Compton Thick Research Articles

AGN populations in the local Universe: Their alignment with the main sequence, stellar population characteristics, accretion efficiency, and the impact of AGN feedback

In this study, we used a sample of 338 galaxies – within the redshift range of 0.02 < z < 0.1 drawn from the Sloan Digital Sky Survey (SDSS) – for which there are available classifications based on their emission line ratios. We identified and selected Compton-thick (CT) AGN through the use of X-ray and infrared luminosities at 12 μm. We constructed the spectral energy distributions (SEDs) for all sources and fit them using the CIGALE code to derive properties related to both the AGN and host galaxies. Employing stringent criteria to ensure the reliability of SED measurements, our final sample comprises 14 CT AGN, 118 Seyfert 2 (Sy2), 82 composite, and 124 low-ionization nuclear emission-line regions (LINER) galaxies. Our analysis reveals that, irrespective of their classification, the majority of the sources lie below the star-forming main sequence (MS). Additionally, a lower level of AGN activity is associated with a closer positioning to the MS. Using the Dn4000 spectral index as a proxy for the age of stellar populations, we observe that, compared to other AGN classes, LINERs exhibit the oldest stellar populations. Conversely, CT sources are situated in galaxies with the youngest stellar populations. Furthermore, LINER and composite galaxies tend to show the lowest accretion efficiency, while CT AGN, on average, display the most efficient accretion among the four AGN populations. Our findings are consistent with a scenario in which the different AGN populations might not originate from the same AGN activity burst. Early triggers in gas-rich environments can create high-accretion-rate supermassive black holes (SMBHs), leading to a progression from CT to Sy2, while later triggers in gas-poor stages result in low-accretion-rate SMBHs like those found in LINERs.

X-ray spectral properties of dust-obscured galaxies in the XMM-SERVS coverage of the XMM-LSS field

ABSTRACT With an aim to unveil the population of obscured active galactic nuclei (AGN) hosted in high-z dust-obscured galaxies (DOGs), we performed X-ray spectral study of 34 DOGs (0.59 ≤ z ≤ 4.65) lying within 5.3 square degrees of the XMM-Spitzer Extragalactic Representative Volume Survey (XMM-SERVS) coverage in the XMM-Large Scale Structure (XMM-LSS) field. To improve the spectral quality of individual sources, we combined all the existing XMM−Newton data and also included Chandra/Advanced CCD Imaging Spectrometer (ACIS) data, whenever available. We find that the X-ray spectra of our DOGs can be fitted with a simple absorbed power law or with a physically motivated borus02 model. The line-of-sight column densities (NH) in our sources span across a wide range (1.02 × 1022 cm−2 ≤ NH ≤ 1.21 × 1024 cm−2), with a substantial fraction (∼17.6 per cent) of them being heavily obscured (NH ≥ 1023 cm−2). We also identified one new Compton-thick (CT)-AGN candidate, yielding the CT-AGN fraction in our sample to be only 3 per cent. The absorption-corrected 2.0–10 keV X-ray luminosities of our sources (2.00 × 1043 erg s−1 ≤ $L_{\rm 2-10~keV}^{\rm int}$ ≤ 6.17 × 1045 erg s−1) suggest them to be luminous quasars. The NH versus Eddington ratio diagnostic plot infers that our sample consists of a heterogeneous population that includes a small fraction (∼12 per cent) of DOGs belonging to an early phase (Hot DOGs) during which accretion and obscuration peaks, while the remaining DOGs belong to an intermediate or late phase during which radiative feedback from the dominant AGN blows away surrounding obscuring material.

TeV Neutrinos and Hard X-Rays from Relativistic Reconnection in the Corona of NGC 1068

The recent discovery of astrophysical neutrinos from the Seyfert galaxy NGC 1068 suggests the presence of nonthermal protons within a compact “coronal” region close to the central black hole. The acceleration mechanism of these nonthermal protons remains elusive. We show that a large-scale magnetic reconnection layer, of the order of a few gravitational radii, may provide such a mechanism. In such a scenario, rough energy equipartition between magnetic fields, X-ray photons, and nonthermal protons is established in the reconnection region. Motivated by recent 3D particle-in-cell simulations of relativistic reconnection, we assume that the spectrum of accelerated protons is a broken power law, with the break energy being constrained by energy conservation (i.e., the energy density of accelerated protons is at most comparable to the magnetic energy density). The proton spectrum is dnp/dEp∝Ep−1 below the break and dnp/dEp∝Ep−s above the break, with IceCube neutrino observations suggesting s ≃ 3. Protons above the break lose most of their energy within the reconnection layer via photohadronic collisions with the coronal X-rays, producing a neutrino signal in good agreement with the recent observations. Gamma rays injected in photohadronic collisions are cascaded to lower energies, sustaining the population of electron–positron pairs that makes the corona moderately Compton thick.

