We investigate the dependence of black hole accretion rate (BHAR) on host-galaxy star formation rate (SFR) and stellar mass (M*) in the CANDELS/GOODS-South field in the redshift range of . Our sample ...consists of galaxies, allowing us to probe galaxies with and/or . We use sample-mean BHAR to approximate long-term average BHAR. Our sample-mean BHARs are derived from the Chandra Deep Field-South 7 Ms observations, while the SFRs and M* have been estimated by the CANDELS team through spectral energy distribution fitting. The average BHAR is correlated positively with both SFR and M*, and the BHAR-SFR and BHAR-M* relations can both be described acceptably by linear models with a slope of unity. However, BHAR appears to be correlated more strongly with M* than SFR. This result indicates that M* is the primary host-galaxy property related to supermassive black hole (SMBH) growth, and the apparent BHAR-SFR relation is largely a secondary effect due to the star-forming main sequence. Among our sources, massive galaxies ( ) have significantly higher BHAR/SFR ratios than less massive galaxies, indicating that the former have higher SMBH fueling efficiency and/or higher SMBH occupation fraction than the latter. Our results can naturally explain the observed proportionality between and M* for local giant ellipticals and suggest that their is higher than that of local star-forming galaxies. Among local star-forming galaxies, massive systems might have higher compared to dwarfs.
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 $ \left(\log\frac{M_{\mathrm{BH}}}{M_\odot}=8.5{-}9.6\right) $ . 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 LUV − LX relation, which is traced by the αox 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 αox values and bolometric corrections (Kbol = Lbol/LX). We compared the resulting αox and Kbol distributions with the results found for QSO samples at lower redshift, and ran several statistical tests to check for a possible evolution of the LUV − LX 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 αox 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 ⟨ Γ ⟩ = 2 . 20 − 0.34 + 0.39 $ \langle\Gamma\rangle=2.20_{-0.34}^{+0.39} $ and ⟨Γ⟩ = 2.13−0.13+0.13 ⟨ Γ ⟩ = 2 . 13 − 0.13 + 0.13 $ \langle\Gamma\rangle=2.13_{-0.13}^{+0.13} $ for sources with < 30 and > 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.
We investigate the population of high-redshift (3 ≤ z < 6) active galactic nuclei (AGN) selected in the two deepest X-ray surveys, the 7 Ms Chandra Deep Field-South and 2 Ms Chandra Deep Field-North. ...Their outstanding sensitivity and spectral characterization of faint sources allow us to focus on the sub-L* regime (logLX ≲ 44), poorly sampled by previous works using shallower data, and the obscured population. Taking fully into account the individual photometric-redshift probability distribution functions, the final sample consists of ≈102 X-ray-selected AGN at 3 ≤ z < 6. The fraction of AGN obscured by column densities logNH > 23 is ∼0.6-0.8, once incompleteness effects are taken into account, with no strong dependence on redshift or luminosity. We derived the high-redshift AGN number counts down to F0.5-2 keV = 7 × 10-18 erg cm-2 s-1, extending previous results to fainter fluxes, especially at z > 4. We put the tightest constraints to date on the low-luminosity end of AGN luminosity function at high redshift. The space density, in particular, declines at z > 3 at all luminosities, with only a marginally steeper slope for low-luminosity AGN. By comparing the evolution of the AGN and galaxy densities, we suggest that such a decline at high luminosities is mainly driven by the underlying galaxy population, while at low luminosities there are hints of an intrinsic evolution of the parameters driving nuclear activity. Also, the black hole accretion rate density and star formation rate density, which are usually found to evolve similarly at z ≲ 3, appear to diverge at higher redshifts.
We exploit the 7 Ms Chandra observations in the Chandra Deep Field-South (CDF-S), the deepest X-ray survey to date, coupled with CANDELS/GOODS-S data, to measure the total X-ray emission arising from ...2076 galaxies at 3.5 ≤ z < 6.5. This aim is achieved by stacking the Chandra data at the positions of optically selected galaxies, reaching effective exposure times of ≥109s. We detect significant (>3.7σ) X-ray emission from massive galaxies at z ≈ 4. We also report the detection of massive galaxies at z ≈ 5 at a 99.7 per cent confidence level (2.7σ), the highest significance ever obtained for X-ray emission from galaxies at such high redshifts. No significant signal is detected from galaxies at even higher redshifts. The stacking results place constraints on the BHAD associated with the known high-redshift galaxy samples, as well as on the SFRD at high redshift, assuming a range of prescriptions for X-ray emission due to X- ray binaries. We find that the X-ray emission from our sample is likely dominated by processes related to star formation. Our results show that low-rate mass accretion on to SMBHs in individually X-ray-undetected galaxies is negligible, compared with the BHAD measured for samples of X-ray detected AGN, for cosmic SMBH mass assembly at high redshift. We also place, for the first time, constraints on the faint-end of the AGN X-ray luminosity function (logLX ∼ 42) at z > 4, with evidence for fairly flat slopes. The implications of all of these findings are discussed in the context of the evolution of the AGN population at high redshift.
