Munch: Monday, October 23, 2006

                               


 

    usual time and place:  12:30, 6th floor conference room

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


FAQ

What is Munch?
       Munch Archive
---------

16 Oct 2006

First hints of large scale structures in the ultra-high energy sky?

Authors: A. Cuoco, G. Miele, P.D. Serpico
Comments: 4 pages, 2 eps figures
Report-no: DSF-33/2006, FERMILAB-PUB-06-372-A
The result of the recent publication [M. Kachelriess and D. V. Semikoz Astropart. Phys. 26, 10 (2006)] of a broad maximum around 25 degrees in the two-point autocorrelation function of ultra-high energy cosmic ray arrival directions has been intriguingly interpreted as the first imprint of the large scale structures (LSS) of baryonic matter in the near universe. We analyze this suggestion in light of the clustering properties expected from the PSCz astronomical catalogue of LSS. The chance probability of the signal is consistent within 2 sigma with our expectations. No evidence for a significant cross-correlation of the observed events with known overdensities in the LSS is found, which may be due to the role of the galactic and extragalactic magnetic fields, and is however consistent with the limited statistics. The larger statistics to be collected by the Pierre Auger Observatory is needed to answer definitely the question.

Full-text: PostScript, PDF, or Other formats

Collisionally Regenerated Dark Matter Structures in Galactic Nuclei

Authors: David Merritt, Stefan Harfst, Gianfranco Bertone
Comments: 11 pages, uses revtex4
We show that the presence of a rho~1/r^{3/2} dark matter overdensity can be robustly predicted at the center of any galaxy old enough to have grown a power-law density cusp in the stars via the Bahcall-Wolf mechanism. Using both Fokker-Planck and direct N-body integrations, we demonstrate collisional generation of these dark matter "crests" (Collisionally REgenerated STtructures) even in the extreme case that the density of both stars and dark matter were previously lowered by slingshot ejection from a binary supermassive black hole. The time scale for collisional growth of the crest is approximately the two-body relaxation time as defined by the stars, which is < 10 Gyr at the centers of stellar spheroids with luminosities comparable to that of the Milky Way bulge or less. The presence of crests can robustly be predicted in such galaxies, unlike the steeper enhancements, called "spikes," produced by the adiabatic growth of black holes. We discuss special cases where the prospects for detecting dark matter annihilations from the centers of galaxy haloes are significantly affected by the formation of crests.

Full-text: PostScript, PDF, or Other formats

A New Force in the Dark Sector?

Authors: Glennys R. Farrar, Rachel A. Rosen
Comments: 4 pp
We study the kinematics of dark matter using the massive cluster of galaxies 1E0657-56. The velocity of the "bullet" subcluster has been measured by X-ray emission from the shock front, and the masses and separation of the main and sub-clusters have been measured by gravitational lensing. The velocity with gravity alone is calculated in a variety of models of the initial conditions, mass distribution and accretion history; it is much higher than expected, by at least 2.4 sigma. The probability of so large a subcluster velocity in cosmological simulations is <~ 10^{-7}. A long range force with strength ~ 0.4 - 0.8 times that of gravity would provide the needed additional acceleration.

Full-text: PostScript, PDF, or Other formats

The nonlinear matter power spectrum

Authors: Zhaoming Ma
Comments: 13 pages, 20 figures, apj submitted
We modify the public PM code developed by Anatoly Klypin and Jon Holtzman to simulate cosmology with arbitrary initial power spectrum and equation of state of dark energy. With this tool in hand, we perform the following studies on the matter power spectrum.
With an artificial sharp peak at k~0.2 h/Mpc in the initial power spectrum, we find that the position of the peak is not shifted by nonlinear evolution. We also find that the existence of a peak in the linear power spectrum would boost the nonlinear power at all scales evenly. This is contrary to what HKLM scaling relation predicts, but roughly consistent with that of halo model.
We construct two dark energy models with the same linear power spectra today but different linear growth histories. We demonstrate that their nonlinear power spectra differ at the level of the maximum deviation of the corresponding linear power spectra in the past. Similarly, two constructed dark energy models with the same growth histories result in consistent nonlinear power spectra. This is hinting, not a proof, that linear power spectrum together with linear growth history uniquely determine the nonlinear power spectrum. Based on these results, we propose that linear growth history be included in the next generation fitting formulas of nonlinear power spectrum. (abridged)

Full-text: PostScript, PDF, or Other formats

H.E.S.S. observations of the Galactic Center region and their possible dark matter interpretation

