The Chandra deep field north survey. VI. The nature of the optically faint X-ray source population

D. M. Alexander, William Nielsen Brandt, A. E. Hornschemeier, G. P. Garmire, Donald P. Schneider, F. E. Bauer, R. E. Griffiths

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Abstract

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 +400-350 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+14-10%) 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 x 1044) 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+24-17% 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., f850μm > 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.

Original languageEnglish (US)
Pages (from-to)2156-2176
Number of pages21
JournalAstronomical Journal
Volume122
Issue number5
DOIs
StatePublished - Nov 1 2001

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x rays
active galactic nuclei
erg
radio
background radiation
stacking
star formation
luminosity
energy
spectral energy distribution
quasars
confidence
wavelength
estimates
wavelengths

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

Alexander, D. M. ; Brandt, William Nielsen ; Hornschemeier, A. E. ; Garmire, G. P. ; Schneider, Donald P. ; Bauer, F. E. ; Griffiths, R. E. / The Chandra deep field north survey. VI. The nature of the optically faint X-ray source population. In: Astronomical Journal. 2001 ; Vol. 122, No. 5. pp. 2156-2176.
@article{c4b1237f4f6740ed9244b14ca584533f,
title = "The Chandra deep field north survey. VI. The nature of the optically faint X-ray source population",
abstract = "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 +400-350 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+14-10{\%}) 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 x 1044) 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+24-17{\%} 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., f850μm > 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.",
author = "Alexander, {D. M.} and Brandt, {William Nielsen} and Hornschemeier, {A. E.} and Garmire, {G. P.} and Schneider, {Donald P.} and Bauer, {F. E.} and Griffiths, {R. E.}",
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Alexander, DM, Brandt, WN, Hornschemeier, AE, Garmire, GP, Schneider, DP, Bauer, FE & Griffiths, RE 2001, 'The Chandra deep field north survey. VI. The nature of the optically faint X-ray source population', Astronomical Journal, vol. 122, no. 5, pp. 2156-2176. https://doi.org/10.1086/323540

The Chandra deep field north survey. VI. The nature of the optically faint X-ray source population. / Alexander, D. M.; Brandt, William Nielsen; Hornschemeier, A. E.; Garmire, G. P.; Schneider, Donald P.; Bauer, F. E.; Griffiths, R. E.

In: Astronomical Journal, Vol. 122, No. 5, 01.11.2001, p. 2156-2176.

Research output: Contribution to journalArticle

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T1 - The Chandra deep field north survey. VI. The nature of the optically faint X-ray source population

AU - Alexander, D. M.

AU - Brandt, William Nielsen

AU - Hornschemeier, A. E.

AU - Garmire, G. P.

AU - Schneider, Donald P.

AU - Bauer, F. E.

AU - Griffiths, R. E.

PY - 2001/11/1

Y1 - 2001/11/1

N2 - 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 +400-350 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+14-10%) 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 x 1044) 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+24-17% 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., f850μm > 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.

AB - 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 +400-350 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+14-10%) 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 x 1044) 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+24-17% 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., f850μm > 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.

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