arXiv subjects

N. Cappelluti

Publications and source records attributed to N. Cappelluti.

At least 91 records · Page 5Linked to original sources

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.

astro-ph.CO↗

arXiv2009

A Weak Lensing Study of X-ray Groups in the COSMOS survey: Form and Evolution of the Mass-Luminosity Relation

Measurements of X-ray scaling laws are critical for improving cosmological constraints derived with the halo mass function and for understanding the physical processes that govern the heating and cooling of the intracluster medium. In this paper, we use a sample of 206 X-ray selected galaxy groups to investigate the scaling relation between X-ray luminosity (Lx) and halo mass (M00) where M200 is derived via stacked weak gravitational lensing. This work draws upon a broad array of multi-wavelength COSMOS observations including 1.64 square degrees of contiguous imaging with the Advanced Camera for Surveys (ACS) and deep XMM-Newton/Chandra imaging. The combined depth of these two data-sets allows us to probe the lensing signals of X-ray detected structures at both higher redshifts and lower masses than previously explored. Weak lensing profiles and halo masses are derived for nine sub-samples, narrowly binned in luminosity and redshift. The COSMOS data alone are well fit by a power law, M200 ~ Lx^a, with a slope of a=0.66+-0.14. These results significantly extend the dynamic range for which the halo masses of X-ray selected structures have been measured with weak gravitational lensing. As a result, tight constraints are obtained for the slope of the M-Lx relation. The combination of our group data with previously published cluster data demonstrates that the M-Lx relation is well described by a single power law, a=0.64+-0.03, over two decades in mass, 10^13.5-10^15.5 h72^-1 Msun. These results are inconsistent at the 3.7 level with the self-similar prediction of a=0.75. We examine the redshift dependence of the M-Lx relation and find little evidence for evolution beyond the rate predicted by self-similarity from z ~ 0.25 to z ~ 0.8.

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

PKS 0537-286, carrying the information of the environment of SMBHs in the early Universe

We present the results of a multifrequency campaign on the high-redshift (z = 3.1) blazar PKS 0537-286. The source was observed at different epochs from 2006 to 2008 with INTEGRAL and Swift, and nearly simultaneously with ground-based near-IR/optical telescopes. The SEDs are compatible with a model based on synchrotron radiation and external inverse Compton scattering. The campaign gives an insight into the physical environment of the blazar.

astro-ph.HE↗

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

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↗

arXiv2009

A candidate tidal disruption event in the Galaxy cluster Abell 3571

Tidal disruption events are possible sources of temporary nuclear activity in galactic nuclei and can be considered as good indicators of the existence of super massive black holes in the centers of galaxies. A new X-ray source has been serendipitously detected with ROSAT in a PSPC pointed observation of the galaxy cluster A3571. Given the strong flux decay of the object in subsequent detections, the tidal disruption scenario is investigated as possible explanationof the event. We followed up the evolution of the X-ray transient with ROSAT, XMM-Newton and Chandra for a total period of ~13 years. We also obtained 7-band optical/NIR photometry with GROND at the ESO/MPI 2.2m telescope. We report a very large decay of the X-ray flux of ROSAT source identified with the galaxy LEDA 095953 a member of the cluster Abell 3571. We measured a maximum 0.3-2.4 keV luminosity Log(L_X)=42.8 erg s^-1. The high state of the source lasted at least 150 ks, afterwards L_X declined as ~t^-2. The spectrum of the brightest epoch is consistent with a black body with temperature kT ~0.12 keV. The total energy released by this event in 10 yr was estimated to be Delta_E>2x10^50 erg. We interpret this event as a tidal disruption of a solar type star by the central supermassive black hole (i.e. ~10^7 M_sun) of the galaxy.

astro-ph.CO↗

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.

