5659355
SYSTEMATIC ERRORS IN THE DETERMINATION OF HUBBLE CONSTANT DUE TO THE ASPHERICITY AND NONISOTHERMALITY OF CLUSTERS OF GALAXIES
We present statistical studies on the systematic errors in the determination of the Hubble constant H0 from joint analyses of X-ray and the Sunyaev-Zel'dovich (SZ) effect of clusters of galaxies. We focus on the effects of their triaxiality and nonisothermality. From the triaxial model of dark matter halos obtained from numerical simulations, we derive the distribution of the intracluster gas under the assumption of the hydrodynamic equilibrium. Both the isothermal and the polytropic gases are investigated. We run Monte Carlo simulations to generate samples of clusters according to the distributions of their masses, axial ratios, concentration parameters, and line-of-sight directions. The estimation of H0 is done by fitting X-ray and SZ profiles of a triaxial cluster with the isothermal and spherical β model. We find that for a sample of clusters with M = 1014 h-1 M☉ and z = 0.1, the estimated H0 is positively biased with H(estimated) ≈ 1.05H0(true) and H(estimated) ≈ 1.05H0(true) for the isothermal case. For the polytropic case with γ = 1.15, the bias is large with H(estimated) ≈ 1.35H0(true) and H(estimated) ≈ 3H0(true). For a mass-limited sample of clusters with Mlim = 1013 h-1 M☉, the results are similar. On the other hand, such a large overestimation has not been seen in real observations. It is noticed that the β-value for observed clusters is within the range of 0.5-0.8. Considering only the subsample in Monte Carlo simulations with β in the range of 0.5-0.8, we have H(estimated) = 1.002H0(true) and H(estimated) = 0.994H0(true) for the isothermal and polytropic cases, respectively. We further find that the value of β is more sensitive to the intrinsic asphericity of clusters than the axial ratio of two-dimensional X-ray images η is. Limiting to clusters with β ≥ 0.5 essentially excludes highly aspherical clusters from the sample. From this subsample of clusters, we can get a fair estimate on H0.
6120041
N-body simulations for testing the stability of triaxial galaxies in MOND
We perform a stability test of triaxial models in Modified Newtonian Dynamics (MOND) using N-body simulations. The triaxial models considered here have densities that vary with r −1 in the centre and r −4 at large radii. The total mass of the model varies from 10 8 to 10 10 M� , representing the mass scale of dwarfs to medium-mass elliptical galaxies, respectively, from deep MOND to quasi-Newtonian gravity. We build triaxial galaxy models using the Schwarzschild technique, and evolve the systems for 200 Keplerian dynamical times (at the typical length-scale of 1.0 kpc). We find that the systems are virial overheating, and in quasi-equilibrium with the relaxation taking approximately 5 Keplerian dynamical times (1.0 kpc). For all systems, the change of the inertial (kinetic) energy is less than 10 per cent (20 per cent) after relaxation. However, the central profile of the model is flattened during the relaxation and the (overall) axis ratios change by roughly 10 per cent within 200 Keplerian dynamical times (at 1.0 kpc) in our simulations. We further find that the systems are stable once they reach the equilibrium state.
15633882
SELF-CONSISTENT MODELS OF TRIAXIAL GALAXIES IN MOND GRAVITY
The Bekenstein-Milgrom gravity theory with a modified Poisson equation is tested here for the existence of triaxial equilibrium solutions. Using the nonnegative least-square method, we show that self-consistent triaxial galaxies exist for baryonic models with a mild density cusp, -->ρ ~ Σ/r. Self-consistency is achieved for a wide range of central concentrations, -->Σ ~ 10-1000 M☉ pc−2, representing low to high surface brightness galaxies. Our results demonstrate for the first time that the orbit superposition technique is fruitful for constructing galaxy models beyond Newtonian gravity, and triaxial cuspy galaxies might exist without the help of cold dark matter.
119170453
A new model for the Milky Way bar
We use Schwarzschild's orbit-superposition technique to construct self-consistent models of the Galactic bar. Using χ2 minimization, we find that the best-fitting Galactic bar model has a pattern speed Ωp = 60 km s−1 kpc−1, disc mass Md = 1.0 × 1011 M⊙ and bar angle θbar = 20° for an adopted bar mass Mbar = 2 × 1010 M⊙. The model can reproduce not only the three-dimensional and projected density distributions but also velocity and velocity dispersion data from the Bulge Radial Velocity Assay (BRAVA) survey. We also predict the proper motions in the range l = [−12°, 12°], b = [−10°, 10°], which appear to be higher than observations in the longitudinal direction. The model is stable within a time-scale of 0.5 Gyr, but appears to deviate from steady state on longer time-scales. Our model can be further tested by future observations such as those from GAIA.
