Publications (118)77.63 Total impact
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Article: On the AGN radio luminosity distribution and the black hole fundamental plane
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ABSTRACT: We have studied the dependence of the AGN nuclear radio (1.4 GHz) luminosity on both the AGN 2-10 keV X-ray and the host-galaxy K-band luminosity. A complete sample of 1268 X-ray selected AGN (both type 1 and type 2) has been used, which is the largest catalogue of AGN belonging to statistically well defined samples where radio, X and K band information exists. At variance with previous studies, radio upper limits have been statistically taken into account using a Bayesian Maximum Likelihood fitting method. It resulted that a good fit is obtained assuming a plane in the 3D L_R-L_X-L_K space, namely logL_R= xi_X logL_X + xi_K logL_K + xi_0, having a ~1 dex wide (1 sigma) spread in radio luminosity. As already shown, no evidence of bimodality in the radio luminosity distribution was found and therefore any definition of radio loudness in AGN is arbitrary. Using scaling relations between the BH mass and the host galaxy K-band luminosity, we have also derived a new estimate of the BH fundamental plane (in the L_5GHz -L_X-M_BH space). Our analysis shows that previous measures of the BH fundamental plane are biased by ~0.8 dex in favor of the most luminous radio sources. Therefore, many AGN studies, where the BH fundamental plane is used to investigate how AGN regulate their radiative and mechanical luminosity as a function of the accretion rate, or many AGN/galaxy co-evolution models, where radio-feedback is computed using the AGN fundamental plane, should revise their conclusions.11/2012; -
Article: The M_BH-M_star relation of obscured AGNs at high redshift
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ABSTRACT: We report the detection of broad Halpha emission in three X-ray selected obscured AGNs at z=1-2. By exploiting the Halpha width and the intrinsic X-ray luminosity, we estimate their black hole masses, which are in the range 0.1-3x10^9 Msun. By means of multi-band photometric data, we measure the stellar mass of their host galaxy and, therefore, infer their M_BH/M_star ratio. These are the first obscured AGNs at high-z, selected based on their black hole accretion (i.e. on the basis of their X-ray luminosity), that can be located on the M_BH-M_star relation at high-z. All of these obscured high-z AGNs are fully consistent with the local M_BH-M_star relation. This result conflicts with those for other samples of AGNs in the same redshift range, whose M_BH/M_star ratio departs significantly from the value observed in local galaxies. We suggest that the obscured AGNs in our sample are in an advanced evolutionary stage, have already settled onto the local M_BH-M_star relation, and whose nuclear activity has been temporarily revived by recent galaxy interactions. Comment: 4 pages, 2 figures, accepted for publication in A&A Letters, slightly revised discussion on SMGs10/2010; -
Article: Tools for computing the AGN feedback: radio-loudness distribution and the kinetic luminosity function
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ABSTRACT: We studied the Active Galactic Nuclei (AGN) radio emission from a compilation of hard X-ray selected samples, all observed in the 1.4 GHz band. A total of more than 1600 AGN with 2-10 keV de-absorbed luminosities higher than 10^42 erg/s were used. For a sub-sample of about 50 z\lsim 0.1 AGN it was possible to reach a ~80% fraction of radio detections and therefore, for the first time, it was possible to almost completely measure the probability distribution function of the ratio between the radio and the X-ray luminosity Rx=log[L(1.4)/Lx]. The probability distribution function of Rx was functionally fitted as dependent on the X-ray luminosity and redshift, P(Rx|Lx,z). It roughly spans over 6 decades (-7<Rx<-1), and does not show any sign of bi-modality. It resulted that the probability of finding large values of the Rx ratio increases with decreasing X-ray luminosities and (possibly) with increasing redshift. No statistical significant difference