16138806
Estimating Ω from Galaxy Redshifts: Linear Flow Distortions and Nonlinear Clustering
We propose a method to determine the cosmic mass density Ω from redshift-space distortions induced by large-scale flows in the presence of nonlinear clustering. Nonlinear structures in redshift space, such as fingers of God, can contaminate distortions from linear flows on scales as large as several times the small-scale pairwise velocity dispersion σv. Following Peacock & Dodds, we work in the Fourier domain and propose a model to describe the anisotropy in the redshift-space power spectrum; tests with high-resolution numerical data demonstrate that the model is robust for both mass and biased galaxy halos on translinear scales and above. On the basis of this model, we propose an estimator of the linear growth parameter β = Ω0.6/b, where b measures bias, derived from sampling functions that are tuned to eliminate distortions from nonlinear clustering. The measure is tested on the numerical data and found to recover the true value of β to within ~10%. An analysis of IRAS 1.2 Jy galaxies yields β=0.8+0.4-0.3 at a scale of 1000 km s-1, which is close to optimal given the shot noise and finite size of the survey. This measurement is consistent with dynamical estimates of β derived from both real-space and redshift-space information. The importance of the method presented here is that nonlinear clustering effects are removed to enable linear correlation anisotropy measurements on scales approaching the translinear regime. We discuss implications for analyses of forthcoming optical redshift surveys in which the dispersion is more than a factor of 2 greater than in the IRAS data.
118706760
Stellar Collapse and Gravitational Waves
The new generation of gravitational wave (GW) detectors have the potential to open a new window onto the violent dynamics of core collapse. Although it is certain that core collapse events generate gravitational radiation, understanding the characteristics of the radiation—whether it can be measured with the new detectors, and the best way to go about doing so—is a challenging problem. In this chapter we review the promise of GWs as observational probes, including a discussion of the current state of new detectors, and discuss the status of work to understand the waves generated by stellar core collapse.
118967389
On the Luminosity Dependence of the Galaxy Pairwise Velocity Dispersion
We make predictions for the pairwise velocity dispersion (PVD) of galaxies with models that are constrained to match the projected correlation function and luminosity function of galaxies in the Two Degree Field Galaxy Redshift Survey (2dFGRS). We use these data to constrain the halo occupation distribution (HOD) of 2dFGRS galaxies, then calculate the PVD by populating the halos of a large-volume, high-resolution N-body simulation. We examine the luminosity and scale dependence of the predicted PVD. At small and large scales, r < 1 and r 3 h-1 Mpc, we find that the PVD decreases with increasing galaxy luminosity. This result is mostly driven by the fraction of satellite galaxies fsat, which is well constrained by the correlation function. We find fsat ~ 25% for galaxies fainter than L*, while for brighter galaxies the satellite fraction rapidly declines, creating the decrease in the PVD with luminosity. Recent measurements of the PVD in Fourier space using the "dispersion model" have reported a strong decline in the PVD with increasing luminosity at k = 1 Mpc-1 h. We test this method with our HOD models, finding a luminosity dependence in k-space that is stronger than that in configuration space at that scale. The luminosity dependence of the HOD results in Fourier space are consistent with those measured at k = 1 Mpc-1 h; thus, the recent measurements of the PVD are fully explainable in the context of halo occupation models. The normalization of the PVD is lower than that predicted by our fiducial model, and reproducing it requires a lower value of Ωm (~0.2 instead of 0.3), a lower value of σ8 (~0.7 instead of 0.9), or a significant bias between the velocity dispersion of galaxies and dark matter in massive halos.
