Publication Search Results

ID: 155668
Type: article
Authors: Chang, Yu-Yen; Le Floc'h, Emeric; Juneau, Stéphanie; da Cunha, Elisabete; Salvato, Mara; Dekel, Avishai; Civano, Francesca; Marchesi, Stefano; Suh, Hyewon; Wang, Wei-Hao
Abstract: We present rest-frame far-infrared (FIR) and optical size measurementsof active galactic nucleus (AGN) hosts and star-forming galaxies (SFGs)in the COSMOS field, enabled by high-resolution Atacama LargeMillimeter/submillimeter Array (ALMA)/1 mm (0.″1─0.″4) and Hubble Space Telescope (HST)/F814W imaging (∼0.″1). Our sample includes 27 galaxiesat z < 2.5, classified as infrared-selected AGN (three sources),X-ray selected AGN (four sources), and non-AGN SFGs (20 sources), for which high-resolution Band 6/7 ALMA images are available at 1 mm fromour own observing program as well as archival observations. The sizesand star formation rate surface densities measured from both ALMA/1 mm and HST/F814W images show that obscured AGN host galaxies are morecompact than non-AGN SFGs at similar redshift and stellar mass. Thisresult suggests that the obscured accretion phase may be related to galaxies experiencing a compaction of their gaseous component, whichcould be associated with enhanced central star formation before a subsequent quenching driving the formation of compact passive galaxies.Moreover, most of the detected and stacked rest-frame FIR sizes of AGNsin our sample are similar or more compact than their rest-frame optical sizes, which is consistent with recent results of ALMA-detected sources.This might be explained by the fact that the dusty starbursts take place in the compact regions, and suggests that the star formation mechanismsin the compact regions of AGN hosts are similar to those observed in SFGs observed with ALMA.

ID: 156977
Type: article
Authors: Fornasini, Francesca M.; Civano, Francesca; Suh, Hyewon
Abstract: The integrated X-ray luminosity (L_X) of high-mass X-ray binaries (HMXBs) in a galaxy is correlated with its star formation rate (SFR), and the normalization of this correlation increases with redshift. Population synthesis models suggest that the redshift evolution of L_X/SFR is driven by the metallicity (Z) dependence of HMXBs, and the first direct evidence of this connection was recently presented using galaxies at z ∼ 2. To confirm this result with more robust measurements and better constrain the L_X-SFR-Z relation, we have studied the Z dependence of L_X/SFR at lower redshifts. Using samples of star-forming galaxies at z = 0.1-0.9 with optical spectra from the hCOSMOS and zCOSMOS surveys, we stacked Chandra data from the COSMOS Legacy survey to measure the average L_X/SFR as a function of Z in three redshift ranges: z = 0.1-0.25, 0.25-0.4, and 0.5-0.9. We find no significant variation of the L_X-SFR-Z relation with redshift. Our results provide further evidence that the Z dependence of HMXBs is responsible for the redshift evolution of L_X/SFR. Combining all available z > 0 measurements together, we derive a best-fitting L_X-SFR-Z relation and assess how different population synthesis models describe the data. These results provide the strongest constraints to date on the L_X-SFR-Z relation in the range of 8.0 < 12 + log(O/H) < 9.0.

ID: 157776
Type: article
Authors: Grazian, A.; Giallongo, E.; Fiore, F.; Boutsia, K.; Civano, Francesca; Cristiani, S.; Cupani, G.; Dickinson, M.; Fontanot, F.; Menci, N.; Romano, M.
Abstract: It is widespread opinion that hydrogen reionization is mainly driven by primeval star-forming galaxies, with high-z active galactic nuclei (AGNs) having a minor role. Recent observations, however, challenge this notion, indicating a number of issues related to a galaxy-driven reionization scenario. We provide here an updated assessment of the space density of relatively faint (M₁₄₅₀ ∼ -22.5) AGNs at z_spec ∼ 5.5 in order to improve the estimate of the photoionization rate contribution from accreting supermassive black holes. Exploiting deep UV rest-frame ground-based spectra collected at the Very Large Telescope on the CANDELS/GOODS-South field and deep Chandra X-ray images in the CANDELS/GOODS-North and EGS areas, we find two relatively bright (M₁₄₅₀ ∼ -22.5) AGNs at z_spec ∼ 5.5. We derive an AGN space density of Φ = 1.29 × 10^-6 cMpc^-3 at z ∼ 5.5 and M₁₄₅₀ ∼ -22.5 by simply dividing their observed number by the cosmological volume in the range 5.0 ∼ -22.5 by simply dividing their observed number by the cosmological volume in the range 5.0 5, in contrast with previous claims mostly based on standard color selection, possibly affected by significant incompleteness. Our estimate for the AGN photoionization rate at z ∼ 5.5 is in agreement with the observed values at similar redshifts, which are needed to keep the intergalactic medium highly ionized. Upcoming James Webb Space Telescope and giant ground-based telescopes observations will improve the study of high-z AGNs and their contribution to the reionization of the universe.

