint sources down to 0.5–2.0 keV and 2.0–8.0 keV fluxes of ≈3 × 10-17 ergs cm-2 s-1 and ≈2 × 10-16 ergs cm-2 s-1, respectively; these sources contribute ≈14% and ≈21% of the 0.5–2.0 keV and 2.0–8.0 keV X-ray background radiation, respectively. The fraction of optically faint X-ray sources is approximately constant (at ≈35%) for 0.5–8.0 keV fluxes from 3 × 10-14 ergs cm-2 s-1 down to the X-ray flux limit. A considerable fraction (30%) of the optically faint X-ray sources are Very Red Objects (I-K ≥ 4). Analysis of the optical and X-ray properties suggests a large number of optically faint X-ray sources are likely to host obscured active galactic nucleus (AGN) activity at z = 1–3. From these results we calculate that a significant fraction (≈5%–45%) of the optically faint X-ray source population could be obscured QSOs (rest-frame unabsorbed 0.5–8.0 keV luminosity >3 × 1044 ergs s-1) at z ≤ 3. Given the number of X-ray sources without I-band counterparts, there are unlikely to be more than ≈15 sources at z > 6. We provide evidence that the true number of z > 6 sources is considerably lower. We investigate the multiwavelength properties of optically faint X-ray sources. Nine optically faint X-ray sources have μJy radio counterparts; ≈ 53% of the optically faint μJy radio sources in this region. The most likely origin of the X-ray emission in these X-ray detected, optically faint μJy radio sources is obscured AGN activity. However, two of these sources have been previously detected at submillimeter wavelengths, and the X-ray emission from these sources could be due to luminous star formation activity. Assuming the spectral energy distribution of NGC 6240, we estimate the 175 μm flux of a typical optically faint X-ray source to be less than 10 mJy; however, those sources with detectable submillimeter counterparts (i.e., f850 μm > 3 mJy) could be substantially brighter. Hence, most optically faint X-ray sources are unlikely to contribute significantly to the far-IR (140–240 μm) background radiation. However, as expected for sources with AGN activity, the two optically faint X-ray sources within the most sensitive area of the ISOCAM HDF-N region have faint (≲50 μJy) 15 μm counterparts. We also provide constraints on the average X-ray properties of classes of optically faint sources not individually detected at X-ray energies. Stacking analyses of optically faint μJy radio sources not individually detected with X-ray emission yields a possible detection (at 98.3% confidence) in the 0.5–2.0 keV band; this X-ray emission could be produced by star formation activity at z = 1–3. None of the optically faint AGN-candidate sources in the HDF-N itself are detected at X-ray energies either individually or with stacking analyses, showing that these sources have low X-ray luminosities if they are indeed AGNs.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

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Citations in the corpus

THE CHANDRA COSMOS SURVEY. III. OPTICAL AND INFRARED IDENTIFICATION OF X-RAY POINT SOURCES (2012, methodology): “With the advent ofChandraand XMM-Newton, the same relation has been used in harder bands (Hornschemeier et al. 2001, Alexander et al. 2001, Fiore et al. 2003, Della Ceca et al. 2004, Cocchia et al. 2007).”
Bayesian Estimation of Hardness Ratios: Modeling and Computations (2006, background): “Because these faint sources have only a few counts, hardness ratios are commonly used to study properties of their X-ray emission and absorption (e.g., Alexander et al. 2001, Brandt et al. 2001a, Brandt et al. 2001b, Giacconi et al. 2001, Silverman et al. 2005).”
What drives the growth of black holes (2011, background): “…photometric data; e.g., Barger et al. 2003b; Szokoly et al. 2004; Zheng et al. 2004; Cardamone et al. 2010; Luo et al. 2010; Salvato et al. 2011), since the optical/near-IR counterparts for many of the AGNs are very faint (e.g., Alexander et al., 2001; Mainieri et al., 2005b; Rovilos et al., 2010).”
Chandra Deep Field South: The 1 Ms Catalog (2002, result): “This population of optically faint sources (R > 25) is consistent with a mix of obscured AGN at z ¼ 1 3 and evolved, high-z galaxies (see Alexander et al. 2001; Cowie et al. 2001).”
GALAXY ZOO: THE FUNDAMENTALLY DIFFERENT CO-EVOLUTION OF SUPERMASSIVE BLACK HOLES AND THEIR EARLY- AND LATE-TYPE HOST GALAXIES (2010, methodology): “…account for the the observed peak at z ∼ 0.7 in the number density of moderate-luminosity X-ray selected AGN at z ∼ 0.7, while the mode at work in late-types is responsible for the high redshift peak of luminous AGN (Giacconi et al. 2001; Alexander et al. 2001; Ueda et al. 2003; Gilli et al. 2005).”
Spectral Energy Distributions of Hard X-Ray Selected Active Galactic Nuclei in the XMM-Newton Medium Deep Survey (2007, unknown): “As previously found (see, e.g., Alexander et al. 2001a, 2001b, 2002; Fadda et al. 2002; Perola et al. 2004; Szokoly et al. 2004; Franceschini et al. 2005; Polletta et al. 2006), the multiwavelength SEDs of X-rayYselected AGNs show a wide variety of properties.”
