arXiv subjects

D. M. Alexander

Publications and source records attributed to D. M. Alexander.

At least 163 records · Page 9Linked to original sources

arXiv2014

NuSTAR J033202-2746.8: direct constraints on the Compton reflection in a heavily obscured quasar at z~2

We report NuSTAR observations of NuSTAR J033202-2746.8, a heavily obscured, radio-loud quasar detected in the Extended Chandra Deep Field-South, the deepest layer of the NuSTAR extragalactic survey (~400 ks, at its deepest). NuSTAR J033202-2746.8 is reliably detected by NuSTAR only at E>8 keV and has a very flat spectral slope in the NuSTAR energy band (Gamma=0.55^{+0.62}_{-0.64}; 3-30 keV). Combining the NuSTAR data with extremely deep observations by Chandra and XMM-Newton (4 Ms and 3 Ms, respectively), we constrain the broad-band X-ray spectrum of NuSTAR J033202-2746.8, indicating that this source is a heavily obscured quasar (N_H=5.6^{+0.9}_{-0.8}x10^23 cm^-2) with luminosity L_{10-40 keV}~6.4x10^44 erg s^-1. Although existing optical and near-infrared (near-IR) data, as well as follow-up spectroscopy with the Keck and VLT telescopes, failed to provide a secure redshift identification for NuSTAR J033202-2746.8, we reliably constrain the redshift z=2.00+/-0.04 from the X-ray spectral features (primarily from the iron K edge). The NuSTAR spectrum shows a significant reflection component (R=0.55^{+0.44}_{-0.37}), which was not constrained by previous analyses of Chandra and XMM-Newton data alone. The measured reflection fraction is higher than the R~0 typically observed in bright radio-loud quasars such as NuSTAR J033202-2746.8, which has L_{1.4 GHz}~10^27 W Hz^-1. Constraining the spectral shape of AGN, including bright quasars, is very important for understanding the AGN population, and can have a strong impact on the modeling of the X-ray background. Our results show the importance of NuSTAR in investigating the broad-band spectral properties of quasars out to high redshift.

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arXiv2014

{\it NuSTAR} Reveals an Intrinsically X-ray Weak Broad Absorption Line Quasar in the Ultraluminous Infrared Galaxy Markarian 231

We present high-energy (3--30 keV) {\it NuSTAR} observations of the nearest quasar, the ultraluminous infrared galaxy (ULIRG) Markarian 231 (Mrk 231), supplemented with new and simultaneous low-energy (0.5--8 keV) data from {\it Chandra}. The source was detected, though at much fainter levels than previously reported, likely due to contamination in the large apertures of previous non-focusing hard X-ray telescopes. The full band (0.5--30 keV) X-ray spectrum suggests the active galactic nucleus (AGN) in Mrk 231 is absorbed by a patchy and Compton-thin (N$_{\rm H} \sim1.2^{+0.3}_{-0.3}\times10^{23}$ cm$^{-2}$) column. The intrinsic X-ray luminosity (L$_{\rm 0.5-30 keV}\sim1.0\times10^{43}$ erg s$^{1}$) is extremely weak relative to the bolometric luminosity where the 2--10 keV to bolometric luminosity ratio is $\sim$0.03% compared to the typical values of 2--15%. Additionally, Mrk 231 has a low X-ray-to-optical power law slope ($α_{\rm OX}\sim-1.7$). It is a local example of a low-ionization broad absorption line (LoBAL) quasar that is intrinsically X-ray weak. The weak ionizing continuum may explain the lack of mid-infrared [O IV], [Ne V], and [Ne VI] fine-structure emission lines which are present in sources with otherwise similar AGN properties. We argue that the intrinsic X-ray weakness may be a result of the super-Eddington accretion occurring in the nucleus of this ULIRG, and may also be naturally related to the powerful wind event seen in Mrk 231, a merger remnant escaping from its dusty cocoon.

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arXiv2014

NuSTAR Observations of Heavily Obscured Quasars at z ~ 0.5

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.

