The Chandra Deep Field–North Survey. XVI. The X-Ray Properties of Moderate-Luminosity Active Galaxies at [CLC][ITAL]z[/ITAL][/CLC] ] 4
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We present X-ray spectral analyses of the three z > 4 active galactic nuclei (AGNs) thus far spectroscopically identified in the Chandra Deep Field-North Survey, at redshifts of 5.186, 4.424, and 4.137. These analyses are made possible by the extremely deep exposure (≈2 Ms) and the low Chandra background. The rest-frame ≈2.5-40 keV spectra are the first for optically faint (two of the three sources have I > 24) z > 4 AGNs. The z = 5.186 quasar is well fitted by a power-law model with photon index Γ = 1.8 ± 0.3, consistent with those of lower-redshift, unobscured AGNs. The other two AGNs have flatter effective X-ray photon indices (Γ ≈ 1.1-1.5), suggesting the presence of intrinsic absorption (provided their underlying X-ray continua are similar to those of lower redshift AGNs). It is possible that the flat X-ray continuum of the z = 4.424 AGN is partially related to its radio loudness. If the z = 4.137 AGN suffers from X-ray absorption, the implied column density is NH ≈ 2 × 1023 cm-2.Cite
We extend our earlier work on X-ray source stacking in the deep XMM—Newton observation of the Lockman Hole, to the 2-Ms Chandra Deep Field North (CDF-N) and the 1-Ms Chandra Deep Field South (CDF-S). The XMM—Newton work showed the resolved fraction of the X-ray background (XRB) to be ∼80–100 per cent at ≲ 2 keV but this decreased to only ∼50 per cent above ∼8 keV. The CDF-N and CDF-S probe deeper, and are able to fill in some of the missing fraction in the 4–6 keV range, but the resolved fraction in the 6–8 keV band remains only ∼60 per cent, confirming the trend seen with XMM—Newton. The missing XRB component has a spectral shape that is consistent with a population of highly obscured active galactic nuclei (AGN) at redshifts ∼0.5–1.5 and with absorption column densities of ∼1023–1024 cm−2.
X-ray background
Source counts
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Deep Chandra X-ray exposures provide an efficient route for locating optically faint active galactic nuclei (AGN) at high redshifts. We use deep multicolor optical data to search for z>5 AGN in the 2 Ms X-ray exposure of the Chandra Deep Field-North. Of the 423 X-ray sources bright enough (z'<25.2) for a color-color analysis, at most one lies at z=5-6 and none at z>6. The z>5 object is spectroscopically confirmed at z=5.19. Only 31 of the 77 sources with z'>25.2 are undetected in the B or V bands at the 2-sigma level and could lie at z>5. There are too few moderate luminosity AGN at z=5-6.5 to ionize the intergalactic medium.
Optical spectra
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Abstract 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, erg s −1 ) identification still poses a challenge because of significant contamination from host galaxies. Based on the 7 Ms Chandra Deep Field-South (CDF-S) survey, the longest-timescale (∼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 that 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 × 10 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 anticorrelation between X-ray luminosity and variability amplitude for high-luminosity AGNs, but as the luminosity drops to ≲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.
Black hole (networking)
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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 (LBol) has been presented in the literature, based on spectral energy distribution fitting, or, for sources where these data do not exist, we calculate LBol using a bolometric correction to the X-ray luminosity, derived from a relationship between the bolometric correction and LX/L3000. From a sample of 69 X-ray bright sources (>250 counts), where Γ can be measured with greatest precision, with an estimate of LBol, 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.
Spectral index
Spectral energy distribution
Eddington luminosity
Black hole (networking)
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We present X-ray spectral analyses of the three z > 4 active galactic nuclei (AGNs) thus far spectroscopically identified in the Chandra Deep Field-North Survey, at redshifts of 5.186, 4.424, and 4.137. These analyses are made possible by the extremely deep exposure (≈2 Ms) and the low Chandra background. The rest-frame ≈2.5-40 keV spectra are the first for optically faint (two of the three sources have I > 24) z > 4 AGNs. The z = 5.186 quasar is well fitted by a power-law model with photon index Γ = 1.8 ± 0.3, consistent with those of lower-redshift, unobscured AGNs. The other two AGNs have flatter effective X-ray photon indices (Γ ≈ 1.1-1.5), suggesting the presence of intrinsic absorption (provided their underlying X-ray continua are similar to those of lower redshift AGNs). It is possible that the flat X-ray continuum of the z = 4.424 AGN is partially related to its radio loudness. If the z = 4.137 AGN suffers from X-ray absorption, the implied column density is NH ≈ 2 × 1023 cm-2.
