arXiv subjects

P. Rosati

Publications and source records attributed to P. Rosati.

At least 253 records · Page 14Linked to original sources

arXiv2009

The VLA survey of the Chandra Deep Field South III: X-ray spectral properties of radio sources

We discuss the X-ray properties of the radio sources detected in a deep 1.4 and 5 GHz VLA Radio survey of the Extended Chandra Deep Field South (E-CDFS). Among the 266 radio sources detected, we find 89 sources (1/3 of the total) with X-ray counterparts in the catalog of the 1Ms exposure of the central 0.08 deg^2 (Giacconi et al. 2002; Alexander et al. 2003) or in the catalog of the 250 ks exposure of the 0.3 deg^2 E-CDFS field (Lehmer et al. 2005). For 76 (85%) of these sources we have spectroscopic or photometric redshifts, and therefore we are able to derive their intrinsic properties from X-ray spectral analysis, namely intrinsic absorption and total X-ray luminosities. We find that the population of submillijansky radio sources with X-ray counterparts is composed of a mix of roughly 1/3 star forming galaxies and 2/3 AGN. The distribution of intrinsic absorption among X-ray detected radio sources is different from that of the X-ray selected sample. Namely, the fraction of low absorption sources is at least two times larger than that of X-ray selected sources in the CDFS. This is mostly due to the larger fraction of star forming galaxies present among the X-ray detected radio sources. If we investigate the distribution of intrinsic absorption among sources with L_X>10^42 erg s^-1 in the hard 2-10 keV band (therefore in the AGN luminosity regime), we find agreement between the X-ray population with and without radio emission. In general, radio detected X-ray AGN are not more heavily obscured than the non radio detected AGN. This argues against the use of radio surveys as an efficient way to search for the missing population of strongly absorbed AGN.

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arXiv2009

Spectroscopic Observations of Lyman-Break Galaxies at Redshift ~ 4, 5 and 6 in the GOODS-South Field

We report on observations of Lyman-break galaxies (LBGs) selected from the Great Observatories Origins Deep Survey at mean redshift z~4, 5 and 6, obtained with FORS2 spectrograph at the ESO-VLT.This program has yielded spectroscopic identifications for 114 galaxies of which 51 are at z~4, 31 at z~5, and 32 at z~6. The adopted selection criteria are effective, identifying galaxies at the expected redshift with minimal foreground contamination. Once selection effects are properly accounted for, the rest-frame UV spectra of the higher-redshift LBGs appear to be similar to their counterparts at z~3. As at z~3, LBGs at z~4 and z~5 show Lya both in emission and in absorption; when in absorption, strong interstellar lines and red UV spectra are observed, a fact also measured at z~2 and 3. At z~6, sensitivity issues bias our sample towards galaxies with Lya in emission; nevertheless, these spectra appear to be similar to their lower-redshift counterparts. As in other studies at similar redshifts, we find clear evidence that brighter LBGs tend to have weaker Lya emission lines. At fixed rest-frame UV luminosity, the equivalent width of the Lya emission line is larger at higher redshifts. At all redshifts where the measurements can be reliably made, the redshift of the Lya emission line turns out to be larger than that of the interstellar absorption lines,with a median velocity difference DeltaV~400 km/s at z~4 and 5, consistent with results at lower redshifts. This shows that powerful, large-scale winds are common at high redshift.In general,there is no strong correlation between the morphology of the UV light and the spectroscopic properties. However, galaxies with deep interstellar absorption lines and strong Lya absorption appear to be more diffuse than galaxies with Lya in emission.(abridged)

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arXiv2008

The VLA Survey of the Chandra Deep Field South. IV. Source Population

We present a detailed analysis of 256 radio sources from our deep (flux density limit of 42 microJy at the field centre at 1.4 GHz) Chandra Deep Field South 1.4 and 5 GHz VLA survey. The radio population is studied by using a wealth of multi-wavelength information in the radio, optical, and X-ray bands. The availability of redshifts for ~ 80% of the sources in our complete sample allows us to derive reliable luminosity estimates for the majority of the objects. X-ray data, including upper limits, for all our sources turn out to be a key factor in establishing the nature of faint radio sources. Due to the faint optical levels probed by this study, we have uncovered a population of distant Active Galactic Nuclei (AGN) systematically missing from many previous studies of sub-millijansky radio source identifications. We find that, while the well-known flattening of the radio number counts below 1 mJy is mostly due to star forming galaxies, these sources and AGN make up an approximately equal fraction of the sub-millijansky sky, contrary to some previous results. The AGN include radio galaxies, mostly of the low-power, Fanaroff-Riley I type, and a significant radio-quiet component, which amounts to approximately one fifth of the total sample. The ratio of radio to optical luminosity depends more on radio luminosity, rather than being due to optical absorption.