was found between the radio properties of the X-ray absorbed and unabsorbed AGN. The measure of the probability distribution function of Rx allowed us to compute the kinetic luminosity function and the kinetic energy density which, at variance with what assumed in many galaxy evolution models, is observed to decrease of about a factor of five at redshift below 0.5. About half of the kinetic energy density results to be produced by the more radio quiet (Rx<-4) AGN. In agreement with previous estimates, the AGN efficiency in converting the accreted mass energy into kinetic power is, on average, ~5x10-3. Comment: 13 pages, ApJsty; ApJ in press06/2010; -
Article: Spectroscopic Identifications of Spitzer Sources in the SWIRE/XMM-Newton/ELAIS-S1 Field: A Large Fraction of Active Galactic Nucleus with High F(24 μm)/F(R) Ratio
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ABSTRACT: We present a catalog of optical spectroscopic identifications of sources detected by Spitzer at 3.6 or 24 μm down to ~10 and ~280 μJy, respectively, in the SWIRE/XMM-Newton/ELAIS-S1 field and classified via line width analysis and diagnostic diagrams. A total of 1376 sources down to R ~ 24.2 mag have been identified (1362 detected at 3.6 μm, 419 at 24 μm, and 405 at both) by low-resolution optical spectroscopy carried out with FORS2, VIMOS, and EFOSC2 at the Very Large Telescope and 3.6 m ESO telescope. The spectroscopic campaigns have been carried out over the central 0.6 deg2 area of ELAIS-S1 which, in particular, has also been observed by XMM-Newton and Chandra. We find the first direct optical spectroscopic evidence that the fraction of active galactic nuclei (AGNs; mostly AGN2) increases with increasing F(24 μm)/F(R) ratio, reaching values of 70(±20)% in the range 316 < F(24 μm)/F(R) < 1000. We present an Infrared Array Camera-Multiband Imaging Photometer color-color diagram able to separate AGN1 from obscured AGN2 candidates. After having corrected for the spectroscopic incompleteness of our sample, the result is that the AGN fraction at F(24 μm) ~0.8 mJy is ~22(±7)% and decreases slowly to ~19(±5)% down to F(24 μm) ~ 0.3 mJy.The Astrophysical Journal 09/2009; 703(2):1778. · 6.02 Impact Factor -
Article: Spectroscopic identifications of Spitzer sources in the SWIRE/XMM-NEWTON/ELAIS-S1 field: a large fraction of Active Galactic Nuclei with high F(24micron)/F(R) ratio
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ABSTRACT: We present a catalog of optical spectroscopic identifications of sources detected by Spitzer at 3.6 or 24 micron down to 10 and 280 microJy, respectively, in the SWIRE/XMM-Newton/ELAIS-S1 field and classified via line width analysis and diagnostic diagrams. A total of 1376 sources down to R~24.2 mag have been identified (1362 detected at 3.6 micron, 419 at 24 micron, and 405 at both) by low-resolution optical spectroscopy carried out with FORS2, VIMOS, and EFOSC2 at the Very Large Telescope and 3.6m ESO telescopes. The spectroscopic campaigns have been carried out over the central 0.6 square degrees area of ELAIS-S1 which, in particular, has also been observed by XMM-Newton and Chandra. We find the first direct optical spectroscopic evidence that the fraction of active galactic nuclei (AGN; mostly AGN2) increases with increasing F(24 micron)/F(R) ratio, reaching values of 70(+/-20)% in the range 316<F(24 micron)F(R)<1000. We present an IRAC-MIPS color-color diagram able to separate AGN1 from obscured AGN2 candidates. After having corrected for the spectroscopic incompleteness of our sample, it results that the AGN fraction at F(24 micron)=0.8 mJy is ~22(+/-7)% and decreases slowly to ~19(+/-5)% down to F(24 micron)=0.3 mJy. Comment: Accepted for publication in the Astrophysical Journal. 15 pages, 17 figures, 2 tables09/2009; -
Article: The HELLAS2XMM survey – XII. The infrared/submillimetre view of an X‐ray selected type 2 quasar at z≈ 2
