arXiv subjects

A. Comastri

Publications and source records attributed to A. Comastri.

At least 199 records · Page 11Linked to original sources

arXiv2010

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

We present a detailed study of a peculiar source in the COSMOS survey at z=0.359. Source CXOCJ100043.1+020637 (CID-42) presents two compact optical sources embedded in the same galaxy. The distance between the 2, measured in the HST/ACS image, is 0.495" that, at the redshift of the source, corresponds to a projected separation of 2.46 kpc. A large (~1200 km/s) velocity offset between the narrow and broad components of Hbeta has been measured in three different optical spectra from the VLT/VIMOS and Magellan/IMACS instruments. CID-42 is also the only X-ray source having in its X-ray spectra a strong redshifted broad absorption iron line, and an iron emission line, drawing an inverted P-Cygni profile. The Chandra and XMM data show that the absorption line is variable in energy by 500 eV over 4 years and that the absorber has to be highly ionized, in order not to leave a signature in the soft X-ray spectrum. That these features occur in the same source is unlikely to be a coincidence. We envisage two possible explanations: (1) a gravitational wave recoiling black hole (BH), caught 1-10 Myr after merging, (2) a Type 1/ Type 2 system in the same galaxy where the Type 1 is recoiling due to slingshot effect produced by a triple BH system. The first possibility gives us a candidate gravitational waves recoiling BH with both spectroscopic and imaging signatures. In the second case, the X-ray absorption line can be explained as a BAL-like outflow from the foreground nucleus (a Type 2 AGN) at the rearer one (a Type 1 AGN), which illuminates the otherwise undetectable wind, giving us the first opportunity to show that fast winds are present in obscured AGN.

astro-ph.CO↗

arXiv2010

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

Using Chandra observations we have 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. Using the multi-wavelength coverage available in the COSMOS field, we study the properties of the host galaxies of these ULXs. In detail, we derived their star formation rate from H_alpha 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 divide the host galaxies in different morphological classes using the available ACS/HST imaging. We find that our ULXs candidates are located in regions of the SFR versus M$_star$ 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 x 10^39 erg s^-1 to L[0.5-2 keV]= 2 x 10^40 erg s^-1.

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arXiv2010

Probing BH mass and accretion through X-ray variability in the CDFS

Recent work on nearby AGNs has shown that X-ray variability is correlated with the mass and accretion rate onto the central SMBH. Here we present the application of the variability-luminosity relation to high redshift AGNs in the CDFS, making use of XMM-Newton observations. We use Monte Carlo simulations in order to properly account for bias and uncertainties introduced by the sparse sampling and the very low statistics. Our preliminary results indicate that BH masses span over the range from 10^5 to 10^9 solar mass while accretion rates range from 10^-3 up to values greater than 1, in unit of Eddington accretion rate.

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arXiv2009

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 ($0.04<\z<4.25$) and X-ray luminosities ($40.6 \leq \Log \Lhard \leq 45.3$). 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 $\alphaox$ 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 $\alphaox$ and $\lo$ is found, in agreement with previous investigations of optically selected samples. We calculate bolometric corrections, $\kbol$, for the whole sample using hard X-ray luminosities ($\Lhard$), 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 $\alphaox$ and $\kbol$, which can be used to estimate accurate bolometric corrections using only optical and X-ray data. We find a significant correlation between $\alphaox$ and Eddington ratio, in which $\alphaox$ increases for increasing Eddington ratios.