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arXiv2013

A wide search for obscured Active Galactic Nuclei using XMM-Newton and WISE

We use a combination of the XMM-Newton serendipitous X-ray survey with the optical SDSS, and the infrared WISE all-sky survey in order to check the efficiency of the low X-ray to infrared luminosity selection method in finding heavily obscured AGN. We select sources in the 2-8 keV X-ray band which have a redshift determination in the SDSS catalogue. We match this sample with the WISE catalogue, and fit the SEDs of the 2844 sources which have three, or more, photometric data-points in the infrared. We then select the heavily obscured AGN candidates by comparing their 12 micron AGN luminosity to the observed 2-10 keV X-ray luminosity and their expected intrinsic relation. With this approach we find 20 candidates, and we examine their X-ray and optical spectra. Of the 20 initial candidates, we find nine (64%; out of the 14, for which X-ray spectra could be fit) based on the X-ray spectra, and seven (78%; out of the nine detected spectroscopically in the SDSS) based on the [OIII] line fluxes. Combining all criteria, we determine the final number of heavily obscured AGN to be 12-19, and the number of Compton-thick AGN to be 2-5, showing that the method is reliable in finding obscured AGN, but not Compton-thick. However those numbers are smaller than what would be expected from X-ray background population synthesis models, which demonstrates how the optical-infrared selection and the scatter of the L_x-L_MIR relation introduced by observational constraints limit the efficiency of the method. Finally, we test popular obscured AGN selection methods based on mid-infrared colours, and find that the probability of an AGN to be selected by its mid-infrared colours increases with the X-ray luminosity. However, a selection scheme based on a relatively low X-ray luminosity and mid-infrared colours characteristic of QSOs would not select ~25% of the heavily obscured AGN of our sample. (abridged)

astro-ph.CO↗

arXiv2013

An ALMA Survey of Submillimeter Galaxies in the Extended Chandra Deep Field South: The AGN Fraction and X-ray Properties of Submillimeter Galaxies

The large gas and dust reservoirs of submm galaxies (SMGs) could potentially provide ample fuel to trigger an Active Galactic Nucleus (AGN), but previous studies of the AGN fraction in SMGs have been controversial largely due to the inhomogeneity and limited angular resolution of the available submillimeter surveys. Here we set improved constraints on the AGN fraction and X-ray properties of the SMGs with ALMA and Chandra observations in the Extended Chandra Deep Field-South (E-CDF-S). This study is the first among similar works to have unambiguously identified the X-ray counterparts of SMGs; this is accomplished using the fully submm-identified, statistically reliable SMG catalog with 99 SMGs from the ALMA LABOCA E-CDF-S Submillimeter Survey (ALESS). We found 10 X-ray sources associated with SMGs (median redshift z = 2.3), of which 8 were identified as AGNs using several techniques that enable cross-checking. The other 2 X-ray detected SMGs have levels of X-ray emission that can be plausibly explained by their star-formation activity. 6 of the 8 SMG-AGNs are moderately/highly absorbed, with N_H > 10e23 cm-2. An analysis of the AGN fraction, taking into account the spatial variation of X-ray sensitivity, yields an AGN fraction of 17+16-6% for AGNs with rest-frame 0.5-8 keV absorption-corrected luminosity >7.8x10e42 erg s-1; we provide estimated AGN fractions as a function of X-ray flux and luminosity. ALMA's high angular resolution also enables direct X-ray stacking at the precise positions of SMGs for the first time, and we found 4 potential SMG-AGNs in our stacking sample.

