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TOPIC: Dark matter
Blobrana


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RE: Dark matter
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Title: Dark matter in the Milky Way, II. the HI gas distribution as a tracer of the gravitational potential
Authors: P.M.W. Kalberla, L. Dedes, J. Kerp, U. Haud

Context. Gas within a galaxy is forced to establish pressure balance against gravitational forces. The shape of an unperturbed gaseous disk can be used to constrain dark matter models.
Aims. We derive the 3-D HI volume density distribution for the Milky Way out to a galactocentric radius of 40 kpc and a height of 20 kpc to constrain the Galactic mass distribution.
Methods. We used the Leiden/Argentine/Bonn all sky 21-cm line survey. The transformation from brightness temperatures to densities depends on the rotation curve. We explored several models, reflecting different dark matter distributions. Each of these models was set up to solve the combined Poisson-Boltzmann equation in a self-consistent way and optimised to reproduce the observed flaring.
Results. Besides a massive extended halo of M ~ 1.8 10^{12} Msun, we find a self-gravitating dark matter disk with M=2 to 3 10^{11} Msun, including a dark matter ring at 13 < R < 18.5 kpc with M = 2.2 to 2.8 10^{10} Msun. The existence of the ring was previously postulated from EGRET data and coincides with a giant stellar structure that surrounds the Galaxy. The resulting Milky Way rotation curve is flat up to R~27 kpc and slowly decreases outwards. The \hi gas layer is strongly flaring. The HWHM scale height is 60 pc at R = 4 kpc and increases to ~2700 pc at R=40 kpc. Spiral arms cause a noticeable imprint on the gravitational field, at least out to R = 30 kpc.
Conclusions. Our mass model supports previous proposals that the giant stellar ring structure is due to a merging dwarf galaxy. The fact that the majority of the dark matter in the Milky Way for R \la 40 kpc can be successfully modelled by a self-gravitating isothermal disk raises the question of whether this massive disk may have been caused by similar merger events in the past.

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Blobrana


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Posts: 131433
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ZEPLIN-II dark matter experiment
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The ZEPLIN-II (ZonEd Proportional scintillation in LIquid Noble gases) dark matter experiment is a 30 kg two-phase xenon detector currently being commissioned at the Boulby Underground Laboratory, in the Boulby Mine, near Whitby, Yorkshire, by the UKDMC.
ZEPLIN-II consists of an approximate cylinder of liquid xenon of diameter 30 cm and height 13 cm. A 2 cm high gas phase is maintained above the liquid xenon level. An electric field strength of 1.8 kV/cm is applied across the liquid phase, and a higher electric field of 2.0 kV/cm is applied across the liquid-gas boundary.

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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. Ara´ujo, 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. L¨uscher, 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
(Version V2)

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 60Co -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 kg×days. 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 28.6±4.3 -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 6.6×107 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 2 × 107 pb for similar exposures as the first run.

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Blobrana


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Weakly Interacting Massive Particles
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The tiny, wimpy particles that might make up the Universe's dark matter must be even wimpier than some theories suggest.
The first results from an experimental technology designed to detect Weakly Interacting Massive Particles (WIMPs) predicted candidates for dark matter help to pin down the strength of their interactions with ordinary matter in the Universe. The work rules out the stronger estimates predicted by some models, making the WIMPS look particularly weak, and narrowing down the places that researchers still have to look for them.
The XENON collaboration announced their results at the American Physical Society's April meeting in Jacksonville, Florida, on 14 April. Their work pushes the maximum possible strength of WIMP interactions down by a factor of six from the previous record, set by the Cold Dark Matter Search (CDMS-II) experiment at the University of California, Berkeley, in 2005. The new limit is stringent enough to test some particle-physics theories of supersymmetry.

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Blobrana


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Cosmic rays
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Our planet is constantly bombarded with cosmic rays. Most collide with atoms in our atmosphere, producing sprays of particles that fall to the ground, thousands striking each square meter every second. Cosmic rays with extremely high energies are infrequent, but their interactions in the atmosphere open a tiny but important window to ultra-high-energy physics a window that even the most advanced particle accelerators are incapable of achieving. Recently, three physicists took a peek in.
The interaction of a cosmic ray from outer space (typically a proton) with the atmosphere may result in the production of an exotic massive particle. This particle would be inside a shower of thousands of other particles. However, if it is long-lived, it will survive the shower without decaying and may be detected in a neutrino telescope or other detector. Here, the researchers focused on two theoretical exotic massive particles they believe are likely produced during these interactions: the gluino, a heavy twin of the gluon, and the weakly interacting massive particle (WIMP), which scientists believe could be a candidate for dark matter.

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Blobrana


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Posts: 131433
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RE: Dark matter
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Title: White Noise from Dark Matter: 21 cm Observations of Early Baryon Collapse
Authors: Kathryn M. Zurek, Craig J. Hogan

 In concordance cosmology, dark matter density perturbations generated by inflation lead to nonlinear, virialised minihalos, into which baryons collapse at redshift z ~ 20. We survey here novel baryon evolution produced by a modification of the power spectrum from white noise density perturbations at scales below k ~ 10 h Mpc^-1 (the smallest scales currently measured with the Lyman-\alpha forest). Exotic dark matter dynamics, such as would arise from scalar dark matter with a late phase transition (similar to an axion, but with lower mass), create such an amplification of small scale power. The dark matter produced in such a phase transition collapses into minihalos, with a size given by the dark matter mass within the horizon at the phase transition. If the mass of the initial minihalos is larger than \sim 10^-3 solar masses, the modified power spectrum is found to cause widespread baryon collapse earlier than standard LambdaCDM, leading to earlier gas heating. It also results in higher spin temperature of the baryons in the 21 cm line relative to LambdaCDM at redshifts z > 20 if the mass of the minihalo is larger than 1 solar masses. It is estimated that experiments probing 21 cm radiation at high redshift will contribute a significant constraint on dark matter models of this type for initial minihalos larger than \sim 10 solar masses. Early experiments reaching to z\approx 15 will constrain minihalos down to ~ 10^3 solar masses.