astro-ph↗

arXiv2008

The spatial clustering of X-ray selected AGN in the XMM-COSMOS field

We study the spatial clustering of 538 X-ray selected AGN in the 2 deg^2 XMM-COSMOS field that are spectroscopically identified to I_{AB}<23 and span the redshift range z=0.2-3.0. The median redshift and luminosity of the sample are z = 0.98 and L_{0.5-10}=6.3 x 10^{43} erg/s, respectively. A strong clustering signal is detected at ~18sigma level, which is the most significant measurement obtained to date for clustering of X-ray selected AGN. By fitting the projected correlation function w(r_p) with a power law on scales of r_p=0.3-40 Mpc/h, we derive a best fit comoving correlation length of r_0 = 8.6 +- 0.5 Mpc/h and slope of gamma=1.88 +- 0.07 (Poissonian errors; bootstrap errors are about a factor of 2 larger). An excess signal is observed in the range r_p~5-15 Mpc/h, which is due to a large scale structure at z ~ 0.36 containing about 40 AGN. When removing the z ~ 0.36 structure, or computing w(r_p) in a narrower range around the peak of the redshift distribution (e.g. z=0.4-1.6), the correlation length decreases to r_0 ~ 5-6 Mpc/h, which is consistent with that observed for bright optical QSOs at the same redshift. We investigate the clustering properties of obscured and unobscured AGN separately. Within the statistical uncertainties, we do not find evidence that AGN with broad optical lines (BLAGN) cluster differently from AGN without broad optical lines (non-BLAGN). The correlation length measured for XMM-COSMOS AGN at z~1 is similar to that of massive galaxies (stellar mass M_*> 3 x 10^{10} M_sun) at the same redshift. This suggests that AGN at z~1 are preferentially hosted by massive galaxies, as observed both in the local and in the distant (z~2) Universe. (shortened)

astro-ph↗

arXiv2008

Ongoing and co-evolving star formation in zCOSMOS galaxies hosting Active Galactic Nuclei

We present a study of the host galaxies of AGN selected from the zCOSMOS survey to establish if accretion onto supermassive black holes and star formation are explicitly linked up to z~1. We identify 152 galaxies that harbor AGN, based on XMM observations of 7543 galaxies (i<22.5). Star formation rates (SFRs), including those weighted by stellar mass, are determined using the [OII]3727 emission-line, corrected for an AGN contribution. We find that the majority of AGN hosts have significant levels of star formation with a distribution spanning ~1-100 Msun yr^-1. The close association between AGN activity and star formation is further substantiated by an increase in the AGN fraction with the youthfulness of their stars as indicated by the rest-frame color (U-V) and spectral index Dn(4000); we demonstrate that mass-selection alleviates an artifical peak falling in the transition region when using luminosity-limited samples. We also find that the SFRs of AGN hosts evolve with cosmic time in a manner that closely mirrors the overall galaxy population and explains the low SFRs in AGNs (z<0.3) from the SDSS. We conclude that the conditions most conducive for AGN activity are a massive host galaxy and a large reservoir of gas. Furthermore, a direct correlation between mass accretion rate onto SMBHs and SFR is shown to be weak although the average ratio is constant with redshift, effectively shifting the evidence for a co-evolution scenario in a statistical manner to smaller physical scales. Our findings illustrate an intermittent scenario with an AGN lifetime substantially shorter than that of star formation and underlying complexities regarding fueling over vastly different physical scales yet to be determined [Abridged].

astro-ph↗

arXiv2008

Chasing highly obscured QSOs in the COSMOS field

(abridged) We take advantage of the deep Chandra and Spitzer coverage of a large area (more than 10 times the area covered by the Chandra deep fields, CDFs in the COSMOS field, to extend the search of highly obscured, Compton-thick active nuclei to higher luminosity. These sources have low surface density and large samples can be provided only through large area surveys, like the COSMOS survey. We analyze the X-ray properties of COSMOS MIPS sources with 24$μ$m fluxes higher than 550$μ$Jy. For the MIPS sources not directly detected in the Chandra images we produce stacked images in soft and hard X-rays bands. To estimate the fraction of Compton-thick AGN in the MIPS source population we compare the observed stacked count rates and hardness ratios to those predicted by detailed Monte Carlo simulations including both obscured AGN and star-forming galaxies. The density of lower luminosity Compton-thick AGN (logL(2-10keV)=43.5-44) at z=0.7--1.2 is $(3.7\pm1.1) \times10^{-5}$ Mpc$^{-3}$, corresponding to $\sim67%$ of that of X-ray selected AGN. The comparison between the fraction of infrared selected, Compton thick AGN to the X-ray selected, unobscured and moderately obscured AGN at high and low luminosity suggests that Compton-thick AGN follow a luminosity dependence similar to that discovered for Compton-thin AGN, becoming relatively rarer at high luminosities. We estimate that the fraction of AGN (unobscured, moderately obscured and Compton thick) to the total MIPS source population is $49\pm10%$, a value significantly higher than that previously estimated at similar 24$μ$m fluxes. We discuss how our findings can constrain AGN feedback models.

astro-ph↗

arXiv2008

High redshift quasars in the COSMOS survey: the space density of z>3 X-ray selected QSOs