5692175
Effects of correlation between merging steps on the global halo formation
The excursion set theory of halo formation is modified by adopting the fractional Brownian motion, to account for possible correlation between merging steps. We worked out analytically the conditional mass function, halo merging rate and formation time distribution in the spherical collapse model. We also developed an approximation for the ellipsoidal collapse model and applied it to the calculation of the conditional mass function and the halo formation time distribution. For models in which the steps are positively correlated, the halo merger rate is enhanced when the accreted mass is less than similar to 25M*, while for the negatively correlated case this rate is reduced. Compared with the standard model in which the steps are uncorrelated, the models with positively correlated steps produce more aged population in small mass haloes and more younger population in large mass haloes, while for the models with negatively correlated steps the opposite is true. An examination of simulation results shows that a weakly positive correlation between successive merging steps appears to fit best. We have also found a systematic effect in the measured mass function due to the finite volume of simulations. In future work, this will be included in the halo model to accurately predict the three-point correlation function estimated from simulations.
125646338
Alignment between the host and the nearest neighbor Groups
We measure four different types of alignment signals using the galaxy groups of the Sloan Digital Sky Survey Data Release 4 to probe the impact of large scale environment on the distribution of satellite galaxies, on the the orientation of the central and satellite galaxies: (1) the alignment between the distributions of the satellites relative to the direction of the nearest neighbor group (NNG); (2) the alignment between the major axis direction of the central galaxy of the host group (HG) and the direction of the NNG; (3) the alignment between the two major axes of the central galaxies of the HG and NNG; and (4) the alignment between the major axes of the satellites of the HG and the direction of the NNG. We find strong alignment signals of satellite distribution and orientation of central galaxy relative to the direction of the NNG even when the NNG locates beyond $3r_{\rm vir}$ of the host group. The alignment signals are stronger for groups that are more massive and with early type central galaxies. For the orientation of satellite galaxies, however, we do not find any significant alignment signals relative to the direction of the NNG. From these four types of the alignment measures, we conclude that the large scale environment traced by the nearby group mainly impacts the shape of the host dark matter halo, hence impacts the distribution of satellite galaxies and the orientation of central galaxies. Apart from these, there is additional impacts by the large scale environment directly onto the distribution of satellite galaxies.
119226794
Observational constraints to Ricci dark energy model by using: SN, BAO, OHD, fgas data sets
We perform a global constraint on the Ricci dark energy model with both the flat case and the non-flat case, using the Markov Chain Monte Carlo (MCMC) method and the combined observational data from the cluster X-ray gas mass fraction, Supernovae of type Ia (397), baryon acoustic oscillations, current Cosmic Microwave Background, and the observational Hubble function. In the flat model, we obtain the best fit values of the parameters in 1σ,2σ regions: Ωm0 = 0.2927+0.0420+0.0542−0.0323−0.0388, α = 0.3823+0.0331+0.0415−0.0418−0.0541, Age/Gyr = 13.48+0.13+0.17−0.16−0.21, H0 = 69.09+2.56+3.09−2.37−3.39. In the non-flat model, the best fit parameters are found in 1σ,2σ regions:Ωm0 = 0.3003+0.0367+0.0429−0.0371−0.0423, α = 0.3845+0.0386+0.0521−0.0474−0.0523, Ωk = 0.0240+0.0109+0.0133−0.0130−0.0153, Age/Gyr = 12.54+0.51+0.65−0.37−0.49, H0 = 72.89+3.31+3.88−3.05−3.72. Compared to the constraint results in the ΛCDM model by using the same datasets, it is shown that the current combined datasets prefer the ΛCDM model to the Ricci dark energy model.
12882522
THE CLUSTERING OF SDSS GALAXY GROUPS: MASS AND COLOR DEPENDENCE
We use a large sample of galaxy groups selected from the SDSS data release 4 with an adaptive halo-based group finder to probe how the clustering strength of groups depends on their masses and colors. In particular, we determine the relative biases of groups of different masses, as well as that of groups with the same mass but with different colors. In agreement with previous studies, we find that more massive groups are more strongly clustered, and the inferred mass dependence of the halo bias is in good agreement with predictions for the ΛCDM concordance cosmology. Regarding the color dependence, we find that groups with red centrals are more strongly clustered than groups of the same mass but with blue centrals. Similar results are obtained when the color of a group is defined to be the total color of its member galaxies. The color dependence is more prominent in less massive groups and becomes insignificant in groups with masses 1014 h−1 M☉. These results are consistent with those obtained by Yang et al. from an analysis of the 2dFGRS, but inconsistent with those obtained by Berlind et al., who also used an SDSS group catalog. We construct a mock galaxy redshift survey from the large Millennium N-body simulation. Applying our group finder to this mock survey, and analyzing the mock data in exactly the same way as the true data, we are able to accurately recover the intrinsic mass and color dependencies of the halo bias in the model. Interestingly, the semianalytical model reveals the same color dependence of the halo bias as we find in our group catalog. We discuss these results in light of the assembly bias of dark matter halos and the star formation histories of galaxies.
