C. Vignali

The X-ray to optical-UV luminosity ratio of X-ray selected type 1 AGN in XMM-COSMOS

We present a study of the X-ray to optical properties of a sample of 545 X-ray selected type 1 AGN, from the XMM-COSMOS survey, over a wide range of redshifts (0.04 textless z textless 4.25) and X-ray luminosities (40.6 textless= Log L([2-10]) (keV) textless= 45.3). About 60% of them are spectroscopically identified type 1 AGN, while the others have a reliable photometric redshift and are classified as type 1 AGN on the basis of their multi-band Spectral Energy Distributions. We discuss the relationship between UV and X-ray luminosity, as parameterized by the alpha(ox) spectral slope, and its dependence on redshift and luminosity. We compare our findings with previous investigations of optically selected broad-line AGN (mostly from SDSS). A highly significant correlation between alpha(ox) and L(2500) angstrom is found, in agreement with previous investigations of optically selected samples. We calculate bolometric corrections, k(bol), for the whole sample using hard X-ray luminosities (L([2-10] keV)), and the Eddington ratios for a subsample of 150 objects for which black hole mass estimates are available. We confirm the trend of increasing bolometric correction with increasing Eddington ratio as proposed in previous works. A tight correlation is found between alpha(ox) and k(bol), which can be used to estimate accurate bolometric corrections using only optical and X-ray data. We find a significant correlation between alpha(ox) and Eddington ratio, in which the ratio between X-ray and optical flux decreases with increasing Eddington ratio.

THE EXTENDED CHANDRA DEEP FIELD-SOUTH SURVEY: OPTICAL SPECTROSCOPY OF FAINT X-RAY SOURCES WITH THE VLT* AND KECK **

We present the results of a program to acquire high-quality optical spectra of X-ray sources detected in the Extended-Chandra Deep Field-South (E-CDF-S) and its central 2 Ms area. New spectroscopic redshifts, up to z = 4, are measured for 283 counterparts to Chandra sources with deep exposures (t ~ 2-9 hr per pointing) using multi-slit facilities on both VLT (VIMOS) and Keck (DEIMOS), thus bringing the total number of spectroscopically identified X-ray sources to over 500 in this survey field. Since our new spectroscopic identifications are mainly associated with X-ray sources in the shallower 250 ks coverage, we provide a comprehensive catalog of X-ray sources detected in the E-CDF-S including the optical and near-infrared counterparts, determined by a likelihood routine, and redshifts (both spectroscopic and photometric), that incorporate published spectroscopic catalogs, thus resulting in a final sample with a high fraction (80%) of X-ray sources having secure identifications. We demonstrate the remarkable coverage of the luminosity-redshift plane now accessible from our data while emphasizing the detection of active galactic nuclei (AGNs) that contribute to the faint end of the luminosity function (L_(0.5-8 keV) ~ 10^(43)-10^(44) erg s^(–1)) at 1.5 ≲ z ≲ 3 including those with and without broad emission lines. Our redshift catalog includes 17 type-2 QSOs at 1 ≲ z ≲ 3.5 that significantly increases such samples (2×). Based on our deepest (9 hr) VLT/VIMOS observation, we identify elusive optically faint galaxies (R_(mag) ~ 25) at z ~ 2-3 based upon the detection of interstellar absorption lines (e.g., O II+Si IV, C II], C IV); we highlight one such case, an absorption-line galaxy at z = 3.208 having no obvious signs of an AGN in its optical spectrum. In addition, we determine accurate distances to eight galaxy groups with extended X-ray emission detected both by Chandra and XMM-Newton. Finally, we measure the physical extent of known large-scale structures (z ~ 0.7) evident in the CDF-S. While a thick sheet (a radial size of 67.7 Mpc) at z ~ 0.67 extends over the full field, the z ~ 0.73 structure is thin (18.8 Mpc) and filamentary as traced by both AGNs and galaxy groups. In the Appendix, we provide spectroscopic redshifts for 49 counterparts to fainter X-ray sources detected only in the 1 and 2 Ms catalogs, and 48 Very Large Array radio sources not detected in X-rays.

