Munch: Monday, Oct 27th, 2008

arXiv:0810.0273 [ps, pdf, other]

Halo stochasticity in global clustering analysis

Authors: Silvia Bonoli (MPA), Ue-Li Pen (CITA)
(Submitted on 1 Oct 2008)
Abstract: In the present work we study the statistics of haloes, which in the halo model determines the distribution of galaxies. Haloes are known to be biased tracer of dark matter, and at large scales it is usually assumed there is no intrinsic stochasticity between the two fields. Following the work of Seljak & Warren (2004), we explore how correct this assumption is and, moving a step further, we try to qualify the nature of stochasticity. We use Principal Component Analysis applied to the outputs of a cosmological N-body simulation to: (1) explore the behaviour of stochasticity in the correlation between haloes of different masses; (2) explore the behaviour of stochasticity in the correlation between haloes and dark matter. We show results obtained using a catalogue with 2.1 million haloes, from a PMFAST simulation with box size of 1000h^{-1}Mpc. In the relation between different populations of haloes we find that stochasticity is not-negligible even at large scales. In agreement with the conclusions of Tegmark & Bromley (1999) who studied the correlations of different galaxy populations, we found that the shot-noise subtracted stochasticity is qualitatively different from `enhanced' shot noise and, specifically, it is dominated by a single stochastic eigenvalue. We call this the `minimally stochastic' scenario, as opposed to shot noise which is `maximally stochastic'. In the correlation between haloes and dark matter, we find that stochasticity is minimized, as expected, near the dark matter peak (k ~ 0.02 h Mpc^{-1} for a LambdaCDM cosmology) and, even at large scales, it is of the order of 15 per cent above the shot noise. Moreover, we find that the reconstruction of the dark matter distribution is improved when we use eigenvectors as tracers of the bias. [Abridged]
arXiv:0810.2177 [ps, pdf, other]

The Fate of Substructures in Cold Dark Matter Haloes

(Submitted on 13 Oct 2008 (v1), last revised 13 Oct 2008 (this version, v2))
Abstract: We use the Millennium Simulation, a large, high resolution N-body simulation of the evolution of structure in a LambdaCDM cosmology, to study the properties and fate of substructures within a large sample of dark matter haloes. We find that the subhalo mass function departs significantly from a power law at the high mass end. We also find that the radial and angular distributions of substructures depend on subhalo mass. In particular, high mass subhaloes tend to be less radially concentrated and to have angular distributions closer to the direction perpendicular to the spin of the host halo than their less massive counterparts. We find that mergers between subhaloes occur. These tend to be between substructures that were already dynamically associated before accretion into the main halo. For subhaloes larger than 0.001 times the mass of the host halo, it is more likely that the subhalo will merge with the central or main subhalo than with another subhalo larger than itself. For lower masses, subhalo-subhalo mergers become equally likely to mergers with the main subhalo. Our results have implications for the variation of galaxy properties with environment and for the treatment of mergers in galaxy formation models.
arXiv:0810.2036 [ps, pdf, other]

Parity Dependence in Strong Lens Systems as a Probe of Dark Matter Substructure

Authors: Jacqueline Chen (AIfA)
(Submitted on 11 Oct 2008)
Abstract: The amount of mass in small, dark matter clumps within galaxies (substructure) is an important test of cold dark matter. One approach to measuring the substructure mass fraction is to analyze the fluxes of images that have been strongly lensed by a galaxy. Flux ratios between images that are anomalous with respect to smooth (no substructure) models have previously suggested that there is a greater amount of substructure than found in dark matter simulations. One measure of anomalous flux ratios is parity dependence -- that the fluxes of different images of a source are perturbed differently. In this paper, we discuss parity dependence as a probe of dark matter substructure. We find that reproducing the observed parity dependence requires a significant alignment between concentrated dark matter clumps and images. The results may imply a larger fraction of mass in substructures than suggested by some dark matter simulations and that the observed parity dependence is unlikely to be reproduced by luminous satellites of lens galaxies.
arXiv:0810.1960 [ps, pdf, other]

Covariance of cross-correlations: towards efficient measures for large-scale structure

