Search

We have determined the relation between the AGN luminosities at rest-frame 6 {\mu}m associated to the dusty torus emission and at 2-10 keV energies using a complete, X-ray flux limited sample of 232 AGN drawn from the Bright Ultra-hard XMM-Newton Survey. The objects have intrinsic X-ray luminosities between 10^42 and 10^46 erg/s and redshifts from 0.05 to 2.8. The rest-frame 6 {\mu}m luminosities were computed using data from the Wide-Field Infrared Survey Explorer and are based on a spectral energy distribution decomposition into AGN and galaxy emission. The best-fit relationship for the full sample is consistent with being linear, L_6 {\mu}m $\propto$ L_2-10 keV^0.99$\pm$0.032, with intrinsic scatter, ~0.35 dex in log L_6 {\mu}m. The L_6 {\mu}m/L_2-10 keV luminosity ratio is largely independent on the line-of-sight X-ray absorption. Assuming a constant X-ray bolometric correction, the fraction of AGN bolometric luminosity reprocessed in the mid-IR decreases weakly, if at all, with the AGN luminosity, a finding at odds with simple receding torus models. Type 2 AGN have redder mid-IR continua at rest-frame wavelengths <12 {\mu}m and are overall ~1.3-2 times fainter at 6 {\mu}m than type 1 AGN at a given X-ray luminosity. Regardless of whether type 1 and type 2 AGN have the same or different nuclear dusty toroidal structures, our results imply that the AGN emission at rest-frame 6 {\mu}m is not isotropic due to self-absorption in the dusty torus, as predicted by AGN torus models. Thus, AGN surveys at rest-frame 6 {\mu}m are subject to modest dust obscuration biases., Comment: Accepted for publication in MNRAS. 24 pages, 10 figures and 5 tables. This version includes minor changes to the text and Table 2 in response to comments from the referee

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), Comment: Accepted for publication in MNRAS

Understanding the astrophysics of feedback in active galactic nuclei (AGN) is key to understanding the growth and co-evolution of supermassive black holes and galaxies. AGN-driven winds/outflows are potentially the most effective way of transporting energy and momentum from the nuclear scales to the host galaxy, quenching star formation by sweeping away the gas reservoir. Key questions in this field are: 1) how do accretion disks around black holes launch winds/outflows, and how much energy do these carry? 2) How are the energy and metals accelerated in winds/outflows transferred and deposited into the circumgalactic medium? X-ray observations are a unique way to address these questions because they probe the phase of the outflows which carries most of the kinetic energy. We show how a high throughput, high spectral resolution instrument like the X-ray Integral Field Unit (X-IFU) on Athena+ will allow us to address these questions by determining the physical parameters (ionization state, density, temperature, abundances, velocities, geometry, etc.) of the outflows on a dynamical time-scale, in a broad sample of nearby bright AGN. The X-IFU will also allow direct spectral imaging of the impact of these winds on the host galaxy for local AGN, forming a template for understanding AGN at higher redshifts where wind shocks cannot be resolved., Comment: Supporting paper for the science theme "The Hot and Energetic Universe" to be implemented by the Athena+ X-ray observatory (http://www.the-athena-x-ray-observatory.eu). 10 pages, 6 figures

X-ray spectroscopy of active galactic nuclei (AGN) offers the opportunity to directly probe the inner regions of the accretion disk. We present the results of our analysis of average AGN XMM-Newton X-ray spectra in the Chandra Deep Field South observation (XMM CDFS). We computed the average spectrum of a sample of 54 AGN with spectroscopic redshifts and signal-to-noise ratio S/N > 15 in the 2-12 keV rest-frame band in at least one EPIC camera. We have taken the effects of combining spectra from sources at different redshifts and both EPIC cameras into account, as well as their spectral resolution; we checked our results using thorough simulations. We explored the Fe line components of distant AGN focusing on the narrow core which arises from regions far from the central engine and on the putative relativistic component (from the accretion disk). The average spectrum shows a highly significant Fe feature. Our model-independent estimates of the equivalent width (EW) suggest a higher EW in a broader range. The line, modelled as an unresolved Gaussian, is significant at 6.8 sigma and has an EW=95 eV (full sample). The current data can be fitted equally well adding a relativistic profile to the narrow component (in the full sample, EW=140 eV and 67 eV respectively for the relativistic and narrow lines). Thanks to the high quality of the XMM CDFS spectra and to the detailed modelling of the continuum and instrumental effects, we have shown that the most distant AGN exhibit a highly significant Fe emission feature. It can be modelled both with narrow and broad lines. We found tantalising evidence for reflection by material both very close and far away from the central engine. The EW of both features are similar to those observed in individual nearby AGN, hence they must be a widespread characteristic of AGN, since otherwise the average values would be smaller than observed., Comment: Accepted by A&A. 15 pages, 12 figures