A Runaway Black Hole in COSMOS: Gravitational Wave or Slingshot Recoil?

We present a detailed study of a peculiar source detected in the COSMOS survey at z = 0.359. Source nCXOC J100043.1+020637, also known as CID-42, has two compact optical sources embedded in the same galaxy. nThe distance between the two, measured in the HST/ACS image, is 0.495 ± 0.005 that, at the redshift of the source, ncorresponds to a projected separation of 2.46 ± 0.02 kpc. A large (~1200 km s^(−1)) velocity offset between the nnarrow and broad components of Hβ has been measured in three different optical spectra from the VLT/VIMOS nand Magellan/IMACS instruments. CID-42 is also the only X-ray source in COSMOS, having in its X-ray spectra na strong redshifted broad absorption iron line and an iron emission line, drawing an inverted P-Cygni profile. The nChandra and XMM-Newton data show that the absorption line is variable in energy by ΔE = 500 eV over four years nand that the absorber has to be highly ionized in order not to leave a signature in the soft X-ray spectrum. That nthese features—the morphology, the velocity offset, and the inverted P-Cygni profile—occur in the same source nis unlikely to be a coincidence. We envisage two possible explanations, both exceptional, for this system: (1) a ngravitational wave (GW) recoiling black hole (BH), caught 1–10 Myr after merging; or (2) a Type 1/Type 2 system nin the same galaxy where the Type 1 is recoiling due to the slingshot effect produced by a triple BH system. The first npossibility gives us a candidate GW recoiling BH with both spectroscopic and imaging signatures. In the second ncase, the X-ray absorption line can be explained as a BAL-like outflow from the foreground nucleus (a Type 2 AGN) nat the rearer one (a Type 1 AGN), which illuminates the otherwise undetectable wind, giving us the first opportunity nto show that fast winds are present in obscured active galactic nuclei (AGNs), and possibly universal in AGNs.

The X-ray to [Ne V]3426 flux ratio: discovering heavily obscured AGN in the distant Universe

We investigate the possibility of using the ratio between the 2−10 keV flux and the [Ne V]3426 emission line flux (X/NeV) as a diagnostic diagram to discover heavily obscured, possibly Compton-thick active galactic nuclei (AGN) in the distant Universe. While it is on average about one order of magnitude fainter than the more commonly used [O III]5007 emission line, the [Ne V]3426 line can be observed with optical spectroscopy up to z ∼ 1.5, whereas the [O III]5007 line is redshifted out of the optical bands already at z ∼ 0.8. First, we calibrated a relation between X/NeV and the cold absorbing column density NH using a sample of 74 bright, nearby Seyferts with both X-ray and [Ne V] data available in the literature and for which the column density is determined unambiguously. Similar to what is found for the X-ray to [O III]5007 flux ratio (X/OIII), we found that the X/NeV ratio decreases towards high column densities, as expected if [Ne V]3426 emission is a good tracer of the AGN intrinsic power. Essentially all local Seyferts with X/NeV values below 15 are found to be Compton-thick objects. At X/NeV values below 100, the percentage of Compton-thick nuclei decreases to ∼50%, but ∼80% of the considered sample is still absorbed with NH > 10 23 cm −2 . Second, we applied this diagnostic diagram to different samples of distant obscured and unobscured QSOs in the Sloan Digital Sky Survey (SDSS). SDSS blue, unobscured, type-1 QSOs in the redshift range z = [0.1−1.5] indeed show X/NeV values typical of unobscured Seyfert 1s in the local Universe. Conversely, SDSS type-2 QSOs at z ∼ 0.5 classified either as Compton-thick or Compton-thin on the basis of their X/OIII ratio, would have mostly been classified in the same way based on the X/NeV ratio. We applied the X/NeV diagnostic diagram to 9 SDSS obscured QSOs in the redshift range z = [0.85−1.31], selected by means of their prominent [Ne V]3426 line (rest EW > 4 A) and observed with Chandra ACIS-S for 10ks each (8 of them as part of our proprietary program). Based on the X/NeV ratio, complemented by X-ray spectral analysis, 2 objects appear good Compton-thick QSO candidates, 4 objects appear as Compton-thin QSOs, while 3 have an ambiguous classification. When excluding broad-lined QSOs with a red continuum from the sample and thus considering only genuine narrow-line objects, the efficiency in selecting Compton-thick QSOs through the [Ne V] line is about 50% (with large errors, though), more similar to what is achieved with [O III] selection. We discuss the possibility of applying the X/NeV diagnostic to deep X-ray surveys to search for Compton-thick Seyferts at z ∼ 1, i.e., those objects that are thought to be responsible for the “missing” X-ray background. Finally, we compared the optical spectral properties of [Ne V]-selected QSOs with those of other SDSS populations of obscured and unobscured QSOs. By restricting the analysis to objects in the same redshift (and luminosity) range z = [0.4−1.5], we found evidence that, at any given [Ne V] luminosity, increasing obscuration is accompanied by increasing [O II]3727 emission. This correlation is interpreted as evidence of enhanced star formation in obscured QSOs, which is consistent with current popular scenarios of BH-galaxy coevolution.