20673458
Characterization and Tuning of A Microwave Gun Cavity
A microwave electron gun system is being constructed to evaluate the feasibility of using such system as the electron source for the linac pre-injector in storage ring synchrotron radiation sources and for novel linac-based light source R&D at NSRRC. A 2856 MHz SSRL-type microwave gun cavity has been fabricated in house for this purpose. Properties of the cavity resonant modes have been characterized as a function of side-coupled cell plunger position. Special tuning plunger design for this cell allows re-adjustment of field ratio even after brazing. Field amplitude ratio of the cavity full cell to half cell has now been tuned to 2:1. That is the optimized ratio for low energy bunch compression by an alpha magnet.
2637296
Small-angle X-ray scattering of reduced ribonuclease A: effects of solution conditions and comparisons with a computational model of unfolded proteins.
The disulfide-reduced form of bovine ribonuclease A, with the Cys thiols irreversibly blocked, was characterized by small-angle x-ray scattering. To help resolve the conflicting results and interpretations from previous studies of this model unfolded protein, we measured scattering profiles using a range of solution conditions and compared them with the profiles predicted by a computational model for a random-coil polypeptide. Analysis of the simulated and experimental profiles reveals that scattering intensities at intermediate angles, corresponding to interatomic distances in the range of 5-20 A, are particularly sensitive to changes in solvation and can be used to assess the internal scaling behavior of the polypeptide chain, expressed as a mass fractal dimension, D(m). This region of the scattering curve is also much less sensitive to experimental artifacts than is the very small angle regime (the Guinier region) that has been more typically used to characterize unfolded proteins. The experimental small-angle x-ray scattering profiles closely matched those predicted by the computational model assuming relatively small solvation energies. The scaling behavior of the polypeptide approaches that of a well-solvated polymer under conditions where it has a large net charge and at high urea concentrations. At lower urea concentrations and neutral pH, the behavior of the chain approaches that expected for theta-conditions, where the effects of slightly unfavorable interactions with solvent balance those of excluded volume, leading to scaling behavior comparable to that of an idealized random walk chain. Though detectable, the shift toward more compact conformations at lower urea concentrations does not correspond to a transition to a globule state and is associated with little or no reduction in conformational entropy. This type of collapse, therefore, is unlikely to greatly reduce the conformational search for the native state.
122999629
The ALICE experiment at the CERN LHC
ALICE (A Large Ion Collider Experiment) is a general-purpose, heavy-ion detector at the CERN LHC which focuses on QCD, the strong-interaction sector of the Standard Model. It is designed to address the physics of strongly interacting matter and the quark-gluon plasma at extreme values of energy density and temperature in nucleus-nucleus collisions. Besides running with Pb ions, the physics programme includes collisions with lighter ions, lower energy running and dedicated proton-nucleus runs. ALICE will also take data with proton beams at the top LHC energy to collect reference data for the heavy-ion programme and to address several QCD topics for which ALICE is complementary to the other LHC detectors. The ALICE detector has been built by a collaboration including currently over 1000 physicists and engineers from 105 Institutes in 30 countries. Its overall dimensions are 161626 m3 with a total weight of approximately 10 000 t. The experiment consists of 18 different detector systems each with its own specific technology choice and design constraints, driven both by the physics requirements and the experimental conditions expected at LHC. The most stringent design constraint is to cope with the extreme particle multiplicity anticipated in central Pb-Pb collisions. The different subsystems were optimized to provide high-momentum resolution as well as excellent Particle Identification (PID) over a broad range in momentum, up to the highest multiplicities predicted for LHC. This will allow for comprehensive studies of hadrons, electrons, muons, and photons produced in the collision of heavy nuclei. Most detector systems are scheduled to be installed and ready for data taking by mid-2008 when the LHC is scheduled to start operation, with the exception of parts of the Photon Spectrometer (PHOS), Transition Radiation Detector (TRD) and Electro Magnetic Calorimeter (EMCal). These detectors will be completed for the high-luminosity ion run expected in 2010. This paper describes in detail the detector components as installed for the first data taking in the summer of 2008.