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arXiv2009

Black hole growth and starburst activity at z = 0.6-4 in the Chandra Deep Field South. Host galaxies properties of obscured AGN

The co-evolution of host galaxies and the active black holes which reside in their centre is one of the most important topics in modern observational cosmology. Here we present a study of the properties of obscured Active Galactic Nuclei (AGN) detected in the CDFS 1Ms observation and their host galaxies. We limited the analysis to the MUSIC area, for which deep K-band observations obtained with ISAAC@VLT are available, ensuring accurate identifications of the counterparts of the X-ray sources as well as reliable determination of photometric redshifts and galaxy parameters, such as stellar masses and star formation rates. In particular, we: 1) refined the X-ray/infrared/optical association of 179 sources in the MUSIC area detected in the Chandra observation; 2) studied the host galaxies observed and rest frame colors and properties. We found that X-ray selected (L_X>10^{42} erg s^{-1}) AGN show Spitzer colors consistent with both AGN and starburst dominated infrared continuum; the latter would not have been selected as AGN from infrared diagnostics. The host galaxies of X-ray selected obscured AGN are all massive (M_*>10^{10} M_sun) and, in 50% of the cases, are also actively forming stars (1/SSFR 1 and M_*>3x10^{11} M_sun, a fraction significantly higher than in the local Universe for AGN of similar luminosities.

astro-ph.CO↗

arXiv2009

On the cosmic evolution of the scaling relations between black holes and their host galaxies: Broad Line AGN in the zCOSMOS survey

(Abriged) We report on the measurement of the rest frame K-band luminosity and total stellar mass of the hosts of 89 broad line Active Galactic Nuclei detected in the zCOSMOS survey in the redshift range 1<z<2.2. The unprecedented multiwavelength coverage of the survey field allows us to disentangle the emission of the host galaxy from that of the nuclear black hole in their Spectral Energy Distributions. We derive an estimate of black hole masses through the analysis of the broad Mg II emission lines observed in the medium-resolution spectra taken with VIMOS/VLT as part of the zCOSMOS project. We found that, as compared to the local value, the average black hole to host galaxy mass ratio appears to evolve positively with redshift, with a best fit evolution of the form (1+z)^{0.68 \pm0.12 +0.6 -0.3}, where the large asymmetric systematic errors stem from the uncertainties in the choice of IMF, in the calibration of the virial relation used to estimate BH masses and in the mean QSO SED adopted. A thorough analysis of observational biases induced by intrinsic scatter in the scaling relations reinforces the conclusion that an evolution of the MBH-M* relation must ensue for actively growing black holes at early times: either its overall normalization, or its intrinsic scatter (or both) appear to increase with redshift. This can be interpreted as signature of either a more rapid growth of supermassive black holes at high redshift, a change of structural properties of AGN hosts at earlier times, or a significant mismatch between the typical growth times of nuclear black holes and host galaxies.

astro-ph.CO↗

arXiv2009

The Chandra survey of the COSMOS field II: source detection and photometry

The Chandra COSMOS Survey (C-COSMOS) is a large, 1.8 Ms, Chandra program, that covers the central contiguous ~0.92 deg^2 of the COSMOS field. C-COSMOS is the result of a complex tiling, with every position being observed in up to six overlapping pointings (four overlapping pointings in most of the central ~0.45 deg^2 area with the best exposure, and two overlapping pointings in most of the surrounding area, covering an additional ~0.47 deg^2). Therefore, the full exploitation of the C-COSMOS data requires a dedicated and accurate analysis focused on three main issues: 1) maximizing the sensitivity when the PSF changes strongly among different observations of the same source (from ~1 arcsec up to ~10 arcsec half power radius); 2) resolving close pairs; and 3) obtaining the best source localization and count rate. We present here our treatment of four key analysis items: source detection, localization, photometry, and survey sensitivity. Our final procedure consists of a two step procedure: (1) a wavelet detection algorithm, to find source candidates, (2) a maximum likelihood Point Spread Function fitting algorithm to evaluate the source count rates and the probability that each source candidate is a fluctuation of the background. We discuss the main characteristics of this procedure, that was the result of detailed comparisons between different detection algorithms and photometry tools, calibrated with extensive and dedicated simulations.

astro-ph.IM↗

arXiv2009

AGN unified scheme and evolution: a Suzaku view

We present broad band Suzaku}observations of a small sample of hard X-ray selected (> 10 keV), nearby Seyfert 2 galaxies and discuss the results in the context of AGN unified model. We also review the issues related to the space density of heavily obscured, Compton Thick AGN in the local Universe and the perspectives for the search of these objects at high redshifts

