High Active Galactic Nuclei Research Articles

REST-FRAME OPTICAL EMISSION LINES IN FAR-INFRARED-SELECTED GALAXIES AT z < 1.7 FROM THE FMOS-COSMOS SURVEY

We have used FMOS on Subaru to obtain near-infrared spectroscopy of 123 far-infrared-selected galaxies in COSMOS and the key rest-frame optical emission lines. This is the largest sample of infrared galaxies with near-infrared spectroscopy at these redshifts. The far-infrared selection results in a sample of galaxies that are massive systems that span a range of metallicities in comparison with previous optically selected surveys, and thus has a higher active galactic nucleus (AGN) fraction and better samples the AGN branch. We establish the presence of AGNs and starbursts in this sample of (U)LIRGs selected as Herschel-PACS and Spitzer-MIPS detections in two redshift bins (z ~ 0.7 and z ~ 1.5) and test the redshift dependence of diagnostics used to separate AGNs from star formation dominated galaxies. In addition, we construct a low-redshift (z ~ 0.1) comparison sample of infrared-selected galaxies and find that the evolution from z ~ 1.5 to today is consistent with an evolving AGN selection line and a range of ISM conditions and metallicities from the models of Kewley et al. We find that a large fraction of (U)LIRGs are BPT-selected AGNs using their new redshift-dependent classification line. We compare the position of known X-ray-detected AGNs (67 in total) with the BPT selection and find that the new classification line accurately selects most of these objects (>70%). Furthermore, we identify 35 new (likely obscured) AGNs not selected as such by their X-ray emission. Our results have direct implications for AGN selection at higher redshift with either current (MOSFIRE, KMOS) or future (PFS, MOONS) spectroscopic efforts with near-infrared spectral coverage.

SUZAKUOBSERVATIONS OF THE TYPE 2 QSO IN THE CENTRAL GALAXY OF THE PHOENIX CLUSTER

We report the \Suzaku/XIS and HXD and \Chandra/ACIS-I results on the X-ray spectra of the Phoenix cluster at the redshift $z=0.596$. The spectrum of the intracluster medium (ICM) is well-reproduced with the emissions from a low temperature ($\sim3.0$\,keV and $\sim0.76$\,solar) and a high temperature ($\sim11$\,keV and $\sim0.33$\,solar) plasmas; the former is localized at the cluster core, while the latter distributes over the cluster. In addition to these ICM emissions, a strongly absorbed power-law component is found, which is due to an active galactic nucleus (AGN) in the cluster center. The absorption column density and unobscured luminosity of the AGN are $\sim3.2\times10^{23}$\,cm$^{-2}$ and $\sim4.7\times10^{45}$\,ergs\,s$^{-1}$ (2-10\,keV), respectively. Furthermore, a neutral iron (\ion{Fe}{1}) K-shell line is discovered for the first time with the equivalent width (EW) of $\sim150$\,eV at the rest frame. The column density and the EW of the \ion{Fe}{1} line are exceptionally large for such a high luminosity AGN, and hence the AGN is classified as a type 2 quasi-stellar object (QSO). We speculate that the significant fraction of the ICM cooled gas would be consumed to maintain the torus and to activate the type 2 QSO. The Phoenix cluster has a massive starburst in the central galaxy, indicating suppression in the cooling flow is less effective. This may be because the onset of the latest AGN feedback has occurred recently and it has not yet been effective. Alternatively, the AGN feedback is predominantly in radiative-mode not in kinetic-mode and the torus may work as a shield to reduce its effect.

