arXiv subjects

M. Brusa

Publications and source records attributed to M. Brusa.

At least 217 records · Page 12Linked to original sources

arXiv2007

The XMM-Newton view of IRAS 09104+4109: evidence for a changing-look Type 2 quasar?

We analyzed the spectroscopic data from the PN and the MOS cameras in the 0.4-10 keV band. We also used an archival BeppoSAX 1-50 keV observation of IRAS 09104+4109 to investigate possible variations of the quasar emission. The X-ray emission in the EPIC band is dominated by the intra-cluster medium thermal emission. We found that the quasar contributes ~35% of the total flux in the 2-10 keV band. Both a transmission- (through a Compton-thin absorber with a Compton optical depth of τ_C~0.3, i.e. Nh~5 x 10^{23} cm^-2) and a reflection-dominated (τ_C>1) model provide an excellent fit to the quasar continuum emission. However, the value measured for the EW of Fe Kalpha emission line is only marginally consistent with the presence of a Compton-thick absorber in a reflection-dominated scenario, which had been suggested by a previous, marginal (i.e. 2.5σ) detection with the hard X-ray (15-50 keV), non-imaging BeppoSAX/PDS instrument. Moreover, the value of luminosity in the 2-10 keV band measured by the transmission-dominated model is fully consistent with that expected on the basis of the bolometric luminosity of IRAS 09104+4109. From the analysis of the XMM-Newton data we therefore suggest the possibility that the absorber along the line of sight to the nucleus of IRAS 09104+4109 is Compton-thin. Alternatively, the absorber column density could have changed from Compton-thick to -thin in the five years elapsed between the observations. If this is the case, then IRAS 09104+4109 is the first 'changing-look' quasar ever detected.

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arXiv2007

The First Release COSMOS Optical and Near-IR Data and Catalog

We present imaging data and photometry for the COSMOS survey in 15 photometric bands between 0.3um and 2.4um. These include data taken on the Subaru 8.3m telescope, the KPNO and CTIO 4m telescopes, and the CFHT 3.6m telescope. Special techniques are used to ensure that the relative photometric calibration is better than 1% across the field of view. The absolute photometric accuracy from standard star measurements is found to be 6%. The absolute calibration is corrected using galaxy spectra, providing colors accurate to 2% or better. Stellar and galaxy colors and counts agree well with the expected values. Finally, as the first step in the scientific analysis of these data we construct panchromatic number counts which confirm that both the geometry of the universe and the galaxy population are evolving.

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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.

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arXiv2007

The HELLAS2XMM survey. X. The bolometric output of luminous obscured quasars: The Spitzer perspective

Aims: We aim at estimating the spectral energy distributions (SEDs) and the physical parameters related to the black holes harbored in eight high X-ray-to-optical (F_X/F_R>10) obscured quasars at z>0.9 selected in the 2--10 keV band from the HELLAS2XMM survey. Methods: We use IRAC and MIPS 24 micron observations, along with optical and Ks-band photometry, to obtain the SEDs of the sources. The observed SEDs are modeled using a combination of an elliptical template and torus emission (using the phenomenological templates of Silva et al. 2004) for six sources associated with passive galaxies; for two point-like sources, the empirical SEDs of red quasars are adopted. The bolometric luminosities and the M_BH-L_K relation are used to provide an estimate of the masses and Eddington ratios of the black holes residing in these AGN. Results: All of our sources are detected in the IRAC and MIPS (at 24 micron) bands. The SED modeling described above is in good agreement with the observed near- and mid-infrared data. The derived bolometric luminosities are in the range ~10^45-10^47 erg s^-1, and the median 2--10 keV bolometric correction is ~25, consistent with the widely adopted value derived by Elvis et al. (1994). For the objects with elliptical-like profiles in the K_s band, we derive high stellar masses (0.8-6.2)X10^11 Mo, black hole masses in the range (0.2-2.5)X10^9 Mo, and Eddington ratios L/L_Edd<0.1, suggesting a low-accretion phase.

