Wide area X-ray and far infrared surveys are a fundamental tool to investigate the link between AGN growth and star formation, especially in the low-redshift universe (z<1). The Herschel Terahertz Large Area survey (H-ATLAS) has covered 550 deg^2 in
five far-infrared and sub-mm bands, 16 deg^2 of which have been presented in the Science Demonstration Phase (SDP) catalogue. Here we introduce the XMM-Newton observations in H-ATLAS SDP area, covering 7.1 deg^2 with flux limits of 2e-15, 6e-15 and 9e-15 erg/s/cm^2 in the 0.5--2, 0.5--8 and 2--8 keV bands, respectively. We present the source detection and the catalogue, which includes 1700, 1582 and 814 sources detected by Emldetect in the 0.5--8, 0.5--2 and 2--8 keV bands, respectively; the number of unique sources is 1816. We extract spectra and derive fluxes from power-law fits for 398 sources with more than 40 counts in the 0.5--8 keV band. We compare the best-fit fluxes with the catalogue ones, obtained by assuming a common photon index of Gamma=1.7; we find no bulk difference between the fluxes, and a moderate dispersion of s=0.33 dex. Using wherever possible the fluxes from the spectral fits, we derive the 2--10 keV LogN-LogS, which is consistent with a Euclidean distribution. Finally, we release computer code for the tools developed for this project.
The XMM-Newton spectral-fit database (XMMFITCAT) is a catalogue of spectral fitting results for the source detections within the XMM-Newton Serendipitous source catalogue with more than 50 net (background-subtracted) counts per detector in the 0.5-10
keV energy band. Its most recent version, constructed from the latest version of the XMM-Newton catalogue, the 3XMM Data Release 4 (3XMM-DR4), contains spectral-fitting results for $gtrsim$ 114,000 detections, corresponding to $simeq$ 78,000 unique sources. Three energy bands are defined and used in the construction of XMMFITCAT: Soft (0.5-2 keV), Hard (2-10 keV), and Full (0.5-10 keV) bands. Six spectral models, three simple and three more complex models, were implemented and applied to the spectral data. Simple models are applied to all sources, whereas complex models are applied to observations with more than 500 counts (30%). XMMFITCAT includes best-fit parameters and errors, fluxes, and goodness of fit estimates for all fitted models. XMMFITCAT has been conceived to provide the astronomical community with a tool to construct large and representative samples of X-ray sources by allowing source selection according to spectral properties, as well as characterise the X-ray properties of samples selected in different wavelengths. We present in this paper the main details of the construction of this database, and summarise its main characteristics.
Heavily obscured, Compton Thick (CT, NH>10^24 cm^-2) AGN may represent an important phase in AGN/galaxy co-evolution and are expected to provide a significant contribution to the cosmic X-ray background (CXB). Through direct X-ray spectra analysis, w
e selected 39 heavily obscured AGN (NH>3x10^23 cm^-2) in the 2 deg^2 XMM-COSMOS survey. After selecting CT AGN based on the fit of a simple absorbed two power law model to the XMM data, the presence of CT AGN was confirmed in 80% of the sources using deeper Chandra data and more complex models. The final sample of CT AGN comprises 10 sources spanning a large range of redshift and luminosity. We collected the multi-wavelength information available for all these sources, in order to study the distribution of SMBH and host properties, such as BH mass (M_BH), Eddington ratio (lambda_Edd), stellar mass (M*), specific star formation rate (sSFR) in comparison with a sample of unobscured AGN. We find that highly obscured sources tend to have significantly smaller M_BH and higher lambda_edd with respect to unobscured ones, while a weaker evolution in M* is observed. The sSFR of highly obscured sources is consistent with the one observed in the main sequence of star forming galaxies, at all redshift. We also present optical spectra, spectral energy distribution (SED) and morphology for the sample of 10 CT AGN: all the available optical spectra are dominated by the stellar component of the host galaxy, and a highly obscured torus component is needed in the SED of the CT sources. Exploiting the high resolution Hubble-ACS images available, we conclude that these highly obscured sources have a significantly larger merger fraction with respect to other X-ray selected samples of AGN. Finally we discuss implications in the context of AGN/galaxy co-evolutionary models, and compare our results with the predictions of CXB synthesis models.