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ABSTRACT: We present multiwavelength observations (from optical to submillimetre, including Spitzer and Submillimetre Common-User Bolometer Array) of H2XMMJ 003357.2−120038 (also GD 158_19), an X-ray selected, luminous narrow-line (type 2) quasar at z= 1.957 selected from the HELLAS2XMM survey. Its broad-band properties can be reasonably well modelled assuming three components: a stellar component to account for the optical and near-infrared (IR) emission; an active galactic nucleus (AGN) component (i.e. dust heated by an accreting active nucleus), dominant in the mid-IR, with an optical depth at 9.7 along the line of sight (close to the equatorial plane of the obscuring matter) of τ(9.7) = 1 and a full covering angle of the reprocessing matter (torus) of 140° and a far-IR starburst component (i.e. dust heated by star formation) to reproduce the wide bump observed longward of 70 .The derived star formation rate is ≈1500 M⊙ yr−1. The overall modelling indicates that GD 158_19 is a high-redshift X-ray luminous, obscured quasar with coeval powerful AGN activity and intense star formation. It is probably caught before the process of expelling the obscuring gas has started, thus quenching the star formation.Monthly Notices of the Royal Astronomical Society 05/2009; 395(4):2189 - 2195. · 4.90 Impact Factor -
Article: The Infrared View of Luminous X‐ray Selected Type 2 Quasars, and Coeval Nuclear Activity and Star Formation at z = 2
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ABSTRACT: Here we present some preliminary results obtained for a sample of X‐ray selected, luminous and obscured (Type 2) quasars at z ≈ 1–2 observed in the near‐ and mid‐IR with Spitzer. In particular, adopting a torus configuration for the obscuring (and reprocessing) matter and fitting the quasar Spectral Energy Distributions (SEDs), we estimate some geometric parameters for the torus like τ(9.7 μm) and full covering angle, and provide AGN bolometric luminosities and corrections. We also present the interesting case of a z ≈ 2 quasar for which the inclusion of sub‐mm (SCUBA) data allows us to reveal and parameterize coeval nuclear activity and star formation.AIP Conference Proceedings. 05/2009; 1126(1):227-230. -
Article: Mid‐infrared spectroscopy of infrared‐luminous galaxies at z∼ 0.5–3
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ABSTRACT: We present results on low-resolution mid-infrared (MIR) spectra of 70 IR-luminous galaxies obtained with the infrared spectrograph (IRS) onboard Spitzer. We selected sources from the European Large Area Infrared Survey with S15 > 0.8 mJy and photometric or spectroscopic z > 1. About half of the samples are quasi-stellar objects (QSOs) in the optical, while the remaining sources are galaxies, comprising both obscured active galactic nuclei (AGN) and starbursts. Redshifts were obtained from optical spectroscopy, photometric redshifts and the IRS spectra. The later turn out to be reliable for obscured and/or star-forming sources, thus becoming an ideal complement to optical spectroscopy for redshift estimation.We estimate monochromatic luminosities at several rest-frame wavelengths, equivalent widths and luminosities for the polycyclic aromatic hydrocarbon (PAH) features, and strength of the silicate feature in individual spectra. We also estimate integrated 8–1000 μm IR luminosities via spectral energy distribution fitting to MIR and far-IR (FIR) photometry from the Spitzer Wide-Area Infrared Extragalactic survey and the MIR spectrum. Based on these measurements, we classify the spectra using well-known IR diagnostics, as well as a new one that we propose, into three types of source: those dominated by an unobscured AGN, mostly corresponding to optical quasars (QSOs), those dominated by an obscured AGN and starburst-dominated sources. Starbursts concentrate at z∼ 0.6–1.0 favoured by the shift of the 7.7-μm PAH band into the selection 15-μm band, while AGN spread over the 0.5 < z < 3.1 range.Star formation rates (SFR) are estimated for individual sources from the luminosity of the PAH features. An estimate of the average PAH luminosity in QSOs and obscured AGN is obtained from the composite spectrum of all sources with reliable redshifts. The estimated mean SFR in the QSOs is 50–100 M⊙ yr−1, but the implied FIR luminosity is 3–10 times lower than that obtained from stacking analysis of the FIR photometry, suggesting destruction of the PAH carriers by energetic photons from the AGN. The SFR estimated in obscured AGN is two to three times higher than in QSOs of similar MIR luminosity. This discrepancy might not be due to luminosity effects or selection bias alone, but could instead indicate a connection between obscuration and star formation. However, the observed correlation between silicate absorption and the slope of the NIR to MIR spectrum is compatible with the obscuration of the AGN emission in these sources being produced in a dust torus.Monthly Notices of the Royal Astronomical Society 04/2009; 395(3):1695 - 1722. · 4.90 Impact Factor -