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arXiv2007

The obscured X-ray source population in the HELLAS2XMM survey: the Spitzer view

Recent X-ray surveys have provided a large number of high-luminosity, obscured Active Galactic Nuclei (AGN), the so-called Type 2 quasars. Despite the large amount of multi-wavelength supporting data, the main parameters related to the black holes harbored in such AGN are still poorly known. Here we present the results obtained for a sample of eight Type 2 quasars in the redshift range 0.9-2.1 selected from the HELLAS2XMM survey, for which we used Ks-band, Spitzer IRAC and MIPS data at 24 micron to estimate bolometric corrections, black hole masses, and Eddington ratios.

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arXiv2007

Unveiling the oldest and most massive galaxies at very high redshift

(Abridged) This work explores the existence of high redshift massive galaxies unveiled with Spitzer+IRAC, but missed by conventional selection techniques based on optical and near-infrared observations. To this end, we use the multi-wavelength imaging data available for the GOODS-South field, and select a flux-limited sample from the IRAC 3.6um image to m(AB)<23.26. We confine our study to the galaxies undetected by the optical HST+ACS imaging and close to the detection limit of the K-band image (K>23.5 AB). Our selection unveiled 20 galaxies on which we performed a detailed photometric analysis. For each galaxy, we built an SED based on optical-to-8um photometry to estimate the photo-z and to derive the main galaxies physical properties. The majority of the sample sources show degenerate/bimodal solutions for the photometric redshifts (Abridged). These can either be heavily dust-enshrouded (Av~2-4) starbursts at 2 10^12 Lsun, or massive post-starburst galaxies in the redshift interval 4 4 which appear to include significant AGN emissions. These sources may be the oldest stellar systems at z~4. These, previously unrecognized, optically obscured objects might provide an important contribution to the massive-end (M>10^11 sun) of the high-z stellar mass function and they would almost double it (Abridged).

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arXiv2007

The Cosmic Evolution Survey (COSMOS): a large-scale structure at z=0.73 and the relation of galaxy morphologies to local environment

We have identified a large-scale structure at z~0.73 in the COSMOS field, coherently described by the distribution of galaxy photometric redshifts, an ACS weak-lensing convergence map and the distribution of extended X-ray sources in a mosaic of XMM observations. The main peak seen in these maps corresponds to a rich cluster with Tx= 3.51+0.60/-0.46 keV and Lx=(1.56+/-0.04) x 10^{44} erg/s ([0.1-2.4] keV band). We estimate an X-ray mass within $r500$ corresponding to M500~1.6 x 10^{14} Msun and a total lensing mass (extrapolated by fitting a NFW profile) M(NFW)=(6+/-3) x 10^15 Msun. We use an automated morphological classification of all galaxies brighter than I_AB=24 over the structure area to measure the fraction of early-type objects as a function of local projected density Sigma_10, based on photometric redshifts derived from ground-based deep multi-band photometry. We recover a robust morphology-density relation at this redshift, indicating, for comparable local densities, a smaller fraction of early-type galaxies than today. Interestingly, this difference is less strong at the highest densities and becomes more severe in intermediate environments. We also find, however, local "inversions'' of the observed global relation, possibly driven by the large-scale environment. In particular, we find direct correspondence of a large concentration of disk galaxies to (the colder side of) a possible shock region detected in the X-ray temperature map and surface brightness distribution of the dominant cluster. We interpret this as potential evidence of shock-induced star formation in existing galaxy disks, during the ongoing merger between two sub-clusters.

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arXiv2007

The XMM-Newton wide-field survey in the COSMOS field II: X-ray data and the logN-logS

We present the data analysis and the X-ray source counts for the first season of XMM-Newton observations in the COSMOS field. 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. A maximum likelihood source detection was performed in the 0.5-2 keV, 2-4.5 keV and 4.5-10 keV energy bands and 1390 point-like sources were detected in at least one band. Detailed Monte Carlo simulations were performed to fully test the source detection method and to derive the sky coverage to be used in the computation of the logN-logS relations. The 0.5--2 keV and 2--10 keV differential logN-logS were fitted with a broken power-law model which revealed a Euclidean slope (alpha~2.5) at the bright end and a flatter slope (alpha~1.5) at faint fluxes. In the 5--10 keV energy band a single power-law provides an acceptable fit to the observed source counts with a slope alpha~2.4. A comparison with the results of previous surveys shows good agreement in all the energy bands under investigation in the overlapping flux range. We also notice a remarkable agreement between our logN-logS relations and the most recent model of the XRB. 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.