Discovery of Compton-thick quasars in the Sloan Digital Sky Survey

We present new and archival Chandra snapshot (≈10 ks each) observations of 15 optically identified [from the Sloan Digital Sky Survey (SDSS)] Type 2 quasars at z = 0.40-0.73. When combined with existing X-ray data, this work provides complete X-ray coverage for all 25 radio-quiet Type 2 quasars with log L [O III] > 9.28 L ☉ from Zakamska et al. Two targets out of 15 were not detected by Chandra and most of the remaining sources are X-ray weak, with nine having less than 10 counts in the 0.5-8 keV band. Low- to moderate-quality spectral analysis was limited to three sources, whose properties are consistent with the pres- ence of column densities in the range N H ≈ 10 22 -10 23 cm -2 in the source rest frame. If the [O III] luminosity is a reliable proxy for the intrinsic X-ray luminosity, the current X-ray data indicate that Compton-thick quasars may hide among ≈65 per cent of the SDSS Type 2 quasar population (L X,meas/ L X,[O III] < 0.01); however, since the Type 2 quasar sample is selected on [O III] luminosity, the estimated Compton-thick quasar fraction may be overestimated. Using archival Spitzer observations, we find that ≈50 per cent of SDSS Type 2 quasars appear to be obscured by Compton-thick material based on both the L X,meas /L X,mid-IR (where mid-IR corresponds to rest frame 12.3 μm) and L X,meas /L X,[O III] ratios. We use this information to provide an estimate of the Compton-thick quasar number density at z ≈ 0.3-0.8, which we find is in broad agreement with the expectations from X-ray background models.

Suzaku Observations of Hard X-ray-selected Seyfert 2 Galaxies

We present Suzaku observations of five hard X-ray-selected nearby Seyfert 2 galaxies. All the sources were clearly detected with the PIN Hard X-ray Detector up to several tens of keV, allowing for a fairly good characterization of the broadband X-ray continuum. We find that a unique model, even including multiple components, fails to represent the spectra of all the sources. Heavy obscuration manifests itself in different ways. For two sources, there is evidence for a reflection-dominated continuum; among the other three, one is mildly Compton thick (CT; NH ~ 1024 cm–2), while the remaining two are heavily obscured (NH 1023.5 cm–2) but Compton thin. Strong, narrow, iron Kα lines (EW~ 1-2 keV) due to neutral or mildly ionized gas are detected in CT active galactic nuclei (AGNs). In all of them, the Kα line is accompanied by the Kβ. The intensity and shape of the soft X-ray spectrum are different from object to object. Soft X-rays may originate from a nuclear component scattered off, or leaking through, the X-ray absorber plus thermal X-rays from the host galaxy. Emission from circumnuclear gas photoionized by the active nucleus, parameterized with a power law plus individual narrow Gaussian lines, also provides an acceptable description of the soft X-ray spectra. The limited Suzaku XIS CCD energy resolution does not allow us to draw firm conclusions about the origin of the soft X-ray emission. We briefly discuss our findings in the light of the AGN unified model and the geometry of the obscuring gas.

