arXiv subjects

S. Bardelli

Publications and source records attributed to S. Bardelli.

At least 469 records · Page 26Linked to original sources

arXiv2004

Radio halos in merging clusters of galaxies

We present the preliminary results of 235 MHz, 327 MHz and 610 MHz observations of the galaxy cluster A3562 in the core of the Shapley Concentration. The purpose of these observations, carried out with the Giant Metrewave Radio Telescope (GMRT, Pune, India) was to study the radio halo located at the centre of A3562 and determine the shape of its radio spectrum at low frequencies, in order to understand the origin of this source. In the framework of the re--acceleration model, the preliminary analysis of the halo spectrum suggests that we are observing a young source (few $10^8$ yrs) at the beginning of the re--acceleration phase.

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arXiv2004

Bias in the Estimation of Global Luminosity Functions

We discuss a bias present in the calculation of the global luminosity function (LF) which occurs when analysing faint galaxy samples. This effect exists because of the different spectral energy distributions of galaxies, which are in turn quantified by the k-corrections. We demonstrate that this bias occurs because not all galaxy types are visible in the same absolute magnitude range at a given redshift and it mainly arises at high redshift since it is related to large k-corrections. We use realistic simulations with observed LFs to investigate the amplitude of the bias. We also compare our results to the global LFs derived from Hubble Deep Field-North and -South (HDF) surveys. We conclude that, as expected, there is no bias in the global LF measured in the absolute magnitude range where all galaxy types are observable. Beyond this range the faint-end slope of the global LF can be over/under-estimated depending on the adopted LF estimator. The effect is larger when the reference filter in which the global LF is measured, is far from the rest-frame filter in which galaxies are selected. The fact that LF estimators are differently affected by this bias implies that the bias is minimal when the different LF estimators give measurements consistent with one another at the faint-end. For instance, we show that the estimators are discrepant in the same way both in the simulated and HDF LFs. This suggests that the HDF LFs are affected by the presently studied bias. The best solution to avoid this bias is to derive the global LF in the reference filter closest to the rest-frame selection filter.

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arXiv2003

The VIRMOS deep imaging survey: III. ESO/WFI deep U-band imaging of the 0226-04 deep field

In this paper we describe the U-band imaging of the F02 deep field, one of the fields in the VIRMOS Deep Imaging Survey. The observations were done at the ESO/MPG 2.2m telescope at La Silla (Chile) using the 8k x 8k Wide-Field Imager (WFI). The field is centered at alpha(J2000)=02h 26m 00s and delta(J2000)=-04deg 30' 00", the total covered area is 0.9 deg**2 and the limiting magnitude (50% completeness) is U(AB) ~ 25.4 mag. Reduction steps, including astrometry, photometry and catalogue extraction, are first discussed. The achieved astrometric accuracy (RMS) is ~ 0.2" with reference to the I-band catalog and ~ 0.07" internally (estimated from overlapping sources in different exposures). The photometric accuracy including uncertainties from photometric calibration, is < 0.1 mag. Various tests are then performed as a quality assessment of the data. They include: (i) the color distribution of stars and galaxies in the field, done together with the BVRI data available from the VIMOS survey; (ii) the comparison with previous published results of U-band magnitude-number counts of galaxies.

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arXiv2003

Early results from the VIMOS VLT Deep Survey

The VIMOS VLT Deep Survey (VVDS) is underway to study the evolution of galaxies, large scale structures and AGNs, from the measurement of more than 100000 spectra of faint objects. We present here the results from the first epoch observations of more than 20000 spectra. The main challenge of the program, the redshift measurements, is described, in particular entering the ``redshift desert'' in the range $1.5<z<3$ for which only very weak features are observed in the observed wavelength range. The redshift distribution of a magnitude limited sample brighter than $I_{AB}=24$ is presented for the first time, showing a peak at a low redshift $z\sim0.7$, and a tail extending all the way above $z=4$. The evolution of the luminosity function out to $z=1.5$ is presented, with the LF of blue star forming galaxies carrying most of the evolution, with L* changing by more than two magnitudes for this sub-sample.