ID: 158791
Type: article
Authors: Lusso, E.; Risaliti, G.; Nardini, E.; Bargiacchi, G.; Benetti, M.; Bisogni, S.; Capozziello, S.; Civano, Francesca; Eggleston, L.; Elvis, Martin; Fabbiano, Giuseppina; Gilli, R.; Marconi, A.; Paolillo, M.; Piedipalumbo, E.; Salvestrini, F.; Signorini, M.; Vignali, C.
Abstract: We present a new catalogue of ˜2400 optically selected quasars with spectroscopic redshifts and X-ray observations from either Chandra or XMM-Newton. The sample can be used to investigate the non-linear relation between the ultraviolet (UV) and X-ray luminosity of quasars as well as to build a Hubble diagram up to a redshift of z ˜ 7.5. We selected sources that are neither reddened by dust in the optical and UV nor obscured by gas in the X-rays, and whose X-ray fluxes are free from flux-limit-related biases. After checking for any possible systematics, we confirm, in agreement with our previous works, that the X-ray to UV relation provides distance estimates matching those from supernovae up to z ˜ 1.5, and its slope shows no redshift evolution up to z ˜ 5. We provide a full description of the methodology for testing cosmological models, further supporting a trend whereby the Hubble diagram of quasars is well reproduced by the standard flat cold dark matter model up to z ˜ 1.5-2, but strong deviations emerge at higher redshifts. Since we have minimised all non-negligible systematic effects and proven the stability of the L_X - L_UV relation at high redshifts, we conclude that an evolution of the expansion rate of the Universe should be considered as a possible explanation for the observed deviation, rather than some systematic (redshift-dependent) effect associated with high-redshift quasars.
Full Table 3 is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/642/A150

ID: 156469
Type: article
Authors: Shankar, Francesco; Weinberg, David H.; Marsden, Christopher; Grylls, Philip J.; Bernardi, Mariangela; Yang, Guang; Moster, Benjamin; Fu, Hao; Carraro, Rosamaria; Alexander, David M.; Allevato, Viola; Ananna, Tonima T.; Bongiorno, Angela; Calderone, Giorgio; Civano, Francesca; Daddi, Emanuele; Delvecchio, Ivan; Duras, Federica; La Franca, Fabio; Lapi, Andrea; Lu, Youjun; Menci, Nicola; Mezcua, Mar; Ricci, Federica; Rodighiero, Giulia; Sheth, Ravi K.; Suh, Hyewon; Villforth, Carolin; Zanisi, Lorenzo
Abstract: The masses of supermassive black holes at the centres of local galaxies appear to be tightly correlated with the mass and velocity dispersions of their galactic hosts. However, the local M_bh-M_star relation inferred from dynamically measured inactive black holes is up to an order-of-magnitude higher than some estimates from active black holes, and recent work suggests that this discrepancy arises from selection bias on the sample of dynamical black hole mass measurements. In this work, we combine X-ray measurements of the mean black hole accretion luminosity as a function of stellar mass and redshift with empirical models of galaxy stellar mass growth, integrating over time to predict the evolving M_bh-M_star relation. The implied relation is nearly independent of redshift, indicating that stellar and black hole masses grow, on average, at similar rates. Matching the de-biased local M_bh-M_star relation requires a mean radiative efficiency ∊ ≳ 0.15, in line with theoretical expectations for accretion on to spinning black holes. However, matching the `raw' observed relation for inactive black holes requires ∊ ∼ 0.02, far below theoretical expectations. This result provides independent evidence for selection bias in dynamically estimated black hole masses, a conclusion that is robust to uncertainties in bolometric corrections, obscured active black hole fractions, and kinetic accretion efficiency. For our fiducial assumptions, they favour moderate-to-rapid spins of typical supermassive black holes, to achieve ∊ ∼ 0.12-0.20. Our approach has similarities to the classic Soltan analysis, but by using galaxy-based data instead of integrated quantities we are able to focus on regimes where observational uncertainties are minimized.