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arXiv2013

The NuSTAR Extragalactic Survey: A First Sensitive Look at the High-Energy Cosmic X-ray Background Population

We report on the first ten identifications of sources serendipitously detected by the NuSTAR to provide the first sensitive census of the cosmic X-ray background (CXB) source population at >10 keV. We find that these NuSTAR-detected sources are ~100x fainter than those previously detected at >10 keV and have a broad range in redshift and luminosity (z=0.020-2.923 and L_10-40 keV~4x10^{41}-5x10^{45} erg/s); the median redshift and luminosity are z~0.7 and L_10-40 keV~3x10^{44} erg/s, respectively. We characterize these sources on the basis of broad-band ~0.5-32 keV spectroscopy, optical spectroscopy, and broad-band ultraviolet-to-mid-infrared SED analyzes. We find that the dominant source population is quasars with L_10-40 keV>10^{44} erg/s, of which ~50% are obscured with N_H>10^{22} cm^{-2}. However, none of the ten NuSTAR sources are Compton thick (N_H>10^{24} cm^{-2}) and we place a 90% confidence upper limit on the fraction of Compton-thick quasars (L_10-40 keV>10^{44} erg/s) selected at >10 keV of ~33% over the redshift range z=0.5-1.1. We jointly fitted the rest-frame ~10-40 keV data for all of the non-beamed sources with L_10-40 keV>10^{43} erg/s to constrain the average strength of reflection; we find R<1.4 for Γ=1.8, broadly consistent with that found for local AGNs observed at >10 keV. We also constrain the host galaxy masses and find a median stellar mass of ~10^{11} M_sun, a factor ~5 times higher than the median stellar mass of nearby high-energy selected AGNs, which may be at least partially driven by the order of magnitude higher X-ray luminosities of the NuSTAR sources. Within the low source-statistic limitations of our study, our results suggest that the overall properties of the NuSTAR sources are broadly similar to those of nearby high-energy selected AGNs but scaled up in luminosity and mass.

astro-ph.HE↗

arXiv2013

Weak Hard X-ray Emission from Two Broad Absorption Line Quasars Observed with NuSTAR: Compton-thick Absorption or Intrinsic X-ray Weakness?

We present NuSTAR hard X-ray observations of two X-ray weak broad absorption line (BAL) quasars, PG 1004+130 (radio loud) and PG 1700+518 (radio quiet). Many BAL quasars appear X-ray weak, probably due to absorption by the shielding gas between the nucleus and the accretion-disk wind. The two targets are among the optically brightest BAL quasars, yet they are known to be significantly X-ray weak at rest-frame 2-10 keV (16-120 times fainter than typical quasars). We would expect to obtain ~400-600 hard X-ray (>10 keV) photons with NuSTAR, provided that these photons are not significantly absorbed (NH<1E24 cm^{-2}). However, both BAL quasars are only detected in the softer NuSTAR bands (e.g., 4-20 keV) but not in its harder bands (e.g., 20-30 keV), suggesting that either the shielding gas is highly Compton-thick or the two targets are intrinsically X-ray weak. We constrain the column densities for both to be NH~7E24 cm^{-2} if the weak hard X-ray emission is caused by obscuration from the shielding gas. We discuss a few possibilities for how PG 1004+130 could have Compton-thick shielding gas without strong Fe K? line emission; dilution from jet-linked X-ray emission is one likely explanation. We also discuss the intrinsic X-ray weakness scenario based on a coronal-quenching model relevant to the shielding gas and disk wind of BAL quasars. Motivated by our NuSTAR results, we perform a Chandra stacking analysis with the Large Bright Quasar Survey BAL quasar sample and place statistical constraints upon the fraction of intrinsically X-ray weak BAL quasars; this fraction is likely 17-40%.

astro-ph.CO↗

arXiv2013

New insights on the distant AGN population

Current X-ray surveys have proved to be essential tools in order to identify and study AGNs across cosmic time. However, there is evidence that the most heavily obscured AGNs are largely missing even in the deepest surveys. The search for these obscured AGNs is one of the most outstanding issues of extragalactic astronomy, since they are expected to make a major contribution to the high energy peak of the X-ray background (XRB) and might constitute a particularly active and dusty phase of black hole and galaxy evolution. Using a newly developed SED fitting technique to decompose the AGN from the star-formation emission in the infrared band we identify a sample of 17 IR bright quasars at z=1-3. For the majority of these sources the X-ray spectrum is well characterised by an absorbed power law model, revealing that ~25% of the sources are unabsorbed; the remainder of these IR quasars have moderate-to-high absorption (N_H>10^23 cm^-2), and ~25% are not detected in the X-rays and are likely to be Compton thick (N_H>10^24 cm^-2). We therefore find a much higher fraction of obscured than unobscured quasars, indicating that a large fraction of the luminous black hole accretion was very heavily obscured at z~2.

