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Register a new userRevealing a Population of Heavily Obscured Active Galactic Nuclei at z=0.5-1 in the Chandra Deep Field-South
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(abridged) We identify a numerically significant population of heavily obscured AGNs at z~0.5-1 in the Chandra Deep Field-South (CDF-S) and Extended Chandra Deep Field-South by selecting 242 X-ray undetected objects with infrared-based star formation rates (SFRs) substantially higher (a factor of 3.2 or more) than their SFRs determined from the UV after correcting for dust extinction. An X-ray stacking analysis of 23 candidates in the central CDF-S region using the 4 Ms Chandra data reveals a hard X-ray signal with an effective power-law photon index of Gamma=0.6_{-0.4}^{+0.3}, indicating a significant contribution from obscured AGNs. Based on Monte Carlo simulations, we conclude that 74+-25% of the selected galaxies host obscured AGNs, within which ~95% are heavily obscured and ~80% are Compton-thick (CT; NH>1.5x10^{24} cm^{-2}). The heavily obscured objects in our sample are of moderate intrinsic X-ray luminosity [ ~ (0.9-4)x10^{42} erg/s in the 2-10 keV band]. The space density of the CT AGNs is (1.6+-0.5)x10^{-4} Mpc^{-3}. The z~0.5-1 CT objects studied here are expected to contribute ~1% of the total XRB flux in the 10-30 keV band, and they account for ~5-15% of the emission in this energy band expected from all CT AGNs according to population-synthesis models. In the 6--8 keV band, the stacked signal of the 23 heavily obscured candidates accounts for <5% of the unresolved XRB flux, while the unresolved ~25% of the XRB in this band can probably be explained by a stacking analysis of the X-ray undetected optical galaxies in the CDF-S (a 2.5 sigma stacked signal). We discuss prospects to identify such heavily obscured objects using future hard X-ray observatories.
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We investigate early black hole (BH) growth through the methodical search for $zgtrsim5$ AGN in the $Chandra$ Deep Field South. We base our search on the $Chandra$ 4-Ms data with flux limits of $9.1times 10^{-18}$ (soft, 0.5 - 2 keV) and $5.5times 10^{-17} mathrm{erg} mathrm{s}^{-1} mathrm{cm}^{-2}$ (hard, 2 - 8 keV). At $zsim5$ this corresponds to luminosities as low as $sim10^{42}$ ($sim10^{43}$) $mathrm{erg} mathrm{s}^{-1}$ in the soft (hard) band and should allow us to detect Compton-thin AGN with $M_mathrm{BH}>10^7 M_{odot}$ and Eddington ratios > 0.1. Our field ($0.03~mathrm{deg}^2$) contains over 600 $zsim5$ Lyman Break Galaxies. Based on lower redshift relations we would expect $sim20$ of them to host AGN. After combining the $Chandra$ data with GOODS/ACS, CANDELS/WFC3 and $Spitzer$/IRAC data, the sample consists of 58 high-redshift candidates. We run a photometric redshift code, stack the GOODS/ACS data, apply colour criteria and the Lyman Break Technique and use the X-ray Hardness Ratio. We combine our tests and using additional data find that all sources are most likely at low redshift. We also find five X-ray sources without a counterpart in the optical or infrared which might be spurious detections. We conclude that our field does not contain any convincing $zgtrsim5$ AGN. Explanations for this result include a low BH occupation fraction, a low AGN fraction, short, super-Eddington growth modes, BH growth through BH-BH mergers or in optically faint galaxies. By searching for $zgtrsim5$ AGN we are setting the foundation for constraining early BH growth and seed formation scenarios.
The 4 Ms Chandra Deep Field-South (CDF-S) and other deep X-ray surveys have been highly effective at selecting active galactic nuclei (AGN). However, cosmologically distant low-luminosity AGN (LLAGN) have remained a challenge to identify due to significant contribution from the host galaxy. We identify long-term X-ray variability (~month-years, observed frame) in 20 of 92 CDF-S galaxies spanning redshifts z~0.08-1.02 that do not meet other AGN selection criteria. We show that the observed variability cannot be explained by X-ray binary populations or ultraluminous X-ray sources, so the variability is most likely caused by accretion onto a supermassive black hole. The variable galaxies are not heavily obscured in general, with a stacked effective power-law photon index of Gamma_stack~1.93+/-0.13, and are therefore likely LLAGN. The LLAGN tend to lie a factor of ~6-80 below the extrapolated linear variability-luminosity relation measured for luminous AGN. This may be explained by their lower accretion rates. Variability-independent black-hole mass and accretion-rate estimates for variable galaxies show that they sample a significantly different black-hole mass-accretion rate space, with masses a factor of 2.4 lower and accretion rates a factor of 22.5 lower than variable luminous AGN at the same redshift. We find that an empirical model based on a universal broken power-law PSD function, where the break frequency depends on SMBH mass and accretion rate, roughly reproduces the shape, but not the normalization, of the variability-luminosity trends measured for variable galaxies and more luminous AGN.
