Munch

Galaxy clusters are the most massive gravitational structures in the universe, and also provide unique probes of its expansion. Their strong X-ray emission and unique cosmic microwave background scattering signature afford an independent method to obtain distances based on the physics of ionized plasmas. In this article we determine the distance to 39 clusters of galaxies in the redshift range 0.14<z<0.89 using Chandra X-ray data and radio observations from the OVRO and BIMA interferometric arrays. We analyze the plasma and dark matter distribution in clusters using a hydrostatic equilibrium model that accounts for radial variations in density, temperature and abundance, and quantify the statistical and systematic errors of this method. The analysis is performed via a Markov chain Monte Carlo technique that provides simultaneous estimation of all model parameters. We measure a Hubble constant of H_0= 77.1 +3.8-3.4 +10.0-8.0 Km/s/Mpc (statistical followed by systematic uncertainty at 68% confidence) for an Omega_M=0.3, Omega_Lambda=0.7 cosmology. We also analyze the data using an isothermal beta model that is free of the hydrostatic equilibrium assumption, and find H_0=73.4 +4.5-3.8 +9.5-7.6 Km/s/Mpc; to avoid effects from cool cores in clusters, we repeated this analysis excluding the central 100 kpc from the X-ray data, and find H_0= 77.2 +4.8-4.2 +10.1-8.2 km/s/Mpc (statistical followed by systematic uncertainty at 68% confidence). The consistency between the models illustrates the relative insensitivity of SZE/X-ray determinations of H_0 to the details of the cluster model. Our determination of the Hubble parameter in the distant universe agrees with the recent measurement from the Hubble Space Telescope key project that probes the nearby universe.

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The Impact of Galaxy Formation on the Sunyaev-Zeldovich Effect of Galaxy Clusters

Authors: Daisuke Nagai (U.Chicago, KICP)
Comments: submitted to ApJ, 12 pages, 7 figures

We study the effects of radiative cooling and galaxy formation on the Sunyaev-Zel'dovich (SZ) observable-mass relations using high-resolution cosmological simulations performed with the shock-capturing eulerian adaptive mesh refinement N-body+gasdynamics ART code. To assess the impact of galaxy formation, we compare two sets of simulations performed in the adiabatic regime and with radiative cooling and several physical processes critical to various aspects of galaxy formation: star formation, metal enrichment and stellar feedback. We show that the SZ signal integrated to sufficiently large fraction of the cluster volume correlates strongly with the enclosed cluster mass, regardless of the details of the cluster physics or dynamical state of the cluster. The slope and redshift evolution of the SZ flux-mass relation are also insensitive to the details of the cluster gas physics, and they are well characterized by the simple self-similar cluster model. While the tightness, slope and redshift evolution are relatively unaffected, the radiative cooling and galaxy formation significantly modify the normalization of the SZ scaling relations. The effect is due to the decrease in the hot gas fraction, which is offset slightly by the increase in the gas temperature. The baryon dissipation also causes the increase in the total cluster mass and modifies the normalization by a few percent. Finally, we show that the simulations that include gas cooling and star formation are in good agreement with the recent observational results on the SZ scaling relations, highlighting the importance of galaxy formation in theoretical modelling of clusters.

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Cosmology of Mass-Varying Neutrinos Driven by Quintessence: Theory and Observations

Authors: A. W. Brookfield, C. van de Bruck, D. F. Mota, D. Tocchini-Valentini
Comments: 14 pages, revtex, 15 figures

The effects of mass-varying neutrinos on cosmic microwave background (CMB) anisotropies and large scale structures (LSS) are studied. In these models, dark energy and neutrinos are coupled such that the neutrino masses are functions of the scalar field playing the role of dark energy. We begin by describing the cosmological background evolution of such a system. It is pointed out that, similar to models with a dark matter/dark energy interaction, the apparent equation of state measured with SNIa can be smaller than -1. We then discuss the effect of mass-varying neutrinos on the CMB anisotropies and the matter power spectrum. A suppression of power in the CMB power spectrum at large angular scales is usually observed. We give an explanation for this behaviour and discuss different couplings and quintessence potentials to show the generality of the results obtained. We perform a likelihood analysis using wide-ranging SNIa, CMB and LSS observations to assess whether such theories are viable. Treating the neutrino mass as a free parameter we find that the constraints on the coupling are weak, since CMB and LSS surveys give only upper bounds on the neutrino mass. However, fixing a priori the neutrino masses, we find that there is some evidence that the existence of such a coupling is actually preferred by current cosmological data over the standard LambdaCDM cosmology.