The Chandra Deep Field is the region of the sky with the highest concentration of X-ray data available: 4Ms of Chandra and 3Ms of XMM data, allowing excellent quality spectra to be extracted even for faint sources. We take advantage of this in order to compile a sample of heavily obscured Active Galactic Nuclei (AGN) using X-ray spectroscopy. We select our sample among the 176 brightest XMM sources, searching for either a) flat X-ray spectra (Photon index<1.4 at the 90% confidence level) suggestive of a reflection dominated continuum or b) an absorption turn-over suggestive of a column density higher than ~10^{24} cm-2. We find a sample of nine candidate heavily obscured sources satisfying the above criteria. Four of these show statistically significant FeKalpha lines with large equivalent widths (three out of four have EW consistent with 1 keV) suggesting that these are the most secure Compton-thick AGN candidates. Two of these sources are transmission dominated while the other two are most probably reflection dominated Compton-thick AGN. Although this sample of four sources is by no means statistically complete, it represents the best example of Compton-thick sources found at moderate-to-high redshift with three sources at z=1.2-1.5 and one source at z=3.7. Using Spitzer and Herschel observations, we estimate with good accuracy the X-ray to mid-IR (12 micron) luminosity ratio of our sources. These are well below the average AGN relation, independently suggesting that these four sources are heavily obscured., Comment: To appear in A&A. 14 pages

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., Comment: 28 pages, 13 figures, (abstract abridged). Accepted for publication in A&A

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., Comment: Published: ApJL 760:L15 (2012); 6 pages; 3 figures; Email: c.m.harrison@durham.ac.uk

In this paper we investigate the power spectrum of the unresolved 0.5-2 keV CXB with deep Chandra 4 Ms observations in the CDFS. We measured a signal which, on scales >30", is significantly higher than the Shot-Noise and is increasing with the angular scale. We interpreted this signal as the joint contribution of clustered undetected sources like AGN, Galaxies and Inter-Galactic-Medium (IGM). The power of unresolved cosmic sources fluctuations accounts for \sim 12% of the 0.5-2 keV extragalactic CXB. Overall, our modeling predicts that \sim 20% of the unresolved CXB flux is made by low luminosity AGN, \sim 25% by galaxies and \sim 55% by the IGM (Inter Galactic Medium). We do not find any direct evidence of the so called Warm Hot Intergalactic Medium (i.e. matter with 10^5K7.5 and compared it with SMBH evolution models., Comment: 15 pages, 9 figures, accepted by MNRAS

Models of galaxy evolution assume some connection between the AGN and star formation activity in galaxies. We use the multi-wavelength information of the CDFS to assess this issue. We select the AGNs from the 3Ms XMM-Newton survey and measure the star-formation rates of their hosts using data that probe rest-frame wavelengths longward of 20 um. Star-formation rates are obtained from spectral energy distribution fits, identifying and subtracting an AGN component. We divide the star-formation rates by the stellar masses of the hosts to derive specific star-formation rates (sSFR) and find evidence for a positive correlation between the AGN activity (proxied by the X-ray luminosity) and the sSFR for the most active systems with X-ray luminosities exceeding Lx=10^43 erg/s and redshifts z~1. We do not find evidence for such a correlation for lower luminosity systems or those at lower redshifts. We do not find any correlation between the SFR (or the sSFR) and the X-ray absorption derived from high-quality XMM-Newton spectra either, showing that the absorption is likely to be linked to the nuclear region rather than the host, while the star-formation is not nuclear. Comparing the sSFR of the hosts to the characteristic sSFR of star-forming galaxies at the same redshift we find that the AGNs reside mostly in main-sequence and starburst hosts, reflecting the AGN - sSFR connection. Limiting our analysis to the highest X-ray luminosity AGNs (X-ray QSOs with Lx>10^44 erg/s), we find that the highest-redshift QSOs (with z>2) reside predominantly in starburst hosts, with an average sSFR more than double that of the "main sequence", and we find a few cases of QSOs at z~1.5 with specific star-formation rates compatible with the main-sequence, or even in the "quiescent" region. (abridged), Comment: Accepted for publication in A&A

