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Register a new userHard X-ray emitting Active Galactic Nuclei selected by the Chandra Multi-wavelength Project
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J. Silverman
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We present X-ray and optical analysis of 188 AGN identified from 497 hard X-ray (f (2.0-8.0 keV) > 2.7x10^-15 erg cm^-2 s^-1) sources in 20 Chandra fields (1.5 deg^2) forming part of the Chandra Multi-wavelength Project. These medium depth X-ray observations enable us to detect a representative subset of those sources responsible for the bulk of the 2-8 keV Cosmic X-ray Background. Brighter than our optical spectroscopic limit, we achieve a reasonable degree of completeness (77% of X-ray sources with counter-parts r< 22.5 have been classified): broad emission line AGN (62%), narrow emission line galaxies (24%), absorption line galaxies (7%), stars (5%) or clusters (2%). We find that most X-ray unabsorbed AGN (NH<10^22 cm^-2) have optical properties characterized by broad emission lines and blue colors, similiar to optically-selected quasars from the Sloan Digital Sky Survey but with a slighly broader color distribution. However, we also find a significant population of redder (g-i>1.0) AGN with broad optical emission lines. Most of the X-ray absorbed AGN (10^22<NH<10^24 cm^-2) are associated with narrow emission line galaxies, with red optical colors characteristically dominated by luminous, early type galaxy hosts rather than from dust reddening of an AGN. We also find a number of atypical AGN; for instance, several luminous AGN show both strong X-ray absorption (NH>10^22 cm^-2) and broad emission lines. Overall, we find that 81% of X-ray selected AGN can be easily interpreted in the context of current AGN unification models. Most of the deviations seem to be due to an optical contribution from the host galaxies of the low luminosity AGN.
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We use highly spectroscopically complete deep and wide-area Chandra surveys to determine the cosmic evolution of hard X-ray-selected AGNs. We determine hard X-ray luminosity functions (HXLFs) for all spectral types and for broad-line AGNs (BLAGNs) alone. At z<1.2, both are well described by pure luminosity evolution. Thus, all AGNs drop in luminosity by almost an order of magnitude over this redshift range. We show that this observed drop is due to AGN downsizing. We directly compare our BLAGN HXLFs with the optical QSO LFs and find that the optical QSO LFs do not probe faint enough to see the downturn in the BLAGN HXLFs. We rule out galaxy dilution as a partial explanation for the observation that BLAGNs dominate the number densities at the higher X-ray luminosities, while optically-narrow AGNs (FWHM<2000 km/s) dominate at the lower X-ray luminosities by measuring the nuclear UV/optical properties of the Chandra sources using the HST ACS GOODS-North data. The UV/optical nuclei of the optically-narrow AGNs are much weaker than expected if they were similar to the BLAGNs. We therefore postulate the need for a luminosity dependent unified model. Alternatively, the BLAGNs and the optically-narrow AGNs could be intrinsically different source populations. We cover both interpretations by constructing composite spectral energy distributions--including long-wavelength data from the MIR to the submillimeter--by spectral type and by X-ray luminosity. We use these to infer the bolometric corrections (from hard X-ray luminosities to bolometric luminosities) needed to map the accretion history. We determine the accreted supermassive black hole mass density for all spectral types and for BLAGNs alone using the observed evolution of the hard X-ray energy density production rate and our inferred bolometric corrections.
We present results of near-infrared photometry (J, H, K_S) for a sample of active galactic nuclei (AGNs) obtained from hard X-ray surveys with ASCA. The sample covers the AGNs at z=0.1-1 with L(2-10keV)=10^42-10^46 erg/s with very high completeness. The fraction of red (J-K_S>2 mag) AGNs in our sample is 2(+-1)%, which is comparable to that for optically- or UV-selected quasi-stellar objects (QSOs, i.e. luminous AGNs). The number of red AGNs found in our sample is also consistent with that expected from the surface density of red AGNs found in 2MASS by Cutri et al. (2001). We find that the anomalously-small dust-to-gas ratios in circumnuclear gas, which is seen in some AGNs with Seyfert-class luminosity, also occur in the QSOs (AGNs with luminosity of L(2-10keV) > 10^44.5 erg/s). For all the QSOs with an X-ray absorption of N_H > 10^22 /cm2 in our sample, the values of A_V/N_H are smaller than the Galactic value by a factor of 5 to 100. Since a fraction of this population among the QSOs in our sample is about 30%, such fraction of optical/UV-selected type 1 QSOs known to date may show type 2 nature in X-ray.