The majority of active galactic nuclei (AGN) are obscured by large amounts of absorbing material that makes them invisible at many wavelengths. X-rays, given their penetrating power, provide the most secure way for finding these AGN. The XMM-Newton s
erendipitous source catalog is the largest catalog of X-ray sources ever produced; it contains about half a million detections. These sources are mostly AGN. We have derived X-ray spectral fits for very many 3XMM-DR4 sources ($gtrsim$ 114 000 observations, corresponding to $sim$ 77 000 unique sources), which contain more than 50 source photons per detector. Here, we use a subsample of $simeq$ 1000 AGN in the footprint of the SDSS area (covering 120 deg$^2$) with available spectroscopic redshifts. We searched for highly obscured AGN by applying an automated selection technique based on X-ray spectral analysis that is capable of efficiently selecting AGN. The selection is based on the presence of either a) flat rest-frame spectra; b) flat observed spectra; c) an absorption turnover, indicative of a high rest-frame column density; or d) an Fe K$alpha$ line with an equivalent width > 500 eV. We found 81 highly obscured candidate sources. Subsequent detailed manual spectral fits revealed that 28 of them are heavily absorbed by column densities higher than 10$^{23}$ cm$^{-2}$. Of these 28 AGN, 15 are candidate Compton-thick AGN on the basis of either a high column density, consistent within the 90% confidence level with N$_{rm H}$ $>$10$^{24}$ cm$^{-2}$, or a large equivalent width (>500 eV) of the Fe K$alpha$ line. Another six are associated with near-Compton-thick AGN with column densities of $sim$ 5$times$10$^{23}$ cm$^{-2}$. A combination of selection criteria a) and c) for low-quality spectra, and a) and d) for medium- to high-quality spectra, pinpoint highly absorbed AGN with an efficiency of 80%.
The XMM-Newton spectral-fit database is an ongoing ESA funded project aimed to construct a catalogue of spectral-fitting results for all the sources within the XMM-Newton serendipitous source catalogue for which spectral data products have been pipel
ine-extracted (~ 120,000 X-ray source detections). The fundamental goal of this project is to provide the astronomical community with a tool to construct large and representative samples of X-ray sources by allowing source selection according to spectral properties.
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. W
e 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)
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.
This paper investigates what constraints can be placed on the fraction of Compton-thick (CT) AGN in the Universe from the modeling of the spectrum of the diffuse X-ray background (XRB). We present a model for the synthesis of the XRB that uses as inp
ut a library of AGN X-ray spectra generated by the Monte Carlo simulations described by Brightman & Nandra. This is essential to account for the Compton scattering of X-ray photons in a dense medium and the impact of that process on the spectra of obscured AGN. We identify a small number of input parameters to the XRB synthesis code which encapsulate the minimum level of uncertainty in reconstructing the XRB spectrum. These are the power-law index and high energy cutoff of the intrinsic X-ray spectra of AGN, the level of the reflection component in AGN spectra and the fraction of CT AGN in the Universe. We then map the volume of the space allowed to these parameters by current observations of the XRB spectrum in the range 3-100 keV. One of the least constrained parameters is the fraction of CT AGN. Statistically acceptable fits to the XRB spectrum at the 68% confidence level can be obtained for CT fractions in the range 5-50%. This is because of degeneracies among input parameters to the XRB synthesis code and uncertainties in the modeling of AGN spectra (e.g. reflection). The most promising route for constraining the fraction of CT AGN in the Universe is via the direct detection of those sources in high energy (>10keV) surveys. It is shown that the observed fraction of CT sources identified in the SWIFT/BAT survey, limits the intrinsic fraction of CT AGN, at least at low redshift, to 10-20% (68% confidence level). We also make predictions on the number density of CT sources that current and future X-ray missions are expected to discover. Testing those predictions will constrain the intrinsic fraction of CT AGN as a function of redshift.
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 exami
ning 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.
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 obs
ervations 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.