ID: 158660
Type: article
Authors: Sicilian, Dominic; Cappelluti, Nico; Bulbul, Esra; Civano, Francesca; Moscetti, Massimo; Reynolds, Christopher S.
Abstract: We present a comprehensive search for the 3.5 keV line using ˜51 Ms of archival Chandra observations peering through the Milky Way's dark matter halo from across the entirety of the sky, gathered via the Chandra Source Catalog Release 2.0. We consider the data's radial distribution, organizing observations into four data subsets based on angular distance from the Galactic center. All data are modeled using both background-subtracted and background-modeled approaches to account for the particle instrument background, demonstrating the statistical limitations of the currently available ˜1 Ms of particle background data. A nondetection is reported in the total data set, allowing us to set an upper limit on 3.5 keV line flux and constrain the sterile neutrino dark matter mixing angle. The upper limit on sin²(2?) is $2.58\times {10}^{-11}$ (though systematic uncertainty may increase this by a factor of ˜2), corresponding to an upper limit on 3.5 keV line flux of $2.34\times {10}^{-7}$ photons s^-1 cm^-2. These limits show consistency with recent constraints and several prior detections. Nondetections are reported in all radial data subsets, allowing us to constrain the spatial profile of 3.5 keV line intensity, which does not conclusively differ from Navarro-Frenk-White predictions. Thus, while offering heavy constraints, we do not entirely rule out the sterile neutrino dark matter scenario or the more general decaying dark matter hypothesis for the 3.5 keV line. We have also used the nondetection of any unidentified emission lines across our continuum to further constrain the sterile neutrino parameter space.

ID: 155705
Type: article
Authors: Suh, Hyewon; Civano, Francesca; Trakhtenbrot, Benny; Shankar, Francesco; Hasinger, Günther; Sanders, David B.; Allevato, Viola
Abstract: We investigate the cosmic evolution of the ratio between black hole (BH) mass (M_BH) and host galaxy total stellar mass (M_stellar) out to z ∼ 2.5 for a sample of 100 X-ray-selected moderate-luminosity, broad-line active galactic nuclei (AGNs) in the Chandra-COSMOS Legacy Survey. By taking advantage of the deep multiwavelength photometry and spectroscopy in the COSMOS field, we measure in a uniform way the galaxy total stellar mass using an spectral energy distribution decomposition technique and the BH mass based on broad emission line measurements and single-epoch virial estimates. Our sample of AGN host galaxies has total stellar masses of 10¹⁰⁻¹²M_☉, and BH masses of 10^7.0─9.5M_☉. Combining our sample with the relatively bright AGN samples from the literature, we find no significant evolution of the M_BH─M_stellar relation with the BH-to-host total stellar mass ratio of M_BH/M_stellar ∼ 0.3% at all redshifts probed. We conclude that the average BH-to-host stellar mass ratio appears to be consistent with the local value within the uncertainties, suggesting a lack of evolution of the M_BH─M_stellar relation up to z ∼ 2.5.