119224104
The dark matter of galaxy voids
How do observed voids relate to the underlying dark matter distribution? To examine the spatial distribution of dark matter contained within voids identified in galaxy surveys, we apply Halo Occupation Distribution models representing sparsely and densely sampled galaxy surveys to a high-resolution N-body simulation. We compare these galaxy voids to voids found in the halo distribution, low-resolution dark matter, and high-resolution dark matter. We find that voids at all scales in densely sampled surveys - and medium- to large-scale voids in sparse surveys - trace the same underdensities as dark matter, but they are larger in radius by ~20%, they have somewhat shallower density profiles, and they have centers offset by ~0.4Rv rms. However, in void-to-void comparison we find that shape estimators are less robust to sampling, and the largest voids in sparsely sampled surveys suffer fragmentation at their edges. We find that voids in galaxy surveys always correspond to underdensities in the dark matter, though the centers may be offset. When this offset is taken into account, we recover almost identical radial density profiles between galaxies and dark matter. All mock catalogs used in this work are available at this http URL
16668358
EXTENDING RECOVERY OF THE PRIMORDIAL MATTER POWER SPECTRUM
The shape of the primordial matter power spectrum encodes critical information on cosmological parameters. At large scales, in the linear regime, the observable galaxy power spectrum P obs(k) is expected to follow the shape of the linear matter power spectrum P lin(k), but on smaller scales the effects of nonlinearity and galaxy bias make the ratio P obs(k)/P lin(k) scale dependent. We develop a method that can extend the dynamic range of the primordial matter power spectrum recovery, taking full advantage of precision measurements on quasi-linear scales, by incorporating additional constraints on the galaxy halo occupation distribution (HOD) from the projected galaxy correlation function wp (rp ). We devise an analytic model to calculate observable galaxy power spectrum P obs(k) in real space and redshift space, given P lin(k) and HOD parameters, and we demonstrate its accuracy at the few percent level with tests against a suite of populated N-body simulations. Once HOD parameters are determined by fitting wp (rp ) measurements for a given cosmological model, galaxy bias is completely specified, and our analytic model predicts both the shape and normalization of P obs(k). Applying our method to the main galaxy redshift samples from the Sloan Digital Sky Survey (SDSS), we find that the real-space galaxy power spectrum follows the shape of the nonlinear matter power spectrum at the 1%-2% level up to k = 0.2 h Mpc–1 and that current observational uncertainties in HOD parameters leave only few percent uncertainties in our scale-dependent bias predictions up to k = 0.5 h Mpc–1. These uncertainties can be marginalized over in deriving cosmological parameter constraints, and they can be reduced by higher precision wp (rp ) measurements. When we apply our method to the SDSS luminous red galaxy (LRG) samples, we find that the linear bias approximation is accurate to 5% at k ≤ 0.08 h Mpc–1, but the strong scale dependence of LRG bias prevents the use of linear theory at k ≥ 0.08 h Mpc–1. Our HOD model prediction is in good agreement with the recent SDSS LRG power spectrum measurements at all measured scales (k ≤ 0.2 h Mpc–1), naturally explaining the observed shape of P obs(k) in the quasi-linear regime. The phenomenological "Q-model" prescription is a poor description of galaxy bias for the LRG samples, and it can lead to biased cosmological parameter estimates when measurements at k ≥ 0.1 h Mpc–1 are included in the analysis. We quantify the potential bias and constraints on cosmological parameters that arise from applying linear theory and Q-model fitting, and we demonstrate the utility of HOD modeling of high-precision measurements of P obs(k) on quasi-linear scales, which will be obtainable from the final SDSS data set.
119025968
The Distribution of Dark Matter in the Universe on Scales of 10^10 M_sun to 10^15 M_sun
The use of parallel computers and increasingly sophisticated software has allowed us to perform a large suite of N-body simulations using from $10^8$,to $10^9$ particles. We will report on our recent convergence tests of the halo mass function, N-point correlation functions, power spectrum and pairwise velocity from very large high resolution treecode N-body simulations. Rather than basing results on just one or two large simulations, now one can investigate the role of different numerical and physical effects on the statistics used to characterize the mass distribution of the Universe.
108962109
Development of a modular and scalable data acquisition system for calorimeters at a linear collider
A data acquisition (DAQ) system has been developed which will read out and control calorimeters serving as prototype systems for a future detector at an electron-positron linear collider. This is a modular, flexible and scalable DAQ system in which the hardware and signals are standards-based, using FPGAs and serial links. The idea of a backplaneless system was also pursued with a commercial development board housed in a PC and a chain of concentrator cards between it and the detector forming the basis of the system. As well as describing the concept and performance of the system, its merits and disadvantages are discussed.