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Durham University

Department of Physics

Staff profile

Publication details for Prof David Alexander

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

Author(s) from Durham


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.