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 < 1.14 net counts in the 0.5–2 keV band, although the current positional uncertainty does not allow a conclusive association with either PSO167–13 or its companion galaxy. From X-ray photometry and hardness-ratio arguments, we estimated an obscuring column density of NH > 2 × 1024 cm−2 and NH > 6 × 1023 cm−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.
We present an investigation into how well the properties of the accretion flow on to a supermassive black hole may be coupled to those of the overlying hot corona. To do so, we specifically measure ...the characteristic spectral index, Γ, of a power-law energy distribution, over an energy range of 2-10 keV, for X-ray selected, broad-lined radio-quiet active galactic nuclei (AGN) up to z ∼ 2 in Cosmic Evolution Survey (COSMOS) and Extended Chandra Deep Field South (E-CDF-S). We test the previously reported dependence between Γ and black hole mass, full width at half-maximum (FWHM) and Eddington ratio using a sample of AGN covering a broad range in these parameters based on both the Mg ii and Hα emission lines with the later afforded by recent near-infrared spectroscopic observations using Subaru/Fibre Multi Object Spectrograph. We calculate the Eddington ratios, λEdd, for sources where a bolometric luminosity (L
Bol) has been presented in the literature, based on spectral energy distribution fitting, or, for sources where these data do not exist, we calculate L
Bol using a bolometric correction to the X-ray luminosity, derived from a relationship between the bolometric correction and L
X/L
3000. From a sample of 69 X-ray bright sources (>250 counts), where Γ can be measured with greatest precision, with an estimate of L
Bol, we find a statistically significant correlation between Γ and λEdd, which is highly significant with a chance probability of 6.59× 10−8. A statistically significant correlation between Γ and the FWHM of the optical lines is confirmed, but at lower significance than with λEdd indicating that λEdd is the key parameter driving conditions in the corona. Linear regression analysis reveals that Γ = (0.32 ± 0.05) log10λEdd + (2.27 ± 0.06) and Γ = (−0.69 ± 0.11) log10(FWHM/km s−1) + (4.44 ± 0.42). Our results on Γ-λEdd are in very good agreement with previous results. While the Γ-λEdd relationship means that X-ray spectroscopy may be used to estimate black hole accretion rate, considerable dispersion in the correlation does not make this viable for single sources, however could be valuable for large X-ray spectral samples, such as those to be produced by eROSITA.
ABSTRACT We perform long-term ( 15 years, observed-frame) X-ray variability analyses of the 68 brightest radio-quiet active galactic nuclei (AGNs) in the 6 Ms Chandra Deep Field-South survey; the ...majority are in the redshift range of 0.6-3.1, providing access to penetrating rest-frame X-rays up to 10-30 keV. Of the 68 sources, 24 are optical spectral type I AGNs, and the rest (44) are type II AGNs. The timescales probed in this work are among the longest for X-ray variability studies of distant AGNs. Photometric analyses reveal widespread photon flux variability: 90% of AGNs are variable above a 95% confidence level, including many X-ray obscured AGNs and several optically classified type II quasars. We characterize the intrinsic X-ray luminosity ( ) and absorption ( ) variability via spectral fitting. Most (74%) sources show variability; the variability amplitudes are generally smaller for quasars. A Compton-thick candidate AGN shows variability of its high-energy X-ray flux, indicating the size of reflecting material to be 0.3 pc. variability is also detected in a broad absorption line quasar. The variability amplitude for our sample appears to rise as time separation increases. About 16% of sources show variability. One source transitions from an X-ray unobscured to obscured state, while its optical classification remains type I; this behavior indicates the X-ray eclipsing material is not large enough to obscure the whole broad-line region.
