type-II Active Galactic Nuclei Research Articles

The Distance of the Narrow FeKalpha Emitting Region for a Sample of Compton-thin Type-II Active Galactic Nuclei

Abstract I have studied the variability of the FeKα emission line flux in response to changes in the 2–10 keV unabsorbed flux in a sample of Compton thin type-II active galactic nuclei. All of the 20 sources in the sample studied by Laha et al., exhibit the presence of a narrow FeKα emission line. Except for one source (NGC 2992), all other sources show no correlations between the FeKα emission line flux and 2–10 keV unabsorbed flux. This implies that although most of the sources have shown significant variations in their 2–10 keV flux, the FeKα emission line did not get enough time to respond to those changes. This helps us put lower limits on the distance of the FeKα emitters to >10 lt-yr from the central engine, equivalent to ∼3 pc which is consistent with the standard torus/narrow-line-region distance. I conclude that the FeKα emission lines of these sources arise from reflection of the hard X-ray photons off neutral materials located beyond pcscale.

The Nature of the Absorbed Soft X-Ray Variations in Compton-thin Type-II Active Galactic Nuclei

Abstract The absorbed soft X-ray (<1 keV) emission from Compton thin type-II active galactic nuclei (AGNs) with neutral absorption columns in the range N H = 1021−23 cm−2 may contain emission from star formation or scattered photons from the primary X-ray emission in Compton-thin circumnuclear gas extending out to ∼1 kpc, or leaky-absorbers along the line of sight allowing a part of the primary AGN emission to manifest in the soft X-rays. In a recent sample study of 20 Compton thin type-II AGN, Laha et al. detected variability in the soft X-ray spectra in 5 sources with timescales <20 years, indicating possibly that the primary AGN emission has leaked through the absorption column. I found that the sources with soft X-ray variability exhibit the lowest absorption column densities (N H ∼ 0.7–1.3 × 1022 cm−2) in the sample.

Galaxies hosting an active galactic nucleus: a view from the CALIFA survey

ABSTRACT We study the presence of optically-selected active galactic nuclei (AGNs) within a sample of 867 galaxies extracted from the extended Calar-Alto Legacy Integral Field spectroscopy Area (eCALIFA) spanning all morphological classes. We identify 10 Type-I and 24 Type-II AGNs, amounting to ∼4 per cent of our sample, similar to the fraction reported by previous explorations in the same redshift range. We compare the integrated properties of the ionized and molecular gas, and stellar population of AGN hosts and their non-active counterparts, combining them with morphological information. The AGN hosts are found in transitory parts (i.e. green-valley) in almost all analysed properties which present bimodal distributions (i.e. a region where reside star-forming galaxies and another with quiescent/retired ones). Regarding morphology, we find AGN hosts among the most massive galaxies, with enhanced central stellar-mass surface density in comparison to the average population at each morphological type. Moreover, their distribution peaks at the Sab-Sb classes and none are found among very late-type galaxies (>Scd). Finally, we inspect how the AGN could act in their hosts regarding the quenching of star-formation. The main role of the AGN in the quenching process appears to be the removal (or heating) of molecular gas, rather than an additional suppression of the already observed decrease of the star-formation efficiency from late-to-early type galaxies.

NuSTARobservations of heavily obscuredSwift/BAT AGNs: Constraints on the Compton-thick AGNs fraction

The evolution of the accretion history of the Universe has been studied in unprecedented detail owing to recent X-ray surveys performed byChandraandXMM-Newton. A focus on the most heavily obscured or Compton-thick active galactic nuclei (AGNs) is missing in these studies. These AGNs evade detection even in X-ray surveys owing to their extreme hydrogen column densities, which exceed 1024 cm−2. Recently, the all-sky hard X-ray survey performed bySwift/BAT brought a breakthrough, allowing the detection of many of these AGNs. This is because of the very high energy bandpass (14–195 keV) of this instrument, which helps to minimise attenuation effects. In our previous work, we identified more than 50 candidate Compton-thick AGNs in the local Universe, corresponding to an observed fraction of about 7% of the total AGNs population. This number can only be converted to the intrinsic Compton-thick AGNs number density if we know their exact selection function. This function sensitively depends on the form of the Compton-thick AGN spectrum, that is the energy of their absorption turnover, photon-index and its cut-off energy at high energies, and the strength of the reflection component on the matter surrounding the nucleus. For example, the reflection component at hard energies 20–40 keV antagonises the number density of missing Compton-thick AGNs in the sense that the stronger the reflection the easier these sources are detected in the BAT band. In order to constrain their number density, we analysed the spectra of 19 Compton-thick AGNs that have been detected withSwift/BAT and have been subsequently observed withNuSTARin the 3–80 keV band. We analysed their X-ray spectra using the MYTORUS models which properly take into account the Compton scattering effects. These were combined with physically motivated Comptonisation models, which accurately describe the primary coronal X-ray emission. We derived absorbing column densities that are consistent with those derived by the previousSwift/BAT analyses. We estimate the coronal temperatures to be roughly between 25 and 80 keV corresponding to high energy cut-offs roughly between 75 and 250 keV. Furthermore, we find that the majority of our AGNs lack a strong reflection component in the 20–40 keV band placing tighter constraints on the intrinsic fraction of Compton-thick AGNs. Combining these results with our X-ray background synthesis models, we estimate a percentage of Compton-thick AGNs in the local Universe of ≈20 ± 3 % relative to the type-II AGNs population.