DEEP EXTRAGALACTIC X-RAY SURVEYS (2005, background): “…obscured AGN at z ≈ 1–4 when their X-ray properties, optical photometric properties (including photometric redshifts), and multiwavelength properties are considered (e.g., Alexander et al. 2001; Barger et al. 2003a; Fiore et al. 2003; Treister et al. 2004; Zheng et al. 2004; Mainieri et al. 2005).”
The Fall of Active Galactic Nuclei and the Rise of Star-forming Galaxies: A Close Look at the Chandra Deep Field X-Ray Number Counts (2004, methodology): “We note that detailed studies of optically faint X-ray sources indicate that they often have hard X-ray colors typical of obscured AGNs and red optical colors typical of early-type galaxies at z 1–3 (e.g., Alexander et al. 2001; Koekemoer et al. 2004; Treister et al. 2004).”
The Chandra Deep Field North Survey. XIII. 2 Ms Point-Source Catalogs (2003, background): “Following x 3.3 of Alexander et al. (2001) we find an average band ratio of 0.28 0.02, corresponding to an average effective photon index of C = 2.0 0.1.”
The Chandra Deep Field North Survey. XIV. X-Ray-detected Obscured AGNs and Starburst Galaxies in the Bright Submillimeter Source Population (2002, methodology): “This source was also detected in the 1 Ms Chandraexposure at a 10−5 false-positive probability threshold (Alexander et al. 2001) and is probably not detected here at a higher probability threshold since it lies within the bright extended X-ray source CXOHDFN J123620.”
The anti‐hierarchical growth of supermassive black holes (2004, background): “With the deep Chandra and XMM surveys revealing the redshift distribution of the obscured sources that most contribute to the XRB (Alexander et al. 2001; Barger et al. 2002; Mainieri et al. 2002; Rosati et al. 2002; Hasinger 2003; Fiore et al. 2003), the newest synthesis models seem to suggest that…”
The XMM-Newton Wide-Field Survey in the COSMOS Field. III. Optical Identification and Multiwavelength Properties of a Large Sample of X-Ray-Selected Sources (2006, background): “…between optical obscuration of the nucleus as inferred from ACS (extended morphology), optical to near infrared colors, and the presence of X–ray obscuration as inferred from the hardness ratio (see also Alexander et al. 2001; Giacconi et al. 2002; Brusa et al. 2005; Mainieri et al. 2007).”
THE CHANDRA COSMOS LEGACY SURVEY: OPTICAL/IR IDENTIFICATIONS (2015, background): “Chandra and XMM-Newton studies extended this relation to harder bands (Hornschemeier et al. 2001, 2-8 keV; Alexander et al. 2001, 2-8 keV; Giacconi et al. 2002, 0.5-10 keV; Fiore et al. 2003, 2-10 keV; Della Ceca et al. 2004, 4.5-7.5 keV; Cocchia et al. 2007, 2-10 keV).”
The X-Ray Spectral Properties of SCUBA Galaxies (2005, background): “Further z > 1 obscured AGNs are likely to have been detected in X-ray surveys but lack spectroscopic redshifts due to the faintness of their optical counterparts (e.g., Alexander et al. 2001; Barger et al. 2002; Fiore et al. 2003; Treister et al. 2004; Mainieri et al. 2005b).”
THE XMM-NEWTON WIDE-FIELD SURVEY IN THE COSMOS FIELD (XMM-COSMOS): DEMOGRAPHY AND MULTIWAVELENGTH PROPERTIES OF OBSCURED AND UNOBSCURED LUMINOUS ACTIVE GALACTIC NUCLEI (2010, background): “Since the first source identification campaigns of hard X–rays surveys, a class of sources with high (> 10) X-ray-to-optical flux ratio (X/O6; Hornschemeier et al. 2001, Alexander et al. 2001; Giacconi et al. 2001; F03; Koekemoer et al. 2004; Eckart et al. 2006) has been found.”
THE CHANDRA DEEP FIELD-SOUTH SURVEY: 4 Ms SOURCE CATALOGS (2008, background): “However, as pointed out by Alexander et al. (2001), using a more stringent sourcedetection threshold (e.g., 10−6, 10−7, or 10−8) can lose an appreciable number of real sources.”
The HELLAS2XMM survey IV. Optical identifications and the evolution of the accretion luminosity in the Universe (2003, background): “4, while typical PG QSO and Seyfert 1 galaxies have X/O ∼ 1, see also Alexander et al. 2001).”
Cosmic X-ray surveys of distant active galaxies (2015, unknown): “E; e.g., Alexander et al 2001, 2002; Barger et al 2001b, 2002; Cowie et al 2001; Severgnini et al 2003; Gandhi et al 2004).”
The HELLAS2XMM Survey. II. Multiwavelength Observations of P3: An X-Ray-bright, Optically Inactive Galaxy (2002, background): “Such an object would not belong to the optically faint X– ray source population investigated by Alexander et al. (2001), but rather to the class of Extremely Red Objects (EROs; R–”