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arXiv2009

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

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.

astro-ph.CO↗

arXiv2009

Evolution of the X-ray Luminosity in Young HII Galaxies

In an effort to understand the correlation between X-ray emission and present star formation rate (SFR), we obtained XMM-Newton data to estimate the X-ray luminosities of a sample of actively starforming HII galaxies. The obtained X-ray luminosities are compared to other well known tracers of star formation activity such as the far infrared and the ultraviolet luminosities. We also compare the obtained results with empirical laws from the literature and with recently published analysis applying synthesis models. We use the time delay between the formation of the stellar cluster and that of the first X-ray binaries, in order to put limits on the age of a given stellar burst. We conclude that the generation of soft X-rays, as well as the Ha or infrared luminosities is instantaneous. The relation between the observed radio and hard X-ray luminosities, on the other hand, points to the existence of a time delay between the formation of the stellar cluster and the explosion of the first massive stars and the consequent formation of supernova remnants and high mass X-ray binaries (HMXB) which originate the radio and hard X-ray fluxes respectively. When comparing hard X-rays with a star formation indicator that traces the first million years of evolution (e.g. Ha luminosities) we found a deficit in the expected X-ray luminosity. This deficit is not found when the X-ray luminosities are compared with infrared luminosities, a star formation tracer that represents an average over the last 10^8 years. The results support the hypothesis that hard X-rays are originated in X-ray binaries which, as supernova remnants, have a formation time delay of a few mega years after the starforming burst.

astro-ph.CO↗

arXiv2009

On the nature of red galaxies: the Chandra perspective

We present the X-ray properties of the extremely red objects (ERO) population observed by Chandra with three partially overlapping pointings (up to ~90 ks) over an area of ~500 arcmin^2, down to a 0.5-8 keV flux limit of ~10-15 erg cm-2 s-1. We selected EROs using a multi-band photometric catalog down to a KS-band magnitude of ~19.3 (Vega system); 14 EROs were detected in X-rays, corresponding to ~9% of the overall X-ray source population (149 X-ray sources) and to ~5% of the ERO population (288). The X-ray emission of all X-ray detected EROs is consistent with that of an active galactic nucleus (AGN) (>=3.5x10^{42} erg s-1 at photometric redshifts z > 1), in agreement with previous X-ray observations, with an indication of increasing absorption between the three X-ray brightest EROs and the 11 X-ray faintest EROs.We take advantage of the good spatial resolution and limited background provided by Chandra to place constraints on the population of the X-ray undetected EROs by a stacking analysis. Their stacked emission, whose statistical significance is 5.7sigma in the observed 0.5-8 keV band, provides an upper limit to the average intrinsic absorption at z=1 of 2.5x10^{22} cm^{-2} and corresponds to a rest-frame 0.5-8 keV luminosity of 8.9x10^{41} erg s^{-1} . We estimate that any accretion-related X-ray emission to the stacked signal is likely "diluted" by emission due to hot gas in normal galaxies and star-formation activity in dust-enshrouded galaxies at high redshift.

astro-ph.HE↗

arXiv2009

The Growth of Supermassive Black Holes Across Cosmic Time

One of the main themes in extragalactic astronomy for the next decade will be the evolution of galaxies over cosmic time. Many future observatories, including JWST, ALMA, GMT, TMT and E-ELT will intensively observe starlight over a broad redshift range, out to the dawn of the modern Universe when the first galaxies formed. It has, however, become clear that the properties and evolution of galaxies are intimately linked to the growth of their central black holes. Understanding the formation of galaxies, and their subsequent evolution, will therefore be incomplete without similarly intensive observations of the accretion light from supermassive black holes (SMBH) in galactic nuclei. To make further progress, we need to chart the formation of typical SMBH at z>6, and their subsequent growth over cosmic time, which is most effectively achieved with X-ray observations. Recent technological developments in X-ray optics and instrumentation now bring this within our grasp, enabling capabilities fully matched to those expected from flagship observatories at longer wavelengths.