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arXiv2008

HAWK-I imaging of the X-ray luminous galaxy cluster XMMU J2235.3-2557: The red sequence at z=1.39

We use HAWK-I, the recently-commissioned near-IR imager on Yepun (VLT-UT4), to obtain wide-field, high-resolution images of the X-ray luminous galaxy cluster XMMU J2235.3-2557 in the J and Ks bands, and we use these images to build a colour-magnitude diagram of cluster galaxies. Galaxies in the core of the cluster form a tight red sequence with a mean J-Ks colour of 1.9 (Vega system). The intrinsic scatter in the colour of galaxies that lie on the red sequence is similar to that measured for galaxies on the red sequence of the Coma cluster. The slope and location of the red sequence can be modelled by passively evolving the red sequence of the Coma cluster backwards in time. Using simple stellar population (SSP) models, we find that galaxies in the core of XMMU J2235.3-2557 are, even at z=1.39, already 3 Gyr old, corresponding to a formation redshift of z ~ 4. Outside the core, the intrinsic scatter and the fraction of galaxies actively forming stars increase substantially. Using SSP models, we find that most of these galaxies will join the red sequence within 1.5 Gyr. The contrast between galaxies in the cluster core and galaxies in the cluster outskirts indicates that the red sequence of XMMU J2235.3-2557 is being built from the dense cluster core outwards.

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arXiv2008

The Environmental Dependence of Properties of Galaxies around the RDCSJ0910+54 Cluster at z=1.1

(Abridged) We report on the environmental dependence of properties of galaxies around the RDCSJ0910+54 cluster at z=1.1. We have obtained multi-band wide-field images of the cluster with Suprime-Cam and MOIRCS on Subaru and WFCAM on UKIRT. Also, an intensive spectroscopic campaign has been carried out using LRIS on Keck and FOCAS on Subaru. We discover a possible large-scale structure around the cluster in the form of three clumps of galaxies. This is potentially one of the largest structures found so far in the z>1 Universe. We then examine stellar populations of galaxies in the structure. Red galaxies have already become the dominant population in the cores of rich clusters at z~1, and the fraction of red galaxies has not strongly changed since then. The red fraction depends on richness of clusters in the sense that it is higher in rich clusters than in poor groups. The luminosity function of red galaxies in rich clusters is consistent with that in local clusters. On the other hand, luminosity function of red galaxies in poor groups shows a deficit of faint red galaxies. This confirms our earlier findings that galaxies follow an environment-dependent down-sizing evolution. There seems to be a large variation in the evolutionary phases of galaxies in groups with similar masses. Further studies of high-z clusters will be a promising way of addressing the role of nature and nurture effects on galaxy evolution.

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arXiv2008

Coupling MOAO with Integral Field Spectroscopy: specifications for the VLT and the E-ELT

[Abridged] We have developed an end-to-end simulation to specify the science requirements of a MOAO-fed integral field spectrograph on either an 8m or 42m telescope. Our simulations re-scales observations of local galaxies or results from numerical simulations of disk or interacting galaxies. For the current analysis, we limit ourselves to a local disk galaxy which exhibits simple rotation and a simulation of a merger. We have attempted to generalize our results by introducing the simple concepts of "PSF contrast" which is the amount of light polluting adjacent spectra which we find drives the smallest EE at a given spatial scale. The choice of the spatial sampling is driven by the "scale-coupling", i.e., the relationship between the IFU pixel scale and the size of the features that need to be recovered by 3D spectroscopy in order to understand the nature of the galaxy and its substructure. Because the dynamical nature of galaxies are mostly reflected in their large-scale motions, a relatively coarse spatial resolution is enough to distinguish between a rotating disk and a major merger. Although we used a limited number of morpho-kinematic cases, our simulations suggest that, on a 42m telescope, the choice of an IFU pixel scale of 50-75 mas seems to be sufficient. Such a coarse sampling has the benefit of lowering the exposure time to reach a specific signal-to-noise as well as relaxing the performance of the MOAO system. On the other hand, recovering the full 2D-kinematics of z~4 galaxies requires high signal-to-noise and at least an EE of 34% in 150 mas (2 pixels of 75 mas). Finally, we carried out a similar study at z=1.6 with a MOAO-fed spectrograph for an 8m, and find that at least an EE of 30% at 0.25 arcsec spatial sampling is required to understand the nature of disks and mergers.