Authors: Robert E. Smith (ITP, UZurich)
(Submitted on 13 Oct 2008)
Abstract: We study the covariance of the cross-power spectrum (CP) of different tracers for large-scale structure. We use the counts-in-cells framework to derive expressions for the full non-Gaussian covariance, including all contributions from the discreteness of matter. We pay attention to the assumed sampling distribution: besides the usual Poisson model, we also consider a toy-model where one tracer is a sub-sample of the other. This is instructive, since it is likely that not all galaxies are equally good tracers of the mass -- in particular those hosted in the same halo. We then compare the efficiency of the CP with the simple auto-statistic and find that the CP approach can out perform the standard auto-spectrum, provided one is cross-correlating a high-density sample with a rare sample. We then test the theory by measuring the fractional errors in the mass-mass, halo-mass, and halo-halo power spectra from the zHORIZON-I simulations, total volume ~100 [Gpc/h]^3. Good agreement is found on large-scales k<0.07 h/Mpc and there is no obvious advantage gained from the different estimators, since fractional errors all scale simply with the number of modes and survey volume. On smaller scales, there is an increase in the errors for all spectra. This can be attributed to increased importance of Poisson sampling fluctuations and the generation of non-Gaussian error terms. However, for cluster studies, there is a factor ~2 advantage to be gained from using the CP approach. All of the analysis was repeated in configuration space, and the main difference is that, on very large scales, there is a factor ~2 improvement in the S/N for this method. This work points the way towards the design of improved estimators and is expected to be of most use in studies of primordial non-Gaussianity. (Abridged)
arXiv:0810.3562 [pdf]

The Fermi Gamma Ray Space Telescope discovers the Pulsar in the Young Galactic Supernova-Remnant CTA 1

(Submitted on 20 Oct 2008)
Abstract: Energetic young pulsars and expanding blast waves (supernova remnants, SNRs) are the most visible remains after massive stars, ending their lives, explode in core-collapse supernovae. The Fermi Gamma-Ray Space Telescope has unveiled a radio quiet pulsar located near the center of the compact synchrotron nebula inside the supernova remnant CTA 1. The pulsar, discovered through its gamma-ray pulsations, has a period of 316.86 ms, a period derivative of 3.614 x 10-13 s s-1 . Its characteristic age of 104 years is comparable to that estimated for the SNR. It is conjectured that most unidentified Galactic gamma ray sources associated with star-forming regions and SNRs are such young pulsars.
arXiv:0810.3504 [ps, pdf, other]

Limits on Low-Mass WIMP Dark Matter with an Ultra-Low-Energy Germanium Detector at 220 eV Threshold

(Submitted on 20 Oct 2008)
Abstract: An energy threshold of (220$\pm$10) eV was achieved at an efficiency of 50% with a four-channel ultra-low-energy germanium detector each with an active mass of 5 g\cite{wimppaper}. This provides a unique probe to WIMP dark matter with mass below 10 GeV. With low background data taken at the Kuo-Sheng Laboratory, constraints on WIMPs in the galactic halo were derived. Both spin-independent WIMP-nucleon and spin-dependent WIMP-neutron bounds improve over previous results for WIMP mass between 3$-$6 GeV. These results, together with those on spin-dependent couplings, will be presented. Sensitivities for full-scale experiments were projected. This detector technique makes the unexplored sub-keV energy window accessible for new neutrino and dark matter experiments.
arXiv:0810.3475 [ps, pdf, other]

Limits on an Energy Dependence of the Speed of Light from a Flare of the Active Galaxy PKS 2155-304

(Submitted on 20 Oct 2008)
Abstract: In the past few decades, several models have predicted an energy-dependence of the speed of light in the context of quantum gravity. For cosmological sources such as active galaxies, this minuscule effect can add up to measurable photon-energy dependent time lags. In this paper a search for such time lags during the H.E.S.S. observations of the exceptional very high energy flare of the active galaxy PKS 2155-304 on 28 July in 2006 is presented. Since no significant time lag is found, lower limits on the energy scale of speed of light modifications are derived.
arXiv:0810.3913 [ps, pdf, other]

Constraints on generating the primordial curvature perturbation and non-Gaussianity from instant preheating

Authors: Christian T. Byrnes (Heidelberg U.)
(Submitted on 22 Oct 2008)
Abstract: We analyse models of inflation in which isocurvature perturbations present during inflation are converted into the primordial curvature perturbation during instant preheating. This can be due to an asymmetry between the fields present either during inflation or during preheating. We consider all the constraints that the model must satisfy in order to be theoretically valid and to satisfy observations. We show that the constraints are very tight in all of the models proposed and special initial conditions are required for the models to work. In the case where the symmetry is strongly broken during inflation the non-Gaussianity parameter f_NL is generally large and negative.
arXiv:0810.2893 [ps, pdf, other]

Gamma Ray burst afterglows in the infrared: a standard candle?

