Hy Trac

Can sterile neutrinos be the dark matter

We use the Ly-alpha forest power spectrum measured by the Sloan Digital Sky Survey and high-resolution spectroscopy observations in combination with cosmic microwave background and galaxy clustering constraints to place limits on a sterile neutrino as a dark matter candidate in the warm dark matter scenario. Such a neutrino would be created in the early Universe through mixing with an active neutrino and would suppress structure on scales smaller than its free-streaming scale. We ran a series of high-resolution hydrodynamic simulations with varying neutrino masses to describe the effect of a sterile neutrino on the Ly-alpha forest power spectrum. We find that the mass limit is m(s) >13 keV at 95% C.L. (9 keV at 99.9%), which is above the upper limit allowed by x-ray constraints, excluding this candidate from being all of the dark matter in this model.

Radiative Transfer Simulations of Cosmic Reionization. I. Methodology and Initial Results

We present a new hybrid code for large-volume, high-resolution simulations of cosmic reionization, which utilizes an N-body algorithm for dark matter, physically motivated prescriptions for baryons and star formation, and an adaptive ray-tracing algorithm for radiative transfer of ionizing photons. Two test simulations, each with 3 billion particles and 400 million rays in a 50 Mpc h-1 box, have been run to give initial results. Halos are resolved down to virial temperatures of 104 K for the redshift range of interest in order to robustly model star formation and clumping factors. This is essential to correctly account for ionization and recombination processes. We find that the halos and sources are strongly biased with respect to the underlying dark matter, re-enforcing the requirement of large simulation boxes to minimize cosmic variance and to obtain a qualitatively correct picture of reionization. We model the stellar initial mass function (IMF) by following the spatially dependent gas metallicity evolution, and distinguish between the first generation, Population III (PopIII) stars and the second generation, Population II (PopII) stars. The PopIII stars with a top-heavy IMF produce an order of magnitude more ionizing photons at high redshifts z 10, resulting in a more extended reionization. In our simulations, complete overlap of H II regions occurs at z ≈ 6.5, and the computed mass- and volume-weighted residual H I fractions at 5 z 6.5 are both in good agreement with high-redshift quasar absorption measurements from the Sloan Digital Sky Survey (SDSS). The values for the Thomson optical depth are consistent within 1 - σ of the current best-fit value from third-year Wilkinson Microwave Anisotropy Probe (WMAP) results.

The Atacama Cosmology Telescope: a measurement of the cosmic microwave background power spectrum at 148 and 218 GHz from the 2008 southern survey

We present measurements of the cosmic microwave background (CMB) power spectrum made by the Atacama Cosmology Telescope at 148 GHz and 218 GHz, as well as the cross-frequency spectrum between the two channels. Our results clearly show the second through the seventh acoustic peaks in the CMB power spectrum. The measurements of these higher-order peaks provide an additional test of the ΛCDM cosmological model. At l>3000, we detect power in excess of the primary anisotropy spectrum of the CMB. At lower multipoles 500 < l < 3000, we find evidence for gravitational lensing of the CMB in the power spectrum at the 2.8σ level. We also detect a low level of Galactic dust in our maps, which demonstrates that we can recover known faint, diffuse signals.

Cosmic Reionization and the 21-cm signal: Comparison between an analytical model and a simulation

We measure several properties of the reionization process and the corresponding low-frequency 21 cm signal associated with the neutral hydrogen distribution, using a large volume, high-resolution simulation of cosmic reionization. The brightness temperature of the 21 cm signal is derived by postprocessing this numerical simulation with a semianalytical prescription. Our study extends to high redshifts (z ~ 25) where, in addition to collisional coupling, our postprocessed simulations take into account the inhomogeneities in the heating of the neutral gas by X-rays and the effect of an inhomogeneous Ly α radiation field. Unlike the well-studied case in which spin temperature is assumed to be significantly greater than the temperature of the cosmic microwave background due to uniform heating of the gas by X-rays, spatial fluctuations in both the Ly α radiation field and X-ray intensity affect predictions related to the brightness temperature at z > 10, during the early stages of reionization and gas heating. The statistics of the 21 cm signal from our simulation are then compared to existing analytical models in the literature, and we find that these analytical models provide a reasonably accurate description of the 21 cm power spectrum at z 10, and, with upcoming interferometric data, these differences in return can provide a way to better understand the astrophysical processes during reionization.

The cosmic code comparison project

Current and upcoming cosmological observations allow us to probe structures on smaller and smaller scales, entering highly nonlinear regimes. In order to obtain theoretical predictions in these regimes, large cosmological simulations have to be carried out. The promised high accuracy from observations makes the simulation task very demanding: the simulations have to be at least as accurate as the observations. This requirement can only be fulfilled by carrying out an extensive code verification program. The first step of such a program is the comparison of different cosmology codes including gravitational interactions only. In this paper, we extend a recently carried out code comparison project to include five more simulation codes. We restrict our analysis to a small cosmological volume which allows us to investigate properties of halos. For the matter power spectrum and the mass function, the previous results hold, with the codes agreeing at the 10% level over wide dynamic ranges. We extend our analysis to the comparison of halo profiles and investigate the halo count as a function of local density. We introduce and discuss ParaView as a flexible analysis tool for cosmological simulations, the use of which immensely simplifies the code comparison task.

