arXiv subjects

Piero Rosati

Publications and source records attributed to Piero Rosati.

123 records · Page 7Linked to original sources

arXiv2002

The Evolution of X-ray Clusters of Galaxies

Considerable progress has been made over the last decade in the study of the evolutionary trends of the population of galaxy clusters in the Universe. In this review we focus on observations in the X-ray band. X-ray surveys with the ROSAT satellite, supplemented by follow-up studies with ASCA and Beppo-SAX, have allowed an assessment of the evolution of the space density of clusters out to z~1, and the evolution of the physical properties of the intra-cluster medium out to z~0.5. With the advent of Chandra and Newton-XMM, and their unprecedented sensitivity and angular resolution, these studies have been extended beyond redshift unity and have revealed the complexity of the thermodynamical structure of clusters. The properties of the intra-cluster gas are significantly affected by non-gravitational processes including star formation and Active Galactic Nucleus (AGN) activity. Convincing evidence has emerged for modest evolution of both the bulk of the X-ray cluster population and their thermodynamical properties since redshift unity. Such an observational scenario is consistent with hierarchical models of structure formation in a flat low density universe with Omega_m=0.3 and sigma_8=0.7-0.8 for the normalization of the power spectrum. Basic methodologies for construction of X-ray-selected cluster samples are reviewed and implications of cluster evolution for cosmological models are discussed.

astro-ph↗

arXiv2002

SPICES II. Optical and Near-Infrared Identifications of Faint X-Ray Sources from Deep Chandra Observations of Lynx

We present our first results on field X-ray sources detected in a deep, 184.7 ks observation with the ACIS-I camera on Chandra. The observations target the Lynx field of SPICES, and contains three known X-ray-emitting clusters out to z=1.27. Not including the known clusters, in the 17'x17' ACIS-I field we detect 132 sources in the 0.5-2 keV (soft) X-ray band down to a limiting flux of \~1.7e-16 erg/cm2/s and 111 sources in the 2-10 keV (hard) X-ray band down to a limiting flux of ~1.3e-15 erg/cm2/s. The combined catalog contains a total of 153 sources, of which 42 are detected only in the soft band and 21 are detected only in the hard band. Confirming previous Chandra results, we find that the fainter sources have harder X-ray spectra, providing a consistent solution to the long-standing `spectral paradox'. From deep optical and near-infrared follow-up data, 77% of the X-ray sources have optical counterparts to I=24 and 71% of the X-ray sources have near-infrared counterparts to K=20. Four of the 24 sources in the near-IR field are associated with extremely red objects (EROs; I-K>4). We have obtained spectroscopic redshifts with the Keck telescopes of 18 of the Lynx Chandra sources. These sources comprise a mix of broad-lined active galaxies, apparently normal galaxies, and two late-type Galactic dwarfs. Intriguingly, one Galactic source is identified with an M7 dwarf exhibiting non-transient, hard X-ray emission. We review non-AGN mechanisms to produce X-ray emission and discuss properties of the Lynx Chandra sample in relation to other samples of X-ray and non-X-ray sources.

astro-ph↗

arXiv2001

Distant Cluster Hunting I: A Comparison Between the Optical and X-ray Luminosity Functions from an Optical/X-ray Survey