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arXiv2006

The XMM-Newton wide-field survey in the COSMOS field. IV: X-ray spectral properties of Active Galactic Nuclei

We present a detailed spectral analysis of point-like X-ray sources in the XMM-COSMOS field. Our sample of 135 sources only includes those that have more than 100 net counts in the 0.3-10 keV energy band and have been identified through optical spectroscopy. The majority of the sources are well described by a simple power-law model with either no absorption (76%) or a significant intrinsic, absorbing column (20%).As expected, the distribution of intrinsic absorbing column densities is markedly different between AGN with or without broad optical emission lines. We find within our sample four Type-2 QSOs candidates (L_X > 10^44 erg/s, N_H > 10^22 cm^-2), with a spectral energy distribution well reproduced by a composite Seyfert-2 spectrum, that demonstrates the strength of the wide field XMM/COSMOS survey to detect these rare and underrepresented sources.

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arXiv2006

The XMM-Newton wide-field survey in the COSMOS field: III. optical identification and multiwavelength properties of a large sample of X-ray selected sources

[ABRIGED] We present the optical identification of a sample of 695 X-ray sources detected in the first 1.3 deg^2 of the XMM-COSMOS survey, down to a 0.5-2 keV (2-10 keV) limiting flux of ~10^-15 erg cm-2 s-1 (~5x10^-15 erg cm^-2 s-1). We were able to associate a candidate optical counterpart to ~90% (626) of the X-ray sources, while for the remaining ~10% of the sources we were not able to provide a unique optical association due to the faintness of the possible optical counterparts (I_AB>25) or to the presence of multiple optical sources within the XMM-Newton error circles. We also cross-correlated the candidate optical counterparts with the Subaru multicolor and ACS catalogs and with the Magellan/IMACS, zCOSMOS and literature spectroscopic data; the spectroscopic sample comprises 248 objects (~40% of the full sample). Our analysis reveals that for ~80% of the counterparts there is a very good agreement between the spectroscopic classification, the morphological parameters as derived from ACS data, and the optical to near infrared colors. About 20% of the sources show an apparent mismatch between the morphological and spectroscopic classifications. All the ``extended'' BL AGN lie at redshift <1.5, while the redshift distribution of the full BL AGN population peaks at z~1.5. Our analysis also suggests that the Type 2/Type 1 ratio decreases towards high luminosities, in qualitative agreement with the results from X-ray spectral analysis and the most recent modeling of the X-ray luminosity function evolution.

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arXiv2006

The XMM-Newton wide-field survey in the COSMOS field: VI. Statistical properties of clusters of galaxies

We present the results of a search for galaxy clusters in the first 36 XMM-Newton pointings on the COSMOS field. We reach a depth for a total cluster flux in the 0.5-2 keV band of 3x10-15 ergs cm-2 s-1, having one of the widest XMM-Newton contiguous raster surveys, covering an area of 2.1 square degrees. Cluster candidates are identified through a wavelet detection of extended X-ray emission. Verification of the cluster candidates is done based on a galaxy concentration analysis in redshift slices of thickness of 0.1-0.2 in redshift, using the multi-band photometric catalog of the COSMOS field and restricting the search to z<1.3 and i_AB < 25. We identify 72 clusters and derive their properties based on the X-ray cluster scaling relations. A statistical description of the survey in terms of the cumulative log(N>S)-lg(S) distribution compares well with previous results, although yielding a somewhat higher number of clusters at similar fluxes. The X-ray luminosity function of COSMOS clusters matches well the results of nearby surveys, providing a comparably tight constraint on the faint end slope of alpha=1.93+/-0.04. For the probed luminosity range of 8x10+42 - 2x10+44 ergs s-1, our survey is in agreement with and adds significantly to the existing data on the cluster luminosity function at high redshifts and implies no substantial evolution at these luminosities to z=1.3.