ABSTRACT We present an X-ray and multiwavelength study of 33 weak emission-line quasars (WLQs) and 18 quasars that are analogs of the extreme WLQ, PHL 1811, at -2.9. New Chandra 1.5-9.5 ks ...exploratory observations were obtained for 32 objects while the others have archival X-ray observations. Significant fractions of these luminous type 1 quasars are distinctly X-ray weak compared to typical quasars, including 16 (48%) of the WLQs and 17 (94%) of the PHL 1811 analogs with average X-ray weakness factors of 17 and 39, respectively. We measure a relatively hard ( ) effective power-law photon index for a stack of the X-ray weak subsample, suggesting X-ray absorption, and spectral analysis of one PHL 1811 analog, J1521+5202, also indicates significant intrinsic X-ray absorption. We compare composite Sloan Digital Sky Survey spectra for the X-ray weak and X-ray normal populations and find several optical-UV tracers of X-ray weakness, e.g., Fe ii rest-frame equivalent width (REW) and relative color. We describe how orientation effects under our previously proposed "shielding-gas" scenario can likely unify the X-ray weak and X-ray normal populations. We suggest that the shielding gas may naturally be understood as a geometrically thick inner accretion disk that shields the broad line region from the ionizing continuum. If WLQs and PHL 1811 analogs have very high Eddington ratios, the inner disk could be significantly puffed up (e.g., a slim disk). Shielding of the broad emission-line region by a geometrically thick disk may have a significant role in setting the broad distributions of C iv REW and blueshift for quasars more generally.
ABSTRACT We present X-ray source catalogs for the 7 Ms exposure of the Chandra Deep Field-South (CDF-S), which covers a total area of 484.2 arcmin2. Utilizing wavdetect for initial source detection ...and ACIS Extract for photometric extraction and significance assessment, we create a main source catalog containing 1008 sources that are detected in up to three X-ray bands: 0.5-7.0 keV, 0.5-2.0 keV, and 2-7 keV. A supplementary source catalog is also provided, including 47 lower-significance sources that have bright ( ) near-infrared counterparts. We identify multiwavelength counterparts for 992 (98.4%) of the main-catalog sources, and we collect redshifts for 986 of these sources, including 653 spectroscopic redshifts and 333 photometric redshifts. Based on the X-ray and multiwavelength properties, we identify 711 active galactic nuclei (AGNs) from the main-catalog sources. Compared to the previous 4 Ms CDF-S catalogs, 291 of the main-catalog sources are new detections. We have achieved unprecedented X-ray sensitivity with average flux limits over the central 1 arcmin2 region of 1.9 × 10−17, 6.4 × 10−18, and 2.7 × 10−17 erg cm−2 s−1 in the three X-ray bands, respectively. We provide cumulative number-count measurements observing, for the first time, that normal galaxies start to dominate the X-ray source population at the faintest 0.5-2.0 keV flux levels. The highest X-ray source density reaches 50,500 deg−2, and 47% 4% of these sources are AGNs ( 23,900 deg−2).
Utilizing 21 new Chandra observations as well as archival Chandra, ROSAT, and XMM-Newton data, we study the X-ray properties of a representative sample of 59 of the most optically luminous quasars in ...the universe (M sub(f) approximately -29.3 to -30.2) spanning a redshift range of z approximately 1.5 4.5. Our full sample consists of 32 quasars from the Sloan Digital Sky Survey (SDSS) Data Release 3 (DR3) quasar catalog, two additional objects in the DR3 area that were missed by the SDSS selection criteria, and 25 comparably luminous quasars at z unk 4. This is the largest X-ray study of such luminous quasars to date. By jointly fitting the X-ray spectra of our sample quasars, excluding radio-loud and broad absorption line (BAL) objects, we find a mean X-ray power-law photon index of Gamma = 1.92 super(+) sub(-) super(0) sub(0) super(.) sub(.) super(0) sub(0) super(9) sub(8) and constrain any neutral intrinsic absorbing material to have a mean column density of N sub(H) unk 2 x 10 super(21) cm super(-2). We find, consistent with other studies, that Gamma does not change with redshift, and we constrain the amount of allowed Gamma evolution for the most luminous quasars. Our sample, excluding radio-loud and BAL quasars, has a mean X-ray-to-optical spectral slope of alpha sub(ox) = -1.80 plus or minus 0.02, as well as no significant evolution of alpha sub(ox) with redshift. We also comment on the X-ray properties of a number of notable quasars, including an X-ray-weak quasar with several strong narrow absorption line systems, a mildly radio-loud BAL quasar, and a well-studied gravitationally lensed quasar.
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