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Blobrana


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Title: Do Mergers Spin up Dark Matter Halos?
Authors: Elena D'Onghia (1), Julio F. Navarro (2) ((1) University of Zurich, (2) University of Victoria)

We use a large cosmological N-body simulation to study the origin of possible correlations between the merging history and spin of cold dark matter halos. In particular, we examine claims that remnants of major mergers tend to have higher-than-average spins, and find that the effect is driven largely by unrelaxed systems: equilibrium dark matter halos show no significant correlation between spin and merger history. Out-of-equilibrium halos have, on average, higher spins than relaxed systems, suggesting that the virialisation process leads to a net decrease in the value of the spin parameter. We find that this decrease is driven by the internal redistribution of mass and angular momentum that occurs during virialisation, a process that is especially efficient during major mergers, when high angular momentum material is pushed beyond the virial radius of the remnant. Since such redistribution likely affects the angular momentum of baryons and dark matter unevenly, our findings question the common practice of identifying the specific angular momentum content of a halo with that of its embedded luminous component. Further work is needed to elucidate the true relation between the angular momentum content of baryons and dark matter in galaxy systems assembled hierarchically.

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Blobrana


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Clumps of dark matter roving unseen through our galaxy could be revealed by careful observations of pulsars, a new study says. In fact, telltale signs of these clouds might already lurk unnoticed in archival data – potentially holding the key to understanding what the mysterious matter is made of.
Astronomers have abundant evidence that some sort of invisible matter permeates the universe. It reveals its presence only by the gravitational tug it exerts on ordinary matter, which it outweighs 6 to 1. What this dark matter is made of is one of the most hotly pursued questions in astronomy and physics.
The prevailing view is that dark matter consists of some sort of exotic subatomic particle, and various theories have come up with a zoo of candidates with names like neutralinos, axions, and gravitinos. Neutralinos, for example, are a product of supersymmetry, a theory that attempts to unify all the known forces of physics excluding gravity and posits that familiar particles like electrons and neutrinos have heavier counterparts.
But astronomical evidence has so far failed to allow scientists to distinguish between the possibilities, and laboratory experiments designed to detect individual dark matter particles have likewise come up empty.
Now, scientists led by Ethan Siegel of the University of Wisconsin, in Madison, US, have come up with a new way to potentially reveal blobs of dark matter drifting nearby and perhaps even pin down what it is once and for all.

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Blobrana


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Our Milky Way galaxy is heavier than it looks, and it's not too much ice cream, or cookies, that is responsible for the extra weight -- it's "dark matter."
Dark matter is one of the greatest mysteries in modern astronomy. Scientists use the term as an umbrella definition for all the invisible "heavy stuff" in the universe. Astronomers currently believe that there are two components to dark matter. One part of dark matter is made up of exotic materials, different from the ordinary particles that make up the familiar world around us. The other part consists of dark celestial bodies -- like planets, black holes, or failed stars -- which do not produce light or are too faint to detect from Earth.
Astronomers suspect that about most of our galaxy's and universe' weight comes from dark matter. For almost a century, they scoured our Milky Way galaxy for both exotic dark matter and dark celestial bodies in hopes of accounting for the missing weight.
Now, research is showing that NASA's Spitzer Space Telescope may be able to play an important role in identifying the "invisible" celestial bodies that are weighing our galaxy down.

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Blobrana


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Title: Dark Matter annihilation in Draco: new considerations of the expected gamma flux
Authors: M. A. Sanchez-Conde (1), F. Prada (1), E. L. Lokas (2), M. E. Gomez (3), R. Wojtak (2), M. Moles (1) ((1) Instituto de Astrofisica de Andalucia - CSIC, (2) Nicolaus Copernicus Astronomical Centre, (3) Departamento de Fisica Aplicada, Facultad de Ciencias Experimentales, Universidad de Huelva)
(revised v2)

A new revision of the gamma flux that we expect to detect in Imaging Atmospheric Cherenkov Telescopes (IACTs) from SUSY dark matter annihilation in the Draco dSph is presented using the dark matter density profiles compatible with the latest observations. This revision takes also into account the important effect of the Point Spread Function (PSF) of the telescope. We show that this effect is crucial in the way we will observe and interpret a possible signal detection. In particular, it could be impossible to discriminate between a cuspy and a cored dark matter density profile due to the fact that both density profiles may yield very similar flux profile observed by the telescope. Finally, we discuss the prospects to detect a possible gamma signal from Draco for current or planned experiments, i.e. MAGIC, GLAST and GAW.

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Blobrana


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The mystery of how the darkest galaxies in the Universe came to exist may have been solved by scientists.
Dwarf spheroidals are galaxies composed almost entirely of dark matter; faint examples have been discovered orbiting the Milky Way and Andromeda galaxies.
Scientists believe these dark systems were once gas-rich, but as they became satellites of larger galaxies, most of their visible matter was stripped away.

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