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arXiv2008

Star formation histories of early-type galaxies at z = 1.2 in cluster and field environments

We derive the star formation histories of early-type galaxies at z = 1.2 in both low and high density environments. To this purpose, we compare the co-added spectroscopic and 8-9 band photometric data of 43 mass selected early-type galaxies in the massive cluster RDCS J1252.9-2927 and the GOODS/CDF-S field with a large grid of composite stellar population models. We find that the cluster early-type galaxies formed the bulk of their stars approximately 0.5 Gyr earlier than early-types in the field, whereas field early-types presumably finish forming their stellar content on a longer time scale. Such a difference is particularly evident at lower masses but becomes negligible for the most massive galaxies. While our differential analysis of the stellar population parameters of cluster and field galaxies in the same mass range convincingly shows distinct star formation histories, the absolute age difference remains model dependent. Using the star formation histories that best fit the SEDs of the red sequence galaxies in RDCS 1252.9-2927, we reconstruct the evolution of the cluster red sequence and find that it was established over 1 Gyr and is expected to dissolve by z ~ 2.

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arXiv2008

The Red Sequence of High-Redshift Clusters: a Comparison with Cosmological Galaxy Formation Models

We compare the results from a semi-analytic model of galaxy formation with spectro-photometric observations of distant galaxy clusters observed in the range 0.8< z< 1.3. We investigate the properties of their red sequence (RS) galaxies and compare them with those of the field at the same redshift. In our model we find that i) a well-defined, narrow RS is obtained already by z= 1.2; this is found to be more populated than the field RS, analogously to what observed and predicted at z=0; ii) the predicted U-V rest-frame colors and scatter of the cluster RS at z=1.2 have average values of 1 and 0.15 respectively, with a cluster-to-cluster variance of 0.2 and 0.06, respectively. The scatter of the RS of cluster galaxies is around 5 times smaller than the corresponding field value; iii) when the RS galaxies are considered, the mass growth histories of field and cluster galaxies at z=1.2 are similar, with 90 % of the stellar mass of RS galaxies at z=1.2 already formed at cosmic times t=2.5 Gyr, and 50 % at t=1 Gyr; v) the predicted distribution of stellar ages of RS galaxies at z=1.2 peaks at 3.7 Gyr for both cluster and field populations; however, for the latter the distribution is significantly skewed toward lower ages. When compared with observations, the above findings show an overall consistency, although the average value 0.07 of the observed cluster RS scatter (U-V colors) at z=1.2 is smaller than the corresponding model central value. We discuss the physical origin and the significance of the above results in the framework of cosmological galaxy formation.

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arXiv2008

ICM properties and AGN distribution in high-z RCS clusters

Here we discuss the X-ray properties of clusters of galaxies optically selected in the Red-Sequence Cluster Survey (RCS) observed with the Chandra satellite, at redshifts 0.6 < z < 1.2. We intended to assess the evolutionary stage of optically selected high-z clusters of galaxies, performing a spectral analysis of the diffuse emission from their ICM. We also investigated the distribution of AGN in their surroundings. The background subtracted spectra were analyzed and fitted with a single temperature model to measure average ICM temperature, X-ray bolometric luminosity and Fe abundance within typical radii between 200 and 350 kpc. We also analyzed the point source number density and spatial distribution in the RCS clusters fields as a function of the X-ray flux. We detected emission for the majority of the clusters, except for three, for which we have only marginal detection at ~3 Sigma. We find that the normalization of the L-T relation for RCS clusters is a factor of ~2 lower than the one for X-ray selected clusters. We confirm that the Fe abundance in the detected objects is consistent with that of X-ray selected clusters at the same redshift. We also found an excess of low-luminosity AGN towards the center of the clusters.

