Munch: Monday, October 2, 2006

                               


 

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

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


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Munch Archive
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The Scale Dependence of Halo and Galaxy Bias: Effects in Real Space

Authors: Robert E. Smith (UPenn), Roman Scoccimarro (NYU), Ravi K. Sheth (UPenn)
Comments: 30 pages, 11 figures. Submitted to PRD. Table I corrected
We examine the scale dependence of dark matter, halo and galaxy clustering on very large scales (0.01<k[h/Mpc]<0.15), due to non-linear effects from dynamics and halo bias. We pursue a two line offensive: high resolution numerical simulations are used to establish several new results, and an analytic model is developed to understand their origins. Our simulations show: (i) that the z=0 dark matter power spectrum is suppressed relative to linear theory by ~5% on scales (0.05<k[h/Mpc]<0.075); (ii) that, indeed, halo bias is non-linear over the scales we probe and that the scale dependence is a strong function of halo mass. High mass haloes show no suppression of power on scales (k<0.07[h/Mpc]), and only show amplification on smaller scales, whereas low mass haloes show strong, ~5-10%, suppression over the range (0.05 <k[h/Mpc] <0.15). Our results have relevance for studies of the baryon acoustic oscillation features. Non-linear mode-mode coupling: (i) damps these features on progressively larger scales as halo mass increases; (ii) produces small shifts in the positions of the peaks and troughs which depend on halo mass. Our analytic model is described in the language of the `halo-model'. However, for the first time the halo-halo clustering term is propagated into the non-linear regime using `1-loop' perturbation theory and a non-linear halo bias model. We show that, with bias parameters derived from simulations, the model predictions are in agreement with the numerical results. We then use the model to explore the scale dependence of galaxies of different colour and find significant differences between the power spectra of the two populations. Thus understanding the scale dependent bias for a given galaxy sample will be crucial for deriving accurate cosmological constraints. (Abridged)

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B-Mode contamination by synchrotron emission from 3-years WMAP data

Authors: E. Carretti, G. Bernardi, S. Cortiglioni
Comments: 5 pages, 3 figures, accepted for publication in MNRAS Letters
We study the contamination of the B-mode of the Cosmic Microwave Background Polarization (CMBP) by Galactic synchrotron in the lowest emission regions of the sky. The 22.8-GHz polarization map of the 3-years WMAP data release is used to identify and analyse such regions. Two areas are selected with signal-to-noise ratio S/N<2 and S/N<3, covering ~16% and ~26% fraction of the sky, respectively. The polarization power spectra of these two areas are dominated by the sky signal on large angular scales (multipoles l < 15), while the noise prevails on degree scales. Angular extrapolations show that the synchrotron emission competes with the CMBP B-mode signal for tensor-to-scalar perturbation power ratio $T/S = 10^{-3}$ -- $10^{-2}$ at 70-GHz in the 16% lowest emission sky (S/N<2 area). These values worsen by a factor ~5 in the S/N<3 region. The novelty is that our estimates regard the whole lowest emission regions and outline a contamination better than that of the whole high Galactic latitude sky found by the WMAP team (T/S>0.3). Such regions allow $T/S \sim 10^{-3}$ to be measured directly which approximately corresponds to the limit imposed by using a sky coverage of 15%. This opens interesting perspectives to investigate the inflationary model space in lowest emission regions.

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Revisiting the Baryon Fractions of Galaxy Clusters: A Comparison with WMAP 3-year Results

Authors: Ian G. McCarthy (ICC, Durham), Richard G. Bower (ICC, Durham), Michael L. Balogh (Waterloo)
Comments: 7 pages, 3 figures, submitted to MNRAS
The baryonic mass fraction, Omega_b/Omega_m, can be sensitively constrained using X-ray observations of galaxy clusters. In this paper, we compare the baryonic mass fraction inferred from measurements of the cosmic microwave background with the gas mass fractions (f_gas) of a sample of 19 clusters taken from the recent literature. In systems cooler than 4 keV, f_gas declines as the system temperature decreases. However, in higher temperature systems, f_gas converges to (0.12 +/- 0.02)(h/0.72)^{-1.5}, where the uncertainty reflects the systematic variations between clusters and the dependence on radius beyond r500. This is significantly lower than the maximum-likelihood value of the baryon fraction from the recently released WMAP 3-year results. We investigate possible reasons for this discrepancy, including the effects of radiative cooling and non-gravitational heating, and conclude that the most likely solution is that Omega_m = 0.28-0.39, higher than the best-fit WMAP value, but consistent at the 2-sigma level. Degeneracies within the WMAP data require that sigma_8 must also be greater than the maximum likelihood value for consistency between the data sets.

