928 research outputs found

    Dark matter and Modified Newtonian Dynamics in a sample of high-redshift galaxy clusters observed with Chandra

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    We compare the measurement of the gravitational mass of 38 high-redshift galaxy clusters observed by Chandra using Modified Newtonian Dynamics (MOND) and standard Newtonian gravity. Our analysis confirms earlier findings that MOND cannot explain the difference between the baryonic mass and the total mass inferred from the assumption of hydrostatic equilibrium. We also find that the baryon fraction at r2500r_{2500} using MOND is consistent with the Wilkinson Microwave Anisotropy Probe (WMAP) value of ΩB/ΩM\Omega_{B}/\Omega_{M}Comment: New Astronomy in pres

    Soft X-ray excess of clusters: a thermal filament model, and the strong lensing of background galaxy groups

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    The observational and theoretical status of the search for missing cosmological baryons is summarized, with a discussion of some indirect methods of detection. The thermal interpretation of the cluster soft X-ray and EUV excess phenomenon is examined in the context of emission filaments, which are the higher density part of the warm hot intergalactic medium (WHIM) residing at the outskirt of clusters. We derived an analytic radial profile of the soft excess surface brightness using a simple filament model, which provided us a means of observationally constraining the WHIM parameters, especially the total mass budget of warm gas associated with a cluster. We then pointed out a new scenario for soft excess emission, viz. a cluster that can strongly lens the soft X-rays from background WHIM knots. If, as seems quite likely, the missing baryons are mostly in the WHIM halos of galaxy groups, the lensing probability will be quite high (\sim 10 %). This way of accounting for at least part of a cluster's soft excess may also explain the absence of O VII absorption at the redshift of the cluster.Comment: ApJ in press, 7 pages, emulateapj styl

    The Effect of Helium Sedimentation on Galaxy Cluster Masses and Scaling Relations

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    Recent theoretical studies predict that the inner regions of galaxy clusters may have an enhanced helium abundance due to sedimentation over the cluster lifetime. If sedimentation is not suppressed (e.g., by tangled magnetic fields), this may significantly affect the cluster mass estimates. We use Chandra X-ray observations of eight relaxed galaxy clusters to investigate the upper limits to the effect of helium sedimentation on the measurement of cluster masses and the best-fit slopes of the Y_X - M_500 and Y_X - M_2500 scaling relations. We calculated gas mass and total mass in two limiting cases: a uniform, un-enhanced abundance distribution and a radial distribution from numerical simulations of helium sedimentation on a timescale of 11 Gyrs. The assumed helium sedimentation model, on average, produces a negligible increase in the gas mass inferred within large radii (r < r500) (1.3 +/- 1.2 per cent) and a (10.2 +/- 5.5) per cent mean decrease in the total mass inferred within r < r500. Significantly stronger effects in the gas mass (10.5 +/- 0.8 per cent) and total mass (25.1 +/- 1.1 per cent) are seen at small radii owing to a larger variance in helium abundance in the inner region, r < 0.1 r500. We find that the slope of the Y_X -M_500 scaling relation is not significantly affected by helium sedimentation.Comment: 11 pages, accepted for publication in Astronomy and Astrophysic

    FUSE Observations of Galactic and Intrinsic Absorption in the Spectrum of the Seyfert 1 Galaxy 2MASX J21362313-6224008

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    We present the far-ultraviolet spectrum of the Seyfert 1 galaxy 2MASX J21362313-6224008 obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE). The spectrum features absorption from Galactic OVI at two velocities and redshifted HI Lyman beta and gamma, CII, CIII, and O VI. The redshifted absorption features represent a single kinematic component blueshifted by ~310 km/s relative to the AGN. We use photoionization models to derive constraints on the physical parameters of the absorbing gas. An alternative interpretation for the absorption lines is also proposed, whereby the absorbing gas is associated with an intervening galaxy cluster.Comment: Astrophysical Journal, in pres

    Dark Matter search with Fermi

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    The Fermi mission is a gamma-ray telescope operating since 2008. One of its goals is the indirect search of Dark Matter (DM) in the Universe. Emission yielded from annihilating DM particles is supposed to produce a characteristic gamma-ray spectrum that could be observed in several astrophysical sites. Results and status updates for different DM searches with Fermi data will be presented