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arXiv2003

AGN and starburst radio activity in the A3558 cluster complex

In this paper we present Very Large Array (VLA) 1.4 GHz (21 cm) observations of the region between the centres of A3558 and A3562, in the majorcluster merger complex of the Shapley Concentration. Our final catalogue includes a total of 174 radio sources above the flux density limit of 0.25 mJy b$^{-1}$. By cross-correlation with optical and spectroscopic catalogues we found 33 optical counterparts belonging to the Shapley Concentration. We investigated the effects of cluster merger on the radio emission properties of the galaxy population by means of the radio source counts and the radio luminosity functions (RLF). We found that the radio source counts are consistent with the field source counts.The RLF of elliptical and S0 galaxies in the region surveyed here, is consistent with the ``universal'' RLF for early--type galaxies.This result suggests that the deficit in radio galaxies found in ourprevious works over the whole A3558 chain, is entirely due to the cluster A3558. A population of faint radio galaxies (logP$_{1.4 GHz}$(W Hz$^{-1}$) \ltsim 22) is also found.Half of these objects are also blue, suggesting that starburst is the main mechanism driving the radio emission. Finally, we detected 14 spiral radio galaxies, whose ratio between radio and optical emission is similar to those found in galaxies located in rich and dynamically evolved clusters. Our results are briefly discussed in the light of the age and stage of the merger in the A3558 cluster complex.

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arXiv2003

Comparison of two optical cluster finding algorithms for the new generation of deep galaxy surveys

We present a comparison between two optical cluster finding methods: a matched filter algorithm using galaxy angular coordinates and magnitudes, and a percolation algorithm using also redshift information. We test the algorithms on two mock catalogues. The first mock catalogue is built by adding clusters to a Poissonian background, while the other is derived from N-body simulations. Choosing the physically most sensible parameters for each method, we carry out a detailed comparison and investigate advantages and limits of each algorithm, showing the possible biases on final results. We show that, combining the two methods, we are able to detect a large part of the structures, thus pointing out the need to search for clusters in different ways in order to build complete and unbiased samples of clusters, to be used for statistical and cosmological studies. In addition, our results show the importance of testing cluster finding algorithms on different kinds of mock catalogues to have a complete assessment of their behaviour.

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arXiv2003

XMM-Newton observation of the interacting cluster Abell 3528

We analyze the XMM dataset of the interacting cluster of galaxies Abell 3528 located westward in the core of the Shapley Supercluster, the largest concentration of mass in the nearby Universe. A3528 is formed by two interacting clumps (A3528-N at North and A3528-S at South) separated by 0.9 $h_{70}^{-1}$ Mpc at redshift 0.053. XMM data describe these clumps as relaxed structure with an overall temperature of $4.14 \pm 0.09$ and $4.29 \pm 0.07$ keV in A3528-N and A3528-S, respectively, and a core cooler by a factor 1.4--1.5 and super-solar metal abundance in the inner 30 arcsec. These clumps are connected by a X-ray soft, bridge-like emission and present asymmetric surface brightness with significant excess in the North--West region of A3528-N and in the North--East area of A3528-S. However, we do not observe any evidence of shock heated gas, both in the surface brightness and in the temperature map. Considering also that the optical light distribution is more concentrated around A3528-N and makes A3528-S barely detectable, we do not find support to the originally suggested head-on pre-merging scenario and conclude that A3528 is in a off-axis post-merging phase, where the closest cores encounter happened about 1--2 Gyrs ago.

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arXiv2003

The origin of X-ray emission of two distant (z>1) cluster candidate with XMM-Newton

We present here a study of XMM-Newton data of two distant galaxy cluster candidates. One of these was discovered serendipitously in near infrared data, CL J0533-2411, the other one corresponds to the cluster EIS J0533-2412 part of the EIS cluster survey. The estimated redshift of CL J0533-2411 is z=1.2-1.7. EIS J0533-2412 is a rich system (Lambda_cl=299), with a spectroscopically confirmed redshift of z=1.3. Both galaxy concentrations show firm X-ray detections, located within 30" of their optical center. However, we cannot resolve the sources with XMM-Newton. If the X-ray emission originates from the X-ray emitting intra-cluster medium (ICM) it would be extremely concentrated which is rather unlikely (core radii below 14 h_{65}^{-1}kpc and 40 h_{65}^{-1}kpc, respectively). We argue that the X-ray sources are more likely AGN members of the galaxy concentrations. We set an upper limit for the bolometric luminosity of a hot ICM in the range ~0.7-2.1 10^{44} h_{65}^{-2}erg/s for CL J0533-2411, depending on the exact redshift. For EIS J0533-2412 the limit is L_bol=(6.2+/-1.4) 10^{43} h_{65}^{-2}erg/s. We interpret our result in the following way: EIS J0533-2412 (and possibly CL J0533-2411) are proto-clusters and show matter overdensities before collapse, which explains the low significance of extended X-ray emission.