We present broad band photometry and photometric redshifts for 187611 sources located in ~0.5deg^2 in the Lockman Hole area. The catalog includes 389 X-ray detected sources identified with the very deep XMM-Newton observations available for an area of 0.2 deg^2. The source detection was performed on the Rc, z' and B band images and the available photometry is spanning from the far ultraviolet to the mid infrared, reaching in the best case scenario 21 bands. Astrometry corrections and photometric cross-calibrations over the entire dataset allowed the computation of accurate photometric redshifts. Special treatment is undertaken for the X-ray sources, the majority of which is active galactic nuclei. Comparing the photometric redshifts to the available spectroscopic redshifts we achieve for normal galaxies an accuracy of \sigma_{\Delta z/(1+z)}=0.036, with 12.7% outliers, while for the X-ray detected sources the accuracy is \sigma_{\Delta z/(1+z)}=0.069, with 18.3% outliers, where the outliers are defined as sources with |z_{phot}-z_{spec}|>0.15 (1+z_{spec})}. These results are a significant improvement over the previously available photometric redshifts for normal galaxies in the Lockman Hole, while it is the first time that photometric redshifts are computed and made public for AGN for this field., Comment: 26 pages, 19 figures. Submitted to ApJ. Revised version according to referee comments

As the mid-IR luminosity represents a good isotropic proxy of the AGN power, a low X-ray to mid-IR luminosity ratio is often claimed to be a reliable indicator for selecting Compton-thick (CT) AGN. We assess the efficiency of this diagnostic by examining the 12mu IRAS AGN sample for which high signal-to-noise XMM observations have been recently become available. We find that the vast majority (10/11) of the AGN that have been classified as CT on the basis the X-ray spectroscopy by Brightman & Nandra present a low Lx/L6 luminosity ratio, i.e. lower than a few percent of the average AGN ratio which is typical of reflection-dominated CT sources. At low Lx/L6 ratios we also find a comparable number of AGN, most of which are heavily absorbed but not CT. This implies that although most Compton-thick AGN present low Lx/L6 ratios, at least in the local, Universe, the opposite is not necessarily true. Next, we extend our analysis to higher redshifts. We perform the same analysis in the CDFS where excellent quality chandra (4 Ms) and xmm (3 Ms) X-ray spectra are available. We derive accurate X-ray luminosities for chandra sources using X-ray spectral fits, as well as 6mu luminosities from SED fits. We find 8 AGN with low Lx/L6 ratios in total, after excluding one source where the 6mu emission primarily comes from star-formation. One of these sources has been already demonstrated to host a CT nucleus, while for another one at a redshift of z=1.22 we argue it is most likely CT on the basis of its combined chandra and xmm spectrum. We find a large number of non CT contaminant with low Lx/L6 ratios. The above suggest that a low Lx/L6 ratio alone cannot ascertain the presence of a CT AGN, albeit the majority of low Lx/L6 AGN are heavily obscured. The two most reliable CT AGN in the high redshift Universe have high Lx/L6 ratios, showing that this method cannot provide complete CT AGN samples., Comment: 11 pages, to appear to A&A

