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

We provide constraints on the nature of the optically faint (I greater than or equal to 24) X-ray source population from a 1 Ms Chandra exposure of a 8.'4 x 8.'4 region within the Hawaii flanking-field area containing the Hub ble Deep Field North region. We detect 47 (2400(-350)(+400) deg(-2)) optica lly faint sources down to 0.5- 2.0 keV and 2.0-8.0 keV fluxes of approximat e to3 x 10(-17) ergs cm(-2) s(-1) and approximate to2 x 10(-16) ergs cm(-2) s(-1), respectively; these sources contribute approximate to 14% and appro ximate to 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 approximat ely constant (at approximate to 35%) for 0.5-8.0 keV fluxes from 3 x 10(-14 ) ergs cm(-2) s(-1) down to the X-ray flux limit. A considerable fraction ( 30(-10)(+14)%) of the optically faint X-ray sources are Very Red Objects (I -K greater than or equal to 4). Analysis of the optical and X-ray propertie s suggests a large number of optically faint X-ray sources are likely to ho st obscured active galactic nucleus (AGN) activity at z = 1-3. From these r esults we calculate that a significant fraction (approximate to5%-45%) of t he optically faint X-ray source population could be obscured QSOs (rest-fra me unabsorbed 0.5-8.0 keV luminosity > 3 x 10(44) ergs s(-1)) at z less tha n or equal to 3. Given the number of X-ray sources without I-band counterpa rts, there are unlikely to be more than approximate to 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 sour ces. Nine optically faint X-ray sources have mu Jy radio counterparts; appr oximate to 53(-17)(+24)%of the optically faint mu Jy radio sources in this region. The most likely origin of the X-ray emission in these X-ray detecte d, optically faint mu Jy radio sources is obscured AGN activity. However, t wo of these sources have been previously detected at submillimeter waveleng ths, and the X-ray emission from these sources could be due to luminous sta r formation activity. Assuming the spectral energy distribution of NGC 6240 , we estimate the 175 mum flux of a typical optically faint X-ray source to be less than 10 mJy; however, those sources with detectable submillimeter counterparts (i.e., f(850 mum) > 3 mJy) could be substantially brighter. He nce, most optically faint X-ray sources are unlikely to contribute signific antly to the far-IR (140-240 mum) background radiation. However, as expecte d for sources with AGN activity, the two optically faint X-ray sources with in the most sensitive area of the ISOCAM HDF-N region have faint (less than or similar to 50 mu Jy) 15 mum counterparts. We also provide constraints on the average X-ray properties of classes of o ptically faint sources not individually detected at X-ray energies. Stackin g analyses of optically faint mu Jy radio sources not individually detected with X-ray emission yields a possible detection (at 98.3% confidence) in t he 0.5-2.0 keV band; this X-ray emission could be produced by star formatio n 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 wit h stacking analyses, showing that these sources have low X-ray luminosities if they are indeed AGNs.