A machine learning algorithm for reliably predicting active galactic nucleus absorbing column densities

We present a new method for predicting the line-of-sight column density (NH) values of active galactic nuclei (AGN) based on mid-infrared (MIR), soft X-ray, and hard X-ray data. We developed a multiple linear regression machine learning algorithm trained with WISE colors, Swift-BAT count rates, soft X-ray hardness ratios, and an MIR–soft X-ray flux ratio. Our algorithm was trained off 451 AGN from the Swift-BAT sample with known NH and has the ability to accurately predict NH values for AGN of all levels of obscuration, as evidenced by its Spearman correlation coefficient value of 0.86 and its 75% classification accuracy. This is significant as few other methods can be reliably applied to AGN with Log(NH < 22.5). It was determined that the two soft X-ray hardness ratios and the MIR–soft X-ray flux ratio were the largest contributors toward accurate NH determinations. We applied the algorithm to 487 AGN from the BAT 150 Month catalog with no previously measured NH values. This algorithm will continue to contribute significantly to finding Compton-thick (CT) AGN (NH ≥ 1024 cm−2), thus enabling us to determine the true intrinsic fraction of CT-AGN in the local Universe and their contribution to the cosmic X-ray background.

A High Fraction of Heavily X-Ray-obscured Active Galactic Nuclei

We present new estimates on the fraction of heavily X-ray-obscured, Compton-thick (CT) active galactic nuclei (AGNs) out to a redshift of z ≤ 0.8. From a sample of 540 AGNs selected by mid-infrared (MIR) properties in observed X-ray survey fields, we forward model the observed-to-intrinsic X-ray luminosity ratio () with a Markov Chain Monte Carlo simulation to estimate the total fraction of CT AGNs (f CT), many of which are missed in typical X-ray observations. We create model N H distributions and convert these to using a set of X-ray spectral models. We probe the posterior distribution of our models to infer the population of X-ray-nondetected sources. From our simulation we estimate a CT fraction of f CT = . We perform an X-ray stacking analysis for sources in Chandra X-ray Observatory fields and find that the expected soft (0.5–2 keV) and hard (2–7 keV) observed fluxes drawn from our model to be within 0.48 and 0.12 dex of our stacked fluxes, respectively. Our results suggests at least 50% of all MIR-selected AGNs, possibly more, are CT (N H ≳ 1024 cm−2), which is in excellent agreement with other recent work using independent methods. This work indicates that the total number of AGNs is higher than can be identified using X-ray observations alone, highlighting the importance of a multiwavelength approach. A high f CT also has implications for black hole (BH) accretion physics and supports models of BH and galaxy coevolution that include periods of heavy obscuration.

Deep Chandra Observations of NGC 5728: Morphology and Spectral Properties of the Extended X-Ray Emission

Recent deep Chandra observations of nearby Compton-thick (CT) active galactic nuclei (AGNs) have produced surprising results, uncovering extended emission not only in soft X-rays but also in hard emission (>3 keV), challenging the long-held belief that the characteristic hard X-ray continuum and fluorescent Fe K lines are associated with the torus in the standard picture of AGNs. In this work, we present an analysis of our deep (∼261 ks) X-ray Chandra ACIS-S observations of NGC 5728, a nearby (z = 0.00932) CT AGN. We find that the diffuse emission is more extended at lower energies, in the bicone direction out to ∼2 kpc radially, but also significantly extended in the direction of the cross-cone, out to ∼1.4 kpc. Our results suggest that the ratio of detected photons in the cross-cone to the bicone region is ∼16%, below 3 keV, decreasing to 5% for energies 3–6 keV. The nuclear spectrum suggests a low-photoionization phase mixed with a more ionized gas component, while the bicone and cross-cone spectra are dominated by a mix of photoionization and shocked gas emission. A mixture of thermal and photoionization models used to fit the spectra indicates the presence of complex gas interactions, consistent with previous observations of other CT AGNs (e.g., ESO 428-G014).