Spitzer/IRS has revealed many sources with very deep Si features at 9.7micron (tau>1). We set out to investigate whether a strong Si absorption feature is a good indicator for the presence of a heavily obscured AGN. We compile X-ray spectroscopic observations available in the literature on the optically-thick,tau(9.7)>1 sources from the IRAS Seyfert sample. We find that the majority of the high-tau optically confirmed Seyferts (6/9) in this sample are probably CT. Thus we provide direct evidence for a connection between mid-IR optically-thick galaxies and CT AGN, with the success rate being close to 70% in the local Universe. This is at least comparable, if not better, than other rates obtained with photometric information in the mid to far-IR, or even mid-IR to Xray. However, this technique cannot provide complete CT AGN samples,ie there are many CT AGN which do not show significant Si absorption, with the most notable example being N1068. Having assessed the validity of the high 9.7micron technique locally, we attempt to construct a sample of candidate CT AGN at higher redshifts. We compile a sample of 7 high-tau sources in the GOODS and 5 in the Spitzer FLS. All these have been selected to have no PAH features EW(6.2)<0.3 in order to maximize the probability that they are AGN. 6 out of 7 sources in the GOODS have been detected in X-rays, while for the five FLS sources only X-ray flux upper limits are available. The high X-ray luminosities of the detected GOODS sources corroborates that these are AGN. For FLS, ancillary optical spectroscopy reveals hidden nuclei in two more sources. SED fitting can support the presence of an AGN in the vast majority of sources. We cannot derive useful X-ray spectroscopy constraints on whether these are CT. However, the low LX/L6 ratios, suggest that at least 4 out of the 6 detected sources in GOODS may be associated with CT AGN., Comment: 12 pages, to appear in A&A; version after language editing

We present the observed-frame optical, near- and mid-infrared properties of X-ray selected AGN in the Lockman Hole. Using a likelihood ratio method on optical, near-infrared or mid-infrared catalogues, we assigned counterparts to 401 out of the 409 X-ray sources of the XMM-Newton catalogue. Accurate photometry was collected for all the sources from U to 24um. We used X-ray and optical criteria to remove any normal galaxies, galactic stars, or X-ray clusters among them and studied the multi-wavelength properties of the remaining 377 AGN. We used a mid-IR colour-colour selection to understand the AGN contribution to the optical and infrared emission. Using this selection, we identified different behaviours of AGN-dominated and host-dominated sources in X-ray-optical-infrared colour-colour diagrams. More specifically, the AGN dominated sources show a clear trend in the f_x/f_R vs. R-K and f_24um/f_R vs. R-K diagrams, while the hosts follow the behaviour of non X-ray detected galaxies. In the optical-near-infrared colour-magnitude diagram we see that the known trend of redder objects being more obscured in X-rays is stronger for AGN-dominated than for host-dominated systems. This is an indication that the trend is more related to the AGN contaminating the overall colours than any evolutionary effects. Finally, we find that a significant fraction (~30%) of the reddest AGN are not obscured in X-rays., Comment: Accepted for publication in A&A

We explore the X-ray properties of the 126 sub-mm galaxies (SMGs) of the LABOCA survey in the CDFS and the eCDFS regions. SMGs are believed to experience massive episodes of star-formation. Our goal is to examine whether star-formation coexists with AGN activity, determine the fraction of highly obscured AGN and finally to obtain an idea of the dominant power-mechanism in these sources. Using Spitzer and radio arc-second positions for the SMGs, we find 14 sources with significant X-ray detections. For most of these there are only photometric redshifts available, with their median redshift being ~2.3. Taking into account only the CDFS area which has the deepest X-ray observations, we estimate an X-ray AGN fraction of <26+/-9 % among SMGs. The X-ray spectral properties of the majority of the X-ray AGN which are associated with SMGs are consistent with high obscuration, 10^23 cm-2, but there is no unambiguous evidence for the presence of Compton-thick sources. Detailed Spectral Energy Distribution fittings show that the bulk of total IR luminosity originates in star-forming processes, although a torus component is usually present. Finally, stacking analysis of the X-ray undetected SMGs reveals a signal in the soft (0.5-2 keV) and marginally in the hard (2-5 keV) X-ray band. The hardness ratio of the stacked signal is relatively soft (-0.40+/-0.10) corresponding to a photon index of ~1.6. This argues against a high fraction of Compton-thick sources among the X-ray undetected SMGs., Comment: 13 pages, to appear in A&A