Starbursting brightest cluster galaxy: a Herschel view of the massive cluster MACS J1931.8−2634

Abstract We investigate the dust-obscured star formation (SF) properties of the massive, X-ray-selected galaxy cluster MACS J1931.8−2634 at z = 0.352. Using far-infrared (FIR) imaging in the range 100–500 μm obtained with the Herschel telescope, we extract 31 sources (2σ) within r ∼ 1 Mpc from the brightest cluster galaxy (BCG). Among these sources, we identify six cluster members for which we perform an analysis of their spectral energy distributions (SEDs). We measure total infrared luminosity (LIR), star formation rate (SFR) and dust temperature. The BCG, with LIR = 1.4 × 1012 L⊙ is an ultraluminous infrared galaxy and hosts a type-II active galactic nuclei (AGN). We decompose its FIR SED into AGN and starburst components and find equal contributions from AGN and starburst. We also recompute the SFR of the BCG finding SFR = 150 ± 15 M⊙ yr−1. We search for an isobaric cooling flow in the cool core using Chandra X-ray data, and find no evidence for gas colder than 1.8 keV in the inner 30 kpc, for an upper limit to the instantaneous mass-deposition rate of 58 M⊙ yr−1 at 95 per cent c.l. This value is 3× lower than the SFR in the BCG, suggesting that the on-going SF episode lasts longer than the intracluster medium cooling events.

Revisiting the Unified Model of Active Galactic Nuclei

This review describes recent developments related to the unified model of active galactic nuclei (AGNs). It focuses on new ideas about the origin and properties of the central obscurer (torus) and the connection to its surroundings. The review does not address radio unification. AGN tori must be clumpy but uncertainties about their properties persist. Today's most promising models involve disk winds of various types and hydrodynamic simulations that link the large-scale galactic disk to the inner accretion flow. Infrared (IR) studies greatly improved our understanding of the spectral energy distribution of AGNs, but they are hindered by various selection effects. X-ray samples are more complete. The dependence of the covering factor of the torus on luminosity is a basic relationship that remains unexplained. There is also much confusion regarding real type-II AGNs, which do not fit into a simple unification scheme. The most impressive recent results are due to IR interferometry, which is not in accord with most torus models, and the accurate mapping of central ionization cones. AGN unification may not apply to merging systems and is possibly restricted to secularly evolving galaxies.

Iron Kα emission in type-I and type-II active galactic nuclei

The narrow Fe K$\alpha$ line is one of the main signatures of the reprocessing of X-ray radiation from the material surrounding supermassive black holes, and it has been found to be omnipresent in the X-ray spectra of active galactic nuclei (AGN). In this work we study the characteristics of the narrow Fe K$\alpha$ line in different types of AGN. Using the results of a large Suzaku study we find that Seyfert 2s have on average lower Fe K$\alpha$ luminosities than Seyfert 1s for the same 10-50 keV continuum luminosity. Simulating dummy Sy1s and Sy2s populations using physical torus models of X-ray reflected emission, we find that this difference can be explained by means of different average inclination angles with respect to the torus, as predicted by the unified model. Alternative explanations include differences in the intensities of Compton humps or in the photon index distributions. We show that the ratio between the flux of the broad and narrow Fe K$\alpha$ line in the 6.35-6.45 keV range depends on the torus geometry considered, and is on average $<25\%$ and $<15\%$ for type I and type II AGN, respectively. We find evidence of absorption of the narrow Fe K$\alpha$ line flux in Compton-thick AGN, which suggests that part of the reflecting material is obscured. We estimate that on average in obscured AGN the reflected radiation from neutral material is seen through a column density which is 1/4 of that absorbing the primary X-ray emission. This should be taken into account in synthesis models of the CXB and when studying the luminosity function of heavily obscured AGN. We detect the first evidence of the X-ray Baldwin effect in Seyfert 2s, with the same slope as that found for Seyfert 1s, which suggests that the mechanism responsible for the decrease of the equivalent width with the continuum luminosity is the same in the two classes of objects.

RADIATION PRESSURE FORCE AND BLACK HOLE MASS DETERMINATION IN LOW-REDSHIFT TYPE-I AND TYPE-II ACTIVE GALACTIC NUCLEI

The distributions of L(oiii 5007), black hole (BH) mass and L/L(Edd) in two large samples of type-I and type-II active galactic nuclei (AGNs) are compared in order to test the suggestion that radiation pressure force is affecting the gas velocity in the broad line region and hence the BH mass determination. The samples are drawn from the SDSS archive and are modified to represent the same parent distribution at 0.1 < z < 0.2. BH masses in type-I sources are calculated in two different ways, one using a simple virial mass assumption and the other by taking into account the effect of radiation pressure force on the gas. The simple virial mass estimate results in good agreement with the sigma*-based BH mass and L/L(Edd) estimates in type-II sources. In contrast, there is a clear disagreement in the L/L(Edd) distributions when radiation pressure-based estimates are used. This indicates that radiation pressure force is not important in 0.1 < z < 0.2 AGNs with L(5100)=10^{42.8-44.8} erg/s. This has important implications to the physics of the gas producing the broad emission lines in AGNs, in particular the existence of extremely large (about 10^24 cm^(-2)) column density clouds.