ID: 154547
Type: article
Authors: Allevato, V.; Viitanen, A.; Finoguenov, A.; Civano, Francesca; Suh, H.; Shankar, F.; Bongiorno, A.; Ferrara, A.; Gilli, R.; Miyaji, T.; Marchesi, S.; Cappelluti, N.; Salvato, M.
Abstract: Aims: We perform clustering measurements of 800 X-ray selected Chandra COSMOS Legacy (CCL) Type 2 active galactic nuclei (AGN) with known spectroscopic redshift to probe the halo mass dependence on AGN host galaxy properties, such as galaxy stellar mass M_star, star formation rate (SFR), and specific black hole accretion rate (BHAR λ_BHAR) in the redshift range z = [0-3]. Methods: We split the sample of AGN with known spectroscopic redshits according to M_star, SFR and λ_BHAR, while matching the distributions in terms of the other parameters, including redshift. We measured the projected two-point correlation function w_p(r_p) and modeled the clustering signal, for the different subsamples, with the two-halo term to derive the large-scale bias b and corresponding typical mass of the hosting halo. Results: We find no significant dependence of the large-scale bias and typical halo mass on galaxy stellar mass and specific BHAR for CCL Type 2 AGN at mean z ̃ 1, while a negative dependence on SFR is observed, i.e. lower SFR AGN reside in richer environment. Mock catalogs of AGN, matched to have the same X-ray luminosity, stellar mass, λ_BHAR, and SFR of CCL Type 2 AGN, almost reproduce the observed M_star - M_h, λ_BHAR - M_h and SFR-M_h relations, when assuming a fraction of satellite AGN f_AGN^sat ̃ 0.15. This corresponds to a ratio of the probabilities of satellite to central AGN of being active Q ̃ 2. Mock matched normal galaxies follow a slightly steeper M_star - M_h relation, in which low mass mock galaxies reside in less massive halos than mock AGN of similar mass. Moreover, matched mock normal galaxies are less biased than mock AGN with similar specific BHAR and SFR, at least for Q > 1.

ID: 150485
Type: article
Authors: Ananna, Tonima Tasnim; Treister, Ezequiel; Urry, C. Megan; Ricci, C.; Kirkpatrick, Allison; LaMassa, Stephanie; Buchner, Johannes; Civano, Francesca; Tremmel, Michael; Marchesi, Stefano
Abstract: As matter accretes onto the central supermassive black holes in active galactic nuclei (AGNs), X-rays are emitted. We present a population synthesis model that accounts for the summed X-ray emission from growing black holes; modulo the efficiency of converting mass to X-rays, this is effectively a record of the accreted mass. We need this population synthesis model to reproduce observed constraints from X-ray surveys: the X-ray number counts, the observed fraction of Compton-thick AGNs [log (N _H/cm^‑2) > 24], and the spectrum of the cosmic X-ray background (CXB), after accounting for selection biases. Over the past decade, X-ray surveys by XMM-Newton, Chandra, NuSTAR, and Swift-BAT have provided greatly improved observational constraints. We find that no existing X-ray luminosity function (XLF) consistently reproduces all these observations. We take the uncertainty in AGN spectra into account and use a neural network to compute an XLF that fits all observed constraints, including observed Compton-thick number counts and fractions. This new population synthesis model suggests that, intrinsically, 50% ± 9% (56% ± 7%) of all AGNs within z ≃ 0.1 (1.0) are Compton-thick.

ID: 154601
Type: article
Authors: Fornasini, Francesca M.; Kriek, Mariska; Sanders, Ryan L.; Shivaei, Irene; Civano, Francesca; Reddy, Naveen A.; Shapley, Alice E.; Coil, Alison L.; Mobasher, Bahram; Siana, Brian; Aird, James; Azadi, Mojegan; Freeman, William R.; Leung, Gene C. K.; Price, Sedona H.; Fetherolf, Tara; Zick, Tom; Barro, Guillermo
Abstract: Population synthesis models predict that high-mass X-ray binary (HMXB) populations produced in low-metallicity environments should be more X-ray luminous, a trend supported by studies of nearby galaxies. This trend may be responsible for the observed increase of the X-ray luminosity (L _X) per star formation rate (SFR) with redshift due to the decrease of metallicity (Z) at fixed stellar mass as a function of redshift. To test this hypothesis, we use a sample of 79 z ̃ 2 star-forming galaxies with oxygen abundance measurements from the MOSDEF survey, which obtained rest-frame optical spectra for ̃1500 galaxies in the CANDELS fields at 1.37 ) per star formation rate (SFR) with redshift due to the decrease of metallicity (Z) at fixed stellar mass as a function of redshift. To test this hypothesis, we use a sample of 79 z ̃ 2 star-forming galaxies with oxygen abundance measurements from the MOSDEF survey, which obtained rest-frame optical spectra for ̃1500 galaxies in the CANDELS fields at 1.37 ) per star formation rate (SFR) with redshift due to the decrease of metallicity (Z) at fixed stellar mass as a function of redshift. To test this hypothesis, we use a sample of 79 z ̃ 2 star-forming galaxies with oxygen abundance measurements from the MOSDEF survey, which obtained rest-frame optical spectra for ̃1500 galaxies in the CANDELS fields at 1.37 _X/SFR of our z ̃ 2 galaxy sample is enhanced by ≈0.4-0.8 dex relative to local HMXB L _X-SFR scaling relations. Splitting our sample by Z, we find that L _X/SFR and Z are anticorrelated with 97% confidence. This observed Z dependence for HMXB-dominated galaxies is consistent with both the local L _X-SFR-Z relation and a subset of population synthesis models. Although the statistical significance of the observed trends is weak owing to the low X-ray statistics, these results constitute the first direct evidence connecting the redshift evolution of L _X/SFR and the Z dependence of HMXBs.