Recently, Papadopoulos et al., 2010 using sub-mm CO molecular line observations of nearby ultra-luminous IRAS galaxies, (U)LIRGs, have found that exceptionally large gas column densities (N_H > 10^25 cm-2) can be present across some of the very dense gaseous disks that are typically found in these objects. They also proposed a diagnostic for finding such sources using CO and HCN molecular lines. Given that such high column densities are expected to absorb any X-ray luminous AGN, yielding Compton-thick sources, we set out toexplore whether this can be discerned using X-ray observations. More specifically we examine X-ray spectral observations of 14 sources in their sample, using public Chandra observations (0.5-10 keV) for eleven sources as well as BeppoSAX results (2-100 keV) from the literature for another three sources. Our goal is to find candidate Compton-thick AGN and to check whether the molecular line selection criterion is successful in selecting such systems. X-ray spectroscopy reveals four candidate Compton-thick AGN of which half fall within the high obscuration region in the molecular line ratio diagnostics. Of the remaining five sources falling into the `high dust obscuration' box, one (Mrk273) is highly obscured (N_H ~4x10^23 cm-2) while in the other four the X-ray emission is most probably associated with star-forming processes rather than an AGN on the basis of their X-ray and mid-infrared properties. Overall, we argue that although this method as expected cannot recover all Compton-thick AGN, there are no examples of X-ray luminous AGN inside that region that have low obscuration, suggesting that this method is efficient in finding heavily obscured AGN in dust-enshrouded star-forming galaxies. The above results bear important implications for future joint ALMA and X-ray observations for the detection of Compton-thick AGN., Comment: To appear in A&A Letters

We examine the properties of the X-ray detected, Infrared Excess AGN or Dust Obscured Galaxies (DOGs) in the Chandra Deep Fields (CDF). We find 26 X-ray selected sources which obey the 24 micron to R-band flux ratio criterion f_24/f_R>1000. These are at a median redshift of 2.3 while their IR luminosities are above 10^12 solar. Their X-ray luminosities are all above a few times 10^42 erg s-1 in the 2-10 keV band unambiguously arguing that these host AGN. Nevertheless, their IR Spectral Energy Distributions are split between AGN (Mrk231) and star-forming templates (Arp220). Our primary goal is to examine their individual X-ray spectra in order to assess whether this X-ray detected DOG population contains heavily obscured or even Compton-thick sources. The X-ray spectroscopy reveals a mixed bag of objects. We find that four out of the 12 sources with adequate photon statistics and hence reliable X-ray spectra, show evidence for a hard X-ray spectral index (~1) or harder,consistent with a Compton-thick spectrum. In total 12 out of the 26 DOGs show evidence for flat spectral indices. However, owing to the limited photon statistics we cannot differentiate whether these are flat because they are reflection-dominated or because they show moderate amounts of absorption. Seven DOGs show relatively steep spectra (>1.4) indicative of small column densities. All the above suggest a fraction of Compton-thick sources that does not exceed 5%. The average X-ray spectrum of all 26 DOGs is hard (~1.1) or even harder (~0.6) when we exclude the brightest sources. These spectral indices are well in agreement with the stacked spectrum of X-ray undetected sources (~0.8 in the CDFN). This could suggest (but not necessarily prove) that X-ray undetected DOGs, in a similar fashion to the X-ray detected ones presented here, are hosting a moderate fraction of Compton-thick sources., Comment: 16 pages To appear in A&A