Extended Hard X-Ray Emission in Highly Obscured AGNs

Kiloparsec-scale hard (>3 keV) X-ray continuum and fluorescent Fe Kα line emission has been recently discovered in nearby Compton-thick (CT) active galactic nuclei (AGNs), which opens new opportunities to improve AGN torus modeling and investigate how the central supermassive black hole interacts with and impacts the host galaxy. Following a pilot Chandra survey of nearby CT AGNs, we present in this paper the results of Chandra spatial analysis of five uniformly selected non-CT but still heavily obscured AGNs to investigate the extended hard X-ray emission by measuring the excess emission counts, excess fractions, and physical scales. Three of these AGNs show extended emission in the 3.0–7.0 keV band detected at >3σ above the Chandra point-spread function with total excess fractions ranging from ∼8% to 20%. The extent of the hard emission ranges from at least ∼250 pc to 1.1 kpc in radius. We compare these new sources with CT AGNs and find that CT AGNs appear to be more extended in the hard band than the non-CT AGNs. Similar to CT AGNs, the amounts of extended hard X-ray emission relative to the total emission of these obscured AGNs are not negligible. Together with other AGNs detected with extended hard X-ray emission in the literature, we further explore potential correlations between the extended hard X-ray component and AGN parameters. We also discuss the implications for torus modeling and AGN feedback. Considering potential contributions from X-ray binaries (XRBs) to the extended emission, we do not see strong XRB contamination in the overall sample.

The Complex X-Ray Obscuration Environment in the Radio-loud Type 2 Quasar 3C 223

3C 223 is a radio-loud, Type 2 quasar at z = 0.1365 with an intriguing X-ray Multi-mirror Mission (XMM)-Newton spectrum that implicated it as a rare, Compton-thick (N H ≳ 1.25 × 1024 cm−2) active galactic nucleus (AGN). We obtained contemporaneous XMM-Newton and Nuclear Spectroscopic Telescope Array (NuSTAR) spectra to fit the broadband X-ray spectrum with the physically motivated MYTorus and borus02 models. We confirm earlier results that the obscuring gas is patchy with both high (though not Compton-thick) levels of obscuration (N H > 1023 cm−2) and gas clouds with column densities up to an order of magnitude lower. The spectral fitting results indicate additional physical processes beyond those modeled in the spectral grids of MYTorus and borus02 impact the emergent spectrum: the Compton-scattering region may be extended beyond the putative torus; a ring of heavy Compton-thick material blocks most X-ray emission along the line of sight; or the radio jet is beamed, boosting the production of Fe Kα line photons in the global medium compared with what is observed along the line of sight. We revisit a recent claim that no radio-loud Compton-thick AGN have yet been conclusively shown to exist, finding three reported cases of radio-loud AGN with global average (but not line-of-sight) column densities that are Compton thick. Now that it is possible to separately determine line-of-sight and global column densities, inhomogeneity in the obscuring medium has consequences for how we interpet the spectrum and classify an AGN as “Compton thick.”

Jet–ISM Interaction in NGC 1167/B2 0258+35, an LINER with an AGN Past

We report the results of joint Chandra/ACIS—NuSTAR deep observations of NGC 1167, the host galaxy of the young radio jet B2 0258+35. In the ACIS data, we detect X-ray emission, extended both along and orthogonal to the jet. At the end of the southeast radio jet, we find lower-energy X-ray emission that coincides with a region of CO turbulence and fast outflow motions. This suggests that the hot interstellar medium (ISM) may be compressed by the jet and molecular outflow, resulting in more efficient cooling. Hydrodynamic simulations of jet–ISM interaction tailored to NGC 1167 are in agreement with this conclusion and with the overall morphology and spectra of the X-ray emission. The faint hard nuclear source detected with Chandra and the stringent NuSTAR upper limits on the harder X-ray emission show that the active galactic nucleus (AGN) in NGC 1167 is in a very low-accretion state. However, the characteristics of the extended X-ray emission are more consonant to those of luminous Compton-thick (CT) AGNs, suggesting that we may be observing the remnants of a past high accretion rate episode, with sustained strong activity lasting ∼2 × 103 yr. We conclude that NGC1167 is presently a Low-Ionization Nuclear Emission-line Region (LINER) , but was an AGN in the past, given the properties of the extended X-ray emission and their similarity with those of CT AGN extended emission.