K+a galaxies in the zCOSMOS survey - physical properties of systems in their post-starburst phase

The identities of the main processes triggering and quenching star-formation in galaxies remain unclear. A key stage in evolution, however, appears to be represented by post-starburst galaxies. To investigate their impact on galaxy evolution, we initiated a multiwavelength study of galaxies with k+a spectral features in the COSMOS field. We examine a mass-selected sample of k+a galaxies at z=0.48-1.2 using the spectroscopic zCOSMOS sample. K+a galaxies occupy the brightest tail of the luminosity distribution. They are as massive as quiescent galaxies and populate the green valley in the colour versus luminosity (or stellar mass) distribution. A small percentage (<8%) of these galaxies have radio and/or X-ray counterparts (implying an upper limit to the SFR of ~8Msun/yr). Over the entire redshift range explored, the class of k+a galaxies is morphologically a heterogeneous population with a similar incidence of bulge-dominated and disky galaxies. This distribution does not vary with the strength of the Hdelta absorption line but instead with stellar mass in a way reminiscent of the well-known mass-morphology relation. Although k+a galaxies are also found in underdense regions, they appear to reside typically in a similarly rich environment as quiescent galaxies on a physical scale of ~2-8Mpc, and in groups they show a morphological early-to-late type ratio similar to the quiescent galaxy class. With the current data set, we do not find evidence of statistical significant evolution in either the number/mass density of k+a galaxies at intermediate redshift with respect to the local values, or the spectral properties. Those galaxies, which are affected by a sudden quenching of their star-formation activity, may increase the stellar mass of the red-sequence by up to a non-negligible level of ~10%.

The HELLAS2XMM survey - XIII. Multi-component analysis of the spectral energy distribution of obscured AGN

Aims. We combine near-to-mid-IR Spitzer data with shorter wavelength observations (optical to X-rays) to get insight into the properties of a sample of luminous, obscured active galactic nuclei (AGN). We aim at modelling their broad-band spectral energy distributions (SEDs) in order to estimate the main parameters related to the dusty torus that is assumed to be responsible for the reprocessed IR emission. Our final goal is to estimate the intrinsic nuclear luminosities and the Eddington ratios for our luminous, obscured AGN. Methods. The sample comprises 16 obscured high-redshift (0.9 < z < 2.1), X-ray luminous quasars (L2−10 keV ∼ 10 44 erg s −1 ) selected from the HELLAS2XMM survey in the 2–10 keV band. The optical-IR SEDs are described by a multi-component model that includes a stellar component to account for the optical and near-IR emission, an AGN component that dominates in the mid-IR (mainly emission from a dusty torus heated by nuclear radiation), and a starburst to reproduce the far-IR bump. A radiative transfer code to compute the spectrum and intensity of dust reprocessed emission was extensively tested against our multiwavelength data. While the torus parameters and the BH accretion luminosities are a direct output of the SED-fitting procedure, the BH masses were estimated indirectly by means of the local Mbulge–MBH relation. Results. The majority (∼80%) of the sources show moderate optical depth (τ9.7 μm ≤ 3), and the derived column densities NH are consistent with the X-ray inferred values (10 22 < NH < 3 × 10 23 cm −2 ) for most of the objects, confirming that the sources are moderately obscured Compton-thin AGN. Accretion luminosities in the range 5 × 10 44 < Lbol < 4 × 10 46 erg s −1 are inferred from the multiwavelength fitting procedure. We compare model luminosities with those obtained by integrating the observed SED, finding that the latter are lower by a factor of ∼2 in the median. The discrepancy can be as high as an order of magnitude for models with high optical depth (τ9.7 μm = 10). The ratio between the luminosities obtained by the fitting procedure and from the observed SED suggest that, at least for type 2 AGN, observed bolometric luminosities are likely to underestimate intrinsic ones and the effect is more severe for highly obscured sources. Bolometric corrections from the hard X-ray band are computed and have a median value of k2−10 keV ∼ 20. The obscured AGN in our sample are characterized by relatively low Eddington ratios (median λEdd ∼ 0.08). On average, they are consistent with the Eddington ratio increasing at increasing bolometric correction.

