819 research outputs found

    A filament of dark matter between two clusters of galaxies

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    It is a firm prediction of the concordance Cold Dark Matter (CDM) cosmological model that galaxy clusters live at the intersection of large-scale structure filaments. The thread-like structure of this "cosmic web" has been traced by galaxy redshift surveys for decades. More recently the Warm-Hot Intergalactic Medium (WHIM) residing in low redshift filaments has been observed in emission and absorption. However, a reliable direct detection of the underlying Dark Matter skeleton, which should contain more than half of all matter, remained elusive, as earlier candidates for such detections were either falsified or suffered from low signal-to-noise ratios and unphysical misalignements of dark and luminous matter. Here we report the detection of a dark matter filament connecting the two main components of the Abell 222/223 supercluster system from its weak gravitational lensing signal, both in a non-parametric mass reconstruction and in parametric model fits. This filament is coincident with an overdensity of galaxies and diffuse, soft X-ray emission and contributes mass comparable to that of an additional galaxy cluster to the total mass of the supercluster. Combined with X-ray observations, we place an upper limit of 0.09 on the hot gas fraction, the mass of X-ray emitting gas divided by the total mass, in the filament.Comment: Nature, in pres

    The galaxy-halo connection from a joint lensing, clustering and abundance analysis in the CFHTLenS/VIPERS field

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    We present new constraints on the relationship between galaxies and their host dark matter halos, measured from the location of the peak of the stellar-to-halo mass ratio (SHMR), up to the most massive galaxy clusters at redshift z∌0.8z\sim0.8 and over a volume of nearly 0.1~Gpc3^3. We use a unique combination of deep observations in the CFHTLenS/VIPERS field from the near-UV to the near-IR, supplemented by ∌60 000\sim60\,000 secure spectroscopic redshifts, analysing galaxy clustering, galaxy-galaxy lensing and the stellar mass function. We interpret our measurements within the halo occupation distribution (HOD) framework, separating the contributions from central and satellite galaxies. We find that the SHMR for the central galaxies peaks at Mh,peak=1.9−0.1+0.2×1012M⊙M_{\rm h, peak} = 1.9^{+0.2}_{-0.1}\times10^{12} M_{\odot} with an amplitude of 0.0250.025, which decreases to ∌0.001\sim0.001 for massive halos (Mh>1014M⊙M_{\rm h} > 10^{14} M_{\odot}). Compared to central galaxies only, the total SHMR (including satellites) is boosted by a factor 10 in the high-mass regime (cluster-size halos), a result consistent with cluster analyses from the literature based on fully independent methods. After properly accounting for differences in modelling, we have compared our results with a large number of results from the literature up to z=1z=1: we find good general agreement, independently of the method used, within the typical stellar-mass systematic errors at low to intermediate mass (M⋆<1011M⊙{M}_{\star} < 10^{11} M_{\odot}) and the statistical errors above. We have also compared our SHMR results to semi-analytic simulations and found that the SHMR is tilted compared to our measurements in such a way that they over- (under-) predict star formation efficiency in central (satellite) galaxies.Comment: 31 pages, 18 figures, 4 table. Accepted for publication in MNRAS. Online material available at http://www.cfhtlens.or

    Delta--Excitation and Exchange Corrections for NN--Bremsstrahlung

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    The role of the relativistic amplitudes for a number of O(k){\cal O}(k) processes usually neglected in potential model calculations of NN--bremsstrahlung is investigated. In particular, we consider the Δ\Delta--excitation pole contributions related to the one--pion and one--rho exchange and in addition include the exchange contributions induced by the radiative ω,â€‰Ïâ†’Ï€Îł\omega,\,\rho \to \pi \gamma decays. The contributions are calculated from relativistic Born amplitudes fitted to Δ\Delta--production and absorption data in the energy range up to 1 GeV and then used to supplement potential model and soft photon calculations for nucleon--nucleon bremsstrahlung. The effects on NNÎłNN\gamma--observables, although moderate in general, are found to be important in some kinematic domains.Comment: 15 pages in LaTex, using Revtex, 6 figures as uufile'd, compressed Postscript file included, TRIUMF preprint TRI-PP-94-9