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arXiv2008

[OIII] emitters in the field of the MRC 0316-257 protocluster

Venemans et al. (2005) found evidence for an overdensity of Ly-alpha emission line galaxies associated with the radio galaxy MRC 0316-257 at z=3.13 indicating the presence of a massive protocluster. Here, we present the results of a search for additional star-forming galaxies and AGN within the protocluster. Narrow-band infrared imaging was used to select candidate [O III] emitters in a 1.1 x 1.1 Mpc^2 region around the radio galaxy. Thirteen candidates have been detected. Four of these are among the previously confirmed sample of Ly-alpha galaxies, and an additional three have been confirmed through follow-up infrared spectroscopy. The three newly confirmed objects lie within a few hundred km/s of each other, but are blueshifted with respect to the radio galaxy and Ly-alpha emitters by ~2100 km/s. Although the sample is currently small, our results indicate that the radio-selected protocluster is forming at the centre of a larger, ~60 co-moving Mpc super-structure. On the basis of an HST/ACS imaging study we calculate dust-corrected star-formation rates and investigate morphologies and sizes of the [O III] candidate emitters. From a comparison of the star formation rate derived from UV-continuum and [O III] emission, we conclude that at least two of the [O III] galaxies harbour an AGN which ionized the O+ gas.

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arXiv2008

Formation epochs, star formation histories and sizes of massive early-type galaxies in cluster and field environments at z=1.2: insights from the rest-frame UV

We derive stellar masses, ages and star formation histories of massive early-type galaxies in the z=1.237 RDCS1252.9-2927 cluster and compare them with those measured in a similarly mass-selected sample of field contemporaries drawn from the GOODS South Field. Robust estimates of these parameters are obtained by comparing a large grid of composite stellar population models with 8-9 band photometry in the rest-frame NUV, optical and IR, thus sampling the entire relevant domain of emission of the different stellar populations. Additionally, we present new, deep $U$-band photometry of both fields, giving access to the critical FUV rest-frame, in order to constrain empirically the dependence on the environment of the most recent star formation processes. We find that early-type galaxies, both in the cluster and in the field, show analogous optical morphologies, follow comparable mass vs. size relation, have congruent average surface stellar mass densities and lie on the same Kormendy relation. We also that a fraction of early-type galaxies in the field employ longer timescales, $τ$, to assemble their mass than their cluster contemporaries. Hence we conclude that, while the formation epoch of early-type only depends on their mass, the environment does regulate the timescales of their star formation histories. Our deep $U$-band imaging strongly supports this conclusions. It shows that cluster galaxies are at least 0.5 mag fainter than their field contemporaries of similar mass and optical-to-infrared colors, implying that the last episode of star formation must have happened more recently in the field than in the cluster.

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arXiv2008

The Chandra Deep Field-South Survey: 2 Ms Source Catalogs

We present point-source catalogs for the ~2 Ms exposure of the Chandra Deep Field-South (CDF-S); this is one of the two most-sensitive X-ray surveys ever performed. The survey covers an area of ~436 arcmin^2 and reaches on-axis sensitivity limits of ~1.9x10^{-17} and ~1.3x10^{-16} ergs/cm^2/s for the 0.5-2.0 and 2-8 keV bands, respectively. Four hundred and sixty-two X-ray point sources are detected in at least one of three X-ray bands that were searched; 135 of these sources are new compared to the previous ~1 Ms CDF-S detections. Source positions are determined using centroid and matched-filter techniques; the median positional uncertainty is ~0.36". The X-ray-to-optical flux ratios of the newly detected sources indicate a variety of source types; ~55% of them appear to be active galactic nuclei while ~45% appear to be starburst and normal galaxies. In addition to the main Chandra catalog, we provide a supplementary catalog of 86 X-ray sources in the ~2 Ms CDF-S footprint that was created by merging the ~250 ks Extended Chandra Deep Field-South with the CDF-S; this approach provides additional sensitivity in the outer portions of the CDF-S. A second supplementary catalog that contains 30 X-ray sources was constructed by matching lower significance X-ray sources to bright optical counterparts (R<23.8); the majority of these sources appear to be starburst and normal galaxies. The total number of sources in the main and supplementary catalogs is 578. R-band optical counterparts and basic optical and infrared photometry are provided for the X-ray sources in the main and supplementary catalogs. We also include existing spectroscopic redshifts for 224 of the X-ray sources. (Abstract abridged)

