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Register a new userObscuration-dependent evolution of Active Galactic Nuclei
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We aim to constrain the evolution of AGN as a function of obscuration using an X-ray selected sample of $sim2000$ AGN from a multi-tiered survey including the CDFS, AEGIS-XD, COSMOS and XMM-XXL fields. The spectra of individual X-ray sources are analysed using a Bayesian methodology with a physically realistic model to infer the posterior distribution of the hydrogen column density and intrinsic X-ray luminosity. We develop a novel non-parametric method which allows us to robustly infer the distribution of the AGN population in X-ray luminosity, redshift and obscuring column density, relying only on minimal smoothness assumptions. Our analysis properly incorporates uncertainties from low count spectra, photometric redshift measurements, association incompleteness and the limited sample size. We find that obscured AGN with $N_{H}>{rm 10^{22}, cm^{-2}}$ account for ${77}^{+4}_{-5}%$ of the number density and luminosity density of the accretion SMBH population with $L_{{rm X}}>10^{43}text{ erg/s}$, averaged over cosmic time. Compton-thick AGN account for approximately half the number and luminosity density of the obscured population, and ${38}^{+8}_{-7}%$ of the total. We also find evidence that the evolution is obscuration-dependent, with the strongest evolution around $N_{H}thickapprox10^{23}text{ cm}^{-2}$. We highlight this by measuring the obscured fraction in Compton-thin AGN, which increases towards $zsim3$, where it is $25%$ higher than the local value. In contrast the fraction of Compton-thick AGN is consistent with being constant at $approx35%$, independent of redshift and accretion luminosity. We discuss our findings in the context of existing models and conclude that the observed evolution is to first order a side-effect of anti-hierarchical growth.
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We study the incidence of nuclear obscuration on a complete sample of 1310 AGN selected on the basis of their rest-frame 2-10 keV X-ray flux from the XMM-COSMOS survey, in the redshift range 0.3<z<3.5. We classify the AGN as obscured or un-obscured on the basis of either the optical spectral properties and the overall SED or the shape of the X-ray spectrum. The two classifications agree in about 70% of the objects, and the remaining 30% can be further subdivided into two distinct classes: at low luminosities X-ray un-obscured AGN do not always show signs of broad lines or blue/UV continuum emission in their optical spectra, most likely due to galaxy dilution effects; at high luminosities broad line AGN may have absorbed X-ray spectra, which hints at an increased incidence of small-scale (sub-parsec) dust-free obscuration. We confirm that the fraction of obscured AGN is a decreasing function of the intrinsic X-ray luminosity, while the incidence of absorption shows significant evolution only for the most luminous AGN, which appear to be more commonly obscured at higher redshift. We find no significant difference between the mean stellar masses and star formation rates of obscured and un-obscured AGN hosts. We conclude that the physical state of the medium responsible for obscuration in AGN is complex, and mainly determined by the radiation environment (nuclear luminosity) in a small region enclosed within the gravitational sphere of influence of the central black hole, but is largely insensitive to the wider scale galactic conditions.
Observations give strong support for the unification scheme of active galactic nuclei. The scheme is premised on toroidal obscuration of the central engine by dusty clouds that are individually very optically thick. These lectures summarize the torus properties, describe the handling and implications of its clumpy nature and present speculations about its dynamic origin.
We study accretion environments of active galactic nuclei when a super-massive black hole wanders in a circum-nuclear region and passes through an interstellar medium there. It is expected that a Bondi-Hoyle-Lyttleton type accretion of the interstellar matter takes place and an accretion stream of matter trapped by the black hole gravitational field appears from a tail shock region. Since the trapped matter is likely to have a certain amount of specific angular momentum, the accretion stream eventually forms an accretion ring around the black hole. According to the recent study, the accretion ring consists of a thick envelope and a thin core, and angular momenta are transfered from the inner side facing to the black hole to the opposite side respectively in the envelope and the core. As a result, a thick accretion flow and a thick excretion flow extend from the envelope, and a thin accretion disk and a thin excretion disk do from the core. The thin excretion disk is predicted to terminate at some distance forming an excretion ring, while the thick excretion flow is considered to become a super-sonic wind flowing to the infinity. The thick excretion flow from the accretion ring is expected to interact with the accretion stream toward the accretion ring and to be collimated to bi-polar cones. These pictures provide a likely guide line to interpret the overall accretion environments suggested from observations.
X-ray variation is a ubiquitous feature of active galactic nuclei (AGNs), however, its origin is not well understood. In this paper, we show that the X-ray flux variations in some AGNs, and correspondingly the power spectral densities (PSDs) of the variations, may be interpreted as being caused by absorptions of eclipsing clouds or clumps in the broad line region (BLR) and the dusty torus. By performing Monte-Carlo simulations for a number of plausible cloud models, we systematically investigate the statistics of the X-ray variations resulting from the cloud eclipsing and the PSDs of the variations. For these models, we show that the number of eclipsing events can be significant and the absorption column densities due to those eclipsing clouds can be in the range from 10^{21} to 10^{24} cm^{-2}, leading to significant X-ray variations. We find that the PSDs obtained from the mock observations for the X-ray flux and the absorption column density resulting from these models can be described by a broken double power law, similar to those directly measured from observations of some AGNs. The shape of the PSDs depend strongly on the kinematic structures and the intrinsic properties of the clouds in AGNs. We demonstrate that the X-ray eclipsing model can naturally lead to a strong correlation between the break frequencies (and correspondingly the break timescales) of the PSDs and the masses of the massive black holes (MBHs) in the model AGNs, which can be well consistent with the one obtained from observations. Future studies of the PSDs of the AGN X-ray (and possibly also the optical-UV) flux and column density variations may provide a powerful tool to constrain the structure of the BLR and the torus and to estimate the MBH masses in AGNs.
Jets associated with Active Galactic Nuclei (AGN) have been observed for almost a century, initially at optical and radio wavelengths. They are now widely accepted as exhausts produced electromagnetically by the central, spinning, massive black hole and its orbiting, accreting gas. Observations at X-ray and, especially, gamma-ray energies have transformed our understanding of how these jets evolve dynamically, accelerate electrons (and positrons) and radiate throughout the entire electromagnetic spectrum. Some new approaches to modeling the powerful and rapidly variable TeV emission observed from many blazars are sketched. Observations at the highest TeV energies, to which the High Altitude Water Cherenkov Gamma-Ray Observatory (HAWC) will contribute, promise crucial discrimination between rival models of AGN jets.
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