astro-ph.CO↗

arXiv2013

Reflection-dominated nuclear X-ray emission in the early-type galaxy ESO 565--G019

We present the discovery of a reflection-dominated active galactic nucleus (AGN) in the early-type radio-quiet galaxy ESO 565--G019 with Suzaku and Swift/BAT. The source X-ray spectrum below 10 keV is characteristic of other Compton-thick (CT) AGN, clearly showing an inverted continuum and prodigious fluorescence iron emission above ~3 keV. A Compton shoulder to the neutral Fe Kalpha line also appears to be present. There is evidence for long-term hard X-ray flux variability which we associate with changes in the intrinsic AGN power-law. The increasing sensitivity of ongoing and new hard X-ray surveys means that more such reflection-dominated AGN ought to be uncovered in the near future. ESO 565--G019 is hosted in an early-type galaxy whose morphology has been variously classified as either type E or type S0. Only about 20 bona fide CT-AGN have been identified in the local universe so far, and all exist in host galaxies with late Hubble types (S0 or later). CT columns of nuclear obscuring gas are uncommon in early-type galaxies in the local universe, so confirmation of the exact morphological class of ESO 565--G019 is important. Infrared photometry also shows the presence of large quantities of cool dust in the host, indicative of significant ongoing star-formation. ESO 565--G019 may be the first identified local example of minor-merger driven CT-AGN growth in an early-type host, or may be the result of interaction with its neighboring galaxy ESO 565--G018 in a wide pair.

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arXiv2013

A statistical relation between the X-ray spectral index and Eddington ratio of active galactic nuclei in deep surveys

We present an investigation into how well the properties of the accretion flow onto a supermassive black hole may be coupled to those of the overlying hot corona. To do so, we specifically measure the characteristic spectral index, Gamma, of a power-law energy distribution, over an energy range of 2 to 10 keV, for X-ray selected, broad-lined radio-quiet AGN up to z~2 in COSMOS and E-CDF-S. We test the previously reported dependence between Gamma and black hole mass, FWHM and Eddington ratio using a sample of AGN covering a broad range in these parameters based on both the Mg ii and H-alpha emission lines with the later afforded by recent near infrared spectroscopic observations using Subaru/FMOS. We calculate the Eddington ratios, lambda_Edd, for sources where a bolometric luminosity (L_Bol) has been presented in the literature, based on SED fitting, or, for sources where these data do not exist, we calculate L_Bol using a bolometric correction to the LX, derived from a relationship between the bolometric correction, and LX/L3000. From a sample of 69 X-ray bright sources (>250 counts), where Gamma can be measured with greatest precision, with an estimate of L_Bol, we find a statistically significant correlation between Gamma and lambda_Edd, which is highly significant with a chance probability of 6.59x10^-8. A statistically significant correlation between Gamma and the FWHM of the optical lines is confirmed, but at lower significance than with lambda_Edd indicating that lambda_Edd is the key parameter driving conditions in the corona. Linear regression analysis reveals that Gamma=(0.32+/-0.05)log10 lambda_Edd+(2.27+/-0.06) and Gamma=(-0.69+/-0.11)log10(FWHM/km/s)+(4.44+/-0.42). Our results on Gamma-lambda_Edd are in very good agreement with previous results. (ABRIDGED)

astro-ph.HE↗

arXiv2013

Narrow-line region gas kinematics of 24,264 optically-selected AGN: the radio connection