Authors: B. Gendre (1), S. Pelisson (1), M. Boer (2), S. Basa (1), A. Mazure ((1) LAM/CNRS/Universite de Provence, (2) OHP/CNRS)
(Submitted on 16 Oct 2008)
Abstract: Clustering in the luminosity of the afterglows of gamma-ray burst has been reported in the optical and X-ray. We investigate the possibility that a clustering in the luminosity of the afterglows of gamma-ray burst exists in near infrared (J, H, K bands). We use observations of events occurring from 1997 to the end of 2007. We correct the gamma-ray burst afterglow light curve for distance effect and time dilation, and replace all light curves to a common distance of z=1. We used only observations of signal emitted in the near infrared (in the burst frame). We observe a clustering identical to the one observed in optical and similar to the one observed in X-ray. We thus confirm the previous works made in optical. We set a constraint on the total energy of the fireball.
arXiv:0810.2787 [ps, pdf, other]

Proving Inflation: A Bootstrap Approach

Authors: Latham Boyle (CITA), Paul J. Steinhardt (Princeton)
(Submitted on 16 Oct 2008)
Abstract: We propose a way to test the essential idea underlying the inflationary paradigm: that the universe underwent a brief period of accelerated expansion followed by a long period of decelerated expansion.
arXiv:0810.3002 [ps, pdf, other]

Cosmic Analogues of the Stern-Gerlach Experiment and the Detection of Light Bosons

(Submitted on 16 Oct 2008)
Abstract: We show that, by studying the arrival times of radio pulses from highly-magnetized pulsars, it may be possible to detect light spin-0 bosons (such as axions and axion-like particles) with a much greater sensitivity, over a broad particle mass range than is currently reachable by terrestrial experiments and indirect astrophysical bounds. In particular, we study the effect of splitting of photon-boson beams under intense magnetic field gradients in magnetars and show that radio pulses (at meter wavelengths) may be split and shift by a discernible phase down to a photon-boson coupling constant of g ~ 1e-14 [1/GeV]; i.e., about four orders of magnitude lower than current upper limits on g. The effect increases linearly with photon wavelength with split pulses having equal fluxes and similar polarizations. These properties make the identification of beam-splitting and beam deflection effects straightforward with currently available data. Better understanding of radio emission from magnetars is, however, required to confidently exclude regions in the parameter space when such effects are not observed.
arXiv:0810.3499 [pdf, other]

Initial Hubble Diagram Results from the Nearby Supernova Factory

(Submitted on 20 Oct 2008 (v1), last revised 21 Oct 2008 (this version, v2))
Abstract: The use of Type Ia supernovae as distance indicators led to the discovery of the accelerating expansion of the universe a decade ago. Now that large second generation surveys have significantly increased the size and quality of the high-redshift sample, the cosmological constraints are limited by the currently available sample of ~50 cosmologically useful nearby supernovae. The Nearby Supernova Factory addresses this problem by discovering nearby supernovae and observing their spectrophotometric time development. Our data sample includes over 2400 spectra from spectral timeseries of 185 supernovae. This talk presents results from a portion of this sample including a Hubble diagram (relative distance vs. redshift) and a description of some analyses using this rich dataset.
arXiv:0810.3337 [ps, pdf, other]

Inflationary Potential from 21 cm Tomography and Planck

(Submitted on 20 Oct 2008)
Abstract: Three-dimensional neutral hydrogen mapping using the redshifted 21 cm line has recently emerged as a promising cosmological probe. Within the framework of slow-roll reconstruction, we analyze how well the inflationary potential can be reconstructed by combining data from 21 cm experiments and cosmic microwave background data from the Planck satellite. We consider inflationary models classified according to the amplitude of their tensor component, and show that 21 cm measurements can significantly improve constraints on the slow-roll parameters and determine the shape of the inflationary potential.
arXiv:0810.3931 [ps, pdf, other]

Measuring Distance Ratios with CMB-Galaxy Lensing Cross-correlations

(Submitted on 21 Oct 2008)
Abstract: We propose a method for cosmographic measurements by combining gravitational lensing of the cosmic microwave background (CMB) with cosmic shear surveys. We cross-correlate the galaxy counts in the lens plane with two different source planes: the CMB at $z \sim 1100$ and galaxies at an intermediate redshift. The ratio of the galaxy count/CMB lensing cross-correlation to the galaxy count/galaxy lensing cross correlation is shown to be a purely geometric quantity, depending only on the distribution function of the source galaxies. By combining Planck, ADEPT and LSST the ratio can be measured to $\sim 4%$ accuracy, whereas a future polarization based experiment like CMBPOL can make a more precise ($\sim 1%$) measurement. For cosmological models where the curvature and the equation of state parameter are allowed to vary, the direction of degeneracy defined by the measurement of this ratio is different from that traced out by Baryon Acoustic Oscillation (BAO) measurements. Combining this method with the stacked cluster mass reconstruction cosmography technique as proposed by Hu, Holz and Vale (2007), the uncertainty in the ratio can be further reduced, improving the constraints on cosmological parameters. We also study the implications of the lensing-ratio measurement for early dark energy models, in context of the parameterization proposed by Doran and Robbers (2006). For models which are degenerate with respect to the CMB, we find both BAO and lensing-ratio measurements to be insensitive to the early component of the dark energy density.