DETERMINATION OF THE POINT-SPREAD FUNCTION FOR THEFERMILARGE AREA TELESCOPE FROM ON-ORBIT DATA AND LIMITS ON PAIR HALOS OF ACTIVE GALACTIC NUCLEI

The Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope is a pair-conversion telescope designed to detect photons with energies from 20 MeV to > 300 GeV. The pre-launch response functions of the LAT were determined through extensive Monte Carlo simulations and beam tests. The point-spread function (PSF) characterizing the angular distribution of reconstructed photons as a function of energy and geometry in the detector is determined here from two years of on-orbit data by examining the distributions of \gamma rays from pulsars and active galactic nuclei (AGN). Above 3 GeV, the PSF is found to be broader than the pre-launch PSF. We checked for dependence of the PSF on the class of \gamma-ray source and observation epoch and found none. We also investigated several possible spatial models for pair-halo emission around BL Lac AGN. We found no evidence for a component with spatial extension larger than the PSF and set upper limits on the amplitude of halo emission in stacked images of low and high redshift BL Lac AGN and the TeV blazars 1ES0229+200 and 1ES0347-121.

A MONTE CARLO MARKOV CHAIN BASED INVESTIGATION OF BLACK HOLE SPIN IN THE ACTIVE GALAXY NGC 3783

The analysis of relativistically broadened X-ray spectral features from the inner accretion disk provides a powerful tool for measuring the spin of supermassive black holes (SMBH) in active galactic nuclei (AGN). However, AGN spectra are often complex and careful analysis employing appropriate and self-consistent models are required if one is to obtain robust results. In this paper, we revisit the deep July-2009 Suzaku observation of the Seyfert galaxy NGC3783 in order to study in a rigorous manner the robustness of the inferred black hole spin parameter. Using Monte Carlo Markov Chain (MCMC) techniques, we identify a (partial) modeling degeneracy between the iron abundance of the disk and the black hole spin parameter. We show that the data for NGC3783 strongly require both supersolar iron abundance (Z_Fe=2-4Zsun) and a rapidly spinning black hole (a>0.88). We discuss various astrophysical considerations that can affect the measured abundance. We note that, while the abundance enhancement inferred in NGC3783 is modest, the X-ray analysis of some other objects has found extreme iron abundances. We introduce the hypothesis that the radiative levitation of iron ions in the innermost regions of radiation-dominated AGN disks can enhance the photospheric abundance of iron. We show that radiative levitation is a plausible mechanism in the very inner regions of high accretion rate AGN disks.

Are supermassive black holes shrouded by ‘super-Oort’ clouds of comets and asteroids?

The last decade has seen a dramatic confirmation that in situ star formation is possible inside the inner parsec of the Milky Way. Here we suggest that giant planets, solid terrestrial-like planets, comets and asteroids may also form in these environments, and that this may have observational implications for active galactic nuclei (AGN). Like in debris discs around main-sequence stars, collisions of large solid objects should initiate strong fragmentation cascades. The smallest particles in such a cascade – the microscopic dust – may provide a significant opacity. We put a number of observational and physical constraints on AGN obscuring tori resulting from such fragmentation cascades. We find that tori fed by fragmenting asteroids disappear at both low and high AGN luminosities. At high luminosities, L∼LEdd, where LEdd is the Eddington limit, the AGN radiation pressure blows out the microscopic dust too rapidly. At low luminosities, on the other hand, the AGN discs may avoid gravitational fragmentation into stars and solids. We also note that these fragmentation cascades may be responsible for astrophysically ‘large’ dust particles of m sizes that were postulated by some authors to explain unusual absorption properties of the AGN tori.

ACTIVE GALACTIC NUCLEUS PAIRS: CHANCE SUPERPOSITIONS OR BLACK HOLE BINARIES?

