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TOPIC: Dark matter


L

Posts: 131433
Date:
Missing satellite problem
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Small galaxies that are exceptionally rich in dark matter may have had most of their visible matter stripped away by larger neighbours, suggest the most detailed computer simulations ever made of the galaxies. The results could help explain a long-standing cosmological mystery called the "missing satellite problem".

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L

Posts: 131433
Date:
Dark Matter and Sterile Neutrinos
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Title: Dark Matter and Sterile Neutrinos
Authors: Peter L. Biermann (1,2,3), Faustin Munyaneza (1) ((1) Max Planck Institute for Radioastronomy, Germany, (2) Department of Physics and Astronomy, University of Bonn, Germany, (3) Department of Physics and Astronomy, University of Alabama, Tuscaloosa, USA)

Dark matter has been recognized as an essential part of matter for over 70 years now, and many suggestions have been made, what it could be. Most of these ideas have centred on Cold Dark Matter, particles that are predicted in extensions of standard particle physics, such as supersymmetry. Here we explore the concept that dark matter is sterile neutrinos, particles that are commonly referred to as Warm Dark Matter. Such particles have keV masses, and decay over a very long time, much longer than the Hubble time. In their decay they produce X-ray photons which modify the ionisation balance in the very early universe, increasing the fraction of molecular Hydrogen, and thus help early star formation. Sterile neutrinos may also help to understand the baryon-asymmetry, the pulsar kicks, the early growth of black holes, the minimum mass of dwarf spheroidal galaxies, as well as the shape and smoothness of dark matter halos. As soon as all these tests have been made quantitative in their various parameters, we may focus on the creation mechanism of these particles, and could predict the strength of the sharp X-ray emission line, expected from any large dark matter assembly. A measurement of this X-ray emission line would be definitive proof for the existence of may be called weakly interacting neutrinos, or WINs.

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L

Posts: 131433
Date:
The Bullet Cluster
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Title: The Bullet Cluster 1E0657-558 evidence shows Modified Gravity in the absence of Dark Matter
Authors: J. R. Brownstein, J. W. Moffat

A detailed analysis of the November 15, 2006 data release (Clowe et al., 2006) X-ray surface density Sigma-map and the strong and weak gravitational lensing convergence kappa-map for the Bullet Cluster 1E0657-558 is performed and the results are compared with the predictions of a modified gravity (MOG) and dark matter. Our surface density Sigma-model is computed using a King beta-model density, and a mass profile of the main cluster and an isothermal temperature profile are determined by the MOG. We find that the main cluster thermal profile is nearly isothermal. The MOG prediction of the isothermal temperature of the main cluster is T = 15.5 3.9 keV, in good agreement with the experimental value T = 14.8{+2.0}{-1.7} keV. Excellent fits to the two-dimensional convergence kappa-map data are obtained without non-baryonic dark matter, accounting for the 8-sigma spatial offset between the Sigma-map and the kappa-map reported in Clowe et al. (2006). The MOG prediction for the kappa-map results in two baryonic components distributed across the Bullet Cluster 1E0657-558 with averaged mass-fraction of 83% intracluster medium (ICM) gas and 17% galaxies. Conversely, the Newtonian dark matter kappa-model has on average 76% dark matter (neglecting the indeterminant contribution due to the galaxies) and 24% ICM gas for a baryon to dark matter mass-fraction of 0.32, a statistically significant result when compared to the predicted Lambda-CDM cosmological baryon mass-fraction of 0.176{+0.019}{-0.012} (Spergel et al., 2006).

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L

Posts: 131433
Date:
Cosmic Evolution Survey
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(old news)
After decades of attempts, astronomers have unveiled the dark side of the Universe.
Dark matter, the ubiquitous yet ethereal stuff filling the cosmos, has been mapped three-dimensionally for the first time by a team of astronomers using a fleet of orbiting and ground-based telescopes. Implementing techniques not even dreamed of when dark matter was first postulated, they have created a map two degrees on a side (roughly 15 times the area of the full Moon) and 6 billion light years deep. The Cosmic Evolution Survey, or COSMOS, reveals the spatial distribution of dark matter stretching back to a time when the Universe was half its present age.

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L

Posts: 131433
Date:
RE: Dark matter
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Title: Universe's Skeleton Sketched
Authors: Eric V. Linder

The deepest and clearest maps yet of the Universe's skeleton of dark matter structure present a picture broadly in concord with favoured models - although puzzling discrepancies remain.

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L

Posts: 131433
Date:
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Title: Kinematics and dynamics of the M51-type galaxy pair NGC 3893/96 (KPG 302)
Authors: Isaura Fuentes-Carrera (IAG-USP, Brazil), Margarita Rosado (IA-UNAM, Mexico), Philippe Amram (LAM, France), Heikki Salo, Eija Laurikainen (University of Oulu, Finland)

We study the kinematics and dynamics of the M51-type interacting galaxy pair KPG 302 (NGC 3893/96). We analyse the distribution of the dark matter (DM) halo of the main galaxy in order to explore possible differences between DM halos of "isolated" galaxies and those of galaxies belonging to a pair. The velocity field of each galaxy was obtained using scanning Fabry-Perot interferometry. A two-dimensional kinematic and dynamical analysis of each galaxy and the pair as a whole is done emphasizing the contribution of circular and non-circular velocities. Non-circular motions can be traced on the rotation curves of each galaxy allowing us to differentiate between motions associated to particular features and motions that reflect the global mass distribution of the galaxy. For the main galaxy of the pair, NGC 3893, optical kinematic information is complemented with HI observations from the literature to build a multi-wavelength rotation curve. We try to fit this curve with a mass-distribution model using different DM halos. We find that the multi-wavelength rotation curve of NGC 3893, "cleaned" from the effect of non-circular motions, cannot be fitted neither by a pseudo-isothermal nor by a NFW DM halo.

