The Chandra Deep Field North Survey. VI. The nature of the optically faint X-ray source population

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

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 84 × 84 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 0.52.0 keV and 2.08.0 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 0.52.0 keV and 2.08.0 keV X-ray background radiation, respectively. The fraction of optically faint X-ray sources is approximately constant (at 35%) for 0.58.0 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 = 13. 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 0.58.0 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 Jy radio counterparts; 53% of the optically faint Jy radio sources in this region. The most likely origin of the X-ray emission in these X-ray detected, optically faint Jy 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 m 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 m > 3 mJy) could be substantially brighter. Hence, most optically faint X-ray sources are unlikely to contribute significantly to the far-IR (140240 m) 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 Jy) 15 m 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 Jy radio sources not individually detected with X-ray emission yields a possible detection (at 98.3% confidence) in the 0.52.0 keV band; this X-ray emission could be produced by star formation activity at z = 13. 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.

Citation

Alexander, D., Brandt, W., Hornschemeier, A., Garmire, G., Schneider, D., Bauer, F., & Griffiths, R. (2001). The Chandra Deep Field North Survey. VI. The nature of the optically faint X-ray source population. Astronomical Journal, 122(5), 2156-2176. https://doi.org/10.1086/323540

• Journal Article Type: Article
• Publication Date: Nov 1, 2001
• Deposit Date: May 12, 2008
• Journal: Astronomical Journal
• Print ISSN: 0004-6256
• Electronic ISSN: 1538-3881
• Publisher: IOP Publishing
• Peer Reviewed: Peer Reviewed
• Volume: 122
• Issue: 5
• Pages: 2156-2176
• DOI: https://doi.org/10.1086/323540
• Keywords: Cosmology, Observations, Galaxies, Active, Nuclei, X-rays, Active galacti nuclei, Infrared galaxy NGC-6240, Isolated neutron stars, Large sky survey, Redshift, Radio-emission, Lockman field, Dust, Quasars, Spectra.
• Public URL: https://durham-repository.worktribe.com/output/1546844
• Publisher URL: http://www.journals.uchicago.edu/AJ/journal/issues/v122n5/201184/brief/201184.abstract.html