MOND predictions of 'halo' phenomenology in disc galaxies     

Mordehai Milgrom  Robert H. Sanders 《Monthly notices of the Royal Astronomical Society》2005,357(1):45-48

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Neutrinos as cluster dark matter     

R. H. Sanders 《Monthly notices of the Royal Astronomical Society》2007,380(1):331-338

The dynamical mass of clusters of galaxies, calculated in terms of MOdified Newtonian Dynamics (MOND), is a factor of 2 or 3 times smaller than the Newtonian dynamical mass but remains significantly larger than the observed baryonic mass in the form of hot gas and stars in galaxies. Here I consider further the suggestion that the undetected matter might be in the form of cosmological neutrinos with mass of the order of 2 eV. If the neutrinos and baryons have comparable velocity dispersions and if the two components maintain their cosmological density ratio, then the electron density in the cores of clusters should be proportional to T ^3/2, as appears to be true in non-cooling flow clusters. This is equivalent to the 'entropy floor' proposed to explain the steepness of the observed luminosity–temperature relation, but here preheating of the medium is not required. Two-fluid (neutrino–baryon) hydrostatic models of clusters, in the context of MOND, reproduce the observed luminosity–temperature relation of clusters. If the β law is imposed on the gas density distribution, then the self-consistent models predict the general form of the observed temperature profile in both cooling and non-cooling flow clusters.  相似文献   

An X-ray absorption analysis of the high-velocity system in NGC 1275     

K. Gillmon  J. S. Sanders  A. C. Fabian 《Monthly notices of the Royal Astronomical Society》2004,348(1):159-164

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Origin of rare earth element anomalies in mangrove sediments,Sepetiba Bay,SE Brazil: used as geochemical tracers of sediment sources     

Emmanoel V. Silva-Filho  Christian J. Sanders  Michel Bernat  Ana M. G. Figueiredo  Silvia M. Sella  Julio Wasserman 《Environmental Earth Sciences》2011,64(5):1257-1267

Elemental contents were determined in two mangrove habitats along Sepetiba Bay, SE Brazil, an area impacted by local industrial activities, as well as hinterland water diversion networks. This study demonstrates how specific REEs (La, Ce, Nd, Sm, Eu, Tb, Yb and Lu) may be used as a sediment source tracer to mangrove-dominated coastlines. From the two stations studied, a pair of cores was collected, one in the mangrove forest and the other in the tidal flat. Station 1 results show a general enrichment in most of the fractioned patterns of the REEs normalised by Post-Archean Australian Shale. The relatively light rare earth elements are similarly enriched in the generally more polluted Station 1. Despite the probable difference in background sediment characteristics, a common sharp increase in mud contents patterns in the upper part of the mangrove sediment core was related to a lower REE content as well as Eu anomalies. With existing knowledge of clockwise water circulation in the bay, these patterns can be explained by man-made water diversion from the São Francisco and Guandu rivers, initiated more than 30 years ago, whereby suspended matter with relatively large contents of REEs and material originating from industrial sources accumulate in the eastern sector of the bay. This is the first comprehensive assessment of REEs as sedimentary tracers in a mangrove ecosystem in Brazil.  相似文献   

Clusters of galaxies with modified Newtonian dynamics     

R. H. Sanders 《Monthly notices of the Royal Astronomical Society》2003,342(3):901-908

I consider X-ray emitting clusters of galaxies in the context of modified Newtonian dynamics (MOND). Self-gravitating isothermal gas spheres are not good representations of rich clusters; the X-ray luminosity at a given temperature is typically an order of magnitude larger than observed, and the predicted X-ray surface brightness distribution is not well-matched by the standard 'β-model' fits to the observations. Pure gas spheres with a density distribution described by a β-model also fail because, with MOND, these objects are far from isothermal and again overluminous. These problems may be resolved by adding an additional dark mass component in the central regions, here modelled by a constant density sphere contained within two core radii and having a mass typically of one to two times the total cluster mass in the gas. With this additional component, the observed luminosity–temperature relation for clusters of galaxies is reproduced, and the typical mass discrepancy in actual clusters is three to four times smaller than implied by Newtonian dynamics. Thus, while MOND significantly reduces the mass of the dark component in clusters it does not remove it completely. I speculate on the nature of the dark component and argue that neutrinos, with mass near the experimental upper limit are a possible candidate.  相似文献   