AGN−Host Interaction in IC 5063. I. Large-scale X-Ray Morphology and Spectral Analysis

Abstract We report the analysis of the deep (∼270 ks) X-ray Chandra data of one of the most radio-loud, Seyfert 2 galaxies in the nearby universe (z = 0.01135), IC 5063. The alignment of the radio structure with the galactic disk and ionized bicone, enables us to study the effects of both radio jet and nuclear irradiation on the interstellar medium (ISM). The nuclear and bicone spectra suggest a low photoionization phase mixed with a more ionized or thermal gas component, while the cross-cone spectrum is dominated by shocked and collisionally ionized gas emission. The clumpy morphology of the soft (<3 keV) X-ray emission along the jet trails, and the large (≃2.4 kpc) filamentary structure perpendicular to the radio jets at softer energies (<1.5 keV), suggest a large contribution of the jet−ISM interaction to the circumnuclear gas emission. The hard X-ray continuum (>3 keV) and the Fe Kα 6.4 keV emission are both extended to kpc size along the bicone direction, suggesting an interaction of nuclear photons with dense clouds in the galaxy disk, as observed in other Compton Thick (CT) active nuclei. The northwest cone spectrum also exhibits an Fe xxv emission line, which appears spatially extended and spatially correlated with the most intense radio hot-spot, suggesting jet−ISM interaction.

Extended X-Ray Emission in Compton Thick AGN with Deep Chandra Observations

Abstract We present the spatial analysis of five Compton thick (CT) active galactic nuclei (AGNs), including MKN 573, NGC 1386, NGC 3393, NGC 5643, and NGC 7212, for which high-resolution Chandra observations are available. For each source, we find hard X-ray emission (>3 keV) extending to ∼kiloparsec scales along the ionization cone, and for some sources, in the cross-cone region. This collection represents the first, high-signal sample of CT AGN with extended hard X-ray emission for which we can begin to build a more complete picture of this new population of AGN. We investigate the energy dependence of the extended X-ray emission, including possible dependencies on host galaxy and AGN properties, and find a correlation between the excess emission and obscuration, suggesting a connection between the nuclear obscuring material and the galactic molecular clouds. Furthermore, we find that the soft X-ray emission extends farther than the hard X-rays along the ionization cone, which may be explained by a galactocentric radial dependence on the density of molecular clouds due to the orientation of the ionization cone with respect to the galactic disk. These results are consistent with other CT AGN with observed extended hard X-ray emission (e.g., ESO 428-G014 and the Ma et al. CT AGN sample), further demonstrating the ubiquity of extended hard X-ray emission in CT AGN.

Is Extended Hard X-Ray Emission Ubiquitous in Compton-thick AGN?

Abstract The recent Chandra discovery of extended ∼kiloparsec-scale hard (>3 keV) X-ray emission in nearby Compton-thick (CT) active galactic nuclei (AGN) opens a new window to improving AGN torus modeling and investigating how the central supermassive black hole interacts with and impacts the host galaxy. Since there are only a handful of detections so far, we need to establish a statistical sample to determine the ubiquity of the extended hard X-ray emission in CT AGN and quantify the amount and extent of this component. In this paper, we present the spatial analysis results of a pilot Chandra imaging survey of seven nearby ( 0.006 < z < 0.013 ) CT AGN selected from the Swift-BAT spectroscopic AGN survey. We find that five out of the seven CT AGN show extended emission in the 3–7 keV band detected at >3σ above the Chandra point-spread function (PSF), with ∼12%–22% of the total emission in the extended components. ESO 137-G034 and NGC 3281 display biconical ionization structures with extended hard X-ray emission reaching kiloparsec scales (∼1.9 and 3.5 kpc in diameter). The other three show extended hard X-ray emission above the PSF out to at least ∼360 pc in radius. We find a trend that a minimum 3–7 keV count rate of 0.01 counts s−1 and total excess fraction >20% are required to detect a prominent extended hard X-ray component. Given that this extended hard X-ray component appears to be relatively common in this uniformly selected CT AGN sample, we further discuss the implications for torus modeling and AGN feedback.