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Anisotropy of the cosmic gamma-ray background from dark matter annihilation

Authors: Shin'ichiro Ando, Eiichiro Komatsu
Comments: 15 pages, 12 figures

High-energy photons from dark matter annihilation contribute to the cosmic gamma-ray background (CGB). Since dark matter particles are weakly interacting, annihilation can happen only in high density regions such as dark matter halos. The precise shape of the energy spectrum of CGB depends on the nature of dark matter particles, as well as the cosmological evolution of dark matter halos. In order to discriminate between the signals from dark matter annihilation and other astrophysical sources, however, the information from the energy spectrum may not be sufficient. We show that dark matter annihilation also produces a characteristic anisotropy of the CGB, which provides a powerful tool for testing the origin. We develop the formalism based on a halo model approach to calculate the three-dimensional power spectrum of dark matter clumping, which determines the power spectrum of annihilation signals. We show that the sensitivity of future gamma-ray detectors such as GLAST should allow us to measure the angular power spectrum of CGB anisotropy, if dark matter particles are supersymmetric neutralinos and they account for most of the observed mean intensity of CGB in GeV region. On the other hand, if dark matter has a relatively small mass, and accounts for most of the CGB in MeV region, then the future Advanced Compton Telescope should be able to measure the anisotropy in MeV region. As the intensity of photons from annihilation is proportional to the density squared, we show that the predicted shape of the angular power spectrum of gamma rays from dark matter annihilation is different from that due to other astrophysical sources such as blazars. Therefore, the angular power spectrum of the CGB provides a smoking-gun signature of dark matter annihilation.

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A covariant approach to braneworld holography

Authors: Antonio Padilla

Exact holography for cosmological branes in an AdS-Schwarzschild bulk was first introduced in hep-th/0204218. We extend this notion to include all co-dimension one branes moving in non-trivial bulk spacetimes. We use a covariant approach, and show that the bulk Weyl tensor projected on to the brane can always be traded in for "holographic" energy-momentum on the brane. More precisely, a brane moving in a non-maximally symmetric bulk has exactly the same geometry as a brane moving in a maximally symmetric bulk, so long as we include the holographic fields on the brane. This correspondence is exact in that it works to all order in the brane energy-momentum tensor.

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Is Our Universe Natural?

Authors: Sean M. Carroll
Comments: Invited review for Nature, 11 pages

It goes without saying that we are stuck with the universe we have. Nevertheless, we would like to go beyond simply describing our observed universe, and try to understand why it is that way rather than some other way. Physicists and cosmologists have been exploring increasingly ambitious ideas that attempt to explain why certain features of our universe aren't as surprising as they might first appear.

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Do measurements of the one-point distribution of aperture-mass improve constraints on cosmology ?

Authors: Dipak Munshi, Patrick Valageas
Comments: 5 pages, submitted

We study the possibility of using the entire probability distribution function (PDF) of the aperture mass Map and its related cumulative probability distribution function (CPDF) to obtain meaningful constraints on cosmological parameters. Deriving completely analytic expressions for the associated covariance matrices, we construct the Fisher matrix and use it to estimate the accuracy with which various cosmological parameters can be recovered from future surveys using such statistics. This formalism also includes the effect of various noises such as intrinsic ellipticity distribution of galaxies and finite survey size. The estimation errors are then compared with the ones derived from low order moments of the PDF (variance and skewness) to check how efficiently the high Map tail can be used to constrain cosmological parameters such as Omega_m, sigma_8 and dark energy equation of state w_de. We find that for future surveys such as JDEM the full PDF does not bring significant tightening of constraints on cosmology beyond what is already achievable by the joint use of second and third order moments.

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DARK ENERGY

An inhomogeneous alternative to dark energy?