ID: 150908
Type: article
Authors: Gozaliasl, Ghassem; Finoguenov, Alexis; Tanaka, Masayuki; Dolag, Klaus; Montanari, Francesco; Kirkpatrick, Charles C.; Vardoulaki, Eleni; Khosroshahi, Habib G.; Salvato, Mara; Laigle, Clotilde; McCracken, Henry J.; Ilbert, Olivier; Cappelluti, Nico; Daddi, Emanuele; Hasinger, Guenther; Capak, Peter; Scoville, Nick Z.; Toft, Sune; Civano, Francesca; Griffiths, Richard E.; Balogh, Michael; Li, Yanxia; Ahoranta, Jussi; Mei, Simona; Iovino, Angela; Henriques, Bruno M. B.; Erfanianfar, Ghazaleh
Abstract: We present the results of a search for galaxy clusters and groups in the ˜2 deg² of the COSMOS field using all available X-ray observations from the XMM-Newton and Chandra observatories. We reach an X-ray flux limit of 3× 10^{-16} erg cm^{-2 s}^{-1} in the 0.5-2 keV range, and identify 247 X-ray groups with M_{ 200c}=8× 10^{12}-3× 10^{14} M_{⊙} at a redshift range of 0.08 ≤ z < 1.53, using the multiband photometric redshift and the master spectroscopic redshift catalogues of the COSMOS. The X-ray centres of groups are determined using high-resolution Chandra imaging. We investigate the relations between the offset of the brightest group galaxies (BGGs) from halo X-ray centre and group properties and compare with predictions from semi-analytic models and hydrodynamical simulations. We find that BGG offset decreases with both increasing halo mass and decreasing redshift with no strong dependence on the X-ray flux and SNR. We show that the BGG offset decreases as a function of increasing magnitude gap with no considerable redshift-dependent trend. The stellar mass of BGGs in observations extends over a wider dynamic range compared to model predictions. At z < 0.5, the central dominant BGGs become more massive than those with large offsets by up to 0.3 dex, in agreement with model prediction. The observed and predicted log-normal scatter in the stellar mass of both low- and large-offset BGGs at fixed halo mass is ˜0.3 dex.

ID: 155307
Type: article
Authors: Lanzuisi, G.; Gilli, R.; Cappi, M.; Dadina, M.; Bianchi, S.; Brusa, M.; Chartas, G.; Civano, Francesca; Comastri, A.; Marinucci, A.; Middei, R.; Piconcelli, E.; Vignali, C.; Brandt, W. N.; Tombesi, F.; Gaspari, M.
Abstract: X-ray emission from the active galactic nucleus (AGN) is believed to be produced via Comptonization of optical/ultraviolet seed photons emitted by the accretion disk, upscattered by hot electrons in a corona surrounding the black hole. A critical compactness versus temperature threshold is predicted above which any increase in the source luminosity, for a fixed size, would then generate positron-electron pairs rather than continue heating the coronal plasma. Current observations seem to confirm that all AGNs populate the region below this critical line. These models, however, have never been probed by observations in the high-luminosity regime, where the critical line is expected to reach low temperatures. To fill this observational gap, we selected two luminous (log(L _Bol) > 47.5 erg s^-1) quasars, 2MASSJ1614346+470420 (z = 1.86) and B1422+231 (z = 3.62), and obtained XMM-Newton and NuSTAR deep observations for them. We performed detailed spectral analysis of their quasisimultaneous soft and hard X-ray data, in order to constrain the parameters of their coronae. Using a phenomenological cutoff power-law model, with the inclusion of a reflection component, we derived rest-frame values of the high-energy cutoff of E _cut = {106}_-37⁺¹⁰² keV and E _cut = {66}_-12⁺¹⁷ keV, respectively. Comptonization models consistently give as best-fit parameters electron temperatures of ∼45 keV and ∼28 keV, respectively, and optically thick coronae (τ > 1). These low coronal temperatures fall in the limited region allowed at these luminosities to avoid runaway pair production.