Article: The HELLAS2XMM Survey. II. Multiwavelength Observations of P3: An X-Ray-bright, Optically Inactive Galaxy
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ABSTRACT: Recent X-ray surveys have clearly demonstrated that a population of optically dull, X-ray-bright galaxies is emerging at 2-10 keV fluxes of the order of 10-14 ergs cm-2 s-1. Although they might constitute an important fraction of the sources responsible for the hard X-ray background, their nature is still unknown. With the aim of better understanding the physical mechanisms responsible for the observed properties, we have started an extensive program of multiwavelength follow-up observations of hard X-ray, optically quiet galaxies discovered with XMM-Newton. Here we report the results of what can be considered the first example of this class of objects: CXOU J031238.9-765134, originally discovered by Chandra, and optically identified by Fiore et al. (2000) with an apparently normal early-type galaxy at z = 0.159, usually known as FIORE P3. Analysis of the broadband energy distribution suggests the presence of a heavily obscured active nucleus.The Astrophysical Journal 12/2008; 571(2):771. · 6.02 Impact Factor -
Article: The HELLAS2XMM Survey. VII. The Hard X-Ray Luminosity Function of AGNs up to z = 4: More Absorbed AGNs at Low Luminosities and High Redshifts
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ABSTRACT: We have determined the cosmological evolution of the density of active galactic nuclei (AGNs) and of their NH distribution as a function of the unabsorbed 2-10 keV luminosity up to redshift 4. We used the HELLAS2XMM sample combined with other published catalogs, yielding a total of 508 AGNs. Our best fit is obtained with a luminosity-dependent density evolution (LDDE) model where low-luminosity (LX ~ 1043 ergs s-1) AGNs peak at z ~ 0.7, while high-luminosity AGNs (LX > 1045 ergs s-1) peak at z ~ 2.0. A pure luminosity evolution model (PLE) can instead be rejected. There is evidence that the fraction of absorbed (NH > 1022 cm-2) AGNs decreases with the intrinsic X-ray luminosity and increases with the redshift. Our best-fit solution provides a good fit to the observed counts, the cosmic X-ray background, and to the observed fraction of absorbed AGNs as a function of the flux in the 10-15 ergs cm-2 s-1 < S2-10 < 10-10 ergs cm-2 s-1 range. We find that the absorbed, high-luminosity (LX > 1044 ergs s-1) AGNs have a density of 267 deg-2 at fluxes S2-10 > 10-15 ergs cm-2 s-1. Using these results, we estimate a density of supermassive black holes in the local universe of ρBH = 3.2 h × 105 M☉ Mpc-3, which is consistent with the recent measurements of the black hole mass function in the local galaxies.The Astrophysical Journal 12/2008; 635(2):864. · 6.02 Impact Factor -
Article: The European Large-Area Infrared Space Observatory Survey V: A BeppoSAX Hard X-Ray Survey of the S1 Region
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ABSTRACT: We present BeppoSAX observations of the southern S1 region in the European Large-Area Infrared Space Observatory (ISO) Survey (ELAIS). These observations cover an area of ~1.7 deg2 and reach an on-axis (~0.7 deg2) 2-10 keV (hard X-ray, or HX) sensitivity of ~10-13 ergs s-1 cm-2. This is the first HX analysis of an ISOCAM survey. We detect nine sources with a signal-to-noise ratio SNRHX > 3, four additional sources with a 1.3-10 keV (total X-ray, or T) SNRT > 3, and two additional sources that seem to be associated with QSOs having SNRT > 2.9. The number densities of the SNRHX > 3 sources are consistent with