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arXiv2006

The XMM-Newton wide-field survey in the COSMOS field: I. Survey description

We present the first set of XMM-Newton EPIC observations in the 2 square degree COSMOS field. The strength of the COSMOS project is the unprecedented combination of a large solid angle and sensitivity over the whole multiwavelength spectrum. The XMM-Newton observations are very efficient in localizing and identifying active galactic nuclei (AGN) and clusters as well as groups of galaxies. One of the primary goals of the XMM-Newton Cosmos survey is to study the co-evolution of active galactic nuclei as a function of their environment in the Cosmic web. Here we present the log of observations, images and a summary of first research highlights for the first pass of 25 XMM-Newton pointings across the field. In the existing dataset we have detected 1416 new X-ray sources in the 0.5-2, 2-4.5 and 4.5-10 keV bands to an equivalent 0.5-2 keV flux limit of 7x10-16 erg cm-2 s-1. The number of sources is expected to grow to almost 2000 in the final coverage of the survey. From an X-ray color color analysis we identify a population of heavily obscured, partially leaky or reflecting absorbers, most of which are likely to be nearby, Compton-thick AGN.

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arXiv2006

The Cosmic Evolution Survey (COSMOS) -- Overview

The Cosmic Evolution Survey (COSMOS) is designed to probe the correlated evolution of galaxies, star formation, active galactic nuclei (AGN) and dark matter (DM) with large-scale structure (LSS) over the redshift range z $> 0.5 $ to 6. The survey includes multi-wavelength imaging and spectroscopy from X-ray to radio wavelengths covering a 2 $\sq$°area, including HST imaging. Given the very high sensitivity and resolution of these datasets, COSMOS also provides unprecedented samples of objects at high redshift with greatly reduced cosmic variance, compared to earlier surveys. Here we provide a brief overview of the survey strategy, the characteristics of the major COSMOS datasets, and summarize the science goals.

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arXiv2006

The HELLAS2XMM survey.VIII. Optical identifications of the extended sample

(Abridged) We present the results of the photometric and spectroscopic identification of 110 hard X-ray selected sources (6e-15 1000, while their Lx, L(R) and Nh are similar. This suggests that while the central engine of narrow-line AGNs and XBONGs looks similar, the narrow-line region in XBONGs could be strongly inhibited or obscured.

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arXiv2006

A wide angle tail radio galaxy in the COSMOS field: evidence for cluster formation

We have identified a complex galaxy cluster system in the COSMOS field via a wide angle tail (WAT) radio galaxy consistent with the idea that WAT galaxies can be used as tracers of clusters. The WAT galaxy, CWAT-01, is coincident with an elliptical galaxy resolved in the HST-ACS image. Using the COSMOS multiwavelength data set, we derive the radio properties of CWAT-01 and use the optical and X-ray data to investigate its host environment. The cluster hosting CWAT-01 is part of a larger assembly consisting of a minimum of four X-ray luminous clusters within ~2 Mpc distance. We apply hydrodynamical models that combine ram pressure and buoyancy forces on CWAT-01. These models explain the shape of the radio jets only if the galaxy's velocity relative to the intra-cluster medium (ICM) is in the range of about 300-550 km/s which is higher than expected for brightest cluster galaxies (BCGs) in relaxed systems. This indicates that the CWAT-01 host cluster is not relaxed, but is possibly dynamically young. We argue that such a velocity could have been induced through subcluster merger within the CWAT-01 parent cluster and/or cluster-cluster interactions. Our results strongly indicate that we are witnessing the formation of a large cluster from an assembly of multiple clusters, consistent with the hierarchical scenario of structure formation. We estimate the total mass of the final cluster to be approximately 20% of the mass of the Coma cluster.