Using a sample of 24264 optically selected AGNs from the SDSS DR7 database, we characterise how the profile of the [OIII] emission line relates to bolometric luminosity (L_Bol), Eddington ratio, radio loudness, radio luminosity (L_Rad) and optical class (Sy 1/2) to determine what drives the kinematics of this kpc-scale line emitting gas. Spectral stacking is used to characterise how the average [OIII] profile changes as a function of these variables. After accounting for the correlation between L_Bol and L_Rad, we report that L_Rad has the strongest influence on the [OIII] profile, with moderate radio luminosity AGNs (log(L_Rad)=23-25 W/Hz) having the broadest [OIII] profiles. When binned according to Eddington ratio, only AGNs in our highest bin (i.e., R_Edd>0.3) show any signs of having broadened [OIII] profiles, although the small numbers of such extreme AGNs mean we cannot rule out that other processes (e.g., radio jets) are responsible for this broadening. We find no significant difference between the [OIII] profiles of broad and narrow-line Seyfert 1s. The [OIII] profiles of Sy 1 and 2 AGNs show the same trends in terms of line width, but Sy 1 AGNs display a much stronger blue wing, which we interpret as evidence of outflowing ionised gas. Using multi-component line fitting we calculate the proportions of AGNs with broad [OIII] profiles, finding that moderate radio luminosity AGNs are roughly 5 times more likely to have extremely broad [OIII] lines (FWHM>1000km/s) compared to lower L_Rad AGNs. Our results are consistent with the most disturbed gas kinematics being induced by compact radio cores (rather than powerful radio jets), although broadened [OIII] lines are also present, but much rarer, in low L_Rad systems (<10^23 W/Hz). Our catalogue of multi-component fits is freely available as an online resource.

astro-ph.CO↗

arXiv2013

An ALMA survey of submillimeter galaxies in the Extended Chandra Deep Field South: Source catalog and multiplicity

We present an Atacama Large Millimeter/submillimeter Array (ALMA) Cycle 0 survey of 126 submillimeter sources from the LABOCA ECDFS Submillimeter Survey (LESS). Our 870 micron survey with ALMA (ALESS) has produced maps ~3X deeper and with a beam area ~200X smaller than the original LESS observations, doubling the current number of interferometrically-observed submillimeter sources. The high resolution of these maps allows us to resolve sources that were previously blended and accurately identify the origin of the submillimeter emission. We discuss the creation of the ALESS submillimeter galaxy (SMG) catalog, including the main sample of 99 SMGs and a supplementary sample of 32 SMGs. We find that at least 35% (possibly up to 50%) of the detected LABOCA sources have been resolved into multiple SMGs, and that the average number of SMGs per LESS source increases with LESS flux density. Using the (now precisely known) SMG positions, we empirically test the theoretical expectation for the uncertainty in the single-dish source positions. We also compare our catalog to the previously predicted radio/mid-infrared counterparts, finding that 45% of the ALESS SMGs were missed by this method. Our ~1.6" resolution allows us to measure a size of ~9 kpc X 5 kpc for the rest-frame ~300 um emission region in one resolved SMG, implying a star formation rate surface density of 80 M_sol yr^-1 kpc^-2, and we constrain the emission regions in the remaining SMGs to be <10 kpc. As the first statistically reliable survey of SMGs, this will provide the basis for an unbiased multiwavelength study of SMG properties.

astro-ph.CO↗

arXiv2013

The Mean Star Formation Rate of X-ray selected Active Galaxies and its Evolution from z=2.5: Results from PEP-Herschel

(Abridged) We study relationships between the SFR and the nuclear properties of X-ray selected AGNs out to z=2.5, using far-IR data in three extragalactic deep fields as part of the PACS Evolutionary Probe (PEP) program. Guided by studies of intrinsic infra-red AGN SEDs, we show that the majority of the FIR emission in AGNs is produced by cold dust heated by star-formation. We uncover characteristic redshift-dependent trends between the mean FIR luminosity (L_fir) and accretion luminosity (L_agn) of AGNs. At low AGN luminosities, accretion and SFR are uncorrelated at all redshifts, consistent with a scenario where most low-luminosity AGNs are primarily fueled by secular processes in their host galaxies. At high AGN luminosities, a significant correlation is observed between L_fir and L_agn, but only among AGNs at low and moderate redshifts (z<1). We interpret this as a signature of the increasing importance of major-mergers in driving both the growth of super-massive black holes (SMBHs) and global star-formation in their hosts at high AGN luminosities. However, we also find that the enhancement of SFR in luminous AGNs weakens or disappears at high redshifts (z>1). This suggests that the role of mergers in SMBH-galaxy co-evolution is less important at these epochs. At all redshifts, we find essentially no relationship between L_fir and nuclear obscuration across five orders of magnitude in obscuring column density, suggesting that various different mechanisms are likely to be responsible for obscuring X-rays in active galaxies. We explain our results within a scenario in which two different modes of SMBH fueling operate among low- and high-luminosity AGNs. We postulate, guided by emerging knowledge about the properties of high redshift galaxies, that the dominant mode of accretion among high-luminosity AGNs evolves with redshift.