the ASCA and BeppoSAX log N-log S functions. Six BeppoSAX sources have reliable ISOCAM 15 μm counterparts within ~60''. All these ISOCAM sources have optical counterparts of R < 20 mag. Five of these sources have been previously optically classified, four as QSOs and one as a broad absorption line (BAL) QSO at z = 2.2. The remaining unclassified source has X-ray and photometric properties consistent with those of a nearby Seyfert galaxy. One further HX source has a 2.6 σ ISOCAM counterpart associated with a galaxy at z = 0.325. If this ISOCAM source is real, the HX/MIR properties suggest either an unusual QSO or a cD cluster galaxy. We have constructed MIR and HX spectral energy distributions to compute the expected HX/MIR ratios for these classes of objects up to z = 3.2 and assess the HX/MIR survey depth. The BAL QSO has an observed X-ray softness ratio and HX/MIR flux ratio similar to those of QSOs but different from those found for low-redshift BAL QSOs. This difference can be explained in terms of absorption, and it suggests that high-redshift BAL QSOs should be comparatively easy to detect in the HX band, allowing their true fraction in the high-redshift QSO population to be determined. The QSOs cover a wide redshift range (0.4 < z < 2.6) and have HX/MIR flux ratios consistent with those found for nearby IRAS and optically selected Palomar-Green QSOs. This suggests that MIR-selected QSOs of R < 20 mag come from the same population as optically selected QSOs. We confirm this with a comparison of the B/MIR flux ratios of MIR and blue-band-selected QSOs.The Astrophysical Journal 12/2008; 554(1):18. · 6.02 Impact Factor -
Article: The Contribution of AGNs and Star-forming Galaxies to the Mid-Infrared as Revealed by Their Spectral Energy Distributions
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ABSTRACT: We present the broadband SEDs of the largest available highly complete (72%) spectroscopic sample of MIR-selected galaxies and AGNs at intermediate redshift. The sample contains 203 extragalactic sources from the 15 μm ELAIS-SWIRE survey, all with measured spectroscopic redshift. Most of these sources have full multiwavelength coverage from the FUV (GALEX) to the FIR (Spitzer) and lie in the redshift range 0.1 < z < 1.3. This large sample allows us for the first time to characterize the spectral properties of sources responsible for the strong evolution observed in the MIR. Based on SED-fitting, we have classified the MIR sources, identifying AGN signatures in about 50% of them. This fraction is significantly higher than that derived from optical spectroscopy (~29%) and is due in particular to the identification of AGN activity in objects spectroscopically classified as galaxies (the spectroscopic classification may be somewhat unreliable because of host galaxy dilution in the optical). It is likely that in most of our objects, the AGN is either obscured or low luminosity, and thus dominates the energetic output only in the MIR, showing up just in the range where the host galaxy SED has a minimum. The fraction of AGNs strongly depends on flux density, with that derived through the SED-fitting about 20% at S15μ m ~ 0.5–1 mJy and gradually increasing to 100% at S15μ m > 10 mJy, while that obtained from optical spectroscopy is never >30%, even at higher flux densities. Our results will be very useful for updating all models aimed at interpreting the deep IR survey data and in particular for constraining the nature and role of dust-obscured systems in the intermediate/high-redshift universe.The Astrophysical Journal 12/2008; 684(1):136. · 6.02 Impact Factor -
Article: The ESO-Spitzer Imaging extragalactic Survey (ESIS) II: VIMOS I,z wide field imaging of ELAIS-S1 and selection of distant massive galaxies