Authors: Havard Alnes, Morad Amarzguioui, Oyvind Gron
Comments: 8 pages (REVTeX4), 4 figures

Recently, there have been suggestions that the apparent accelerated expansion of the universe is not caused by repulsive gravitation due to dark energy, but is rather a result of inhomogeneities in the distribution of matter. In this work, we investigate the behaviour of a dust dominated inhomogeneous Lemaitre-Tolman-Bondi universe model, and confront it with various astrophysical observations. We find that such a model can easily explain the observed luminosity distance-redshift relation of supernovae without the need for dark energy, when the inhomogeneity is in the form of an underdense bubble centered near the observer. With the additional assumption that the universe outside the bubble is approximately described by a homogeneous Einstein-de Sitter model, we find that the position of the first CMB peak can be made to match the WMAP observations. Whether or not it is possible to reproduce the entire CMB angular power spectrum in an inhomogeneous model without dark energy, is still an open question.

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Laboratory tests on dark energy

Authors: Christian Beck
Comments: 7 pages, 1 figure. Invited talk given at the 21 COE symposium 'Astrophysics as Interdisciplinary Science', Waseda University, Tokyo, 1-3 September 2005. To appear in Journal of Physics: Conference Series

The physical nature of the currently observed dark energy in the universe is completely unclear, and many different theoretical models co-exist. Nevertheless, if dark energy is produced by vacuum fluctuations then there is a chance to probe some of its properties by simple laboratory tests based on Josephson junctions. These electronic devices can be used to perform `vacuum fluctuation spectroscopy', by directly measuring a noise spectrum induced by vacuum fluctuations. One would expect to see a cutoff near 1.7 THz in the measured power spectrum, provided the new physics underlying dark energy couples to electric charge. The effect exploited by the Josephson junction is a subtile nonlinear mixing effect and has nothing to do with the Casimir effect or other effects based on van der Waals forces. A Josephson experiment of the suggested type will now be built, and we should know the result within the next 3 years.

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REIONIZATION

Implications for the Cosmic Reionization from the Optical Afterglow Spectrum of the Gamma-Ray Burst 050904 at z = 6.3

Authors: Tomonori Totani (1), Nobuyuki Kawai (2), George Kosugi (3), Kentaro Aoki (3), Toru Yamada (3), Masanori Iye (3), Kouji Ohta (1), Takashi Hattori (3) ((1) Kyoto, (2) TITech, (3) NAO)
Comments: The version with full resolution figures is available at this http URL This is the second paper from the Subaru GRB team for GRB 050904, following Kawai et al. (astro-ph/0512052)

The gamma-ray burst (GRB) 050904 at z = 6.3 provides the first opportunity of probing the intergalactic medium (IGM) by GRBs at the epoch of the reionization. Here we present a spectral modeling analysis of the optical afterglow spectrum taken by the Subaru Telescope, aiming to constrain the reionization history. The spectrum shows a clear damping wing at wavelengths redward of the Lyman break, and the wing shape can be fit either by a damped Ly alpha system with a column density of log N_HI ~ 21.6 at a redshift close to the detected metal absorption lines (z_{metal} = 6.295), or by almost neutral IGM extending to a slightly higher redshift of z_{IGM,u} ~ 6.36. In the latter case, the difference from z_{metal} may be explained by acceleration of metal absorbing shells by the activities of the GRB or its progenitor. However, we exclude this possibility by using the light transmission feature around the Ly beta resonance, leading to a firm upper limit of z_{IGM,u} < 6.314. We then show an evidence that the IGM was largely ionized already at z=6.3, with the best-fit neutral fraction of IGM, x_HI = 0.00 +- 0.17, and an upper limit of x_HI < 0.60 (95% C.L.). This is the first quantitative upper limit on x_HI at z > 6. Various systematic uncertainties are examined, but none of them appears large enough to change this conclusion. To get further information on the reionization, it is important to increase the sample size of z >6 GRBs, to find GRBs with low column densities (log N_HI <~ 20) within their host galaxies, and for statistical studies of Ly alpha line emission from host galaxies.

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The Epoch of Reionization

Authors: A. J. Benson (1), Naoshi Sugiyama (2), Adi Nusser (3), C. G. Lacey (4) ((1) University of Oxford, UK, (2) NAOJ, Tokyo, Japan, (3) Technion, Haifa, Israel, (4) University of Durham (UK))
Comments: 21 pages, submitted to MNRAS