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L

Posts: 131433
Date:
ZEPLIN-I
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Title: Reply to: Critical revision of the ZEPLIN-I sensitivity to WIMP interactions
Authors: N. J. T. Smith, A. S. Murphy, T. J. Sumner

Recent objections (Phys.Lett. B 637, 156) to the published Zeplin I limit (Astropart. Phys 23, 444) are shown to arise from misunderstandings of the calibration data and procedure, and a misreading of the data in one of the referenced papers.

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L

Posts: 131433
Date:
ZEPLIN-II
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Title: First limits on WIMP nuclear recoil signals in ZEPLIN-II: a two phase xenon detector for dark matter detection
Authors: G. J. Alner, H. M. Araujo, A. Bewick, C. Bungau, B. Camanzi, M. J. Carson, R. J. Cashmore, H. Chagani, V. Chepel, D. Cline, D. Davidge, J. C. Davies, E. Daw, J. Dawson, T. Durkin, B. Edwards, T. Gamble, J. Gao, C. Ghag, A. S. Howard, W. G. Jones, M. Joshi, E. V. Korolkova, V. A. Kudryavtsev, T. Lawson, V. N. Lebedenko, J. D. Lewin, P. Lightfoot, A. Lindote, I. Liubarsky, M. I. Lopes, R. Luscher, P. Majewski, K Mavrokoridis, J. E. McMillan, B. Morgan, D. Muna, A. St.J. Murphy, F. Neves, G. G. Nicklin, W. Ooi, S. M. Paling, J. Pinto da Cunha, S. J. S. Plank, R. M. Preece, J. J. Quenby, M. Robinson, F. Sergiampietri, C. Silva, V. N. Solovov, N. J. T. Smith, P. F. Smith, N. J. C. Spooner, T. J. Sumner, C. Thorne, D. R. Tovey, E. Tziaferi, R. J. Walker, H. Wang, J. White, F. L. H. Wolfs

Results are presented from the first underground data run of ZEPLIN-II, a 31 kg two phase xenon detector developed to observe nuclear recoils from hypothetical weakly interacting massive dark matter particles. Discrimination between nuclear recoils and background electron recoils is afforded by recording both the scintillation and ionisation signals generated within the liquid xenon, with the ratio of these signals being different for the two classes of event. This ratio is calibrated for different incident species using an AmBe neutron source and Co-60 gamma-ray sources. From our first 31 live days of running ZEPLIN-II, the total exposure following the application of fiducial and stability cuts was 225 kgxdays. A background population of radon progeny events was observed in this run, arising from radon emission in the gas purification getters, due to radon daughter ion decays on the surfaces of the walls of the chamber. An acceptance window, defined by the neutron calibration data, of 50% nuclear recoil acceptance between 5 keVee and 20 keVee, had an observed count of 29 events, with a summed expectation of 28.64.3 gamma-ray and radon progeny induced background events. These figures provide a 90% c.l. upper limit to the number of nuclear recoils of 10.4 events in this acceptance window, which converts to a WIMP-nucleon spin-independent cross-section with a minimum of 6.6x10^-7 pb following the inclusion of an energy dependent, calibrated, efficiency. A second run is currently underway in which the radon progeny will be eliminated, thereby removing the background population, with a projected sensitivity of 2x10^-7 pb for similar exposures as the first run.

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L

Posts: 131433
Date:
RE: Dark matter
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Title: Axion beams at HERA?
Authors: K. Piotrzkowski

If the recently observed anomaly in the PVLAS experiment is due to the axion, then the powerful beams of synchrotron photons, propagating through high magnetic field of the HERA beamline, become strong axion sources. This gives a unique opportunity of detection of the axion-photon interactions by installing a small detector in the HERA tunnel, and to corroborate the axion hypothesis within a few days of running.

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L

Posts: 131433
Date:
The HERA particle accelerator
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The HERA particle accelerator in Germany is set to call it quits in June, but a lone physicist is now campaigning for HERA to have one last hurrah. He claims it could discover a particle believed by many to account for the unseen dark matter that constitutes the bulk of the universe's mass.
The particle in question is the axion. Proposed in 1977 to solve a problem with the strong nuclear force, these hypothetical entities have been considered as strong candidates for dark matter, because they have very little mass and barely interact with matter.
One of the most tantalising hints of their existence came from the PVLAS experiment at the National Laboratories of Legnaro in Italy. Last July, the PVLAS team announced a slight shift in the polarisation of a laser beam fired through a strong magnetic field. The shift was 10,000 times larger than expected by standard physics, but could be explained if a tiny fraction of photons from the laser had turned into axions.

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