We present results from a spectral analysis of a sample of high-redshift (z>3) X-ray selected AGN in the 4 Ms Chandra Deep Field South (CDF-S), the deepest X-ray survey to date. The sample is selected using the most recent spectroscopic and photometric information available in this field. It consists of 34 sources with median redshift z=3.7, 80 median net counts in the 0.5-7 keV band and median rest-frame absorption-corrected luminosity $L_{2-10 rmn{keV}}approx1.5times10^{44}rmn{erg} rmn{s^{-1}}$. Spectral analysis for the full sample is presented and the intrinsic column density distribution, corrected for observational biases using spectral simulations, is compared with the expectations of X-ray background (XRB) synthesis models. We find that $approx57$ per cent of the sources are highly obscured ($N_H>10^{23}rmn{cm^{-2}}$). Source number counts in the $0.5-2rmn{keV}$ band down to flux $F_{0.5-2 rmn{keV}}approx4times10^{-17}rmn{erg} rmn{s^{-1}cm^{-2}}$ are also presented. Our results are consistent with a decline of the AGN space density at z>3 and suggest that, at those redshifts, the AGN obscured fraction is in agreement with the expectations of XRB synthesis models.
We present NuSTAR hard X-ray (3-79 keV) observations of three Type 2 quasars at z ~ 0.4-0.5, optically selected from the Sloan Digital Sky Survey (SDSS). Although the quasars show evidence for being heavily obscured Compton-thick systems on the basis of the 2-10 keV to [OIII] luminosity ratio and multiwavelength diagnostics, their X-ray absorbing column densities (N_H) are poorly known. In this analysis: (1) we study X-ray emission at >10 keV, where X-rays from the central black hole are relatively unabsorbed, in order to better constrain N_H; (2) we further characterize the physical properties of the sources through broad-band near-UV to mid-IR spectral energy distribution (SED) analyses. One of the quasars is detected with NuSTAR at >8 keV with a no-source probability of <0.1%, and its X-ray band ratio suggests near Compton-thick absorption with N_H gtrsim 5 x 10^23 cm^-2. The other two quasars are undetected, and have low X-ray to mid-IR luminosity ratios in both the low energy (2-10 keV) and high energy (10-40 keV) X-ray regimes that are consistent with extreme, Compton-thick absorption (N_H gtrsim 10^24 cm^-2). We find that for quasars at z ~ 0.5, NuSTAR provides a significant improvement compared to lower energy (<10 keV) Chandra and XMM-Newton observations alone, as higher column densities can now be directly constrained.
In deep X-ray surveys, active galactic nuclei (AGNs) with a broad range of luminosities have been identified. However, cosmologically distant low-luminosity AGN (LLAGN, $L_{mathrm{X}} lesssim 10^{42}$ erg s$^{-1}$) identification still poses a challenge due to significant contamination from host galaxies. Based on the 7 Ms Chandra Deep Field-South (CDF-S) survey, the longest timescale ($sim 17$ years) deep X-ray survey to date, we utilize an X-ray variability selection technique to search for LLAGNs that remain unidentified among the CDF-S X-ray sources. We find 13 variable sources from 110 unclassified CDF-S X-ray sources. Except for one source which could be an ultraluminous X-ray source, the variability of the remaining 12 sources is most likely due to accreting supermassive black holes. These 12 AGN candidates have low intrinsic X-ray luminosities, with a median value of $7 times10^{40}$ erg s$^{-1}$. They are generally not heavily obscured, with an average effective power-law photon index of 1.8. The fraction of variable AGNs in the CDF-S is independent of X-ray luminosity and is only restricted by the total number of observed net counts, confirming previous findings that X-ray variability is a near-ubiquitous property of AGNs over a wide range of luminosities. There is an anti-correlation between X-ray luminosity and variability amplitude for high-luminosity AGNs, but as the luminosity drops to $lesssim 10^{42}$ erg s$^{-1}$, the variability amplitude no longer appears dependent on the luminosity. The entire observed luminosity-variability trend can be roughly reproduced by an empirical AGN variability model based on a broken power-law power spectral density function.
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