We have modelled the process of reionization in the high redshift Universe to determine the epoch of reionization. Reionization is driven by star formation in high redshift galaxies. We employ a semi-analytic model of galaxy formation to track the formation of these galaxies, their influence on the intergalactic medium (IGM) and the back-reaction of the IGM on further galaxy formation. This represents a more complete and physical modelling of reionization than has been conducted in the past. In particular, compared to our previous work our new calculations contain significant improvements in the modelling of the effects of reionization on the galaxy population and in the cooling model used to compute the rate at which galaxies form (our new model includes photoheating from a self-consistently computed ionizing background and also includes cooling due to molecular hydrogen). We find that reionization can be achieved by z~14-15 in a cosmological model motivated by the results from the Wilkinson Microwave Anisotropy Probe (WMAP) satellite, consistent with the optical depth to reionization measured by WMAP. However, a cosmological model with a running spectral index is able to achieve reionization before z~9 only with very extreme assumptions about the physics of feedback at high redshifts. These results assume that all ionizing photons in galaxies are able to escape and ionize the IGM. If this is not the case then the redshift of reionization could be substantially reduced. Under the assumption that early star formation leads to the formation of very massive stars we find that extended periods of partial reionization and double reionizations can occur. Such models do not fully reionize until z~6-7 but obtain an optical depth which is consistent with the results from the WMAP satellite. (abridged)

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Simulating Cosmic Reionization at Large Scales I: the Geometry of Reionization

Authors: Ilian T. Iliev (1), Garrelt Mellema (2,3), Ue-Li Pen (1), Hugh Merz (1), Paul R. Shapiro (4), Marcelo A. Alvarez (4) ((1) CITA, (2) ASTRON, Dwingeloo, (3) Leiden Observatory, (4) Univ. Texas at Austin)
Comments: Comments: 16 pages, 16 figures, submitted to MNRAS. Movies and higher resolution figures can be found at this http URL

We present the first large-scale radiative transfer simulations of cosmic reionization, in a simulation volume of (100/h Mpc)^3, while at the same time capturing the dwarf galaxies which are primarily responsible for reionization. We achieve this by combining the results from extremely large, cosmological, N-body simulations with a new, fast and efficient code for 3D radiative transfer, C^2-Ray. The resulting electron-scattering optical depth is in good agreement with the first-year WMAP polarization data. We show that reionization clearly proceeded in an inside-out fashion, with the high-density regions being ionized earlier, on average, than the voids. Ionization histories of smaller-size (5 to 10 comoving Mpc) subregions exibit a large scatter about the mean and do not describe the global reionization history well. The minimum reliable volume size for such predictions is ~30 Mpc. We derive the power-spectra of the neutral, ionized and total gas density fields and show that there is a significant boost of the density fluctuations in both the neutral and the ionized components relative to the total at arcminute and larger scales. We find two populations of HII regions according to their size, numerous, mid-sized (~10 Mpc) regions and a few, rare, very large regions tens of Mpc in size. We derive the statistical distributions of the ionized fraction and ionized gas density at various scales and for the first time show that both distributions are clearly non-Gaussian. (abridged)

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Formation of an early-type galaxy from cosmological initial conditions?

Authors: Thorsten Naab, Peter H. Johansson, George Efstathiou, Jeremiah P. Ostriker
Comments: submitted to ApJL

We describe a high resolution Smoothed Particle Hydrodynamics (SPH) simulation of a spheroidal galaxy starting from \LCDM initial conditions which is intentionally simple, and includes photoionization and cooling of the intergalactic medium but not feedback from AGN or supernovae. The galaxy undergoes an initial burst of star formation at $z \approx 5$, accompanied by the formation of a bubble of heated gas and does not experience a major merger after $z=3$. Heating from shocks and -PdV work dominates over cooling so that for most of the gas the temperature is an increasing function of time. By $z \approx 1$, 80% of the final stellar spheroid is in place and the spectral energy distribution resembles those of observed extremely red objects (EROs). By the present day, the simulated galaxy is an old ($\approx 10 {\rm Gyrs}$), kinematically hot stellar system with a stellar mass of $\approx 1.2 \times 10^{11} M_{\odot}$, surrounded by a hot gaseous halo containing 40% of the baryonic matter. Stars dominate the mass of the galaxy up to $\approx 4$ effective radii ($\approx 10$ kpc). Projected photometric and kinematic properties are in good agreement with observed field elliptical galaxies. In particular, unlike some recent simulations, the final stellar system has a concentration that is quite typical of real elliptical galaxies. Our simulation shows that a realistic intermediate mass giant elliptical galaxy with a plausible formation history can be formed from \LCDM initial conditions without requiring recent major mergers or feedback from supernovae or AGN.

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