We present a new measurement of the space density of high redshift (3.0 10^{-15} cgs), homogeneous sample of z>3 QSOs for which spectroscopic (22) or photometric (18) redshifts are available. We present the optical (color-color diagrams) and X-ray properties, the number counts and space densities of the z>3 X-ray selected quasars population and compare our findings with previous works and model predictions. We find that the optical properties of X-ray selected quasars are not significantly different from those of optically selected samples. There is evidence for substantial X-ray absorption (logN_H>23 cm^{-2}) in about 20% of the sources in the sample. The comoving space density of luminous (L_X >10^{44} erg s^-1) QSOs declines exponentially (by an e--folding per unit redshift) in the z=3.0-4.5 range, with a behavior similar to that observed for optically bright unobscured QSOs selected in large area optical surveys. Prospects for future, large and deep X-ray surveys are also discussed.

astro-ph↗

arXiv2008

Photometric redshift and classification for the XMM-COSMOS sources

We present photometric redshifts and spectral energy distribution (SED) classifications for a sample of 1542 optically identified sources detected with XMM in the COSMOS field. Our template fitting classifies 46 sources as stars and 464 as non-active galaxies, while the remaining 1032 require templates with an AGN contribution. High accuracy in the derived photometric redshifts was accomplished as the result of 1) photometry in up to 30 bands with high significance detections, 2) a new set of SED templates including 18 hybrids covering the far-UV to mid-infrared, which have been constructed by the combination of AGN and non-active galaxies templates, and 3) multi-epoch observations that have been used to correct for variability (most important for type 1 AGN). The reliability of the photometric redshifts is evaluated using the sub-sample of 442 sources with measured spectroscopic redshifts. We achieved an accuracy of $σ_{Δz/(1+z_{spec})} = 0.014$ for i$_{AB}^*<$22.5 ($σ_{Δz/(1+z_{spec})} \sim0.015$ for i$_{AB}^*<$24.5). The high accuracies were accomplished for both type 2 (where the SED is often dominated by the host galaxy) and type 1 AGN and QSOs out to $z=4.5$. The number of outliers is a large improvement over previous photometric redshift estimates for X-ray selected sources (4.0% and 4.8% outliers for i$_{AB}^*<$22.5 and i$_{AB}^*<$24.5, respectively). We show that the intermediate band photometry is vital to achieving accurate photometric redshifts for AGN, whereas the broad SED coverage provided by mid infrared (Spitzer/IRAC) bands is important to reduce the number of outliers for normal galaxies.

astro-ph↗

arXiv2008

AGN Host Galaxy Morphologies in COSMOS

We use HST/ACS images and a photometric catalog of the COSMOS field to analyze morphologies of the host galaxies of approximately 400 AGN candidates at redshifts 0.3 < z < 1.0. We compare the AGN hosts with a sample of non-active galaxies drawn from the COSMOS field to match the magnitude and redshift distribution of the AGN hosts. We perform 2-D surface brightness modeling with GALFIT to yield host galaxy and nuclear point source magnitudes. X-ray selected AGN host galaxy morphologies span a substantial range that peaks between those of early-type, bulge-dominated and late-type, disk-dominated systems. We also measure the asymmetry and concentration of the host galaxies. Unaccounted for, the nuclear point source can significantly bias results of these measured structural parameters, so we subtract the best-fit point source component to obtain images of the underlying host galaxies. Our concentration measurements reinforce the findings of our 2-D morphology fits, placing X-ray AGN hosts between early- and late-type inactive galaxies. AGN host asymmetry distributions are consistent with those of control galaxies. Combined with a lack of excess companion galaxies around AGN, the asymmetry distributions indicate that strong interactions are no more prevalent among AGN than normal galaxies. In light of recent work, these results suggest that the host galaxies of AGN at these X-ray luminosities may be in a transition from disk-dominated to bulge-dominated, but that this transition is not typically triggered by major mergers.

astro-ph↗

arXiv2008

Galaxy Clusters in the Swift/BAT era: Hard X-rays in the ICM

We report about the detection of 10 clusters of galaxies in the ongoing Swift/BAT all-sky survey. This sample, which comprises mostly merging clusters, was serendipitously detected in the 15--55 keV band. We use the BAT sample to investigate the presence of excess hard X-rays above the thermal emission. The BAT clusters do not show significant (e.g. >2 $σ$) non-thermal hard X-ray emission. The only exception is represented by Perseus whose high-energy emission is likely due to NGC 1275. Using XMM-Newton, Swift/XRT, Chandra and BAT data, we are able to produce upper limits on the Inverse Compton (IC) emission mechanism which are in disagreement with most of the previously claimed hard X-ray excesses. The coupling of the X-ray upper limits on the IC mechanism to radio data shows that in some clusters the magnetic field might be larger than 0.5 $μ$G. We also derive the first logN-logS and luminosity function distribution of galaxy clusters above 15 keV.