Enrichment in the Centaurus cluster of galaxies     

J. S. Sanders  A. C. Fabian 《Monthly notices of the Royal Astronomical Society》2006,371(3):1483-1496

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First Results from ROTES: The ROtse Telescope Eclipsing-binary Survey     

I. Ribas  J. C. Morales  C. Allende Prieto  C. Jordi  D. H. Bradstreet  S. J. Sanders 《Astrophysics and Space Science》2006,304(1-4):231-233

Detached eclipsing binaries (EBs) provide a unique opportunity to carry out stringent tests on stellar evolution models. The value of EBs is enhanced by their membership in open clusters, but the number of known systems is still very scarce. We have started a systematic search for late-type EBs in the nearest open clusters with the fully robotic ROTSE3b telescope at McDonald Observatory in West Texas. On our first campaigns on the Hyades and Collinder 359, we have identified a number of previously unknown eclipsing binary candidates. Some of these stars have been selected for spectroscopic and photometric follow-up. Here we present details of the observing and reduction strategy as well as the first results of this ongoing survey.  相似文献   

The missing soft X-ray luminosity in cluster cooling flows     

A. C. Fabian  S. W. Allen  C. S. Crawford  R. M. Johnstone  R. G. Morris  J. S. Sanders  R. W. Schmidt 《Monthly notices of the Royal Astronomical Society》2002,332(3):L50-L54

The gas temperature in the cores of many clusters of galaxies drops inward by about a factor of 3 or more within the central 100-kpc radius. The radiative cooling time drops over the same region from 5 or more Gyr down to below a few 10⁸ yr. Although this indicates that cooling flows are taking place, XMM-Newton spectra show no evidence for strong mass cooling rates of gas below  1–2 keV  . The soft X-ray luminosity expected from steady cooling flows is missing. Here we outline and test the energetics of a cold mixing model in which gas below  1–2 keV  falls from the flow and is rapidly cooled by mixing with cold gas. The missing X-ray luminosity can emerge in the ultraviolet, optical and infrared bands, where strong emission nebulosities are commonly seen. We explore further the requirements for any heat sources that balance the radiative cooling in cluster cores.  相似文献   

Adaptive binning of X-ray galaxy cluster images     

J.S. Sanders  A.C. Fabian 《Monthly notices of the Royal Astronomical Society》2001,325(1):178-186

We present a simple method for adaptively binning the pixels in an image. The algorithm groups pixels into bins of size such that the fractional error on the photon count in a bin is less than or equal to a threshold value, and the size of the bin is as small as possible. The process is particularly useful for generating surface brightness and colour maps, with clearly defined error maps, from images with a large dynamic range of counts, for example X-ray images of galaxy clusters. We demonstrate the method in application to data from Chandra ACIS-S and ACIS-I observations of the Perseus cluster of galaxies. We use the algorithm to create intensity maps, and colour images that show the relative X-ray intensities in different bands. The colour maps can later be converted, through spectral models, into maps of physical parameters, such as temperature, column density, etc. The adaptive binning algorithm is applicable to a wide range of data, from observations or numerical simulations, and is not limited to two-dimensional data.  相似文献   

Magnetic fields in the centre of the Perseus cluster     

G. B. Taylor  N. E. Gugliucci  A. C. Fabian  J. S. Sanders  G. Gentile  S. W. Allen 《Monthly notices of the Royal Astronomical Society》2006,368(4):1500-1506

We present Very Long Baseline Array (VLBA) observations of the nucleus of NGC 1275, the central, dominant galaxy in the Perseus cluster of galaxies. These are the first observations to resolve the linearly polarized emission from 3C 84, and from them we determine a Faraday rotation measure (RM) ranging from 6500 to 7500 rad m⁻² across the tip of the bright southern jet component. At 22 GHz some polarization is also detected from the central pc of 3C 84, indicating the presence of even more extreme RMs that depolarize the core at lower frequencies. The nature of the Faraday screen is most consistent with being produced by magnetic fields associated with the optical filaments of ionized gas in the Perseus cluster.  相似文献