We study properties of the host galaxies of 15 hard X-ray selected type-2 active galactic nuclei (AGNs) at intermediate redshifts (0.05$<z<$0.6) detected in $ASCA$ surveys. The absorption corrected hard X-ray luminosities $L_{rm 2-10 keV}$ range from 10$^{42}$ erg s$^{-1}$ to $10^{45}$ erg s$^{-1}$. We took the $R$-band image of these AGNs with the University of Hawaii 2.2 m telescope. Thanks to the intrinsic obscuration of nuclear light, we can decompose the galaxies with a spheroid component and a disk component. The resulting spheroid luminosities correlate with $L_{rm 2-10 keV}$; higher (lower) X-ray luminosity AGNs tend to reside in luminous (less luminous) spheroids. It is also found that the hosts of luminous AGNs show a large spheroid-to-disk luminosity ratio ($sim$1), while those of less luminous AGNs spread between 0 and 1. The correlation between $L_{rm 2-10keV}$ and spheroid luminosity indicates that the relation between mass of a supermassive black hole (SMBH) and spheroid luminosity (BS-relation) at the intermediate redshifts. The BS-relation agrees with that in the local universe if the Eddington ratio of 0.24 is adopted, which is a mean value determined from our $ASCA$ type-1 AGN sample at similar redshifts through the broad-line width and continuum luminosity. The present study demonstrates the effectiveness of using type-2 AGNs at high redshifts to study their host properties.
We report the results obtained by a systematic, broadband (0.5--150 keV) X-ray spectral analysis of moderately obscured (Compton-thin; $22 leq log N_{rm H} < 24$) active galactic nuclei (AGNs) observed with Suzaku and Swift/Burst Alert Telescope (BAT). Our sample consists of 45 local AGNs at $z<0.1$ with $log L_{rm 14-1951mmkeV} > 42$ detected in the Swift/BAT 70-month survey, whose Suzaku archival data are available as of 2015 December. All spectra are uniformly fit with a baseline model composed of an absorbed cutoff power-law component, reflected emission accompanied by a narrow fluorescent iron-K$alpha$ line from cold matter (torus), and scattered emission. Main results based on the above analysis are as follows. (1) The photon index is correlated with Eddington ratio, but not with luminosity or black hole mass. (2) The ratio of the iron-K$alpha$ line to X-ray luminosity, a torus covering fraction indicator, shows significant anti-correlation with luminosity. (3) The averaged reflection strength derived from stacked spectra above 14 keV is larger in less luminous ($log L_{rm 10-501mmkeV} leq 43.3$; $R= 1.04^{+0.17}_{-0.19}$) or highly obscured AGNs ($log N_{rm H} > 23$; $R = 1.03^{+0.15}_{-0.17}$) than in more luminous ($log L_{rm 10-501mmkeV} > 43.3$; $R= 0.46^{+0.08}_{-0.09}$) or lightly obscured objects ($log N_{rm H} leq 23$; $R = 0.59^{+0.09}_{-0.10}$), respectively. (4) The [O IV] 25.89 $mu$m line to X-ray luminosity ratio is significantly smaller in AGNs with lower soft X-ray scattering fractions, suggesting that the [O IV] 25.89 $mu$m luminosity underestimates the intrinsic power of an AGN buried in a small opening-angle torus.
We present a uniform broadband X-ray (0.5-100.0 keV) spectral analysis of 12 Swift/Burst Alert Telescope (BAT) selected Compton-thick ($log N_{mathrm{H}}/mathrm{cm}^{-2} geq 24$) Active Galactic Nuclei (CTAGNs) observed with Suzaku. The Suzaku data of 3 objects are published here for the first time. We fit the Suzaku and Swift spectra with models utilizing an analytic reflection code and those utilizing the Monte Carlo based model from an AGN torus by Ikeda et al. 2009. The main results are as follows. (1) The estimated intrinsic luminosity of a CTAGN strongly depends on the model; applying Compton scattering to the transmitted component in an analytic model may largely overestimates the intrinsic luminosity at large column densities. (2) Unabsorbed reflection components are commonly observed, suggesting that the tori are clumpy. (3) Most of CTAGNs show small scattering fractions (<0.5%) implying a buried AGN nature. (4) Comparison with the results obtained for Compton-thin AGNs (Kawamuro et al. 2016) suggests that the properties of these CTAGNs can be understood as a smooth extension from Compton-thin AGNs with heavier obscuration; we find no evidence that the bulk of the population of hard X-ray selected CTAGN is different from less obscured objects.
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