We present a comparison of X-ray and optical luminosities and luminosity functions of cluster candidates from a joint optical/X-ray survey, the ROSAT Optical X-Ray Survey. Completely independent X-ray and optical catalogs of 23 ROSAT fields (4.8 deg2) were created by a matched-filter optical algorithm and by a wavelet technique in the X-ray. We directly compare the results of the optical and X-ray selection techniques. The matched-filter technique detected 74% (26 out of 35) of the most reliable cluster candidates in the X-ray-selected sample; the remainder could be either constellations of X-ray point sources or z>1 clusters. The matched-filter technique identified approximately 3 times the number of candidates (152 candidates) found in the X-ray survey of nearly the same sky (57 candidates). While the estimated optical and X-ray luminosities of clusters of galaxies are correlated, the intrinsic scatter in this relationship is very large. We can reproduce the number and distribution of optical clusters with a model defined by the X-ray luminosity function and by an LX Lambda cl relation if H0=75 km s-1 Mpc-1 and if the LX Lambda cl relation is steeper than the expected LX Lambda 2cl. On statistical grounds, a bimodal distribution of X-ray luminous and X-ray faint clusters is unnecessary to explain our observations. Follow-up work is required to confirm whether the clusters without bright X-ray counterparts are simply X-ray faint for their optical luminosity because of their low mass or youth or are a distinct population of clusters that do not, for some reason, have dense intracluster media. We suspect that these optical clusters are low-mass systems, with correspondingly low X-ray temperatures and luminosities, or that they are not yet completely virialized systems.

astro-ph↗

arXiv2001

Distant Cluster Hunting II: A comparison of X-ray and optical cluster detection techniques and catalogs from the ROX Survey

We present and analyze the optical and X-ray catalogs of moderate-redshift cluster candidates from the ROSAT Optical X-ray Survey, or ROXS. The survey covers 4.8 square degrees (23 ROSAT PSPC pointings). The cross-correlated cluster catalogs were constructed by comparing two independent catalogs extracted from the optical and X-ray bandpasses, using a matched filter technique for the optical data and a wavelet technique for the X-ray data. We cross-id cluster candidates in each catalog. In Paper II we present the cluster catalogs and a numerical simulation of ROXS. We also present color-magnitude plots for several cluster candidates, and examine the prominence of the red sequence in each. We find that the X-ray clusters analyzed in this way do not all have a prominent red sequence. We conclude that while the red sequence may be distinct for massive, virialized clusters, it may be less so for lower-mass clusters at even moderate redshifts. Multiple, complementary methods of selecting and defining clusters may be essential, particularly at high redshift where all methods run into completeness limits, incomplete understanding of physical evolution, and projection effects.

astro-ph↗

arXiv2001

Chandra Detection of a TypeII Quasar at z=3.288

We report on observations of a TypeII quasar at redshift z=3.288, identified as a hard X-ray source in a 185 ks observation with the Chandra X-ray Observatory and as a high-redshift photometric candidate from deep, multiband optical imaging. CXOJ084837.9+445352 (hereinafter CXO52) shows an unusually hard X-ray spectrum from which we infer an absorbing column density N(H) = (4.8+/-2.1)e23 / cm2 (90% confidence) and an implied unabsorbed 2-10 keV rest-frame luminosity of L(2-10) = 3.3e44 ergs/s, well within the quasar regime. Hubble Space Telescope imaging shows CXO52 to be elongated with slight morphological differences between the WFPC2 F814W and NICMOS F160W bands. Optical and near-infrared spectroscopy of CXO52 show high-ionization emission lines with velocity widths ~1000 km/s and flux ratios similar to a Seyfert2 galaxy or radio galaxy. The latter are the only class of high-redshift TypeII luminous AGN which have been extensively studied to date. Unlike radio galaxies, however, CXO52 is radio quiet, remaining undetected at radio wavelengths to fairly deep limits, f(4.8GHz) < 40 microJy. High-redshift TypeII quasars, expected from unification models of active galaxies and long-thought necessary to explain the X-ray background, are poorly constrained observationally with few such systems known. We discuss recent observations of similar TypeII quasars and detail search techniques for such systems: namely (1) X-ray selection, (2) radio selection, (3) multi-color imaging selection, and (4) narrow-band imaging selection. Such studies are likely to begin identifying luminous, high-redshift TypeII systems in large numbers. We discuss the prospects for these studies and their implications to our understanding of the X-ray background.

astro-ph↗

arXiv2001

XMM-Newton Observation of a Distant X-ray Selected Cluster of Galaxies at z=1.26 with Possible Cluster Interaction