astro-ph.CO↗

arXiv2009

The HELLAS2XMM Survey. XII. The infrared/sub-millimeter view of an X-ray selected Type 2 quasar at z=2

We present multi-wavelength observations (from optical to sub-millimeter, including Spitzer and SCUBA) of H2XMMJ 003357.2-120038 (also GD158_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 modeled assuming three components: a stellar component to account for the optical and near-IR emission, an AGN component (i.e., dust heated by an accreting active nucleus), dominant in the mid-IR, with an optical depth at 9.7 micron along the line of sight (close to the equatorial plane of the obscuring matter) of tau(9.7)=1 and a full covering angle of the reprocessing matter (torus) of 140 degrees, and a far-IR starburst component (i.e., dust heated by star formation) to reproduce the wide bump observed longward of 70 micron. The derived star-formation rate is about 1500 solar masses per year. The overall modeling indicates that GD158_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.

astro-ph.GA↗

arXiv2009

Resolving the mid-infrared cores of local Seyferts

We present new photometry of 16 local Seyferts including 6 Compton-thick sources in N-band filters around 12-microns, obtained with the VISIR instrument on the 8-m Very Large Telescope. The near-diffraction-limited imaging provides the least-contaminated core fluxes for these sources to date. Augmenting these with our previous observations and with published intrinsic X-ray fluxes, we form a total sample of 42 sources for which we find a strong mid-infrared:X-ray (12.3 micron:2-10 keV) luminosity correlation. Performing a physically-motivated sub-selection of sources in which the Seyfert torus is likely to be best-resolved results in the correlation L_{MIR} ~ L_X^{1.11+/-0.07}, with a reduction of the scatter in luminosities as compared to the full sample. Consideration of systematics suggests a range of 1.02-1.21 for the correlation slope. The mean 2 keV:12.3 micron spectral index (alpha_IX) is found to be -1.10+/-0.01, largely independent of luminosity. Observed 12-micron bolometric corrections range over ~10-30 if a known luminosity trend of intrinsic X-ray bolometric corrections is assumed. Comparison with ISO data spanning a larger luminosity range suggests that our correlation can be extended into the quasar regime. The fact that unobscured, obscured and Compton-thick sources all closely follow the same luminosity correlation has important implications for the structures of Seyfert cores. The typical resolution-limit of our imaging corresponds to ~70 pc at a median z=0.01, and the tightness of the correlation constrains any residual star-formation within these physical scales, for which we infer a mean upper-limit of <~40% of the remaining unresolved flux. We suggest that uncontaminated mid-IR continuum imaging of AGN is an accurate proxy for their intrinsic power.

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arXiv2009

Multi-wavelength data handling in current and future surveys: the possible role of Virtual Observatory

Here we review some of the main issues related to multi-wavelength source identification and characterization, with particular emphasis on the field of X-ray surveys carried out over the last years. This complex and time-consuming process is going to represent one of the main difficulties over the coming years, when significantly larger surveys, both in area and depth, will be carried out with the new generations of space- and ground-based facilities like e.g. eROSITA, WISE, VISTA, Pan-STARRS, and LSST. The Virtual Observatory can offer a reliable way to approach to a new concept of data handling and multi-wavelength source characterization, provided that uniform and rigorous data analyses and extensive quality checks are performed.

astro-ph.CO↗

arXiv2009

Suzaku observation of the Phoenix Galaxy

In recent years, several Seyfert 2 galaxies have been discovered that change state when observed in X-rays a few years apart, switching from Compton-thin to reflection-dominated or viceversa. We observed a member of this class of "Changing-look" sources, the Phoenix Galaxy, with Suzaku, with the aim of better understanding the nature of the variations. The Suzaku spectrum was analyzed, and the results compared with previous ASCA and XMM-Newton observations. The source was caught in a Compton-thin state, as in XMM-Newton, but differently from ASCA. Comparing the Suzaku and XMM-Newton observations, a variation in the column density of the absorber on a time scale of years is discovered. A similar change, but on much shorter time scales (i.e. ks) may also explain the count-rate variations during the Suzaku observations. A soft excess is also present, likely due to continuum and line emission from photoionized circumnuclear matter.