Nature of Compton-thick Active Galactic Nuclei in “Nonmerging” Luminous Infrared Galaxies UGC 2608 and NGC 5135 Revealed with Broadband X-Ray Spectroscopy

Abstract We have analyzed the broadband X-ray spectra of active galactic nuclei (AGNs) in two “nonmerging” luminous infrared galaxies (LIRGs), UGC 2608 and NGC 5135, utilizing the data of Nuclear Spectroscopic Telescope Array, Suzaku, XMM-Newton, and Chandra. Applying the X-ray clumpy-torus model (XCLUMPY), we find that both sources have similar spectra characterized by Compton-thick (CT) absorption ( ∼ 5–7 × 1024 cm−2) and small torus angular width (σ < 20°). The intrinsic 2–10 keV luminosities are × 1043 erg s−1 (UGC 2608) and × 1043 erg s−1 (NGC 5135). The [O iv]-to-nuclear-12 μm luminosity ratios are larger than those of typical Seyferts, which are consistent with the torus covering factors ( 0.7) estimated from the torus angular widths and column densities by X-ray spectroscopy. The torus covering factors and Eddington ratios ( ∼ 0.1) follow the relation found by Ricci et al. for local AGNs, implying that their tori become geometrically thin, due to significant radiation pressure of the AGN that blows out some part of the tori. These results indicate that the CT AGNs in these “nonmerger” LIRGs are just a normal AGN population seen edge-on through a large line-of-sight column density. They are in contrast to the buried CT AGNs in late-stage mergers that have large torus covering factors even at large Eddington ratios.

NuSTAR observations of four nearby X-ray faint AGNs: low luminosity or heavy obscuration?

ABSTRACT We present NuSTAR (Nuclear Spectroscopic Telescope Array) observations of four active galactic nuclei (AGNs) located within 15 Mpc. These AGNs, namely ESO 121-G6, NGC 660, NGC 3486, and NGC 5195, have observed X-ray luminosities of L2–10 keV,obs ≲ 1039 erg s−1, classifying them as low-luminosity AGN (LLAGN). We perform broad-band X-ray spectral analysis for the AGN by combining our NuSTAR data with Chandra or XMM–Newton observations to directly measure their column densities (NH) and infer their intrinsic power. We complement our X-ray data with archival and new high-angular resolution mid-infrared (mid-IR) data for all objects, except NGC 5195. Based on our X-ray spectral analysis, we found that both ESO 121-G6 and NGC 660 are heavily obscured (NH > 1023 cm−2; L2–10 keV,int ∼ 1041 erg s−1), and NGC 660 may be Compton thick. We also note that the X-ray flux and spectral slope for ESO 121-G6 have significantly changed over the last decade, indicating significant changes in the obscuration and potentially accretion rate. On the other hand, NGC 3486 and NGC 5195 appear to be unobscured and just mildly obscured, respectively, with L2–10 keV,int < 1039 erg s−1, i.e. genuine LLAGN. Both of the heavily obscured AGNs have Lbol > 1041 erg s−1 and λEdd ≳ 10−3, and are detected in high-angular resolution mid-IR imaging, indicating the presence of obscuring dust on nuclear scale. NGC 3486, however, is undetected in high-resolution mid-IR imaging, and the current data do not provide stringent constraints on the presence or absence of obscuring nuclear dust in the AGN.

Local AGN survey (LASr): I. Galaxy sample, infrared colour selection, and predictions for AGN within 100 Mpc