We report on the XMM-Newton (XMM) observation of RXJ1053.7+5735, one of the most distant (z = 1.26) X-ray selected clusters of galaxies, which also shows an unusual double-lobed X-ray morphology, indicative of possible cluster-cluster interaction. The cluster was discovered during our ROSAT deep pointings in the direction of the Lockman Hole. The XMM observations were performed with the European Photon Imaging Camera (EPIC) during the performance verification phase. Total effective exposure time was ~ 100 ksec. The best fit temperature based on a simultaneous fit of spectra from the all EPIC cameras is 4.9(+1.5/-0.9) keV. Metallicity is poorly constrained even using the joint fit of all spectra, with an upper limit on the iron abundance of 0.62 solar. Using the best fit model parameters, we derived a bolometric luminosity of L(bol) = 3.4x10^44 h_{50}^-2 erg /s. Despite the fact that it was observed at fairly large off-axis angle, the temperature errors are much smaller compared with those of typical measurements based on ASCA or Beppo-Sax observations of z > 0.6 clusters, demonstrating the power of the XMM for determining the X-ray temperature for high-z clusters. The measured temperature and luminosity show that one can easily reach the intrinsically X-ray faint and cool cluster regime comparable with those of z ~ 0.4 clusters observed by past satellites. The new cluster temperature and L(bol) we have measured for RXJ1053.7+5735 is consistent with a weak/no evolution of the L(bol) - Tx relation out to z ~ 1.3, which lends support to a low Omega universe, although more data-points of z > 1 clusters are required for a more definitive statement. The caution has to be also exercised in interpreting the result, because of the uncertainty associated with the dynamical status of this cluster.

astro-ph↗

arXiv2000

First Results from the SPICES Survey

We present first results from SPICES, the Spectroscopic, Photometric, Infrared-Chosen Extragalactic Survey. SPICES is comprised of four ~30 square arcminute high Galactic latitude fields with deep BRIzJK imaging reaching depths of ~25th magnitude (AB) in the optical and ~23rd magnitude (AB) in the near-infrared. To date we have 626 spectroscopic redshifts for infrared-selected SPICES sources with K<20 (Vega). The project is poised to address galaxy formation and evolution to redshift z~2. We discuss initial results from the survey, including the surface density of extremely red objects and the fraction of infrared sources at z>1. One of the SPICES fields has been the target of a deep 190 ksec Chandra exposure; we discuss initial results from analysis of that data set. Finally, we briefly discuss a successful campaign to identify high-redshift sources in the SPICES fields.

astro-ph↗

arXiv2000

Predicting the Clustering of X-Ray Selected Galaxy Clusters in Flux-Limited Surveys

(abridged) We present a model to predict the clustering properties of X-ray clusters in flux-limited surveys. Our technique correctly accounts for past light-cone effects on the observed clustering and follows the non-linear evolution in redshift of the underlying DM correlation function and cluster bias factor. The conversion of the limiting flux of a survey into the corresponding minimum mass of the hosting DM haloes is obtained by using theoretical and empirical relations between mass, temperature and X-ray luminosity of clusters. Finally, our model is calibrated to reproduce the observed cluster counts adopting a temperature-luminosity relation moderately evolving with redshift. We apply our technique to three existing catalogues: BCS, XBACs and REFLEX samples. Moreover, we consider an example of possible future space missions with fainter limiting flux. In general, we find that the amplitude of the spatial correlation function is a decreasing function of the limiting flux and that the EdS models always give smaller correlation amplitudes than open or flat models with low matter density parameter. In the case of XBACs, the comparison with previous estimates of the observational spatial correlation shows that only the predictions of models with Omega_0m=0.3 are in good agreement with the data, while the EdS models have too low a correlation strength. Finally, we use our technique to discuss the best strategy for future surveys. Our results show that the choice of a wide area catalogue, even with a brighter limiting flux, is preferable to a deeper, but with smaller area, survey.

