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Register a new userInferring Compton-thick AGN candidates at z>2 with Chandra using the >8 keV restframe spectral curvature
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To fully understand cosmic black hole growth we need to constrain the population of heavily obscured active galactic nuclei (AGN) at the peak of cosmic black hole growth ($zsim$1-3). Sources with obscuring column densities higher than $mathrm{10^{24}}$ atoms $mathrm{cm^{-2}}$, called Compton-thick (CT) AGN, can be identified by excess X-ray emission at $sim$20-30 keV, called the Compton hump. We apply the recently developed Spectral Curvature (SC) method to high-redshift AGN (2<z<5) detected with Chandra. This method parametrizes the characteristic Compton hump feature cosmologically redshifted into the X-ray band at observed energies <10 keV. We find good agreement in CT AGN found using the SC method and bright sources fit using their full spectrum with X-ray spectroscopy. In the Chandra deep field south, we measure a CT fraction of $mathrm{17^{+19}_{-11}%}$ (3/17) for sources with observed luminosity $mathrm{>5times 10^{43}}$ erg $mathrm{s^{-1}}$. In the Cosmological evolution survey (COSMOS), we find an observed CT fraction of $mathrm{15^{+4}_{-3}%}$ (40/272) or $mathrm{32pm11 %}$ when corrected for the survey sensitivity. When comparing to low redshift AGN with similar X-ray luminosities, our results imply the CT AGN fraction is consistent with having no redshift evolution. Finally, we provide SC equations that can be used to find high-redshift CT AGN (z>1) for current (XMM-Newton) and future (eROSITA and ATHENA) X-ray missions.
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We present a new metric that uses the spectral curvature (SC) above 10 keV to identify Compton-thick AGN in low-quality Swift BAT X-ray data. Using NuSTAR, we observe nine high SC-selected AGN. We find that high-sensitivity spectra show the majority are Compton-thick (78% or 7/9) and the remaining two are nearly Compton-thick (NH~5-8x10^23 cm^-2). We find the SC_bat and SC_nustar measurements are consistent, suggesting this technique can be applied to future telescopes. We tested the SC method on well-known Compton-thick AGN and find it is much more effective than broad band ratios (e.g. 100% using SC vs. 20% using 8-24/3-8 keV). Our results suggest that using the >10 keV emission may be the only way to identify this population since only two sources show Compton-thick levels of excess in the OIII to X-ray emission ratio (F_OIII/F_2-10 keV>1) and WISE colors do not identify most of them as AGN. Based on this small sample, we find that a higher fraction of these AGN are in the final merger stage than typical BAT AGN. Additionally, these nine obscured AGN have, on average, four times higher accretion rates than other BAT-detected AGN (Edd ratio=0.068 compared to 0.016). The robustness of SC at identifying Compton-thick AGN implies a higher fraction of nearby AGN may be Compton-thick (~22%) and the sum of black hole growth in Compton-thick AGN (Eddington ratio times population percentage), is nearly as large as mildly obscured and unobscured AGN.
The Circinus galaxy is one of the nearest obscured AGN, making it an ideal target for detailed study. Combining archival Chandra and XMM-Newton data with new NuSTAR observations, we model the 2-79 keV spectrum to constrain the primary AGN continuum and to derive physical parameters for the obscuring material. Chandras high angular resolution allows a separation of nuclear and off-nuclear galactic emission. In the off-nuclear diffuse emission we find signatures of strong cold reflection, including high equivalent-width neutral Fe lines. This Compton-scattered off-nuclear emission amounts to 18% of the nuclear flux in the Fe line region, but becomes comparable to the nuclear emission above 30 keV. The new analysis no longer supports a prominent transmitted AGN component in the observed band. We find that the nuclear spectrum is consistent with Compton-scattering by an optically-thick torus, where the intrinsic spectrum is a powerlaw of photon index $Gamma = 2.2-2.4$, the torus has an equatorial column density of $N_{rm H} = (6-10)times10^{24}$cm$^{-2}$ and the intrinsic AGN $2-10$ keV luminosity is $(2.3-5.1)times 10^{42}$ erg/s. These values place Circinus along the same relations as unobscured AGN in accretion rate-vs-$Gamma$ and $L_X$-vs-$L_{IR}$ phase space. NuSTARs high sensitivity and low background allow us to study the short time-scale variability of Circinus at X-ray energies above 10 keV for the first time. The lack of detected variability favors a Compton-thick absorber, in line with the the spectral fitting results.