astro-ph.CO↗

arXiv2013

Concurrent Supermassive Black Hole and Galaxy Growth: Linking Environment and Nuclear Activity in z = 2.23 H-alpha Emitters

We present results from a ~100 ks Chandra observation of the 2QZ Cluster 1004+00 structure at z = 2.23 (hereafter, 2QZ Clus). 2QZ Clus was originally identified as an overdensity of four optically-selected QSOs at z = 2.23 within a 15x15 arcmin^2 region. Narrow-band imaging in the near-IR revealed that the structure contains an additional overdensity of 22 z = 2.23 Halpha-emitting galaxies (HAEs), resulting in 23 unique z = 2.23 HAEs/QSOs. Our Chandra observations reveal that 3 HAEs in addition to the 4 QSOs harbor powerfully accreting supermassive black holes (SMBHs), with 2-10 keV luminosities of ~(8-60) x 10^43 ergs/s and X-ray spectral slopes consistent with unobscured AGN. Using a large comparison sample of 210 HAEs in Chandra-COSMOS (C-COSMOS), we find suggestive evidence that the AGN fraction increases with local HAE galaxy density. The 2QZ Clus HAEs reside in a moderately overdense environment (a factor of ~2 times over the field), and after excluding optically-selected QSOs, we find the AGN fraction is a factor of ~3.5^+3.8_-2.2 times higher than C-COSMOS HAEs in similar environments. Using stacking analyses of the Chandra data and Herschel SPIRE observations at 250 um, we respectively estimate mean SMBH accretion rates (Mdot) and star-formation rates (SFRs) for the 2QZ Clus and C-COSMOS samples. We find that the mean 2QZ Clus HAE stacked 2-10 keV luminosity is QSO-like (~6-10 x 10^43 ergs/s), and the implied Mdot/SFR \sim (1.6-3.2) x10^-3 is broadly consistent with the local MBH/M* relation and z ~ 2 X-ray selected AGN. The C-COSMOS HAEs are on average an order of magnitude less X-ray luminous and have Mdot/SFR \sim (0.2-0.4) x10^-3, comparable to z ~ 1-2 star-forming galaxies with similar mean X-ray luminosities. We estimate that a periodic QSO phase with duty cycle ~2-8% would be sufficient to bring the star-forming galaxies onto the local MBH/M* relation.

astro-ph.CO↗

arXiv2012

No clear submillimetre signature of suppressed star formation amongst X-ray luminous AGNs

Many theoretical models require powerful active galactic nuclei (AGNs) to suppress star formation in distant galaxies and reproduce the observed properties of today's massive galaxies. A recent study based on Herschel-SPIRE submillimetre observations claimed to provide direct support for this picture, reporting a significant decrease in the mean star-formation rates (SFRs) of the most luminous AGNs (Lx>10^44 erg/s) at z=1-3 in the Chandra Deep Field-North (CDF-N). In this letter we extend these results using Herschel-SPIRE 250um data in the COSMOS and CDF-S fields to achieve an order of magnitude improvement in the number of sources at Lx>10^44 erg/s. On the basis of our analysis, we find no strong evidence for suppressed star formation in Lx>10^44 erg/s AGNs at z=1-3. The mean SFRs of the AGNs are constant over the broad X-ray luminosity range of Lx~10^43-10^45 erg/s (with mean SFRs consistent with typical star-forming galaxies at z~2; ~100-200 Msol/yr). We suggest that the previous CDF-N results were likely due to low number statistics. We discuss our results in the context of current theoretical models.