We investigate the properties of the most optically faint sources in the GOODS-N area (R > 26.5 AB). Such extremely optically faint populations present an uncharted territory despite the fact that they represent an appreciable fraction of the X-ray sources in the GOODS-N field. They are believed to contain either red AGN at moderate redshifts or possibly QSO at very high redshift. We compile our sample by first finding the 3.6um IRAC counterparts of the X-ray sources and searching for the optical counterparts of the IRAC sources. 35 sources do not have counterparts in the R-band Subaru optical images. Of these, 18 have HST-ACS counterparts while the remaining have no optical counterparts. The vast majority of our 35 sources are classified as Extremely Red Objects (EROs) on the basis of their V-K lower limits. Their photometric redshifts show that these populate moderate redshifts (median z~2.8), being markedly different from the already spectroscopically identified population which peaks at z~0.7. The Spitzer-IRAC mid-IR colours of the sources which have no HST counterparts tend to lie within the mid-IR colour diagram AGN "wedge", suggesting either QSO, ULIRG (Mrk231), or early-type galaxy templates at z>3. A large fraction of our sources (17/35), regardless of whether they have HST counterparts, can be classified as mid-IR bright/optically faint sources (Dust Obscured Galaxies) a class which is believed to include many heavily absorbed AGN. The co-added X-ray spectrum of the optically faint sources is very flat having a spectral index of Gamma~0.87, significantly flatter than the spectrum of the X-ray background. The optically faint R>26.5 X-ray sources constitute more than 50% of the total X-ray population at redshifts z>2 bearing important implications for the luminosity function and its evolution; considering X-ray sources with 2

Sensitive Herschel far-infrared observations can break degeneracies that were inherent to previous studies of star formation in high-z AGN hosts. Combining PACS 100 and 160um observations of the GOODS-N field with 2Msec Chandra data, we detect ~20% of X-ray AGN individually at >3sig. The host far-infrared luminosity of AGN with L2-10~10^43erg/s increases with redshift by an order of magnitude from z=0 to z~1. In contrast, there is little dependence of far-infrared luminosity on AGN luminosity, for L2-10<~10^44erg/s AGN at z>~1. We do not find a dependence of far-infrared luminosity on X-ray obscuring column, for our sample which is dominated by L2-10<10^44erg/s AGN. In conjunction with properties of local and luminous high-z AGN, we interpret these results as reflecting the interplay between two paths of AGN/host coevolution. A correlation of AGN luminosity and host star formation is traced locally over a wide range of luminosities and also extends to luminous high z AGN. This correlation reflects an evolutionary connection, likely via merging. For lower AGN luminosities, star formation is similar to that in non-active massive galaxies and shows little dependence on AGN luminosity. The level of this secular, non-merger driven star formation increasingly dominates over the correlation at increasing redshift., Comment: Accepted for publication in A&A Herschel issue. 4 pages, 6 figures

We used the large binocular camera (LBC) mounted on the large binocular telescope (LBT) to observe the Lockman Hole in the U, B, and V bands. Our observations cover an area of 925 sq.arcmin. We reached depths of 26.7, 26.3, and 26.3 mag(AB) in the three bands, respectively, in terms of 50% source detection efficiency, making this survey the deepest U-band survey and one of the deepest B and V band surveys with respect to its covered area. We extracted a large number of sources (~89000), detected in all three bands and examined their surface density, comparing it with models of galaxy evolution. We find good agreement with previous claims of a steep faint-end slope of the luminosity functions, caused by late-type and irregular galaxies at z>1.5. A population of dwarf star-forming galaxies at 1.5

Context: We use the first XMM serendipitous source catalogue (1XMM) to compile a sample of normal X-ray galaxies Aims: We seek to expand the database of X-ray selected normal galaxies at intermediate redshifts and examine the relation between X-ray emission and star formation for late-type systems Methods: The candidates are selected based on their X-ray (soft spectra), X-ray to optical [log(fx/fo)<-2] and optical (extended sources) properties. 44 candidates are found and 35 are spectroscopically observed with the Australian National University's 2.3m telescope to examine their nature. Results: Of the 35 sources observed, 2 are AGN, 11 emission line galaxies, 12 absorption line galaxies, 6 have featureless spectra while 4 are associated with Galactic stars. We combine our emission line sample with earlier works forming the most comprehensive X-ray selected galaxy sample for the study of the X-ray luminosity to the Halpha luminosity - a well-calibrated star-formation indicator - relation. Conclusions: We find that the X-ray luminosity strongly correlates with the Halpha luminosity, suggesting that the X-rays efficiently trace the star-formation., Comment: Accepted for publication in A&A

