X-ray and Multiwavelength Studies of Active Galactic Nuclei in the Chandra Deep Fields

Open Access
Luo, Bin
Graduate Program:
Astronomy and Astrophysics
Doctor of Philosophy
Document Type:
Date of Defense:
August 13, 2010
Committee Members:
  • Niel Brandt, Dissertation Advisor
  • W Nielsen Brandt, Committee Chair
  • Jane Camilla Charlton, Committee Member
  • George Pavlov, Committee Member
  • Donald P Schneider, Committee Member
  • Tyce De Young, Committee Member
  • galaxies: photometry
  • galaxies: distances
  • galaxies: active
  • cosmology: observations
  • surveys
  • X-rays: galaxies
With the advent of the newest generation of X-ray space observatories, chandra and {it XMM-Newton}, X-ray surveys have become the most effective tool to detect Active Galactic Nuclei (AGNs) and explore their physics. The $approx2$~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 hbox{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 hbox{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 hbox{X-ray} luminosity $ga 10^{43}$~erg~${ m s}^{-1}$ and a duration of 6 months, the upper limit on its event rate is $sim 10^{-4}$~galaxy$^{-1}$~yr$^{-1}$, roughly consistent with theoretical predictions. Our results also suggest that the hbox{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 ion{Mg}{ii} $lambda2799$ emission line,with ${ m FWHM}approx11,000$~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 emit ters (Chapter 3). (3) We presented point-source catalogs for the $approx2$~Ms exposure of the CDF-S, including 462 main catalog sources and 116 supplementary catalog sources. The $approx2$~Ms CDF-S achieves on-axis sensitivity limits of $approx1.9 imes10^{-17}$ and $approx1.3 imes10^{-16}$ flux for the hbox{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 hbox{X-ray} sources, with an expected false-match probability of $approx6.2\%$. 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 $approx10$ hbox{high-redshift} ($z>4$) AGN candidates. About 80\% of the X-ray sources are AGNs, among which $approx72\%$ 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 $zapprox0.5$--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 $0.6_{-0.4}^{+0.3}$, indicating a significant contribution from obscured AGNs. Based on Monte Carlo simulations,we conclude that $74pm25\%$ of the galaxies selected host obscured AGNs, within which $approx94\%$ are heavily obscured and $approx79\%$ are CT. The space density of these heavily obscured AGNs is $(2.0pm0.7) imes10^{-4}$~Mpc$^{-3}$, much higher than those from previous discoveries. These heavily obscured objects contribute $approx 3.1\%$ of the total cosmic X-ray background (XRB) flux in the 10--30 keV band, which accounts for $approx10$--30\% of the unresolved XRB in this energy band according to population synthesis models (Chapter 6).