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ABSTRACT: (abridged) The ESIS survey is the optical follow up of the SWIRE/Spitzer in the ELAIS-S1 region of the sky. In the era of observational cosmology, the main efforts are focused on the study of galaxy evolution and its environmental dependence. Wide area, multiwavelength, extragalactic surveys are needed in order to probe sufficiently large volumes, minimize cosmic variance and find significant numbers of rare objects. We present VIMOS I and z band imaging belonging to the ESIS survey. A total of ~4 deg2 were targeted in I and ~1 deg2 in z. More than 300000 galaxies have been detected in the I band and ~50000 in the z band. Object coordinates are defined within an uncertainty of ~0.2 arcsec r.m.s., with respect to GSC 2.2. We reach a 90% average completeness at 23.1 and 22.5 mag (Vega) in the I and z bands, respectively. On the basis of IRAC colors, we identified galaxies having the 1.6 um stellar peak shifted to z=1-3. The new I, z band data provide reliable constraints to avoid low-redshift interlopers and reinforce this selection. Roughly 1000 galaxies between z=2-3 were identified over the ESIS ~4 deg2, at the SWIRE 5.8 um depth (25.8 uJy at 3sigma). These are the best galaxy candidates to dominate the massive tail (M>1e11 Msun) of the z>2 mass function. Comment: Accepted for publication on A&A. The quality of some figures has been degraded for arXiv purposes. More information about the ESIS survey is available at http://www.astro.unipd.it/esis/07/2008; -
Article: ESIS BVR catalog, ELAIS-S1 (Berta+, 2006)
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ABSTRACT: The ESO-Spitzer extragalactic Imaging Survey (ESIS) is the optical follow up of the Spitzer Wide-Area InfraRed Extragalactic (SWIRE) survey in the ELAIS-S1 area. The multiwavelength study of galaxy emission is the key to understand the interplay of the various components of galaxies and to trace their role in cosmic evolution. ESIS provides optical identification and colors of Spitzer IR galaxies and builds the bases for photometric redshift estimates. This paper presents B, V, R Wide Field Imager observations of the first 1.5 square degree of the ESIS survey. Data reduction includes astrometric calibration, illumination and color corrections. Synthetic sources are simulated in scientific and super-sky-flat images, with the purpose of estimating completeness and photometric accuracy for the survey. Number counts and color distributions are compared to literature observational and theoretical data, including non-evolutionary, PLE, evolutionary and semi-analytic Lambda-CDM galaxy models, as well as Milky Way stellar predictions. Object coordinates are defined with an accuracy as good as ~0.15" rms with respect to GSC 2.2; flux uncertainties are ~2, 10, 20% at mag. 20, 23, 24 respectively (Vega); we reach 95% completeness at B, V~25 and R~24.5. ESIS galaxy number counts are in good agreement with previous works and are best reproduced by evolutionary and hierarchical Lambda-CDM scenarios. (1 data file).VizieR Online Data Catalog. 06/2008; 345:10881. -
Article: XMM-Newton survey of the ELAIS-S1 field. II. (Feruglio+, 2008)
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ABSTRACT: Identifications and photometric parameters for 478 sources detected by XMM-Newton in the ELAIS-S1 field are given. For each source we give: X-ray positions and fluxes, optical position and photometry, Spitzer IRAC and MIPS 24 micron positions and fluxes, spectroscopic redshift where available, photometric redshift and SED shape classification. (1 data file).VizieR Online Data Catalog. 06/2008; 348:80417. -
Article: ESIS VIMOS I,z photometry of ELAIS-S1 (Berta+, 2008)
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ABSTRACT: The ESO-Spitzer Imaging extragalactic Survey (ESIS) is the optical follow up of the Spitzer Wide-area Infra-Red Extragalactic survey (SWIRE) in the ELAIS-S1 region of the sky. We present VIMOS I and z band imaging belonging to the ESIS survey. A total of ~4 square degrees were targeted in I and ~1 in z. Accurate data processing includes removal of fringing, and mosaicking of the complex observing pattern. Completeness levels and photometric uncertainties are estimated through simulations. More than 300000 galaxies have been detected in the I band and ~50000 in the z band. Object coordinates are defined within an uncertainty of ~0.2" r.m.s., with respect to GSC 2.2. We reach a 90% average completeness at 23.1 and 22.5mag (Vega) in the I and z bands, respectively. (2 data files).VizieR Online Data Catalog. 06/2008; 348:80533. -