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arXiv2003

The VLA-VIRMOS Deep Field I. Radio observations probing the microJy source population

We have conducted a deep survey (r.m.s noise 17 microJy) with the Very Large Array (VLA) at 1.4 GHz, with a resolution of 6 arcsec, of a 1 square degree region included in the VIRMOS VLT Deep Survey. In the same field we already have multiband photometry down to I(AB)=25, and spectroscopic observations will be obtained during the VIRMOS VLT survey. The homogeneous sensitivity over the whole field has allowed to derive a complete sample of 1054 radio sources (5 sigma limit). We give a detailed description of the data reduction and of the analysis of the radio observations, with particular care to the effects of clean bias and bandwidth smearing, and of the methods used to obtain the catalogue of radio sources. To estimate the effect of the resolution bias on our observations we have modelled the effective angular-size distribution of the sources in our sample and we have used this distribution to simulate a sample of radio sources. Finally we present the radio count distribution down to 0.08 mJy derived from the catalogue. Our counts are in good agreement with the best fit derived from earlier surveys, and are about 50 % higher than the counts in the HDF. The radio count distribution clearly shows, with extremely good statistics, the change in the slope for the sub-mJy radio sources.

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arXiv2003

The radio halo in the merging cluster A3562

we present new VLA observations at 1.4 GHz confirming the presence of a radio halo at the centre of the cluster A3562, in the core of the Shapley Concentration. We also report a detailed multifrequency radio study of the head tail galaxy J1333--3141, which is completely embedded in the halo emission. The radio halo has an irregular shape, and a largest linear size of $\sim$ 620 kpc, which is among the smallest found in the literature. The source has a steep spectrum, i.e. $α_{843 MHz}^{1.4 GHz} \sim 2$, and its total radio power, P$_{1.4 GHz} \sim 2 \times10^{23}$ W Hz$^{-1}$, is the lowest known to date.The radio power of the halo and the X-ray parameters of the cluster, such as L$_X$ and kT, nicely fit the correlations found in the literature for the other halo clusters, extending them to low radio powers. We found that the total number of electrons injected in the cluster environment by the head--tail source is enough to feed the halo, if we assume that the galaxy has been radio active over a large fraction of its crossing time. We discuss possible origins of the radio halo in the light of the two--phase model (Brunetti et al. 2001) and propose that the observed scenario is the result of a young source at the beginning of the reacceleration phase.

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arXiv2002

Abell 3560, a galaxy cluster at the edge of a major merging event

In this paper we study A3560, a rich cluster at the southern periphery of the A3558 complex, a chain of interacting clusters in the central part of the Shapley Concentration supercluster. From a ROSAT-PSPC map we find that the X-ray surface brightness distribution of A3560 is well described by two components, an elliptical King law and a more peaked and fainter structure, which has been modeled with a Gaussian. The main component, corresponding to the cluster, is elongated with the major axis pointing toward the A3558 complex. The second component, centered on the Dumb-bell galaxy which dominates the cluster, appears significantly offset (by about 0.15 Mpc) from the cluster X-ray centroid. From a Beppo-SAX observation we derive the radial temperature profile, finding that the temperature is constant (at kT~3.7 keV) up to 8 arcmin, corresponding to 0.3 Mpc: for larger distances, the temperature significantly drops to kT~1.7 keV. We analyze also temperature maps, dividing the cluster in 4 sectors and deriving the temperature profiles in each sector: we find that the temperature drop is more sudden in the sectors which point towards the A3558 complex. From VLA radio data, at 20 and 6 cm, we find a peculiar bright extended radio source (J1332-3308), composed by a core (centered on the northern component of the Dumb-bell galaxy), two lobes, a "filament" and a diffuse component. The morphology of the source could be interpreted either with a strong interaction of the radio source with the intracluster medium or with the model of intermittency of the central engine.