Authors: H.E.S.S. collaboration: F.A. Aharonian, et al
Comments: 5 pages, 2 figures, accepted in Physical Review Letters
The detection of gamma-rays from the source HESS J1745-290 in the Galactic Center (GC) region with the H.E.S.S. array of Cherenkov telescopes in 2004 is presented. After subtraction of the diffuse gamma-ray emission from the GC ridge, the source is compatible with a point-source with spatial extent less than 1.2'(stat.) (95% CL). The measured energy spectrum above 160 GeV is compatible with a power-law with photon index of 2.25 +/- 0.04(stat.) +/- 0.10 (syst.) and no significant flux variation is detected. These measurements are discussed in the framework of dark matter annihilation. It is found that the bulk of the VHE emission must have non-dark-matter origin. Loose constraints on the velocity-weighted annihilation cross section <sigma.v> are derived assuming the presence of an astrophysical non-dark-matter gamma-ray contribution.

Full-text: PostScript, PDF, or Other formats

A Method to Constrain the Total Mass of Galaxy Groups

Authors: G.C. Baiesi Pillastrini
Comments: 12 pages, 2 figures, to appear in New Astronomy, November 2006, Vol. 12, 2 issues
We present a simple method to constrain the total mass of groups of galaxies. Tidal theory predicts that a limit to the mass of bound groups of galaxies can be obtained by using the fact that the tidal forces due to the external mass distributions are insufficient to disrupt the groups. To illustrate how the method works, we find tidal limits on the mass of eleven nearby galaxy groups. In most cases, tidal limits placed on these groups show that mass estimations obtained from methods based either on the application of the virial theorem or moments of the projected mass are underestimated by a factor of about 2 even if, in many cases, errors are large. Three groups show virial parameters fully concordant with the tidal constraints while two outlier groups show anomalous results. The irregular state of the latter suggests that the reliability of the method depends on the physical properties of the test groups, which should match the fundamental assumptions of spherical symmetry and dynamical equilibrium state.

Full-text: PDF only

DUNE: The Dark Universe Explorer

Authors: A. Refregier, O. Boulade, Y. Mellier, B. Milliard, R. Pain, J. Michaud, F. Safa, A. Amara, P. Astier, E. Barrelet, E. Bertin, S. Boulade, C. Cara, A. Claret, L. Georges, R. Grange, J. Guy, C. Koeck, L. Kroely, C. Magneville, N. Palanque-Delabrouille, N. Regnault, G. Smadja, C. Schimd, Z. Sun
Comments: 12 latex pages, including 7 figures and 2 tables. Procs. of SPIE symposium "Astronomical Telescopes and Instrumentation", Orlando, may 2006
Understanding the nature of Dark Matter and Dark Energy is one of the most pressing issues in cosmology and fundamental physics. The purpose of the DUNE (Dark UNiverse Explorer) mission is to study these two cosmological components with high precision, using a space-based weak lensing survey as its primary science driver. Weak lensing provides a measure of the distribution of dark matter in the universe and of the impact of dark energy on the growth of structures. DUNE will also include a complementary supernovae survey to measure the expansion history of the universe, thus giving independent additional constraints on dark energy. The baseline concept consists of a 1.2m telescope with a 0.5 square degree optical CCD camera. It is designed to be fast with reduced risks and costs, and to take advantage of the synergy between ground-based and space observations. Stringent requirements for weak lensing systematics were shown to be achievable with the baseline concept. This will allow DUNE to place strong constraints on cosmological parameters, including the equation of state parameter of the dark energy and its evolution from redshift 0 to 1. DUNE is the subject of an ongoing study led by the French Space Agency (CNES), and is being proposed for ESA's Cosmic Vision programme.