Imprint of Inhomogeneous Hydrogen Reionization on the Temperature Distribution of the Intergalactic Medium

We study the impact of inhomogeneous hydrogen reionization on the thermal evolution of the intergalactic medium (IGM) using hydrodynamic + radiative transfer simulations where reionization is completed either early ( -->z ~ 9) or late ( -->z ~ 6). In general, we find that low-density gas near large-scale overdensities is ionized and heated earlier than gas in the large-scale, underdense voids. Furthermore, at a later time the IGM temperature is inversely related to the reionization redshift because gas that is heated earlier has more time to cool through adiabatic expansion and Compton scattering. Thus, at the end of reionization the median temperature-density relation is an inverted power law with slope -->γ − 1 ~ − 0.2, in both models. However, at fixed density there is up to order unity scatter in the temperature due to the distribution of reionization redshifts. Because of the complex equation of state, the evolved IGM temperature-density relations for the redshift range -->4 z 6 can still have significant curvature and scatter. These features must be taken into account when interpreting the Lyα absorption in high-redshift quasar spectra.

A Primer on Eulerian Computational Fluid Dynamics for Astrophysics

We present a pedagogical review of some of the methods employed in Eulerian computational fluid dynamics (CFD). Fluid mechanics is governed by the Euler equations, which are conservation laws for mass, momentum, and energy. The standard approach to Eulerian CFD is to divide space into finite volumes or cells and store the cell-averaged values of conserved hydro quantities. The integral Euler equations are then solved by computing the flux of the mass, momentum, and energy across cell boundaries. We review both first- order and second-order flux assignment schemes. All linear schemes are either dispersive or diffusive. The nonlinear, second-order accurate total variation diminishing (TVD) approach provides high-resolution capturing of shocks and prevents unphysical oscillations. We review the relaxing TVD scheme, a simple and robust method to solve systems of conservation laws such as the Euler equations. A three-dimensional relaxing TVD code is applied to the Sedov-Taylor blast-wave test. The propagation of the blast wave is accurately captured and the shock front is sharply resolved. We apply a three-dimensional self-gravitating hydro code to simulating the formation of blue straggler stars through stellar mergers and present some numerical results. A sample three- dimensional relaxing TVD code is provided in the Appendix.

The Atacama Cosmology Telescope: Extragalactic Sources at 148 GHz in the 2008 Survey

We report on extragalactic sources detected in a 455 deg2 map of the southern sky made with data at a frequency of 148 GHz from the Atacama Cosmology Telescope (ACT) 2008 observing season. We provide a catalog of 157 sources with flux densities spanning two orders of magnitude: from 15 mJy to 1500 mJy. Comparison to other catalogs shows that 98% of the ACT detections correspond to sources detected at lower radio frequencies. Three of the sources appear to be associated with the brightest cluster galaxies of low-redshift X-ray-selected galaxy clusters. Estimates of the radio to millimeter-wave spectral indices and differential counts of the sources further bolster the hypothesis that they are nearly all radio sources, and that their emission is not dominated by re-emission from warm dust. In a bright (>50 mJy) 148 GHz selected sample with complete cross-identifications from the Australia Telescope 20 GHz survey, we observe an average steepening of the spectra between 5, 20, and 148 GHz with median spectral indices of α5-20 = –0.07 ± 0.06, α20-148 = –0.39 ± 0.04, and α5-148 = –0.20 ± 0.03. When the measured spectral indices are taken into account, the 148 GHz differential source counts are consistent with previous measurements at 30 GHz in the context of a source count model dominated by radio sources. Extrapolating with an appropriately rescaled model for the radio source counts, the Poisson contribution to the spatial power spectrum from synchrotron-dominated sources with flux density less than 20 mJy is C Sync = (2.8 ± 0.3) × 10–6μK2.

Extended Lyα Emission around Star-forming Galaxies

Ly? photons that escape the interstellar medium of star-forming galaxies may be resonantly scattered by neutral hydrogen atoms in the circumgalactic and intergalactic media, thereby increasing the angular extent of the galaxys Ly? emission. We present predictions of this extended, low surface brightness Ly? emission based on radiative transfer modeling in a cosmological reionization simulation. The extended emission can be detected from stacked narrowband images of Ly? emitters (LAEs) or of Lyman break galaxies (LBGs). Its average surface brightness profile has a central cusp, then flattens to an approximate plateau beginning at an inner characteristic scale below ~0.2?Mpc (comoving), then steepens again beyond an outer characteristic scale of ~1?Mpc. The inner scale marks the transition from scattered light of the central source to emission from clustered sources, while the outer scale marks the spatial extent of scattered emission from these clustered sources. Both scales tend to increase with halo mass, UV luminosity, and observed Ly? luminosity. The extended emission predicted by our simulation is already within reach of deep narrowband photometry using large ground-based telescopes. Such observations would test radiative transfer models of emission from LAEs and LBGs, and they would open a new window on the circumgalactic environment of high-redshift star-forming galaxies.

RADIATIVE TRANSFER MODELING OF Lyα EMITTERS. II. NEW EFFECTS ON GALAXY CLUSTERING

We study the clustering properties of z~5.7 Lyman-alpha emitters (LAEs) in a cosmological reionization simulation with a full Lya radiative transfer calculation. Lya radiative transfer substantially modifies the intrinsic Lya emission properties, compared to observed ones, depending on the density and velocity structure environment around the Lya emitting galaxy. This environment-dependent Lya selection introduces new features in LAE clustering, suppressing (enhancing) the line-of-sight (transverse) density fluctuations and giving rise to scale-dependent galaxy bias. In real space, the contours of the three-dimensional two-point correlation function of LAEs appear to be prominently elongated along the line of sight on large scales, an effect that is opposite to and much stronger than the linear redshift-space distortion effect. The projected two-point correlation function is greatly enhanced in amplitude by a factor of up to a few, compared to the case without the environment dependent selection effect. The new features in LAE clustering can be understood with a simple, physically motivated model, where Lya selection depends on matter density, velocity, and their gradients. We discuss the implications and consequences of the effects on galaxy clustering from Lya selection in interpreting clustering measurements and in constraining cosmology and reionization from LAEs.