X-ray and multiwavelength studies of Active Galactic Nuclei in the Chandra Deep Fields

With the advent of the newest generation of X-ray space observatories, Chandra and XMM-Newton, X-ray surveys have become the most effective tool to detect Active Galactic Nuclei (AGNs) and explore their physics. The ≈ 2 Ms Chandra Deep Field-North (CDF-N) and Chandra Deep Field-South (CDF-S) surveys (jointly the Chandra Deep Fields) are the two deepest X-ray surveys ever performed. We utilized these unprecedented X-ray data along with the superb multiwavelength coverage in these fields to study AGN properties in the distant universe.;(1) We constrained X-ray outbursts from galactic nuclei with harder spectra, higher redshifts, and lower luminosities than have been studied previously. We performed a systematic survey of 24 668 optical galaxies in the Chandra Deep Fields to search for X-ray outbursts. No outbursts were found, and thus we set tight upper limits on the rate of such events in the Universe. For an outburst with X-ray luminosity ≳ 1043 erg s--1 and a duration of 6 months, the upper limit on its event rate is ~ 10--4 galaxy--1 yr--1, roughly consistent with theoretical predictions. Our results also suggest that the X-ray luminosity function for moderate-luminosity active galactic nuclei is not primarily due to stellar tidal disruptions (Chapter 2). (2) We discovered the most-distant double-peaked emitter, CXOECDFS J033115.0--275518, at z = 1.369. A Keck/DEIMOS spectrum shows a clearly double-peaked broad Mg II lambda2799 emission line, with FWHM ≈ 11 000 km s--1 for the line complex. This is one of a handful of double-peaked emitters known to be a luminous quasar, with excellent multiwavelength coverage and a high-quality X-ray spectrum. The local viscous energy released from the line-emitting region of the accretion disk is probably insufficient to power the observed line flux, and external illumination of the disk appears to be required. The illumination cannot arise from a radiatively inefficient accretion flow as suggested for prototype double-peaked emitters (Chapter 3). (3) We presented point-source catalogs for the ≈ 2 Ms exposure of the CDF-S, including 462 main catalog sources and 116 supplementary catalog sources. The ≈ 2 Ms CDF-S achieves on-axis sensitivity limits of ≈ 1.9 x 10--17 and ≈ 1.3 x 10--16 ≈ cm--2 s--1 for the 0.5--2.0 and 2--8 keV bands, respectively. We performed detailed classification of the main catalog sources. Optical-to-radio multiwavelength identifications were carried out using the likelihood-ratio method, resulting in reliable counterparts for 442 (95.7%) of the X-ray sources, with an expected false-match probability of ≈ 6.2%. High-quality photometric redshifts were calculated, which are the best obtained so far for faint X-ray sources. The median redshift is 1.3 for the CDF-S X-ray sources, and we have discovered ≈ 10 high-redshift (z > 4) AGN candidates. About 80% of the X-ray sources are AGNs, among which ≈ 72% are obscured (Chapters 4 and 5). (4) We utilized and improved the relative infrared star formation rate excess (ISX) selection method to search for heavily obscured and Compton-thick (CT) AGN candidates in the CDF-S at z ≈ 0.5--1. We have discovered 242 ISX sources; an X-ray stacking analysis of 23 of the objects resulted in a very hard X-ray signal with an effective photon index of 0.6+0.3-0.4 , indicating a significant contribution from obscured AGNs. Based on Monte Carlo simulations, we conclude that 74+/-25% of the galaxies selected host obscured AGNs, within which ≈ 94% are heavily obscured and ≈ 79% are CT. The space density of these heavily obscured AGNs is (2.0 &...