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arXiv2006

The XMM-Newton survey of the ELAIS-S1 field. I: Number counts, angular correlation function and X-ray spectral properties

We have surveyed with XMM-Newton the central ~0.6 deg2 region of the ELAIS-S1 field down to flux limits of ~5.5X10-16 cgs (0.5-2 keV, S band), ~2X10-15 cgs (2-10 keV, H band), and ~4X10-15 cgs (5-10 keV, HH band). We detect a total of 478 sources, 395 and 205 of which detected in the S and H bands respectively. We identified 7 clearly extended sources and estimated their redshift through X-ray spectral fits with thermal models. In four cases the redshift is consistent with z=0.4. We have computed the angular correlation function of the sources in the S and H bands, finding best fit correlation angles theta_0=5.2+/-3.8 arcsec and theta_0=12.8+/-7.8 arcsec respectively. A rough estimate of the present-day correlation length r_0 can be obtained inverting the Limber equation and assuming an appropriate redshift distribution dN/dz. The results range between 12.8 and 9.8 h-1 Mpc in the S band and between 17.9 and 13.4 h-1 Mpc in the H band, with 30-40% statistical errors, assuming either smooth redshift distributions or redshift distributions with spikes accounting for the presence of a structure at z=0.4. The relative density of the S band sources is higher near the clusters and groups at z~0.4 and extends toward East and toward South/West. This suggests that the structure is complex, with a size comparable to the full XMM-Newton field. Conversely, the highest relative source densities of the H band sources are located in the central-west region of the field.

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arXiv2005

MAMBO 1.2 Millimeter observations of BzK-selected star-forming Galaxies at z~2

We present MAMBO 1.2 mm observations of five BzK-pre-selected vigorous starburst galaxies at z~2. Two of these were detected at more than 99.5% confidence levels, with 1.2 mm fluxes around 1.5 mJy. These millimeter fluxes imply vigorous activity with star-formation rates (SFRs) approx. 500-1500 Msun/yr, confirmed also by detections at 24 microns with the MIPS camera on board of the Spitzer satellite. The two detected galaxies are the ones in the sample with the highest SFRs estimated from the rest-frame UV, and their far-IR- and UV-derived SFRs agree reasonably well. This is different from local ULIRGs and high-z submm/mm selected galaxies for which the UV is reported to underestimate SFRs by factors of 10-100, but similar to the average BzK-ULIRG galaxy at z~2. The two galaxies detected at 1.2 mm are brighter in K than the typical NIR-counterparts of MAMBO and SCUBA sources, implying also a significantly different K-band to submm/mm flux ratio. This suggests a scenario in which z~2 galaxies, after their rapid (sub)mm brightest phase opaque to optical/UV light, evolve into a longer lasting phase of K-band bright and massive objects. Targeting the most UV active BzKs could yield substantial detection rates at submm/mm wavelengths.

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arXiv2005

The HELLAS2XMM survey. IX. Spectroscopic identification of super-EROs hosting AGNs

We present VLT near-IR spectroscopic observations of three X-ray sources characterized by extremely high X-ray-to-optical ratios (X/O>40), extremely red colors (6.3<R-K<7.4, i.e. EROs) and bright infrared magnitudes (17.6<K<18.3). These objects are very faint in the optical, making their spectroscopic identification extremely challenging. Instead, our near-IR spectroscopic observations have been successful in identifying the redshift of two of them (z=2.08 and z=1.35), and tentatively even of the third one (z=2.13). When combined with the X-ray properties, our results clearly indicate that all these objects host obscured QSOs (4e44 < L(2-10keV) < 1.5e45 erg/s, 2e22 < N_H < 4e23 cm-2) at high redshift. The only object with unresolved morphology in the K band shows broad Halpha emission, but not broad Hbeta, implying a type 1.9 AGN classification. The other two objects are resolved and dominated by the host galaxy light in the K band, and appear relatively quiescent: one of them has a LINER-like emission line spectrum and the other presents only a single, weak emission line which we tentatively identify with Halpha. The galaxy luminosities for the latter two objects are an order of magnitude brighter than typical local L* galaxies and the derived stellar masses are well in excess of 10^11 Msun. For these objects we estimate black hole masses higher than 10^9 Msun and we infer that they are radiating at Eddington ratios L/L_Edd < 0.1. We discuss the implications of these findings for the coevolution of galaxies and black hole growth. Our results provide further support that X-ray sources with high X/O ratios and very red colors tend to host obscured QSO in very massive galaxies at high redshift.

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