astro-ph.CO↗

arXiv2012

The high-redshift (z>3) AGN population in the 4 Ms Chandra Deep Field South

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 $L_{2-10 \rmn{keV}}\approx1.5\times10^{44}\rmn{erg} \rmn{s^{-1}}$. 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 $0.5-2\rmn{keV}$ band down to flux $F_{0.5-2 \rmn{keV}}\approx4\times10^{-17}\rmn{erg} \rmn{s^{-1}cm^{-2}}$ 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.

astro-ph.CO↗

arXiv2012

Inverse Compton X-ray halos around high-z radio galaxies: A feedback mechanism powered by far-infrared starbursts or the CMB?

We report the detection of extended X-ray emission around two powerful high-z radio galaxies (HzRGs) at z~3.6 (4C03.24 & 4C19.71) and use these to investigate the origin of extended, Inverse Compton (IC) powered X-ray halos at high z. The halos have X-ray luminosities of Lx~3e44 erg/s and sizes of ~60kpc. Their morphologies are broadly similar to the ~60-kpc long radio lobes around these galaxies suggesting they are formed from IC scattering by relativistic electrons in the radio lobes, of either CMB or FIR photons from the dust-obscured starbursts in these galaxies. These observations double the number of z>3 HzRGs with X-ray detected IC halos. We compare the IC X-ray to radio luminosity ratios for these new detections to the two previously detected z~3.8 HzRGs. Given the similar redshifts, we would expect comparable X-ray IC luminosities if CMB mm photons are the seed field for the IC emission. Instead the two z~3.6 HzRGs, which are ~4x fainter in the FIR, also have ~4x fainter X-ray IC emission. Including a further six z>2 radio sources with IC X-ray halos from the literature, we suggest that in the more compact (lobe sizes <100-200kpc), majority of radio sources, the bulk of the IC emission may be driven by scattering of locally produced FIR photons from luminous, dust-obscured starbursts within these galaxies, rather than CMB photons. The resulting X-ray emission can ionise the gas on ~100-200-kpc scales around these systems and thus form their extended Ly-alpha emission line halos. The starburst and AGN activity in these galaxies are thus combining to produce an effective and wide-spread "feedback" process, acting on the long-term gas reservoir for the galaxy. If episodic radio activity and co-eval starbursts are common in massive, high-z galaxies, then this IC-feedback mechanism may affect the star-formation histories of massive galaxies. [Abridged]

astro-ph.CO↗

arXiv2012

GOODS-Herschel: radio-excess signature of hidden AGN activity in distant star-forming galaxies

We present here a new spectral energy distribution (SED) fitting approach that we adopt to select radio-excess sources amongst distant star-forming galaxies in the GOODS-Herschel (North) field and to reveal the presence of hidden, highly obscured AGN. Through extensive SED analysis of 458 galaxies with radio 1.4 GHz and mid-IR 24 um detections using some of the deepest Chandra X-ray, Spitzer and Herschel infrared, and VLA radio data available to date, we have robustly identified a sample of 51 radio-excess AGN (~1300 deg^-2) out to redshift z~3. These radio-excess AGN have a significantly lower far-IR/radio ratio (q<1.68) than the typical relation observed for star-forming galaxies (q~2.2). We find that ~45% of these radio-excess sources have a dominant AGN component in the mid-IR band, while for the remainders the excess radio emission is the only indicator of AGN activity. The fraction of radio-excess AGN increases with X-ray luminosity reaching ~60% at Lx~10^44-10^45 erg/s, making these sources an important part of the total AGN population. However, almost half (24/51) of these radio-excess AGN are not detected in the deep Chandra X-ray data, suggesting that some of these sources might be heavily obscured. We also find that the specific star formation rates (sSFRs) of the radio-excess AGN are on average lower that those observed for X-ray selected AGN hosts, indicating that our sources are forming stars more slowly than typical AGN hosts, and possibly their star formation is progressively quenching.

astro-ph.CO↗