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Register a new userN-body simulation of a clumpy torus: application to active galactic nuclei
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The gravitational properties of a torus are investigated. It is shown that a torus can be formed from test particles orbiting in the gravitational field of a central mass. In this case, a toroidal distribution is achieved because of the significant spread of inclinations and eccentricities of the orbits. To investigate the self-gravity of the torus we consider the $N$-body problem for a torus located in the gravitational field of a central mass. It is shown that in the equilibrium state the cross-section of the torus is oval with a Gaussian density distribution. The dependence of the obscuring efficiency on torus inclination is found.
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Recent results have suggested that active galactic nuclei (AGN) could provide enough photons to reionise the Universe. We assess the viability of this scenario using a semi-numerical framework for modeling reionisation, to which we add a quasar contribution by constructing a Quasar Halo Occupation Distribution (QHOD) based on Giallongo et al. observations. Assuming a constant QHOD, we find that an AGN-only model cannot simultaneously match observations of the optical depth $tau_e$, neutral fraction, and ionising emissivity. Such a model predicts $tau_e$ too low by $sim 2sigma$ relative to Planck constraints, and reionises the Universe at $zlesssim 5$. Arbitrarily increasing the AGN emissivity to match these results yields a strong mismatch with the observed ionising emissivity at $zsim 5$. If we instead assume a redshift-independent AGN luminosity function yielding an emissivity evolution like that assumed in Madau & Haardt model, then we can match $tau_e$ albeit with late reionisation, however such evolution is inconsistent with observations at $zsim 4-6$ and poorly motivated physically. These results arise because AGN are more biased towards massive halos than typical reionising galaxies, resulting in stronger clustering and later formation times. AGN-dominated models produce larger ionising bubbles that are reflected in $simtimes 2$ more 21cm power on all scales. A model with equal parts galaxies and AGN contribution is still (barely) consistent with observations, but could be distinguished using next-generation 21cm experiments HERA and SKA-low. We conclude that, even with recent claims of more faint AGN than previously thought, AGN are highly unlikely to dominate the ionising photon budget for reionisation.
Dedicated searches generally find a decreasing fraction of obscured Active Galactic Nuclei (AGN) with increasing AGN luminosity. This has often been interpreted as evidence for a decrease of the covering factor of the AGN torus with increasing luminosity, the so-called receding torus models. Using a complete flux-limited X-ray selected sample of 199 AGN, from the Bright Ultra-hard XMM-Newton Survey, we determine the intrinsic fraction of optical type-2 AGN at 0.05$leq$z$leq$1 as a function of rest-frame 2-10 keV X-ray luminosity from 10$^{42}$ to 10$^{45}$ erg/s. We use the distributions of covering factors of AGN tori derived from CLUMPY torus models. Since these distributions combined over the total AGN population need to match the intrinsic type-2 AGN fraction, we reveal a population of X-ray undetected objects with high-covering factor tori, which are increasingly numerous at higher AGN luminosities. When these missing objects are included, we find that Compton-thick AGN account at most for 37$_{-10}^{+9}$% of the total population. The intrinsic type-2 AGN fraction is 58$pm$4% and has a weak, non-significant (less than 2$sigma$) luminosity dependence. This contradicts the results generally reported by AGN surveys, and the expectations from receding torus models. Our findings imply that the majority of luminous rapidly-accreting supermassive black holes at z<1 reside in highly-obscured nuclear environments but most of them are so deeply embedded that they have so far escaped detection in X-rays in <10 keV wide-area surveys.
We construct an X-ray spectral model from the clumpy torus in an active galactic nucleus (AGN), designated as XCLUMPY, utilizing the Monte Carlo simulation for Astrophysics and Cosmology framework (MONACO: Odaka et al. 2011, 2016). The adopted geometry of the torus is the same as that in Nenkova et al. (2008), who assume a power law distribution of clumps in the radial direction and a normal distribution in the elevation direction. We investigate the dependence of the X-ray continuum and Fe K$alpha$ fluorescence line profile on the torus parameters. Our model is compared with other torus models: MYTorus model (Murphy & Yaqoob 2009), Ikeda model (Ikeda et al. 2009), and CTorus model (Liu & Li 2014). As an example, we also present the results applied to the broadband X-ray spectra of the Circinus galaxy observed with XMM-Newton, Suzaku, and NuSTAR. Our model can well reproduce the data, yielding a hydrogen column density along the equatorial plane $N_{mathrm{H}}^{mathrm{Equ}} = 9.08_{-0.08}^{+0.14} times 10^{24}$ cm$^{-2}$, a torus angular width $sigma = 14.7_{-0.39}^{+0.44}$ degree, and a 2--10 keV luminosity $log L_{2-10}/mathrm{erg s^{-1}} = 42.8$. These results are discussed in comparison with the observations in other wavelengths.
The central engines of Active Galactic Nuclei (AGNs) are powered by accreting supermassive black holes, and while AGNs are known to play an important role in galaxy evolution, the key physical processes occur on scales that are too small to be resolved spatially (aside from a few exceptional cases). Reverberation mapping is a powerful technique that overcomes this limitation by using echoes of light to determine the geometry and kinematics of the central regions. Variable ionizing radiation from close to the black hole drives correlated variability in surrounding gas/dust, but with a time delay due to the light travel time between the regions, allowing reverberation mapping to effectively replace spatial resolution with time resolution. Reverberation mapping is used to measure black hole masses and to probe the innermost X-ray emitting region, the UV/optical accretion disk, the broad emission line region and the dusty torus. In this article we provide an overview of the technique and its varied applications.
This whitepaper describes how the VLASS could be designed in a manner to allow the identification of candidate dual active galactic nuclei (AGN) at separations <7 kpc. Dual AGN represent a clear marker of two supermassive black holes within an ongoing merger. A dual AGN survey will provide a wealth of studies in structure growth and gravitational-wave science. Radio wavelengths are ideal for identifying close pairs, as disturbed stellar and gaseous material can obscure their presence in optical and shorter wavelengths. With sufficiently high resolution and sensitivity, a large-scale radio imaging survey like the VLASS will uncover many of these systems and provide the means to broadly study the radio properties of candidate dual systems revealed at other wavelengths. We determine that the ideal survey for our purposes will be at as high a resolution as possible, with significantly more science return in A array at L-band or higher, or B array at C-band or higher. We describe a range of potential survey parameters within this document. Based on the analysis outlined in this whitepaper, our ideal survey would create a catalogue of $gtrsim$100 dual AGN in either: 1) a medium-sensitivity (~1 mJy detection threshold), wide-field (few thousand square degree) survey, or 2) a high-sensitivity (~10 uJy threshold) survey of several hundred square degrees.
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