ID: 154405
Type: article
Authors: Masini, A.; Comastri, A.; Hickox, R. C.; Koss, M.; Civano, Francesca; Brigthman, M.; Brusa, M.; Lanzuisi, G.
Abstract: Active galactic nuclei (AGNs) hosting disk water megamasers are well known to be obscured by large amounts of gas, likely due to the presence along the line of sight of an almost edge-on disky structure orbiting the supermassive black hole. Correcting for the high obscuration is crucial to infer parameters intrinsic to the source, like its luminosity. We present a broadband X-ray spectral analysis of a water megamaser AGN in an early merger (NGC 5765B), combining Chandra and NuSTAR data. NGC 5765B is highly Compton-thick and reflection-dominated, following the general trend among disk megamasers. Combining the exquisite black hole mass from masers with our X-ray spectroscopy, the Eddington ratio of the megamaser is estimated to be in the 2%-14% range, and its robustness is confirmed through SED fitting.

ID: 154391
Type: article
Authors: Mezcua, M.; Suh, H.; Civano, Francesca
Abstract: Dwarf galaxies are thought to host the remnants of the early Universe seed black holes (BHs) and to be dominated by supernova feedback. However, recent studies suggest that BH feedback could also strongly impact their growth. We report the discovery of 35 dwarf galaxies hosting radio active galactic nucleus (AGN) out to redshift ̃3.4, which constitutes the highest redshift sample of AGNs in dwarf galaxies. The galaxies are drawn from the VLA-COSMOS 3 GHz Large Project and all are star forming. After removing the contribution from star formation to the radio emission, we find a range of AGN radio luminosities of L^AGN_1.4 GHz ̃ 10^{37}-10⁴⁰ erg s^-1. The bolometric luminosities derived from the fit of their spectral energy distribution are ≳10⁴² erg s^-1, in agreement with the presence of AGNs in these dwarf galaxies. The 3 GHz radio emission of most of the sources is compact and the jet powers range from Q_jet ̃ 10⁴² to 10⁴⁴ erg s^-1. These values, as well as the finding of jet efficiencies ≥10 per cent in more than 50 per cent of the sample, indicate that dwarf galaxies can host radio jets as powerful as those of massive radio galaxies whose jet mechanical feedback can strongly affect the formation of stars in the host galaxy. We conclude that AGN feedback can also have a very strong impact on dwarf galaxies, either triggering or hampering star formation and possibly the material available for BH growth. This implies that those low-mass AGNs hosted in dwarf galaxies might not be the untouched relics of the early seed BHs, which has important implications for seed BH formation models.

ID: 154538
Type: article
Authors: Nardini, E.; Lusso, E.; Risaliti, G.; Bisogni, S.; Civano, Francesca; Elvis, Martin; Fabbiano, Giuseppina; Gilli, R.; Marconi, A.; Salvestrini, F.; Vignali, C.
Abstract: We present the X-ray analysis of 30 luminous quasars at z ≃ 3.0 - 3.3 with pointed XMM-Newton observations (28-48 ks) originally obtained by our group to test the suitability of active galactic nuclei as standard candles for cosmological studies. The sample was selected in the optical from the Sloan Digital Sky Survey Data Release 7 to be representative of the most luminous, intrinsically blue quasar population, and by construction boasts a high degree of homogeneity in terms of optical and UV properties. In the X-rays, only four sources are too faint for a detailed spectral analysis, one of which is formally undetected. Neglecting one more object later found to be radio-loud, the other 25 quasars are, as a whole, the most X-ray luminous ever observed, with rest-frame 2-10 keV luminosities of 0.5 - 7 × 10⁴⁵ erg s^-1. The continuum photon index distribution, centred at Γ ̃ 1.85, is in excellent agreement with those in place at lower redshift, luminosity, and black-hole mass, confirming the universal nature of the X-ray emission mechanism in quasars. Even so, when compared against the well-known L_X-L_UV correlation, our quasars show an unexpectedly varied behaviour, splitting into two distinct subsets. About two-thirds of the sources are clustered around the relation with a minimal scatter of 0.1 dex, while the remaining one-third appear to be X-ray underluminous by factors of > 3-10. Such a large incidence (≈25%) of X-ray weakness has never been reported in radio-quiet, non- broad absorption line (BAL) quasar samples. Several factors could contribute to enhancing the X-ray weakness fraction among our z ≃ 3 blue quasars, including variability, mild X-ray obscuration, contamination from weak-line quasars, and missed BALs. However, the X-ray weak objects also have, on average, flatter spectra, with no clear evidence of absorption. Indeed, column densities in excess of a few ×10²² cm^-2 can be ruled out for most of the sample. We suggest that, at least in some of our X-ray weak quasars, the corona might experience a radiatively inefficient phase due to the presence of a powerful accretion-disc wind, which substantially reduces the accretion rate through the inner disc and therefore also the availability of seed photons for Compton up-scattering. The origin of the deviations from the L_X-L_UV relation will be further investigated in a series of future studies.