astro-ph↗

arXiv2007

XMM-Newton observations of the Lockman Hole: X-ray source catalogue and number counts

The Lockman Hole field represents the sky area of lowest Galactic line-of-sight column density N_H=5.7X10^19 cm^-2. It was observed by the XMM-Newton X-ray observatory in 18 pointings for a total of 1.16 Msec (raw EPIC pn observing time) constituting the deepest XMM-Newton exposure so far. We present a catalogue of the X-ray sources detected in the central 0.196 deg^2 of the field and discuss the derived number counts and X-ray colours. In the 0.5--2.0 keV band, a sensitivity limit (defined as the faintest detectable source)of 1.9X10^-16 erg cm^-2 s^-1 was reached. The 2.0--10.0 keV band and 5.0--10.0 keV band sensitivity limits were 9X10^-16 erg cm^-2 s^-1 and 1.8X10^-15 erg cm^-2 s^-1, respectively.A total of 409 sources above a detection likelihood of 10 (3.9 sigma) were found within a radius of 15' off the field centre, of which 340, 266, and 98 sources were detected in the soft, hard, and very hard bands, respectively. The number counts in each energy band are in close agreement with results from previous surveys and with the synthesis models of the X-ray background. A 6% of Compton-thick source candidates have been selected from the X-ray colour-colour diagram. This fraction is consistent with the most recent predictions of X-ray background population synthesis models at our flux limits. We also estimated, for the first time, the logN-logS relation for Compton-thick AGN.

astro-ph↗

arXiv2007

The 3D soft X-ray cluster-AGN cross-correlation function in the ROSAT NEP survey

X-ray surveys facilitate investigations of the environment of AGNs. Deep Chandra observations revealed that the AGNs source surface density rises near clusters of galaxies. The natural extension of these works is the measurement of spatial clustering of AGNs around clusters and the investigation of relative biasing between active galactic nuclei and galaxies near clusters.The major aims of this work are to obtain a measurement of the correlation length of AGNs around clusters and a measure of the averaged clustering properties of a complete sample of AGNs in dense environments. We present the first measurement of the soft X-ray cluster-AGN cross-correlation function in redshift space using the data of the ROSAT-NEP survey. The survey covers 9x9 deg^2 around the North Ecliptic Pole where 442 X-ray sources were detected and almost completely spectroscopically identified. We detected a >3sigma significant clustering signal on scales s<50 h70^-1 Mpc. We performed a classical maximum-likelihood power-law fit to the data and obtained a correlation length s_0=8.7+1.2-0.3 h_70-1 Mpc and a slope gamma=1.7$^+0.2_-0.7 (1sigma errors). This is a strong evidence that AGNs are good tracers of the large scale structure of the Universe. Our data were compared to the results obtained by cross-correlating X-ray clusters and galaxies. We observe, with a large uncertainty, that the bias factor of AGN is similar to that of galaxies.

astro-ph↗

arXiv2007

X-ray source counts in the COSMOS field

We present the analysis of the source counts in the XMM-COSMOS survey using data of the first year of XMM-Newton observations. The survey covers ~2 deg^2 within the region of sky bounded by 9^h57.5^m<R.A.<10^h03.5^m; 1^d27.5^m<DEC<2^d57.5^m with a total net integration time of 504 ks. Using a maximum likelihood algorithm we detected a total of 1390 sources at least in one band. Using Monte Carlo simulations to estimate the sky coverage we produced the logN-logS relations. These relations have been then derived in the 0.5--2 keV, 2--10 keV and 5--10 keV energy bands, down to flux limits of 7.2x10^-16 erg cm^-2 s^-1, 4.0x10^-15 erg cm^-2 s^-1 and 9.7x10^-15 erg cm^-2 s^-1, respectively. These relations have been compared to previous X-ray survey and to the most recent X-ray background model finding an excellent agreement. The slightly different normalizations observed in the source counts of COSMOS and previous surveys can be largely explained as a combination of low counting statistics and cosmic variance introduced by the large scale structure.

astro-ph↗