    Measuring the dark side (with weak lensing)

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    We introduce a convenient parametrization of dark energy models that is general enough to include several modified gravity models and generalized forms of dark energy. In particular we take into account the linear perturbation growth factor, the anisotropic stress and the modified Poisson equation. We discuss the sensitivity of large scale weak lensing surveys like the proposed DUNE satellite to these parameters. We find that a large-scale weak-lensing tomographic survey is able to easily distinguish the Dvali-Gabadadze-Porrati model from LCDM and to determine the perturbation growth index to an absolute error of 0.02-0.03.Comment: 19 pages, 11 figure

    Further Evidence for the Decay K+ to pi+ neutrino-antineutrino

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    Additional evidence for the rare kaon decay K+ to pi+ neutrino-antineutrino has been found in a new data set with comparable sensitivity to the previously reported result. One new event was observed in the pion momentum region examined, 211<P<229 MeV/c, bringing the total for the combined data set to two. Including all data taken, the backgrounds were estimated to contribute 0.15 pm 0.05 events. The branching ratio is B=1.57^{+1.75}_{-0.82} 10^{-10}.Comment: 10 pages, 2 figure

    Chronos: A NIR spectroscopic galaxy survey to probe the most fundamental stages of galaxy evolution

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    We propose a dedicated, ultra-deep spectroscopic survey in the near infrared (NIR), that will target a mass-limited sample of galaxies during two of the most fundamental epochs of cosmic evolution: the formation of the first galaxies (at z ≳ 6; comsmic dawn), and at the peak of galaxy formation activity (at redshift z∌1–3; cosmic noon). By way of NIR observations (λ= 0.8–2ÎŒ m), it is possible to study the UV Lyman-α region in the former, and the optical rest-frame in the latter, allowing us to extract fundamental observables such as gas and stellar kinematics, chemical abundances, and ages, providing a unique legacy database covering these two crucial stages of cosmic evolution. The need to work in the NIR at extremely low flux levels makes a ground-based approach unfeasible due to atmospheric emission and absorption. Only with the largest facilities of the future (e.g. ELT) will be possible to observe a reduced set of targets, comprising at most of order thousands of galaxies. Likewise, from space, the small field of view of JWST and its use as a general purpose facility will yield a rather small set of high quality NIR spectra of distant galaxies (in the thousands, at best). Our project (codename Chronos) aims to produce ∌1 million high quality spectra, with a high S/N in the continuum, where information about the underlying stellar populations is encoded. The main science drivers are: - The connection between the star formation history and the mass assembly history. - The role of AGN and supernova feedback in shaping the formation histories of galaxies, with a quantitative estimate of quenching timescales. - The formation of the first galaxies. - The source of reionization. - Evolution of the metallicity-mass relation, including [α/Fe] and individual abundances. - Precision cosmology through detailed studies of the “baryon physics” of galaxy formation, probing the power spectrum over scales k∌1 Mpc− 1

    Experimental study of the atmospheric neutrino backgrounds for proton decay to positron and neutral pion searches in water Cherenkov detectors

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    The atmospheric neutrino background for proton decay to positron and neutral pion in ring imaging water Cherenkov detectors is studied with an artificial accelerator neutrino beam for the first time. In total, about 314,000 neutrino events corresponding to about 10 megaton-years of atmospheric neutrino interactions were collected by a 1,000 ton water Cherenkov detector (KT). The KT charged-current single neutral pion production data are well reproduced by simulation programs of neutrino and secondary hadronic interactions used in the Super-Kamiokande (SK) proton decay search. The obtained proton to positron and neutral pion background rate by the KT data for SK from the atmospheric neutrinos whose energies are below 3 GeV is about two per megaton-year. This result is also relevant to possible future, megaton-scale water Cherenkov detectors.Comment: 13 pages, 16 figure
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