The efficiency of mid-infrared selection methods for finding obscured AGN is investigated using data in the \chandra Deep Field North. It is shown that samples of AGN candidates compiled on the basis of mid-infrared colours only suffer substantial contamination from normal galaxies. X-ray stacking analysis reveals a soft mean X-ray spectrum for these sources, consistent with Gamma~2.1. This suggests that star-forming galaxies and not obscured AGN dominate the stacked signal. In contrast AGN selection methods that combine mid-infrared with optical criteria are more successful in finding heavily obscured AGN candidates. A method similar to the one proposed by Fiore et al. (2008) is adopted to select extremely red objects (R-[3.6]>3.7mag) with high 24micron to optical flux ratio (f_24/f_R>1000). About 80% of these sources are not detected at X-ray wavelengths. Stacking the X-ray photons at the positions of these sources shows a flat mean X-ray spectrum (Gamma~0.8), which suggests Compton-thick sources, low-luminosity and moderately obscured (N_H~8e22) AGN, or a combination of the two. The mid-infrared colours and luminosities of these sources are consistent with ULIRGs at z~2, while HST/ACS images, available for the optically brighter of these sources, show disturbed optical morphologies in many of them. The evidence above suggests that this population includes systems in the process of formation., Comment: Accepted by A&A

We investigate the optical colours of X-ray sources from the Extended Chandra Deep Field South (ECDFS) using photometry from the COMBO-17 survey, aiming to explore AGN - galaxy feedback models. The X-ray sources populate both the ``blue'' and the ``red sequence'' on the colour-magnitude diagram. However, sources in the ``red sequence'' appear systematically more obscured. HST imaging from the GEMS survey demonstrates that the nucleus does not affect significantly the observed colours, and therefore red sources are early-type systems. In the context of AGN feedback models, this means that there is still remaining material after the initial ``blowout''. We argue that this material could not be only left-over from the original merger, but a secondary cold gas supplier (such as minor interactions or self-gravitational instabilities) must also assist., Comment: Submitted to A&A

We explore the nature of the radio emission of X-ray selected AGN by combining deep radio (1.4GHz; 60micro-Jy) and X-ray data with multiwavelength (optical, mid-infrared) observations in the Extended Chandra Deep Field South (E-CDFS). The fraction of radio detected X-ray sources increases from 9% in the E-CDFS to 14% in the central region of this field, which has deeper X-ray coverage from the 1Ms CDFS. We find evidence that the radio emission of up to 60% of the hard X-ray/radio matched AGN is likely associated with star-formation in the host galaxy. Firstly, the mid-IR (24micron) properties of these sources are consistent with the infrared/radio correlation of starbursts. Secondly, most of them are found in galaxies with blue rest-frame optical colours (U-V), suggesting a young stellar population. On the contrary, X-ray/radio matched AGN which are not detected in the mid-infrared have red U-V colours suggesting their radio emission is associated with AGN activity. We also find no evidence for a population of heavily obscured radio-selected AGN that are not detected in X-rays. Finally, we do no confirm previous claims for a correlation between radio emission and X-ray obscuration. Assuming that the radio continuum measures star-formation, this finding is against models where the dust and gas clouds associated with circumnuclear starbursts are spherically blocking our view to the central engine., Comment: Accepted by A&A

In this paper we investigate the power spectrum of the unresolved 0.5-2 keV cosmic X-ray background (CXB) with deep Chandra 4-Msec (Ms) observations in the Chandra Deep Field South (CDFS). We measured a signal that, on scales >30 arcsec, is significantly higher than the shot noise and is increasing with angular scale. We interpreted this signal as the joint contribution of clustered undetected sources like active galactic nuclei (AGN), galaxies and the intergalactic medium (IGM). The power of unresolved cosmic source fluctuations accounts for approximately 12 per cent of the 0.5-2 keV extragalactic CXB. Overall, our modelling predicts that approximately 20 per cent of the unresolved CXB flux is produced by low-luminosity AGN, approximately 25 per cent by galaxies and approximately 55 per cent by the IGM. We do not find any direct evidence of the so-called 'warm hot intergalactic medium' (i.e. matter with 10(exp 5) less than T less than 10(exp 7) K and density contrast delta less than 1000), but we estimated that it could produce about 1/7 of the unresolved CXB. We placed an upper limit on the space density of postulated X-ray-emitting early black holes at z greater than 7.5 and compared it with supermassive black hole evolution models.