Full-text: PostScript, PDF, or Other formats

The dark matter halos of massive, relaxed galaxy clusters observed with Chandra

Authors: R.W. Schmidt (1), S.W. Allen (2) ((1) ARI/Zentrum f. Astronomie Heidelberg, (2) KIPAC/Stanford)
Comments: 14 pages, 4 figures, 5 tables. Submitted to MNRAS
We use the Chandra X-ray Observatory to study the dark matter halos of 34 massive, dynamically relaxed galaxy clusters, spanning the redshift range 0.06<z<0.7. The observed dark matter and total mass (dark-plus-luminous matter) profiles can be approximated by the Navarro Frenk & White (hereafter NFW) model for cold dark matter (CDM) halos; for ~80 per cent of the clusters, the NFW model provides a statistically acceptable fit. In contrast, the singular isothermal sphere model can, in almost every case, be completely ruled out. We observe a well-defined mass-concentration relation for the clusters with a normalization and intrinsic scatter in good agreement with the predictions from simulations. The slope of the mass-concentration relation, c\propto M_vir^a/(1+z)^b with a=-0.41\pm0.11 at 95 per cent confidence, is steeper than the value a~-0.1 predicted by CDM simulations for lower mass halos. With the slope a included as a free fit parameter, the redshift evolution of the concentration parameter, b=0.54\pm0.47 at 95 per cent confidence, is also slower than, but marginally consistent with, the same simulations (b~1). Fixing a~-0.1 leads to an apparent evolution that is significantly slower, b=0.20\pm0.45, although the goodness of fit in this case is significantly worse. Using a generalized NFW model, we find the inner dark matter density slope, alpha, to be consistent with unity at 95 per cent confidence for the majority of clusters. Combining the results for all clusters for which the generalized NFW model provides a good description of the data, we measure alpha=0.88\pm0.29 at 95 per cent confidence, in agreement with CDM model predictions.

Full-text: PostScript, PDF, or Other formats

The supernova Hubble diagram for off-center observers in a spherically symmetric inhomogeneous universe

Authors: Havard Alnes, Morad Amarzguioui
Comments: 7 pages (REVTeX4), 5 figures
We have previously shown that spherically symmetric, inhomogeneous universe models can explain both the supernova data and the location of the first peak in the CMB spectrum without resorting to dark energy. In this work, we investigate whether it is possible to get an even better fit to the supernova data by allowing the observer to be positioned away from the origin in the spherically symmetric coordinate system. In such a scenario, the observer sees an anisotropic relation between redshifts and the luminosity distances of supernovae. The level of anisotropy allowed by the data will then constrain how far away from the origin the observer can be located, and possibly even allow for a better fit. Our analysis shows that the fit is indeed improved, but not by a significant amount. Furthermore, it shows that the supernova data do not place a rigorous constraint on how far off-center the observer can be located.

Full-text: PostScript, PDF, or Other formats

Can the Pioneer anomaly be of gravitational origin?

Authors: Lorenzo Iorio
Comments: Latex2e, 22 pages, 5 tables, 1 figure, 30 references. It is the merging of gr-qc/0608127, gr-qc/0608068, gr-qc/0608101
Subj-class: General Relativity and Quantum Cosmology; Space Physics
In order to satisfy the equivalence principle, any mechanism proposed to gravitationally explain the Pioneer anomaly, in the form in which it is presently known from the so-far analyzed Pioneer 10/11 data, cannot leave out of consideration its impact on the motion of the planets of the Solar System as well. In this paper we, first, use the latest observational determinations of the secular perihelion advances of some planets in order to put on the test two interesting models of modified gravity recently proposed to accommodate the Pioneer anomaly. Second, we use to radio-technical ranging data to Voyager 2 when it encountered Uranus and Neptune to perform a further, independent test of the hypothesis that a Pioneer-like acceleration can also affect the motion of the planets, at least in the regions in which the Pioneer anomaly manifested itself according to our present-day knowledge of it. As in the case of previous tests based on the use of the planetary right ascension and declination, the answer is negative.

Full-text: PostScript, PDF, or Other formats

Non-Gaussianities in Multi-field Inflation

Authors: Thorsten Battefeld, Richard Easther
Comments: 23 pages
Report-no: Brown-HET-1472
We compute the amplitude of the non-Gaussianities in inflationary models with multiple, uncoupled scalar fields. This calculation thus applies to all models of assisted inflation, including N-flation, where inflation is driven by multiple axion fields arising from shift symmetries in a flux stabilized string vacuum. The non-Gaussianities are associated with nonlinear evolution of the field (and density) perturbations, characterized by the parameter $f_{NL}$. We derive a general expression for the nonlinear parameter, incorporating the evolution of perturbations after horizon-crossing. This is valid for arbitrary separable potentials during slow roll. To develop an intuitive understanding of this system and to demonstrate the applicability of the formalism we examine several cases with quadratic potentials: two-field models with a wide range of mass ratios, and a general N-field model with a narrow mass spectrum. We uncover that $f_{NL}$ is suppressed as the number of e-foldings grows, and that this suppression is increased in models with a broad spectrum of masses. On the other hand, we find no enhancement to $f_{NL}$ that increases with the number of fields. We thus conclude that the production of a large non-Gaussian signal in multi-field models of inflation is thus very unlikely. Finally, we compute a spectrum for the scalar spectral index that incorporates the nonlinear corrections to the fields' evolution.

Full-text: PostScript, PDF, or Other formats