ABSTRACT To answer major questions on supermassive black hole (SMBH) and galaxy evolution, a complete census of SMBH growth, i.e. active galactic nuclei (AGN), is required. Thanks to all-sky surveys by the Wide-field Infrared Survey Explorer (WISE) and the Spectrum-Roentgen-Gamma (SRG) missions, this task is now feasible in the nearby Universe. We present a new survey, the Local AGN Survey (LASr), with the goal of identifying AGN unbiased against obscuration and determining the intrinsic Compton-thick (CT) fraction. We construct the most complete all-sky galaxy sample within 100 Mpc ($90{{\ \rm per\ cent}}$ completeness for log (M*/M⊙) ∼ 9.4), four times deeper than the current reference, the Two Micron All-Sky Survey Redshift Survey (2MRS), which misses ${\sim}20{{\ \rm per\ cent}}$ of known luminous AGN. These 49k galaxies serve as parent sample for LASr, called LASr-GPS. It contains 4.3k already known AGN, $\ge 82{{\ \rm per\ cent}}$ of these are estimated to have $L^\mathrm{nuc}(12\, \mu \mathrm{m})\lt 10^{42.3}$ erg s−1, i.e. are low-luminosity AGN. As a first method for identifying Seyfert-like AGN, we use WISE-based infrared colours, finding 221 galaxies at $L^\mathrm{nuc}(12\, \mu \mathrm{m})\ge 10^{42.3}$ erg s−1 to host an AGN at $90{{\ \rm per\ cent}}$ reliability. This includes 61 new AGN candidates and implies an optical type 2 fraction of 50–71 per cent. We quantify the efficiency of this technique and estimate the total number of AGN with $L^\mathrm{int}(\rm {2-10\,keV})\ge 10^{42}$ erg s−1 in the volume to be $362^{+145}_{-116}$ ($8.6^{+3.5}_{-2.8}\, \times$ 10−5 Mpc−3). X-ray brightness estimates indicate the CT fraction to be 40–55 per cent to explain the Swift non-detections of the infrared selected objects. One third of the AGN within 100 Mpc remain to be identified, and we discuss the prospects for the eROSITA all-sky survey to detect them.

Chandra Observations of NGC 7212: Large-scale Extended Hard X-Ray Emission

Abstract Recent observations of nearby Compton thick (CT) active galactic nuclei (AGNs) with Chandra have resolved hard (>3 keV) X-ray emission extending out from the central supermassive black hole to kiloparsec scales, challenging the long-held belief that the characteristic hard X-ray continuum and fluorescent Fe K lines originate in the inner ∼parsec due to the excitation of obscuring material. In this paper we present the results of the most recent Chandra ACIS-S observations of NGC 7212, a CT AGN in a compact group of interacting galaxies, with a total effective exposure of ∼150 ks. We find ∼20% of the observed emission is found outside of the central kiloparsec, with ∼17% associated with the soft X-rays, and ∼3% with hard X-ray continuum and Fe K line. This emission is extended both along the ionization cone and in the cross-cone direction up to ∼3.8 kpc scales. The spectrum of NGC 7212 is best represented by a mixture of thermal and photoionization models that indicate the presence of complex gas interactions. These observations are consistent with what is observed in other CT AGN (e.g., ESO 428–G014, NGC 1068), providing further evidence that this may be a common phenomenon. High-resolution observations of extended CT AGN provide an especially valuable environment for understanding how AGN feedback impacts host galaxies on galactic scales.

The relationship between X-ray and optical absorbers in active galactic nuclei

ABSTRACT We present a large compilation of reddening estimates from broad-line Balmer decrements for active galactic nuclei (AGNs) with measured X-ray column densities. The median reddening is E(B − V) ≈ 0.77 ± 0.10 for type-1 to type-1.9 AGNs with reported column densities. This is notably higher than the median reddening of AGNs from the SDSS. We attribute this to the selection bias of the SDSS towards blue AGNs. For other AGNs, we find evidence of a publication bias against reporting low column densities. We find a significant correlation between NH and E(B − V) but with a large scatter of ±1 dex. On average, the X-ray columns are consistent with those predicted from E(B − V) for a solar neighbourhood dust-to-gas ratio. We argue that the large scatter of column densities and reddenings can be explained by X-ray column density variability. For AGNs with detectable broad-line regions (BLRs) that have undergone significant changes of Seyfert type (‘changing-look’ AGNs), we do not find any statistically significant differences in NH or E(B − V) compared to standard type-1 to type-1.9 AGNs. There is no evidence for any type-1 AGNs being Compton thick. We also analyse type-2 AGNs and find no significant correlation between NH and narrow-line region reddening. We find no evidence for a previously claimed anticorrelation. The median column density of LINERs is 22.68 ± 0.75 compared to a column density of 22.90 ± 0.28 for type-2 AGNs. We find the majority of low column density type-2 AGNs are LINERs, but NH is probably underestimated because of scattered light.