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arXiv2008

The VLA Survey of the Chandra Deep Field South: I. Overview of the Radio Data

We report 20 and 6 cm VLA deep observations of the CDF-S including the Extended CDF-S. We discuss the radio properties of 266 cataloged radio sources, of which 198 are above a 20 cm completeness level reaching down to 43 microJy at the center of the field. Survey observations made at 6 cm over a more limited region covers the original CDF-S to a comparable level of sensitivity as the 20 cm observations. Of 266 cataloged radio sources, 52 have X-ray counterparts in the CDF-S and a further 37 in the E-CDF-S area not covered by the 1 Megasecond exposure. Using a wide range of material, we have found optical or infrared counterparts for 254 radio sources, of which 186 have either spectroscopic or photometric redshifts (Paper II). Three radio sources have no apparent counterpart at any other wavelength. Measurements of the 20 cm radio flux density at the position of each CDF-S X-ray source detected a further 30 radio sources above a conservative 3-sigma detection limit. X-ray and sub-mm observations have been traditionally used as a measure of AGN and star formation activity, respectively. These new observations probe the faint end of both the star formation and radio galaxy/AGN population, as well as the connection between the formation and evolution of stars and SMBHs. Both of the corresponding gravitational and nuclear fusion driven energy sources can lead to radio synchrotron emission. AGN and radio galaxies dominate at high flux densities. Although emission from star formation becomes more prominent at the microjansky levels reached by deep radio surveys, even for the weakest sources, we still find an apparent significant contribution from low luminosity AGN as well as from star formation.

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arXiv2008

A twelve-image gravitational lens system in the z ~ 0.84 cluster Cl J0152.7-1357

Gravitational lens modeling is presented for the first discovered example of a three-component source for which each component is quadruply imaged. The lens is a massive galaxy member of the cluster Cl J0152.7-1357 at z ~ 0.84. Taking advantage of this exceptional configuration and of the excellent angular resolution of the HST/ACS, we measure the properties of the lens. Several parametric macroscopic models were developed for the lens galaxy, starting from pointlike to extended image models. For a lens model in terms of a singular isothermal sphere with external shear, the Einstein radius is found to be R_{E} = (9.54 +/- 0.15) kpc. The external shear points to the cluster's northern mass peak. The unknown redshift of the source is determined to be higher than 1.9 and lower than 2.9. Our estimate of the lensing projected total mass inside the Einstein radius, M_{len}(R < 9.54 kpc), depends on the source distance and lies between 4.6 and 6.2 x 10^{11} M_{Sun}. This result turns out to be compatible with the dynamical estimate based on an isothermal model. By considering the constraint on the stellar mass-to-light ratio that comes from the evolution of the Fundamental Plane, we can exclude the possibility that at more than 4 sigma level the total mass enclosed inside the Einstein ring is only luminous matter. Moreover, the photometric-stellar mass measurement within the Einstein radius gives a minimum value of 50% (1 sigma) for the dark-to-total matter fraction. The lensing analysis has allowed us to investigate the distribution of mass of the deflector, also providing some interesting indications on scales that are larger and smaller than the Einstein radius of the lens galaxy. The combination of different diagnostics has proved to be essential in determining quantities that otherwise would have not been directly measurable with the current data.