astro-ph.HE↗

arXiv2009

The XMM-Newton wide-field survey in the COSMOS field. The point-like X-ray source catalogue

The COSMOS survey is a multiwavelength survey aimed to study the evolution of galaxies, AGN and the large scale structure. The XMM-COSMOS is a deep X-ray survey over the full 2 deg2 of the COSMOS area. It consists of 55 XMM-Newton pointings for a total exposure of ~1.5 Ms with an average vignetting corrected depth of 40 ks across the field of view and a sky coverage of 2.13 deg2. We present the catalogue of point-like X-ray sources detected with the EPIC CCD cameras, the logN-logS relations and the X-ray colour-colour diagrams. The analysis was performed in the 0.5-2 keV, 2-10 keV and 5-10 keV energy bands. The completeness of the catalogue as well as logN-logS have been calibrated using Monte Carlo simulations. The catalogs contains a total of 1887 unique sources detected in at least one band. The survey, that shows unprecedented homogeneity, has a flux limit of ~1.7x10-15 erg cm-2 s-1, ~9.3x10-15 erg cm-2 s-1 and ~1.3x10-14 erg cm-2 s-1 over 90% of the area (1.92 deg2) in the 0.5-2 keV, 2-10 keV and 5-10 keV energy band, respectively. Thanks to the rather homogeneous exposure over a large area, the derived logN-logS relations are very well determined over the flux range sampled by XMM-COSMOS. These relations have been compared with XRB synthesis models, which reproduce the observations with an agreement of ~10% in the 5-10 keV and 2-10 keV band, while in the 0.5-2 keV band the agreement is of the order of ~20%. The hard X-ray colors confirmed that the majority of the extragalactic sources, in a bright subsample, are actually Type I or Type II AGN. About 20% of the sources have X-ray luminosity typical of AGN (L_X >1042 erg/s) although they do not show any clear signature of nuclear activity in the optical spectrum.

astro-ph.HE↗

arXiv2008

The Environments of Active Galactic Nuclei within the zCOSMOS Density Field

The impact of environment on AGN activity up to z~1 is assessed by utilizing a mass-selected sample of galaxies from the 10k catalog of the zCOSMOS spectroscopic redshift survey. We identify 147 AGN by their X-ray emission as detected by XMM-Newton from a parent sample of 7234 galaxies. We measure the fraction of galaxies with stellar mass M_*>2.5x10^10 Msun that host an AGN as a function of local overdensity using the 5th, 10th and 20th nearest neighbors that cover a range of physical scales (~1-4 Mpc). Overall, we find that AGNs prefer to reside in environments equivalent to massive galaxies with substantial levels of star formation. Specifically, AGNs with host masses between 0.25-1x10^11 Msun span the full range of environments (i.e., field-to-group) exhibited by galaxies of the same mass and rest-frame color or specific star formation rate. Host galaxies having M_*>10^11 Msun clearly illustrate the association with star formation since they are predominantly bluer than the underlying galaxy population and exhibit a preference for lower density regions analogous to SDSS studies of narrow-line AGN. To probe the environment on smaller physical scales, we determine the fraction of galaxies (M_*>2.5x10^10 Msun) hosting AGNs inside optically-selected groups, and find no significant difference with field galaxies. We interpret our results as evidence that AGN activity requires a sufficient fuel supply; the probability of a massive galaxy to have retained some sufficient amount of gas, as evidence by its ongoing star formation, is higher in underdense regions where disruptive processes (i.e., galaxy harrassment, tidal stripping) are lessened.

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