astro-ph↗

arXiv1999

Predicting the Clustering of X-Ray Clusters

We present a theoretical model which aims at predicting the clustering properties of X-ray clusters in flux-limited surveys for different cosmological scenarios. The model uses the theoretical and empirical relations between mass, temperature and X-ray luminosity, and fully accounts for the redshift evolution of the underlying dark matter clustering and cluster bias factor. We apply the model to the RASS1 Bright Sample and to the XBACs catalogue. The results show that the Einstein-de Sitter models display too low a correlation length, while models with a matter density parameter Omega_0m=0.3 (with or without a cosmological constant) are successful in reproducing the observed clustering.

astro-ph↗

arXiv1999

The Brera Multi-scale Wavelet (BMW) ROSAT HRI source catalog. I: the algorithm

We present a new detection algorithm based on the wavelet transform for the analysis of high energy astronomical images. The wavelet transform, due to its multi-scale structure, is suited for the optimal detection of point-like as well as extended sources, regardless of any loss of resolution with the off-axis angle. Sources are detected as significant enhancements in the wavelet space, after the subtraction of the non-flat components of the background. Detection thresholds are computed through Monte Carlo simulations in order to establish the expected number of spurious sources per field. The source characterization is performed through a multi-source fitting in the wavelet space. The procedure is designed to correctly deal with very crowded fields, allowing for the simultaneous characterization of nearby sources. To obtain a fast and reliable estimate of the source parameters and related errors, we apply a novel decimation technique which, taking into account the correlation properties of the wavelet transform, extracts a subset of almost independent coefficients. We test the performance of this algorithm on synthetic fields, analyzing with particular care the characterization of sources in poor background situations, where the assumption of Gaussian statistics does not hold. For these cases, where standard wavelet algorithms generally provide underestimated errors, we infer errors through a procedure which relies on robust basic statistics. Our algorithm is well suited for the analysis of images taken with the new generation of X-ray instruments equipped with CCD technology which will produce images with very low background and/or high source density.

astro-ph↗

arXiv1999

An X-ray Selected Galaxy Cluster at z=1.26

We report the discovery of an X-ray luminous galaxy cluster at z=1.26. RXJ0848.9+4452 was selected as an X-ray cluster candidate in the ROSAT Deep Cluster Survey, on the basis of its spatial extent. Deep optical and near-IR imaging have revealed a galaxy overdensity around the peak of the X-ray emission, with a significant excess of red objects with J-K colors typical of elliptical galaxies at z>1. Spectroscopic observations at the Keck II telescope have secured 6 galaxy redshifts in the range 1.257 =1.261), within a 35 arcsec radius around the peak X-ray emission. This system lies only 4.2 arcmin away (5.0 h^{-1}_{50} comoving Mpc, q_0=0.5) from the galaxy cluster ClG J0848+4453, which was identified by Stanford et al. (1997) at z=1.273 in a near-IR field galaxy survey, and is also known to be X-ray luminous. Assuming that the X-ray emission is entirely due to hot intra-cluster gas, both these systems have similar rest frame luminosities L_x ~=1x10^{44} ergs/s (0.5-2.0 keV band). In combination with our spectrophotometric data for the entire 30 arcmin^2 field, this suggests the presence of a superstructure, consisting of two collapsed, possibly virialized clusters, the first detected to date at z>1.

astro-ph↗

arXiv1999

Detection of Evolved High-Redshift Galaxies in Deep NICMOS/VLT Images

A substantial population of high redshift early-type galaxies is detected in very deep UBVRIJHK images towards the HDF-South. Four elliptical profile galaxies are identified in the redshift range z=1-2, all with very red SEDs, implying ages of >2 Gyrs for standard passive evolution. We also find later type IR-luminous galaxies at similarly high redshift, (10 objects with z>1, H<25), whose weak UV emission implies single burst ages of >1 Gyr. The number and luminosity-densities of these galaxies are comparable with the local E/SO-Sbc populations for Ω_m>0.2, and in the absence of a significant cosmological constant, we infer that the major fraction of luminous Hubble-sequence galaxies have evolved little since z~2. A highly complete photometric redshift distribution is constructed to H=25 (69 galaxies) showing a broad spread of redshift, peaking at z~1.5, in reasonable agreement with some analyses of the HDF. Five `dropout' galaxies are detected at z~3.8, which are compact in the IR, ~0.5 kpc/h at rest 3500Å. No example of a blue IR luminous elliptical is found, restricting the star-formation epoch of ellipticals to z>10 for a standard IMF and modest extinction.