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arXiv2008

Discovery of a Very Bright Strongly-Lensed Galaxy Candidate at z ~ 7.6

Using HST and Spitzer IRAC imaging, we report the discovery of a very bright strongly lensed Lyman break galaxy (LBG) candidate at z~7.6 in the field of the massive galaxy cluster Abell 1689. The galaxy candidate, which we refer to as A1689-zD1, shows a strong z-J break of at least 2.2 mag and is completely undetected (<1 sigma) in HST/ACS g, r, i, and z-band data. These properties, combined with the very blue J-H and H-[4.5] colors, are exactly the properties of an z~7.6 LBG and can only be reasonably fit by a star-forming galaxy at z=7.6 +/- 0.4. Attempts to reproduce these properties with a model galaxy at z<4 yield particularly poor fits. A1689-zD1 has an observed (lensed) magnitude of 24.7 AB (8 sigma) in the NICMOS H band and is ~1.3 mag brighter than the brightest-known z-dropout galaxy. When corrected for the cluster magnification of 9.3 at z~7.6, the candidate has an intrinsic magnitude of H=27.1 AB, or about an L* galaxy at z~7.6. The source-plane deprojection shows that the star formation is occurring in compact knots of size ~<300 pc. The best-fit stellar population synthesis models yield a median redshift of 7.6, stellar masses (1.6-3.9) x 10^9 M_sun, stellar ages 45-320 Myr, star-formation rates ~<7.6 M_sun/yr, and low reddening with A_V <= 0.3. These properties are generally similar to those of LBGs found at z~5-6. The inferred stellar ages suggest a formation redshift of z~8-10 (t~<0.63 Gyr). A1689-zD1 is the brightest observed, highly reliable z>7.0 galaxy candidate found to date.

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arXiv2008

The VLA Survey of the Chandra Deep Field South. II. Identification and host galaxy properties of submillijansky sources

We present the optical and infrared identifications of the 266 radio sources detected at 20 cm with the Very Large Array in the Chandra Deep Field South (Kellermann et al. 2008). Using deep i-band Advanced Camera for Surveys, R-band Wide Field Imager, K-band SOFI/NTT, K-band ISAAC/VLT and Spitzer imaging data, we are able to find reliable counterparts for 254 (~95%) VLA sources. Twelve radio sources remain unidentified and three of them are ``empty fields''. Using literature and our own data we are able to assign redshifts to 186 (~70%) radio sources: 108 are spectroscopic redshifts and 78 reliable photometric redshifts. Based on the rest frame colors and morphological distributions of the host galaxies we find evidences for a change in the submillijansky radio source population: a) above ~ 0.08 mJy early-type galaxies are dominating; b) at flux densities below ~0.08 mJy, starburst galaxies become dominant.

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arXiv2008

SPACE: the SPectroscopic All-sky Cosmic Explorer

We describe the scientific motivations, the mission concept and the instrumentation of SPACE, a class-M mission proposed for concept study at the first call of the ESA Cosmic-Vision 2015-2025 planning cycle. SPACE aims to produce the largest three-dimensional evolutionary map of the Universe over the past 10 billion years by taking near-IR spectra and measuring redshifts for more than half a billion galaxies at 0<z<2 down to AB~23 over 3πsr of the sky. In addition, SPACE will also target a smaller sky field, performing a deep spectroscopic survey of millions of galaxies to AB~26 and at 2<z<10+. These goals are unreachable with ground-based observations due to the ~500 times higher sky background. To achieve the main science objectives, SPACE will use a 1.5m diameter Ritchey-Chretien telescope equipped with a set of arrays of Digital Micro-mirror Devices (DMDs) covering a total field of view of 0.4 deg2, and will perform large-multiplexing multi-object spectroscopy (e.g. ~6000 targets per pointing) at a spectral resolution of R~400 as well as diffraction-limited imaging with continuous coverage from 0.8mum to 1.8mum.

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arXiv2008

The VLA 1.4GHz Survey of the Extended Chandra Deep Field South: First Data Release

We have observed the Extended Chandra Deep Field South (E-CDF-S) using a mosaic of six deep Very Large Array (VLA) pointings at 1.4GHz. In this paper, we present the survey strategy, description of the observations, and the first data release. The observations were performed during June through September of 2007 and included from 15 to 17 ``classic'' VLA antennas and 6 to 11 that had been retrofitted for the Expanded VLA (EVLA). The first data release consists of a 34.1' x 34.1' image and the attendant source catalog. The image achieves an rms sensitivity of 6.4 uJy per 2.8" x 1.6" beam in its deepest regions, with a typical sensitivity of 8 uJy. The catalog is conservative in that it only lists sources with peak flux densities greater than seven times the local rms noise, yet it still contains 464 sources. Nineteen of these are complex sources consisting of multiple components. Cross matching of the catalog to prior surveys of the E-CDF-S confirms the linearity of the flux density calibration, albeit with a slight possible offset (a few percent) in scale. Improvements to the data reduction and source catalog are ongoing, and we intend to produce a second data release in January 2009.

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