X-ray imaging of the ionisation cones in NGC 5252

Context. The physical conditions of the gas forming the narrow-line regions (NLR) in active galactic nuclei (AGN) have been extensively studied in the optical band. Recently detailed X-ray studies have shown how the emission in the 0.1–2 keV band detected in Seyfert 2 galaxies is associated to gas lying close to or associated with the NLR. Aims. We take advantage of the spectacular extension (∼15 �� ) of the NLR in the type II Seyfert galaxy NGC 5252 and of the complementary characteristics of XMM-Newton and Chandra to investigate the physical conditions of the gas in this galaxy. Methods. The X-ray data from XMM-Newton are used to define the spectral properties of the ionising nuclear source. The Chandra data are used to trace the spatial characteristics of the soft X-ray emission. This information is then compared to the optical HST characteristics of the NLR in NGC 5252. Results. The X-ray spectrum of the nucleus of NGC 5252 is intrinsically flat (Γ ∼ 1.4–1.5) and absorbed by neutral gas with a column density NH ∼ 10 22 cm −2 .B elow∼1 keV a soft excess is detected. The high-resolution spectrum obtained with the XMM-Newton RGS shows emission lines in the 0.2–1.5 keV range which strongly indicate that the soft X-ray component is essentially due to ionised gas. Moreover, the soft X-ray emission is spatially resolved around the nucleus and well overlaps the images obtained in narrow optical bands centred around the [Oiii] emission line at 5007 A. The [Oiii]/soft-X flux ratios along the ionisation cones are basically constant. This indicates that the electron density does not significantly deviate from the r −2 law (constant ionisation parameter) moving outward from the nucleus. Conclusions. This result combined with previous optical studies suggests two plausible but different scenarios in the reconstruction of the last ∼30 000 years of history of the central AGN. The most promising one is that the source is indeed a “quasar relic” with a steady and inefficient energy release from the accretion of matter onto the central super-massive black-hole. This scenario is suggested also by the flat nuclear X-ray spectrum that evokes an advection dominated accretion flow (ADAF) like emission mechanism.

Ultraluminous X-ray sources out to z 0.3 in the COSMOS field.

Context. Using Chandra observations we identified a sample of seven off-nuclear X-ray sources in the redshift range z = 0.072–0.283, located within optically bright galaxies in the COSMOS Survey. All of them, if associated with their closest bright galaxy, would have L[0.5–7 keV] > 10^(39) erg s^(-1) and therefore can be classified as ultraluminous X-ray sources (ULXs). n nAims. Using the multi-wavelength coverage available in the COSMOS field, we studied the properties of the host galaxies of these ULXs. In detail, we derived their star formation rate from Hα measurements and their stellar masses using SED fitting techniques with the aim to compute the probability to have an off-nuclear source based on the host galaxy properties. We divided the host galaxies in different morphological classes with the available ACS/HST imaging. n nMethods. We selected off-nuclear candidates with the following criteria: 1) the distance between the X-ray and the optical centroid has to be larger than 0.9, larger than 1.8 times the radius of the Chandra positional error circle and smaller than the Petrosian radius of the host galaxy; 2) the optical counterpart is a bright galaxy (R_(AB) < 22); 3) the redshift of the counterpart is lower than z = 0.3; 4) the source has been observed in at least one Chandra pointing at an off-axis angle smaller than 5; 5) the X-ray positional error is smaller than 0.8. We verified each candidate super-imposing the X-ray contours on the optical/IR images. We expect less than one misidentified AGN due to astrometric errors and on average 1.3 serendipitous background source matches. n nResults. We find that our ULXs candidates are located in regions of the SFR versus plane where one or more off-nuclear detectable sources are expected. From a morphological analysis of the ACS imaging and the use of rest-frame colours, we find that our ULXs are hosted both in late and early type galaxies. Finally, we find that the fraction of galaxies hosting a ULX ranges from ≈0.5% to ≈0.2% going from L_(0.5-2 keV) = 3 × 10^(39) ergu2009s^(-1) to L_(0.5-2 keV) = 2 × 10^(40) ergu2009s^(-1).