The correlation of narrow line emission and X-ray luminosity in active galactic nuclei

Aims.We combine emission line and X-ray luminosities for 45 sources from the Chandra Deep Field South (CDF-S), and seven HELLAS sources, to obtain a new sample of 52 X-ray selected type-II active galactic nuclei (AGNs). Eighteen of our sources are very luminous with a typical, absorption-corrected 2–10 keV luminosity of erg s-1 (type-II QSOs).

A Single Intrinsic Luminosity Function for Both Type I and Type II Active Galactic Nuclei

The luminous electromagnetic emission from distant active galactic nuclei (AGNs) including quasars is believed to be powered by accretion onto super-massive black holes (SMBHs). In the standard unification model for AGNs a dusty torus covers a significant portion of the viewing angles to the accretion disk and the BH. The system is classified as a type-I AGN if the accretion disk is viewed through the opening part; otherwise it is called a type-II AGN. Therefore the ratio of type-II to type-I AGNs serves as a sensitive probe to the unification model. A surprising discovery made from several large sky coverage and/or deep AGN surveys has found a significant anti-correlation between the type-II fraction and the observed X-ray luminosity between 2-10 keV. This suggests two different luminosity functions for the two types of AGNs, thus challenging the AGN unification model. However this observed anti-correlation is a natural consequence of the AGN unification model with only one intrinsic luminosity function if the inclination angle effects of the X-ray emitting accretion disk are taken into account. Thus the AGN unification model survived another critical test.

The accretion history of the universe with the SKA

In this paper, we investigate how the Square Kilometre Array (SKA) can aid in determining the evolutionary history of active galactic nuclei (AGN) from redshifts z = 0 → 6. Given the vast collecting area of the SKA, it will be sensitive to both ‘radio-loud’ AGN and the much more abundant ‘radio-quiet’ AGN, namely the radio-quiet quasars and their ‘Type-II’ counterparts, out to the highest redshifts. Not only will the SKA detect these sources but it will also often be able to measure their redshifts via the Hydrogen 21-cm line in emission and/or absorption. We construct a complete radio luminosity function (RLF) for AGN, combining the most recent determinations for powerful radio sources with an estimate of the RLF for radio-quiet objects using the hard X-ray luminosity function of [ApJ 598 (2003) 886], including both Type-I and Type-II AGN. We use this complete RLF to determine the optimal design of the SKA for investigating the accretion history of the Universe for which it is likely to be a uniquely powerful instrument.

Cosmological Evolution of the Hard X-Ray AGN Luminosity Function: Formation History of Supermassive Black Holes

The cosmological evolution of the hard X-ray luminosity function (HXLF) of Active Galactic Nuclei (AGN) is investigated in the 2-10 keV luminosity range of 10 4 1 . 5 - 10 4 6 . 5 erg s - 1 as a function of redshift up to3. From a combination of surveys conducted at photon energies above 2 keV with HEAO1, ASCA and Chandra, we construct a highly complete (>96%) sample consisting of 247 AGNs over the wide flux range of 10 - 1 0 - 3.8 x 10 - 1 5 erg cm - 2 s - 1 (2-10 keV). We find that (i) the fraction of X-ray absorbed AGNs decreases with the intrinsic luminosity and (ii) the evolution of the HXLF of the whole AGNs (including both type-I and type-II AGNs) is best described with a luminosity dependent density evolution where the cutoff redshift increases with the luminosity. Our results directly constrain the evolution of AGNs that produce a major part of the hard X-ray background, thus solving its origin quantitatively. Based on these results, we discuss the formation history of supermassive black holes in galactic centers. The total accreted mass traced by the HXLF with an accretion mass-to-energy conversion factor of 0.1 is consistent with an estimate of SMBH mass density from the demography of galaxies in nearby universe.

X-ray lines as diagnostics of starburst and accretion processes

The X-ray spectra of active galactic nuclei (AGN) and starburst galaxies are characterized by strong emission lines and a large amount of obscuration. The lines in type-II AGN suggest that there are at least two components of highly ionized gas possibly at different locations. Similar X-ray lines are also observed in several ultra-luminous infrared galaxies (ULIRGs), that are very luminous infrared sources powered, most probably, by violent starbursts in colliding galaxies. This raises the question of what distinguish AGN excited gas from hot gas in star-forming regions.X-ray emission lines are the best diagnostics of the hot gas in the centers of starburst galaxies and ULIRGs. High spectral resolution observations, by the coming new X-ray missions, will enable us to distinguish collisionally-excited gas from photoionized gas. They may thus provide the first solid evidence for accretion onto massive black holes in ULIRGs.