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Abstract Arp 187 is one of the fading active galactic nuclei (AGNs), whose AGN activity is currently decreasing in luminosity. We investigate the observational signatures of AGN in Arp 187, which trace various physical scales from less than 0.1 pc to the nearly 10 kpc, to estimate the long-term luminosity change over 10 4 yr. The Very Large Array 5 GHz and 8 GHz images and the ALMA 133 GHz images reveal bimodal jet lobes with ∼5 kpc size and the absence of the central radio core. The 6dF optical spectrum shows that Arp 187 hosts a narrow line region with the estimated size of ∼1 kpc, and the line strengths give the AGN luminosity of L bol = 1.5 × 10 46 erg s −1 . On the other hand, the current AGN activity estimated from the AGN torus emission gives the upper bound of L bol < 2.2 × 10 43 erg s −1 . The absence of the radio core gives the more strict upper bound of the current AGN luminosity of L bol < 8.0 × 10 40 erg s −1 , suggesting that the central engine is already quenched. These multiwavelength signatures indicate that Arp 187 hosts a “dying” AGN: the central engine is already dead, but the large-scale AGN indicators are still observable as the remnant of the past AGN activity. The central engine has experienced the drastic luminosity decline by a factor of ∼10 3–5 fainter within ∼10 4 yr, which is roughly consistent with the viscous timescale of the inner part of the accretion disk within ∼500 yr.
Line (geometry)
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We present near-IR (J and Ks) number counts and colors of galaxies detected in deep VLT-ISAAC images centered on the Chandra Deep Field and Hubble Deep Field-South for a total area of 13.6 arcmin2. The limiting surface brightness obtained is mag/arcsec2 and (1σ) on both fields. A d /dm relation with a slope of ~0.34 in J and ~0.28 in Ks is found in both fields with no evidence of decline near the magnitude limit. The median color of galaxies becomes bluer at magnitudes fainter than , in agreement with the different number counts slope observed in the two bands. We find a fraction (≤5% of the total sample) of sources with color redder than at magnitudes . Most of them appear as isolated sources, possibly elliptical or dusty starburst galaxies at redshift . The comparison of the observed number counts with models shows that our J-band and Ks-band counts are consistent with the prediction of a model based on a small amount of merging in a cosmology. On the other hand, we fail to reproduce the observed counts if we do not consider merging independently of the parameters defining the universe.
Hubble Ultra-Deep Field
Limiting
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Low Luminosity Active Galactic Nuclei (LLAGNs) are contaminated by the light of their host galaxies, thus they cannot be detected by the usual colour techniques. For this reason their evolution in cosmic time is poorly known. Variability is a property shared by virtually all active galactic nuclei, and it was adopted as a criterion to select them using multi epoch surveys. Here we report on two variability surveys in different sky areas, the Selected Area 57 and the Chandra Deep Field South.
Cosmic time
Epoch (astronomy)
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Abstract Even in deep X-ray surveys, Compton-thick active galactic nuclei (CT AGNs, N H ≥ 1.5 × 10 24 cm −2 ) are difficult to identify due to X-ray flux suppression and their complex spectral shapes. However, the study of CT AGNs is vital for understanding the rapid growth of black holes and the origin of the cosmic X-ray background. In the local universe, the fraction of CT AGNs accounts for 30% of the whole AGN population. We expect a higher fraction of CT AGNs in deep X-ray surveys; however, only 10% of AGNs have been identified as CT AGNs in the 7 Ms Chandra Deep Field-South survey. In this work, we select 51 AGNs with abundant multiwavelength data. Using the method of the mid-infrared (mid-IR) excess, we select hitherto unknown eight CT AGN candidates in our sample. Seven of these candidates can be confirmed as CT AGNs based on the multiwavelength identification approach, and a new CT AGN (XID 133) is identified through the mid-IR diagnostics. We also discuss the X-ray origin of these eight CT AGNs and the reason why their column densities were underestimated in previous studies. We find that the multiwavelength approaches of selecting CT AGNs are highly efficient, provided the high quality of observational data. We also find that CT AGNs have a higher Eddington ratio than non-CT AGNs, and that both CT AGNs and non-CT AGNs show similar properties of host galaxies.
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