Article: The XMM-Newton survey of the ELAIS-S1 field II: optical identifications and multiwavelength catalogue of X-ray sources
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ABSTRACT: We present optical identifications and a multi-band catalogue of a sample of 478 X-ray sources in the XMM and Chandra surveys of the central 0.6 deg^2 of the ELAIS-S1 field. The optical/infrared counterpart of each X-ray source was identified using R and IRAC 3.6 um bands. This method was complemented by the precise positions obtained through Chandra observations. Approximately 94% of the counterparts are detected in the R band, while the remaining are blank fields in the optical down to R~24.5, but have a near-infrared counterpart detected by IRAC within 6 arcsec from the XMM centroid. The multi-band catalogue contains photometry in ten photometric bands (B to the MIPS 24 um). We determined redshift and classification for 237 sources (~50% of the sample) brighter than R=24. We classified 47% of the sources with spectroscopic redshift as broad-line active galactic nuclei (BL AGNs) with z=0.1-3.5, while sources without broad-lines are about 46% of the spectroscopic sample and are found up to z=2.6. We identified 11 type 2 QSOs among the sources with X/O>8, with z=0.9-2.6, high 2-10 keV luminosity (log(L2-10keV)>=43.8 erg/s) and hard X-ray colors suggesting large absorbing columns at the rest frame (logN_H up to 23.6 cm^-2). BL AGNs show on average blue optical-to-near-infrared colors, softer X-ray colors and X-ray-to-optical colors typical of optically selected AGNs. Conversely, narrow-line sources show redder optical colors, harder X-ray flux ratio and span a wider range of X-ray-to-optical colors. On average the SEDs of high-luminosity BL AGNs resemble the power-law typical of unobscured AGNs. The SEDs of NOT BL AGNs are dominated by the galaxy emission in the optical/near-infrared, and show a rise in the mid-infrared which suggests the presence of an obscured active nucleus. Comment: 15 pages, A&A accepted, affiliations corrected06/2008; -
Article: The Contribution of AGN and Star-Forming Galaxies to the Mid-Infrared as Revealed by their Spectral Energy Distributions
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ABSTRACT: We present the broad-band Spectral Energy Distributions (SEDs) of the largest available highly (72%) complete spectroscopic sample of mid-infrared (MIR) selected galaxies and AGN at intermediate redshift. The sample contains 203 extragalactic sources from the 15-micron survey in the ELAIS-SWIRE field S1, all with measured spectroscopic redshift. Most of these sources have full multi-wavelength coverage from the far-UV to the far-infrared and lie in the redshift range 0.1<z<1.3. Due to its size, this sample allows us for the first time to characterise the spectral properties of the sources responsible for the strong evolution observed in the MIR. Based on SED-fitting technique we have classified the MIR sources, identifying AGN signatures in about 50% of them. This fraction is significantly higher than that derived from optical spectroscopy (~29%) and is due in particular to the identification of AGN activity in objects spectroscopically classified as galaxies. It is likely that in most of our objects, the AGN is either obscured or of low-luminosity, and thus it does not dominate the energetic output at any wavelength, except in the MIR, showing up just in the range where the host galaxy SED has a minimum. The fraction of AGN strongly depends on the flux density, with that derived through the SED-fitting being about 20% at S(15)~0.5-1 mJy and gradually increasing up to 100% at S(15)>10 mJy, while that obtained from optical spectroscopy never being >30%, even at the higher flux densities. The results of this work will be very useful for updating all the models aimed at interpreting the deep infrared survey data and, in particular, for constraining the nature and the role of dust-obscured systems in the intermediate/high-redshift Universe. Comment: Accepted for publication in The Astrophysical Journal. Scheduled for the ApJ August 20, 2008, v683 n 2 issue. 33 pages: 22 pages of main text + 2 tables + 6 postscript figures, use aastex05/2008; -