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Gamma-ray Bursts as Dark Energy Probes

Authors: O. Bertolami P. T. Silva
Comments: 7 pages, 3 figures. Proceedings of "The Dark Side of The Universe" international workshop, Madrid, 20-24 June 2006
We discuss the prospects of using Gamma Ray Bursts (GRBs) as high-redshift distance estimators, and consider their use in the study of two dark energy models, the Generalized Chaplygin Gas (GCG), a model for the unification of dark energy and dark matter, and the XCDM model, a model where a generic dark energy fluid like component is described by the equation of state, $p= \omega \rho$. We find that this test yields rather disappointing results for the GCG model, being mainly sensitive to the total amount of matter present in the Universe in the case of the XCDM model. We also find that, within the framework of the XCDM model, a large sample of GRBs ($\geq 200$) may turn out to be quite useful to improve the forthcoming type Ia supernovae data.

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The Impact of Baryonic Cooling on Giant Arc Abundances

Authors: Eduardo Rozo (Chicago/Ohio State), Daisuke Nagai (Caltech), Charles Keeton (Rutgers), Andrey Kravtsov (Chicago)
Comments: ApJ Submitted
Using ray tracing for simple analytic profiles, we demonstrate that the lensing cross section for producing giant arcs has distinct contributions due to arcs formed through image distortion only, and arcs form from the merging of two or three images. We investigate the dependence of each of these contributions on halo ellipticity and on the slope of the density profile, and demonstrate that at fixed Einstein radius, the lensing cross section increases as the halo profile becomes steeper. We then compare simulations with and without baryonic cooling of the same cluster for a sample of six clusters, and demonstrate that cooling can increase the overall abundance of giant arcs by factors of a few. The net boost to the lensing probability for individual clusters is mass dependent, and can lower the effective low mass limit of lensing clusters. This last effect can potentially increase the number of lensing clusters by an extra 50%. While the magnitude of these effects may be overestimated due to the well known overcooling problem in simulations, it is evident that baryonic cooling has a non-negligible impact on the expected abundance of giant arcs, and hence cosmological constraints from giant arc abundances may be subject to large systematic errors.

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Quest for circular polarization of gravitational wave background and orbits of laser interferometers in space

Authors: Naoki Seto
Comments: 6 pages, 4 figures
We show that isotropic component of circular polarization of stochastic gravitational wave background can be explored by breaking two dimensional configuration of multiple laser interferometers for correlation analysis. By appropriately selecting orbital parameters for the proposed BBO mission, the circular polarization degree Pi can be measured down to Pi ~ 0.08 (10^{-15}/Omega_{GW})(SNR/5) with slightly (~10%) sacrificing the detection limit for the total intensity Omega_{GW} compared to the standard plane symmetric configuration. This might allow us to detect signature of parity violation in the very early universe.

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Cosmological neutrino mass limit and the dynamics of dark energy

Authors: Jun-Qing Xia, Gong-Bo Zhao, Xinmin Zhang
Comments: 9 pages, 5 figures, 1 table
We investigate the correlation between the neutrino mass limit and dark energy with time evolving equation of state. Parameterizing dark energy as w=w_0+w_1*z/(1+z), we make a global fit using Markov Chain Monte Carlo technique to determine w_0, w_1, neutrino mass as well as other cosmological parameters simultaneously. We pay particular attention to the correlation between neutrino mass \Sigma m_{\nu} and w_1 using current cosmological observations as well as the future simulated datasets such as PLANCK, SNAP and LAMOST.

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Joining the Hubble Flow: Implications for Expanding Space

Authors: Luke A. Barnes, Matthew J. Francis, J. Berian James, Geraint F. Lewis
Comments: Accepted for publication in MNRAS
Report-no: gfl-001
The concept of expanding space has come under fire recently as being inadequate and even misleading in describing the motion of test particles in the universe. Previous investigations have suffered from a number of shortcomings, which we seek to correct. We study the motion of test particles in the universe in detail, solving the geodesic equations of General Relativity for a number of cosmological models. In particular, we use analytic methods to examine whether particles removed from the Hubble flow asymptotically rejoin the Hubble flow, a topic that has caused confusion because of differing definitions and invalid reasoning. We conclude that particles in eternally expanding but otherwise arbitrary universes do not in general rejoin the Hubble flow.

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The effect of inhomogeneous expansion on the supernova observations

Authors: Kari Enqvist (1 and 2), Teppo Mattsson (1) ((1) Helsinki Inst. of Phys., (2) Helsinki U.)
Report-no: HIP-2006-39/TH
We consider an inhomogeneous but spherically symmetric Lemaitre-Tolman-Bondi model to demonstrate that spatial variations of the expansion rate can have a significant effect on the cosmological supernova observations. A model with no dark energy but a local Hubble parameter about 15% larger than its global value fits the supernova data better than the homogeneous model with the cosmological constant. The goodness of the fit is not sensitive to inhomogeneities in the present-day matter density, and our best fit model has Omega_M(r) ~ const. < 0.4, in agreement with galaxy surveys. We also compute the averaged expansion rate, defined by the Buchert equations, of the best fit model and show explicitly that there is no average acceleration.

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