ID: 152892
Type: article
Authors: Riguccini, Laurie A.; Treister, Ezequiel; Menéndez-Delmestre, Karin; Cardamone, Carolin; Civano, Francesca; Gonçalves, Thiago S.; Hasinger, Guenther; Koekemoer, Anton M.; Lanzuisi, Giorgio; Le Floc'h, Emeric; Lusso, Elisabeta; Lutz, Dieter; Marchesi, Stefano; Miyaji, Takamitsu; Pozzi, Francesca; Ricci, Claudio; Rodighiero, Giulia; Salvato, Mara; Sanders, Dave; Schawinski, Kevin; Suh, Hyewon
Abstract: We present the X-ray properties of 108 Dust-Obscured Galaxies (DOGs; F_{24 μm}/F _R > 1000) in the COSMOS field, all of which are detected in at least three far-infrared bands with the Herschel Observatory. Out of the entire sample, 22 are individually detected in the hard 2–8 keV X-ray band by the Chandra COSMOS Legacy survey, allowing us to classify them as AGN. Six (27%) of them are Compton-thick AGN candidates with column densities N _H > 10²⁴ cm^‑2, while 15 are moderately obscured AGNs with 10²² _H ²⁴ cm^‑2. Additionally, we estimate AGN contributions to the IR luminosity (8–1000 μm rest-frame) greater than 20% for 19 DOGs based on SED decomposition using Spitzer/MIPS 24 μm and the five Herschel bands (100–500 μm). Only 7 of these are detected in X-rays individually. We performed an X-ray stacking analysis for the 86 undetected DOGs. We find that the AGN fraction in DOGs increases with 24 μm flux and that it is higher than that of the general 24 μm population. However, no significant difference is found when considering only X-ray detections. This strongly motivates the combined use of X-ray and far-IR surveys to successfully probe a wider population of AGNs, particularly for the most obscured ones.

ID: 150532
Type: article
Authors: Suh, Hyewon; Civano, Francesca; Hasinger, Günther; Lusso, Elisabeta; Marchesi, Stefano; Schulze, Andreas; Onodera, Masato; Rosario, David J.; Sanders, David B.
Abstract: We investigate the multi-wavelength properties of host galaxies of 3701 X-ray-selected active galactic nuclei (AGNs) out to z ∼ 5 in the Chandra-COSMOS Legacy Survey. Thanks to the extensive multi-wavelength photometry available in the COSMOS field, we derive AGN luminosities, host stellar masses, and star formation rates (SFRs) via a multi-component SED fitting technique. Type 1 and Type 2 AGNs follow the same intrinsic L _{2–10 keV}–L _{6 μm} relation, suggesting that mid-infrared emission is a reasonably good measure of the AGN accretion power regardless of obscuration. We find that there is a strong increase in Type 1 AGN fraction toward higher AGN luminosity, possibly due to the fact that Type 1 AGNs tend to be hosted by more massive galaxies. The AGN luminosity and SFR are consistent with an increase toward high stellar mass, while the M _stellar dependence is weaker toward the high-mass end, which could be interpreted as a consequence of quenching both star formation and AGN activity in massive galaxies. AGN host galaxies tend to have SFRs that are consistent with normal star-forming galaxies, independent of AGN luminosities. We confirm that black hole accretion rate and SFR are correlated up to z ∼ 5, when forming stars. The majority (∼73%) of our AGN sample are faint in the far-infrared, implying that the moderate-luminosity AGNs seem to be still active after the star formation is suppressed. It is not certain whether AGN activity plays a role in quenching the star formation. We conclude that both AGN activity and star formation might be more fundamentally related to host stellar mass.