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arXiv2002

Optically-selected clusters at 0.8 < z < 1.3 in the EIS Cluster Survey

This paper presents preliminary results of a spectroscopic survey being conducted at the VLT of fields with optically-selected cluster candidates identified in the EIS I-band survey. Here we report our findings for three candidates selected for having estimated redshifts in the range z=0.8-1.1. New multi-band optical/infrared data were used to assign photometric redshifts to galaxies in the cluster fields and to select possible cluster members in preparation of the spectroscopic observations. Based on the available spectroscopic data, which includes 147 new redshifts for galaxies with Iab<22-23, we confirm the detection of four density enhancements at a confidence level >99%. The detected concentrations include systems with redshifts z=0.81, z=0.95, z=1.14 and the discovery of the first optically-selected cluster at z=1.3. The latter system, with three concordant redshifts, coincides remarkably well with the location of a firm X-ray detection (>5sigma) in a ~80ksec XMM-Newton image taken as part of this program which will be presented in a future paper (Neumann et al. 2002). The z>1 systems presented here are possibly the most distant identified so far by their optical properties alone.

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arXiv2002

Radio emission from the A3571 cluster complex: the final stage of a cluster merger?

In this paper we report and discuss the results of a radio survey in the A3571 cluster complex, a structure located in the Shapley Concentration core, and formed by the three clusters A3571, A3572 and A3575. The survey was carried out simultaneously at 22 cm and 13 cm with the Australia Telescope Compact Array, and led to the detection of 124 radio sources at 22 cm. The radio source counts in this region are in agreement with the background counts. Among the 36 radio sources with optical counterpart, six have measured redshift which places them at the distance of the A3571 cluster complex, and nine radio sources have optical counterparts most likely members of this cluster complex. All radio galaxies emit at low power level, i.e. logP$_{22cm}$ (W Hz$^{-1}$) $\le$ 22.6. A number of them are likely to be starburst galaxies. The radio luminosity function of early type galaxies is in agreement with that derived by Ledlow & Owen (1996) if we restrict our analysis to the A3571 cluster. On the basis of the multiwavelength properties of the A3571 cluster complex, we propose that it is a very advanced merger, and explain the radio properties derived from our study in the light of this hypothesis.

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arXiv2001

SC1327-312 & SC1329-313: two galaxy groups in-between a major merging event observed with Beppo-SAX

We present the results of two Beppo-SAX observations of the poor clusters SC1327-312 and SC1329-313. These objects are located in a huge structure, formed by three ACO clusters, which is probably the remnant of one of the largest and best studied major mergings known. Given the fact that these poor clusters are in between two interacting ACO clusters, the aim of this work is to study the physics of the intracluster medium and to look for the possible presence of shocks. We derived the gas distribution profiles, the global (i.e. within 0.3 Mpc) temperatures and abundances and the temperature profiles and maps for SC1327-312 and SC1329-313. Also the presence of soft excess in LECS and hard excess in PDS have been checked. We do not find evidence of regions where the gas is shocked or significantly heated. The image of SC1327-312 seems rather symmetric, while the gas profile of SC1329-313 shows disturbed, comet-like shaped isophotes, with the tail pointing toward A3562 and a compression toward SC1327-312. The presence of multiphase gas in SC1329-313, as claimed by Hanami et al. (1999) on ASCA data, has been found only at the 2 sigma confidence level. The lack of heating supports the hypothesis that the merging is at a late stage, after the first core--core encounter, when the main shock front had the time to travel to the external regions of the main clusters.