Article: The Nature of the Mid-Infrared Population from Optical Identifications of the ELAIS-S1 Sample
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ABSTRACT: We present a multiwavelength catalog (15 mum, R-band, K-band, and 1.4 GHz flux) plus spectroscopic identifications for 406 15 mum sources detected in the European Large Area ISO Survey (ELAIS) region S1, over the flux density range 0.5<S15mum<150 mJy. Three hundred thirty-two (~82%) sources are optically identified down to R~23.0. Spectra or bona fide stellar identifications are obtained for 290 objects (~88% of the optically identified sources). The areal coverage, mid-infrared (MIR) and optical completeness of the sample are discussed in order to allow statistical and evolutionary analyses. Two main spectroscopic classes have been found to dominate the MIR extragalactic population: z<0.5 star-forming galaxies [from absorbed to extreme starbursts: nuLnu(15mum)~108-1011 Lsolar], which account for ~75% of the sources, and active galactic nuclei (AGNs; both type 1 and 2), which account for ~25% of the sources. About 20% of the extragalactic sources are dust-enshrouded starburst galaxies [e(a) spectra], and all the starburst galaxies appear more dust-extincted in the optical than nearby normal galaxies. We also identified 91 stellar objects (~22% of the MIR sources). The counts for starburst galaxies and AGNs down to 0.6 mJy have been derived. A general trend is found in the optical-MIR spectral energy distribution of the galaxies, where the MIR-luminous objects have larger MIR to optical luminosity ratios. Based on a variety of analyses, we suggest that the ELAIS sources fainter than R~23 are luminous and ultraluminous MIR galaxies (LIG-ULIGs nuLnu(15mum)=1011-1012 Lsolar) at intermediate redshifts (z=0.5-1.5), and that consequently the present sample is virtually 100% spectroscopically complete up to z=0.5.The Astronomical Journal 12/2007; · 4.03 Impact Factor -
Article: The HELLAS2XMM survey: XI. Unveiling the nature of X-ray Bright Optically Normal Galaxies
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ABSTRACT: X-ray Bright Optically Normal Galaxies (XBONGs) constitute a small but not negligible fraction of hard X-ray selected sources in recent Chandra and XMM-Newton surveys. Even though several possibilities were proposed to explain why a relatively luminous hard X-ray source does not leave any significant signature of its presence in terms of optical emission lines, the nature of XBONGs is still subject of debate. We aim to a better understanding of their nature by means of a multiwavelength and morphological analysis of a small sample of these sources. Good-quality photometric near-infrared data (ISAAC/VLT) of four low-redshift (z=0.1-0.3) XBONGs, selected from the HELLAS2XMM survey, have been used to search for the presence of the putative nucleus, applying the surface-brightness decomposition technique through the least-squares fitting program GALFIT. The surface brightness decomposition allows us to reveal a nuclear point-like source, likely to be responsible of the X-ray emission, in two out of the four sources. The results indicate that moderate amounts of gas and dust, covering a large solid angle (possibly 4pi) at the nuclear source, combined with the low nuclear activity, may explain the lack of optical emission lines. The third XBONG is associated with an X-ray extended source and no nuclear excess is detected in the near infrared at the limits of our observations. The last source is associated to a close (d< 1 arcsec) double system and the fitting procedure cannot achieve a firm conclusion. Comment: 20 pages, 12 figures, A&A in press10/2007;
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Institutions
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2009
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European Space Astronomy Centre
Madrid, Madrid, Spain
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5–2009
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Università Degli Studi Roma Tre
- Department of Mathematics and Physics
Roma, Latium, Italy
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