1,721,390 research outputs found
A serendipitous deep cluster survey from ROSAT-PSPC pointed observations
No full textWe present a deep X-ray selected sample of galaxy clusters which has been created from a serendipitous search in ROSAT-PSPC deep pointed observations at high galactic latitude. This survey, hereafter known as the ROSAT Deep Cluster Survey (RDCS), is being carried out utilizing a wavelet-based detection algorithm which, unlike other detection methods, is not biased against extended, low surface brightness sources. It is a flux-diameter Limited sample that extends the X-ray flux limit of previous cluster surveys by more than one order of magnitude (F-X greater than or equal to 1.10(-14)erg cm(-2)s(-1)). The first results of the on-going optical follow-up program indicate a high success rate of identification. At the present, 38 clusters out of 80 candidates have been identified on a 26 deg(2) surveyed area. Recently measured redshifts confirm the nature of these systems as low-moderate redshift groups (z similar or equal to 0.2-0.3) and intermediate to high redshift clusters (z similar or equal to 0.4-0.7). We show X-ray and optical images of several clusters identified to date, discuss the X-ray properties of the sample and present preliminary results on the redshift distribution. The final sample will include similar to 100 clusters covering and area of similar to 40 deg(2)
Riccardo Giacconi (1931-2018)
No full textRiccardo Giacconi, one of the most charismatic and influential figures of astrophysics in the modern era, died on 9 December 2018. He was 87. Giacconi was a co-recipient of the 2002 Nobel Prize in Physics for “pioneering contributions to astrophysics, which have led to the discovery of cosmic x-ray sources.” His contributions to astronomy, however, span the entire electromagnetic spectrum, as he directed the development and operation of major space- and ground-based facilities, which have greatly expanded our scientific boundaries over the past 50 years
The ancestors of most massive galaxy clusters
No full textThe study of the early stages of galaxy cluster formation and their evolutionary path is critical for testing our structure-formation models and cosmological paradigm. Recent observations have pushed the detection of ‘protoclusters’ further back in time
Results from the ROSAT Deep Cluster Survey
No full textDeep X-ray surveys of galaxy clusters covering large solid angles offer ideal samples for studying the evolution of the large scale structure in the Universe. I review here some recent results from the ROSAT Deep Cluster Survey (RDCS), particularly, on the study of the evolution of the abundance of galaxy clusters over a large redshift range. The optical identification of RDCS sources have revealed a significant fraction of clusters at z > 0.5. The observed X-ray Luminosity Function out to z similar or equal to 0.8 and the faint cluster number counts from the RDCS, when combined with previous results of the Einstein Extended Medium Sensitivity Survey, provide a much improved observational picture of the evolution of clusters in the X-ray band, revealing a slowly evolving population of galaxy clusters since at least z similar or equal to 0.8. The theoretical interpretation of these findings in the framework of theories of structure formation will still be subject of much debate in the years to come. The construction of large samples of clusters out to z similar to 1 and beyond appears to be a challenging task even with the next generation of X-ray satellites (AXAF, XMM)
Search and analysis of small scale structures in two X-ray clusters of galaxies
No full textWe present a refinement of the wavelet analysis technique for the detection and characterisation of small scale features embedded in a strongly varying background. This technique handles with particular care the side effects of nonorthogonality in the wavelet space which can cause spurious detections and lead to a biased estimate of source parameters. This novel technique is applied to two ROSAT PSPC pointed observations of nearby clusters of galaxies, A 1367 and A 194. We find evidence that the case of A 1367 is not unique and that galaxy-scale X-ray emission could be a quite common property of clusters of galaxies. We detect 28 sources in the field of A 1367 and 26 in the field of A 194. Since these numbers are significantly larger than those expected from the log N - log S relation in the field, most of the sources are expected to be associated with the cluster itself and indeed several identifications with galaxies are possible. In addition, CCD observations have revealed that two X-ray sources in the field of A 194, classified as extended by the multi-scale analysis, are very likely associated with two background galaxy clusters at intermediate redshift
A hard medium survey with ASCA III. a Type 2 AGN revealed from X-ray spectroscopy
No full textIn this paper we report the discovery of a hard Xray selected Type 2 Seyfert galaxy and we present and discuss its X-ray and optical spectrum together with the radio to X-ray energy distribution. The X-ray source - AXJ2254+1146 - is part of the ASCA Hard Serendipitous Survey (HSS). What makes this discovery particularly noteworthy is the fact that the Spe 2 classification of this Seyfert galaxy has resulted directly from the X-ray data and has been confirmed by optical spectroscopy only subsequently. The X-ray spectrum of AXJ2254+1146 is best described by a model consisting of an unresolved Gaussian line at 6.43 +/- 0.1 keV plus the so called "leaky-absorber" continua having an intrinsic power law photon index of Gamma = 2.51(2.17)(2.76) (1 sigma confidence interval). The best fit values of the absorbing column density (N-H = 1.85(1.47)(2.24) x 10(23) cm(-2)), of the line equivalent width (0.6(0.36)(0.84) keV) and of the scattering fraction (0.7(0.1)(1.4)%), lead us to classify it as a Type 2 AGN from an X-ray point of view. Inspection of the POSS II image reveals the presence, within the ASCA X-ray error circle, of the nearby Sbc spiral galaxy UGC 12237 (m(Bo) = 14.26) that, even on positional ground considerations alone, is the most likely optical counterpart of AXJ2254+1146. Subsequent optical spectroscopy of UGC 12237 has confirmed its Seyfert 2 optical nature
Evidence for X-ray emission from a large-scale filament of galaxies?
No full textCosmological simulations predict that a large fraction of the baryonic mass of the universe exists as 10(5)-10(7) K diffuse, X-ray-emitting gas, tracing low-density filament and sheetlike structures exterior to massive clusters of galaxies. If present, this gas helps reconcile the current shortfall in observed baryon counts relative to the predictions of the standard big bang model. We present here the discovery and analysis of a 5 sigma significance half-degree filamentary structure, which is present in both the I-band salary surface density and the unresolved X-ray emission in a deep ROSAT PSPC field. The estimated diffuse X-ray emission component of this structure has a surface brightness of similar or equal to 1.6 x 10(-16) ergs s(-1) cm(-2) arcmin(-2) (0.5-2 keV), comparable to the predictions for intercluster gas, and may represent a direct detection of this currently unconfirmed baryonic component
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