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arXiv2001

On the Nature of the EIS Candidate Clusters: Confirmation of z<0.6 candidates

We use public V-band imaging data from the wide-angle surveys conducted by the ESO Imaging Survey project (EIS) to further investigate the nature of the EIS galaxy cluster candidates. These were originally identified by applying a matched-filter algorithm which used positional and photometric data of the galaxy sample extracted from the I-band survey images. In this paper, we apply the same technique to the galaxy sample extracted from V-band data and compare the new cluster detections with the original ones. We find that ~75% of the low-redshift cluster candidates (z<0.6) are detected in both passbands and their estimated redshifts show good agreement with the scatter in the redshift differences being consistent with the estimated errors of the method. For the ``robust'' I-band detections the matching frequency approaches ~85%. We also use the available (V-I) color to search for the red sequence of early-type galaxies observed in rich clusters over a broad range of redshifts. This is done by searching for a simultaneous overdensity in the three-dimensional color-projected distance space. We find significant overdensities for ~75% of the ``robust'' candidates with z_I<0.6. We find good agreement between the characteristic color associated to the detected "red sequence" and that predicted by passive evolution galaxy models for ellipticals at the redshift estimated by the matched-filter. The results presented in this paper show the usefulness of color data, even of two-band data, to both tentatively confirm cluster candidates and to select possible cluster members for spectroscopic observations. Based on the present results, we estimate that ~150 EIS clusters with z_I<0.6 are real, making it one of the largest samples of galaxy clusters in this redshift range currently available in the southern hemisphere.

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arXiv2001

Radio-optically selected clusters of galaxies. II. The cluster sample

We present a sample of 171 candidate groups and clusters of galaxies at intermediate redshift over an area of ~550 square degrees at the South Galactic Pole selected by using optically identified radio sources from the NRAO VLA Sky Survey as tracers of dense environment. Out of these 171 candidates, 76 have a counterpart in the literature while 95 of them are previously unknown clusters. This paper presents the cluster selection technique, based on the search of excesses in the optical surface density of galaxies near identified radiogalaxies, and the first spectroscopic results aimed to confirm the presence of a cluster. Spectroscopy for 11 candidates led to the detection of 9 clusters at redshift in the range 0.13 - 0.3, with estimated velocity dispersions ranging from values typical of clusters to those of galaxy groups. These results show that this technique represents a powerful tool for the selection of homogeneous samples of intermediate redshift clusters over a wide range of richness.

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arXiv2001

Radio Properties of the Shapley Concentration. IV. The A3528 Cluster Complex

We report and discuss the results of a 22 cm radio survey carried out with the Australia Telescope Compact Array (ATCA) covering the A3528 complex, a chain formed by the merging ACO clusters A3528-A3530-A3532, located in the central region of the Shapley Concentration. Simultaneous 13 cm observations are also presented. Our final catalogue includes a total of 106 radio sources above the flux density limit of 0.8 mJy. By cross correlation with optical and spectroscopic catalogues we found 32 optical counterparts, nine of them belonging tothe A3528 complex. We explored the effects of cluster mergers on the radio emission properties of radio galaxies by means of the radio luminosity function (RLF) and radio source counts. We found that the radio source counts are consistent with the background counts, as already found for the A3558 complex (Paper III). The RLF for this complex is consistent, both in shape and normalisation, with the general cluster luminosity function for early-type galaxies derived by Ledlow & Owen (LO96). This result is different from what we obtained for the A3558 merging complex, whose RLF is significantly lower than LO96. We propose that the different stage of the merger is responsible for the different RLFs in the two cluster complexes in the core of the Shapley Concentration. The early stage of merger for the A3528 complex, proposed by many authors, may have not yet affected the radio properties of cluster galaxies, while in the more much advanced merger in the A3558 region we actually see the effects of this process on the radio emission.

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arXiv2001

The VIRMOS-VLT Deep Survey

The aim of the VIRMOS VLT Deep Survey (VVDS) is to study of the evolution of galaxies, large scale structures and AGNs from a sample of more than 150,000 galaxies with measured redshifts in the range 0<z<5+. The VVDS will rely on the VIMOS and NIRMOS wide field multi-object spectrographs, which the VIRMOS consortium is delivering to ESO. Together, they offer unprecedented multiplex capability in the wavelength range 0.37-1.8microns, allowing for large surveys to be carried out. The VVDS has several main aspects: (1) a deep multi-color imaging survey over 18deg^2 of more than one million galaxies, (2) a "wide" spectroscopic survey with more than 130,000 redshifts measured for objects brighter than IAB=22.5 over 18deg^2, (3) a "deep" survey with 50,000 redshifts measured to IAB=24, (4) ultra-deep" surveys with several thousand redshifts measured to IAB=25, (5) multi-wavelength observations with the VLA and XMM.

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