The obscured accretion phase in BH growth is a key ingredient in many models linking the AGN activity with the evolution of their host galaxy. At present, a complete census of obscured AGN is still missing. The purpose of this work is to assess the reliability of the [NeV] emission line at 3426 A to pick up obscured AGN up to z~1 by assuming that [NeV] is a reliable proxy of the intrinsic AGN luminosity and using moderately deep X-ray data to characterize the amount of obscuration. A sample of 69 narrow-line (Type 2) AGN at z=0.65-1.20 were selected from the 20k-zCOSMOS Bright galaxy sample on the basis of the presence of the [NeV] emission. The X-ray properties of these galaxies were then derived using the Chandra-COSMOS coverage of the field; the X-ray-to-[NeV] flux ratio, coupled with X-ray spectral and stacking analyses, was then used to infer whether Compton-thin or Compton-thick absorption were present in these sources. Then the [NeV] luminosity function was computed to estimate the space density of Compton-thick (CT) AGN at z~0.8. Twenty-three sources were detected by Chandra, and their properties are consistent with moderate obscuration (on average, ~a few 10^{22} cm^-2). The X-ray properties of the remaining 46 X-ray undetected Type 2 AGN were derived using X-ray stacking analysis. Current data indicate that a fraction as high as ~40% of the present sample is likely to be CT. The space density of CT AGN with logL_2-10keV>43.5 at z=0.83 is (9.1+/-2.1) 10^{-6} Mpc^{-3}, in good agreement with both XRB model expectations and the previously measured space density for objects in a similar redshift and luminosity range. We regard our selection technique for CT AGN as clean but not complete, since even a mild extinction in the NLR can suppress [NeV] emission. Therefore, our estimate of their space density should be considered as a lower limit.
We report a clumpy elongated feature found with deep Chandra ACIS high-resolution imaging of the Fe K{alpha} line emission in the nuclear region of the Compton Thick Active Galactic Nucleus (CT AGN) galaxy NGC 5643. This feature extends for ~65 pc N-S. No corresponding feature is seen in the 3.0-6.0 keV continuum. The Fe K{alpha} feature is spatially consistent with the N-S elongation found in the CO(2-1) high resolution imaging with ALMA (Alonso-Herrero et al 2018), but slightly more extended than the rotating molecular disk of r=26 pc indicated by the kinematics of the CO(2-1) line. The Chandra detection of a corresponding N-S structure in the neutral Fe K{alpha} line, would argue for both CO and Fe K{alpha} emission originating from the obscuring torus.
Current measurements show that the observed fraction of Compton-thick (CT) AGN is smaller than the expected values needed to explain the cosmic X-ray background. Prior fits to the X-ray spectrum of the nearby Seyfert-2 galaxy NGC 5347 ($z=0.00792,, D =35.5 rm ~Mpc $) have alternately suggested a CT and Compton-thin source. Combining archival data from $Suzaku$, $Chandra$, and - most importantly - new data from $NuSTAR$, and using three distinct families of models, we show that NGC 5347 is an obscured CTAGN ($N_{rm H} > 2.23times 10^{24}~rm cm^{-2}$). Its 2-30~keV spectrum is dominated by reprocessed emission from distant material, characterized by a strong Fe K$alpha$ line and a Compton hump. We found a large equivalent width of the Fe K$alpha$ line ($rm EW = 2.3 pm 0.3$ keV) and a high intrinsic-to-observed flux ratio ($sim 100$). All of these observations are typical for bona fide CTAGN. We estimate a bolometric luminosity of $L_{rm bol} simeq 0.014 pm 0.005~L_{rm Edd.}$. The $Chandra$ image of NGC 5347 reveals the presence of extended emission dominating the soft X-ray spectrum ($E < 2,rm keV$), which coincides with the [O III] emission detected in the $Hubble ~Space~ Telescope$ images. Comparison to other CTAGN suggests that NGC 5347 is broadly consistent with the average properties of this source class. We simulated $XRISM$ and $Athena$/X-IFU spectra of the source, showing the potential of these future missions in identifying CTAGN in the soft X-rays.
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