astro-ph↗

arXiv1998

Deep Imaging of AXJ2019+112: The Luminosity of a ``Dark Cluster''

We detect a distant cluster of galaxies centered on the QSO lens and luminous X-ray source AXJ2019+112, a.k.a. ``The Dark Cluster'' (Hattori et al 1997). Using deep V,I Keck images and wide-field K_s imaging from the NTT, a tight red sequence of galaxies is identified within a radius of 0.2 h^{-1} Mpc of the known z=1.01 elliptical lensing galaxy. The sequence, which includes the central elliptical galaxy, has a slope in good agreement with the model predictions of Kodama et al (1998) for z~1. We estimate the integrated rest-frame luminosity of the cluster to be L_V > 3.2 x 10^{11}h^{-2}L_{\sun} (after accounting for significant extinction at the low latitude of this field), more than an order of magnitude higher than previous estimates. The central region of the cluster is deconvolved using the technique of Magain, Courbin & Sohy (1998), revealing a thick central arc coincident with an extended radio source. All the observed lensing features are readily explained by differential magnification of a radio loud AGN by a shallow elliptical potential. The QSO must lie just outside the diamond caustic, producing two images, and the arc is a highly magnified image formed from a region close to the center of the host galaxy, projecting inside the caustic. The mass--to--light ratio within an aperture of 0.4 h ^{-1} Mpc is M_x/L_V= 224^{+112}_{-78}h(M/L_V)_{\sun}, using the X-ray temperature. The strong lens model yields a compatible value, M/L_V= 372^{+94}_{-94}h(M/L_V)_{\sun}, whereas an independent weak lensing analysis sets an upper limit of M/L_V <520 h(M/L_V)_{\sun}, typical of massive clusters.

astro-ph↗

arXiv1998

X-ray Surveys of Distant Galaxy Clusters

I review recent observational progress in the search for and study of distant galaxy clusters in the X-ray band, with particular emphasis on the evolution of the abundance of X-ray clusters out to z~1. Several on-going deep X-ray surveys have led to the discovery of a sizeable population of clusters at z>0.5 and have the sensitivity to detect clusters beyond redshift one. These surveys have significantly improved our understanding of cluster evolution by showing that the bulk of the population of galaxy clusters is not evolving significantly since at least z~=0.8, with some evolution limited to only the most luminous, presumably most massive systems. Thus far, a well defined sample of very high redshift (z>~1) clusters has been difficult to assemble and represents one of the most challenging observational tasks for the years to come.

astro-ph↗

arXiv1997

Infall Regions of Galaxy Clusters

In hierarchical clustering, galaxy clusters accrete mass through the aggregation of smaller systems. Thus, the velocity field of the infall regions of clusters contains significant random motion superimposed on radial infall. Because the purely spherical infall model does not predict the amplitude of the velocity field correctly, methods estimating the cosmological density parameter Omega_0 based on this model yield unreliable biased results. In fact, the amplitude of the velocity field depends on local dynamics and only very weakly on the global properties of the universe. We use N-body simulations of flat and open universes to show that the amplitude of the velocity field of the infall regions of dark matter halos is a direct measure of the escape velocity within these regions. We can use this amplitude to estimate the mass of dark matter halos within a few megaparsecs from the halo center. In this region dynamical equilibrium assumptions do not hold. The method yields a mass estimate with better than 30% accuracy. If galaxies trace the velocity field of the infall regions of clusters reliably, this method provides a straightforward way to estimate the amount of mass surrounding rich galaxy clusters from redshift data alone.

astro-ph↗