Several active galactic nuclei (AGN) with multiple sets of emission lines separated by over 2000 km/s have been observed recently. These have been interpreted as being due to massive black hole (MBH) recoil following a black hole merger, MBH binaries, or chance superpositions of AGN in galaxy clusters. Moreover, a number of double-peaked AGN with velocity offsets of ~ a few 100 km/s have also been detected and interpreted as being due to the internal kinematics of the narrow line regions or MBH binary systems. Here we reexamine the superposition model. Using the Millennium Run we estimate the total number of detectable AGN pairs as a function of the emission line offset. We show that AGN pairs with high velocity line separations up to ~2000 km/s are very likely to be chance superpositions of two AGN in clusters of galaxies for reasonable assumptions about the relative fraction of AGN. No superimposed AGN pairs are predicted for velocity offsets in excess of ~3000 km/s as the required AGN fractions would violate observational constraints. The high velocity AGN pair numbers predicted here are competitive with those predicted from the models relying on MBH recoil or MBH binaries. However, the model fails to account for the largest emission line velocity offsets that require the presence of MBH binaries.

Radio jets in galaxies with actively accreting black holes: new insights from the SDSS

In the local Universe, the majority of radio-loud active galactic nuclei (AGN) are found in massive elliptical galaxies with old stellar populations and weak or undetected emission lines. At high redshifts, however, almost all known radio AGN have strong emission lines. This paper focuses on a subset of radio AGN with emission lines (EL-RAGN) selected from the Sloan Digital Sky Survey. We explore the hypothesis that these objects are local analogues of powerful high-redshift radio galaxies. The probability for a nearby radio AGN to have emission lines is a strongly decreasing function of galaxy mass and velocity dispersion and an increasing function of radio luminosity above 1025 W Hz−1. Emission-line and radio luminosities are correlated, but with large dispersion. At a given radio power, radio galaxies with small black holes have higher [O iii] luminosities (which we interpret as higher accretion rates) than radio galaxies with big black holes. However, if we scale the emission-line and radio luminosities by the black hole mass, we find a correlation between normalized radio power and accretion rate in Eddington units that is independent of black hole mass. There is also a clear correlation between normalized radio power and the age of the stellar population in the galaxy. Present-day EL-RAGN with the highest normalized radio powers are confined to galaxies with small black holes. High-redshift, high radio-luminosity AGN would be explained if big black holes were similarly active at earlier cosmic epochs. To investigate why only a small fraction of emission-line AGN become radio-loud, we create matched samples of radio-loud and radio-quiet AGN and compare their host galaxy properties and environments. The main difference lies in their environments; our local density estimates are a factor of 2 larger around the radio-loud AGN. We propose a scenario in which radio-loud AGN with emission lines are located in galaxies where accretion of both cold and hot gas can occur simultaneously. At the present day, these conditions are only satisfied for low-mass galaxies in dense environments, but they are likely to apply to most galaxies with massive black holes at higher redshifts.

Active galactic nuclei dust tori at low and high luminosities

A cornerstone of active galactic nuclei (AGN) unification schemes is the presence of an optically and a geometrically thick dust torus. It provides the obscuration to explain the difference between type 1 and type 2 AGN. We investigate the influence of the dust distribution on the Eddington limit of the torus. For smooth dust distributions, the Eddingtion limit on the dust alone is five orders of magnitudes below the limit for electron scattering in a fully ionized plasma, while a clumpy dust torus has an Eddington limit slightly larger than the classical one. We study the behaviour of a clumpy torus at low and high AGN luminosities. For low luminosities of the order of ∼10 42 erg s −1 , the torus changes its characteristics and obscuration becomes insufficient. In the high-luminosity regime, the clumpy torus can show a behaviour which is consistent with the ‘receding torus’ picture. The derived luminosity-dependent fraction of type 2 objects agrees with recent observational results. Moreover, the luminosity-dependent covering factor in a clumpy torus may explain the presence of broad-line AGN with high column densities in X-rays.