ID: 154634
Type: article
Authors: Vito, F.; Brandt, W. N.; Bauer, F. E.; Calura, F.; Gilli, R.; Luo, B.; Shemmer, O.; Vignali, C.; Zamorani, G.; Brusa, M.; Civano, Francesca; Comastri, A.; Nanni, R.
Abstract: Context. X-ray emission from quasars (QSOs) has been used to assess supermassive black hole accretion properties up to z ≈ 6. However, at z > 6 only ≈15 QSOs are covered by sensitive X-ray observations, preventing a statistically significant investigation of the X-ray properties of the QSO population in the first Gyr of the Universe.
Aims: We present new Chandra observations of a sample of 10 z > 6 QSOs, selected to have virial black-hole mass estimates from Mg II line spectroscopy (log {M_BH}/{M_☉}=8.5-9.6) . Adding archival X-ray data for an additional 15 z > 6 QSOs, we investigate the X-ray properties of the QSO population in the first Gyr of the Universe. In particular, we focus on the L_UV - L_X relation, which is traced by the α_ox parameter, and the shape of their X-ray spectra.
Methods: We performed photometric analyses to derive estimates of the X-ray luminosities of our z > 6 QSOs, and thus their α_ox values and bolometric corrections (K_bol = L_bol/L_X). We compared the resulting α_ox and K_bol distributions with the results found for QSO samples at lower redshift, and ran several statistical tests to check for a possible evolution of the L_UV - L_X relation. Finally, we performed a basic X-ray spectral analysis of the brightest z > 6 QSOs to derive their individual photon indices, and joint spectral analysis of the whole sample to estimate the average photon index.
Results: We detect seven of the new Chandra targets in at least one standard energy band, while two more are detected discarding energies E > 5 keV, where background dominates. We confirm a lack of significant evolution of α_ox with redshift, which extends the results from previous works up to z > 6 with a statistically significant QSO sample. Furthermore, we confirm the trend of an increasing bolometric correction with increasing luminosity found for QSOs at lower redshifts. The average power-law photon index of our sample (⟨Γ⟩ = 2.20_-0.34^+0.39 and ⟨Γ⟩ = 2.13_-0.13^+0.13 for sources with 30 net counts, respectively) is slightly steeper than, but still consistent with, typical QSOs at z = 1 - 6.
Conclusions: All of these results indicate a lack of substantial evolution of the inner accretion-disk and hot-corona structure in QSOs from low redshift to z > 6. Our data hint at generally high Eddington ratios at z > 6.

ID: 154245
Type: article
Authors: Vito, F.; Brandt, W. N.; Bauer, F. E.; Gilli, R.; Luo, B.; Zamorani, G.; Calura, F.; Comastri, A.; Mazzucchelli, C.; Mignoli, M.; Nanni, R.; Shemmer, O.; Vignali, C.; Brusa, M.; Cappelluti, N.; Civano, Francesca; Volonteri, M.
Abstract: While theoretical arguments predict that most of the early growth of supermassive black holes (SMBHs) happened during heavily obscured phases of accretion, current methods used for selecting z > 6 quasars (QSOs) are strongly biased against obscured QSOs, thus considerably limiting our understanding of accreting SMBHs during the first gigayear of the Universe from an observational point of view. We report the Chandra discovery of the first heavily obscured QSO candidate in the early universe, hosted by a close (≈5 kpc) galaxy pair at z = 6.515. One of the members is an optically classified type-1 QSO, PSO167-13. The companion galaxy was first detected as a [C II] emitter by Atacama large millimeter array (ALMA). An X-ray source is significantly (P = 0.9996) detected by Chandra in the 2-5 keV band, with _H > 2 × 10²⁴ cm^-2 and N_H > 6 × 10²³cm^-2 at 68% and 90% confidence levels, respectively. Thus, regardless of which of the two galaxies is associated with the X-ray emission, this source is the first heavily obscured QSO candidate at z > 6.