NuSTAR Discovery of a Compton-thick, Dust-obscured Galaxy: WISE J0825+3002

Abstract We report the discovery of a Compton-thick (CT), dust-obscured galaxy at z = 0.89, WISE J082501.48+300257.2 (WISE 0825+3002), observed by the Nuclear Spectroscopic Telescope Array. X-ray analysis with the XCLUMPY model revealed that hard X-ray luminosity in the rest-frame 2–10 keV band of WISE 0825+3002 is L X (2–10 keV) = erg s−1 while its hydrogen column density is N H = cm−2, indicating that WISE 0825+3002 is a mildly CT active galactic nucleus (AGN). We performed spectral energy distribution (SED) fitting with CIGALE to derive its stellar mass, star formation rate, and infrared luminosity. The estimated Eddington ratio based on stellar mass and integration of the best-fit SED of the AGN component is λ Edd = 0.70, which suggests that WISE 0825+3002 harbors an actively growing black hole behind a large amount of gas and dust. We found that the relationship between the luminosity ratio of X-ray and 6 μm, and Eddington ratio, follows an empirical relation for AGNs reported by Toba et al.

Compton-thick active galactic nuclei from the 7 Ms observation in the Chandra Deep Field South

We present the X-ray spectroscopic study of the Compton-thick (CT) active galactic nuclei (AGN) population within the Chandra Deep Field South (CDF-S) by using the deepest X-ray observation to date, the Chandra 7 Ms observation of the CDF-S. We combined an optimized version of our automated selection technique and a Bayesian Monte Carlo Markov chains (MCMC) spectral fitting procedure, to develop a method to pinpoint and then characterize candidate CT AGN as less model dependent and/or data-quality dependent as possible. To obtain reliable automated spectral fits, we only considered the sources detected in the hard (2−8 keV) band from the CDF-S 2 Ms catalog with either spectroscopic or photometric redshifts available for 259 sources. Instead of using our spectral analysis to decide if an AGN is CT, we derived the posterior probability for the column density, and then we used it to assign a probability of a source being CT. We also tested how the model-dependence of the spectral analysis, and the spectral data quality, could affect our results by using simulations. We finally derived the number density of CT AGN by taking into account the probabilities of our sources being CT and the results from the simulations. Our results are in agreement with X-ray background synthesis models, which postulate a moderate fraction (25%) of CT objects among the obscured AGN population.

Water masers in Compton-thick AGN

Aims. Investigations of H2O maser galaxies at X-ray energies reveal that most active galactic nuclei (AGN) associated with water masers are characterized by high levels of absorption. With the aim of finding new maser sources for possible interferometric follow-ups, we have searched for water maser emission in a well-defined sample of heavily absorbed AGN (NH > 1023 cm−2), including Compton-thick (CT) sources. Methods. Previous surveys already searched for 22 GHz water maser emission in all the galaxies in this sample. With the goal of providing a detection or a stringent upper limit on the H2O luminosity, we reobserved some of the non-detected sources with the Green Bank Telescope. A new luminous H2O maser (LH2O ∼ 200 L⊙) was detected in the mid-IR-bright Seyfert 2 galaxy IRAS 15480−0344 and then followed-up with the Very Long Baseline Array. In order to shed light on the origin of the maser (jet, outflow, or disk), we recently observed the radio continuum emission in IRAS 15480-0344 with the European VLBI network (EVN) at 1.7 and 5.0 GHz. Results. With the newly discovered megamaser in IRAS 15480−0344 revealing a narrow (∼0.6 km s−1) and a broad (∼90 km s−1) component, the maser detection rate of the CT AGN sample is 50% (18/36), which is one of the highest ever found in maser surveys. The EVN maps show two bright sources (labeled SW and NE) in the nuclear region of IRAS 15480−0344, which we interpret as jet knots tracing regions where the radio plasma impacts dense molecular clouds. The narrow maser feature is approximately at the center of the imaginary line connecting the two continuum sources, likely pinpointing the core, and might be associated with the accretion disk or a nuclear outflow. The location of the broad maser feature, instead, coincides with source NE, suggesting that the maser emission might be produced by a jet-cloud interaction, as it was proposed for NGC 1068 and Mrk 348.