An absorption origin for the soft excess in Seyfert 1 active galactic nuclei

The soft excess seen in the X‐ray spectra of many active galactic nuclei (AGN) can be well modelled by reflection from a partially ionized accretion disc. However, this often requires extreme parameters, both in terms of the underlying space–time and the reflection geometry, and requires that the disc is far from hydrostatic equilibrium. An alternative model uses similarly partially ionized, velocity smeared material but from an accretion disc wind seen in absorption. We explicitly compare these two models for the origin of the soft excess using XMM–Newton data for PG QSOs and narrow‐line Seyfert 1 galaxies (NLS1s). We find that while reflection and absorption give comparably good fits to the data, the absorption model allows a much clearer correspondence with the stellar mass black holes. All the objects are high mass accretion rate AGN, so should be analogous to the high/soft and very high states in black hole binaries. The intrinsic spectral indices derived from the absorption model are all consistent with a one‐to‐one mapping between spectral state and AGN type, with the NLS1s having softer spectra corresponding to the very high state, while the broad‐line AGN have Γ= 2 as expected for the high/soft state. By contrast, a few AGN have intrinsically hard spectra with the reflection model. While this supports an absorption interpretation of the soft excess, we note that the required Gaussian velocity dispersion of ≥0.2c (corresponding to an outflow velocity ≥0.4c) is too fast for a radiatively driven accretion disc wind and instead requires that the material is entrained in a magnetic outflow (jet). We also use the simultaneous optical monitor data to derive the ratio of disc to total accretion power which is another tracer of spectral state in X‐ray binaries. This does not always show that the disc in NLS1s contributes less than 80 per cent of the total power, as expected for a very high state. We suggest that this is an artefact of the standard disc models used to determine the disc luminosity in our fits. The disc seen in the very high state of black hole binaries is often observed to be distorted from the standard shape, and a similar effect in NLS1s could recover the one‐to‐one mapping between black hole binary spectral state and AGN type.

Discovery of a single faint AGN in a large sample of z > 5 Lyman break galaxies

As part of a large spectroscopic survey of z > 5 Lyman break galaxies (LBGs), we have identified a single source which is clearly hosting an active galactic nucleus (AGN). Out of a sample of more than 50 spectroscopically confirmed R-band dropout galaxies at z∼ 5 and above, only J104048.6−115550.2 at z= 5.44 shows evidence for a high ionization potential emission line indicating the presence of a hard ionizing continuum from an AGN. Like most objects in our sample the rest-frame-UV spectrum shows the UV continuum breaking across a Lyα line. Uniquely within this sample of LBGs, emission from N V is also detected, a clear signature of AGN photoionization. The object is spatially resolved in Hubble Space Telescope (HST) imaging. This, and the comparatively high Lyα/N V flux ratio indicates that the majority of the Lyα (and the UV continuum longward of it) originates from stellar photoionization, a product of the ongoing starburst in the LBG. Even without the AGN emission, this object would have been photometrically selected and spectroscopically confirmed as a Lyman break in our survey. The measured optical flux (IAB= 26.1) is therefore an upper limit to that from the AGN and is of order 100 times fainter than the majority of known quasars at these redshifts. The detection of a single object in our survey volume is consistent with the best current models of high redshift AGN luminosity function, providing a substantial fraction of such AGN is found within luminous starbursting galaxies. We discuss the cosmological implications of this discovery.

Statistics of emission-line galaxies in rich clusters

The emission-line characteristics of a sample of 1095 galaxies in rich clusters and 173 field galaxies for which Dressler and Shectman (1985) have obtained spectra are tabulated and analyzed. It is found that the emission-line frequency is much higher in field galaxies than in cluster galaxies. Relatively strong emission is found in the central regions of 31 percent of the field galaxies, but in only 7 percent of the cluster galaxies. The difference in the distribution of morphological types can only partially explain this effect; much of the difference must be attributed to environmental influences. Active galactic nuclei (AGNs) occur at a frequency of about 5 percent in the field sample but only about 1 percent in the cluster sample, the same ratio as found for the emission-line galaxies. The average frequency is an order of magnitude lower than that found for the distant 3C 295 cluster by Dressler and Gunn (1983), but some nearby clusters may also have an unusually high AGN fraction.