Can gravitational infall energy lead to the observed velocity dispersion in DLAs?

The median observed velocity width v_90 of low-ionization species in damped Ly-alpha systems is close to 90 km/s, with approximately 10% of all systems showing v_90 > 210 km/s at z=3. We show that a relative shortage of such high-velocity neutral gas absorbers in state-of-the-art galaxy formation models is a fundamental problem, present both in grid-based and particle-based numerical simulations. Using a series of numerical simulations of varying resolution and box size to cover a wide range of halo masses, we demonstrate that energy from gravitational infall alone is insufficient to produce the velocity dispersion observed in damped Ly-alpha systems, nor does this dispersion arise from an implementation of star formation and feedback in our highest resolution (~ 45 pc) models, if we do not put any galactic winds into our models by hand. We argue that these numerical experiments highlight the need to separate dynamics of different components of the multiphase interstellar medium at z=3.Comment: 12 Pages, 9 Figures, accepted to ApJ, printing in colour recommende

Toward Five-dimensional Core-collapse Supernova Simulations

The computational difficulty of six-dimensional neutrino radiation hydrodynamics has spawned a variety of approximations, provoking a long history of uncertainty in the core-collapse supernova explosion mechanism. Under the auspices of the Terascale Supernova Initiative, we are honoring the physical complexity of supernovae by meeting the computational challenge head-on, undertaking the development of a new adaptive mesh refinement code for self-gravitating, six-dimensional neutrino radiation magnetohydrodynamics. This code--called {\em GenASiS,} for {\em Gen}eral {\em A}strophysical {\em Si}mulation {\em S}ystem--is designed for modularity and extensibility of the physics. Presently in use or under development are capabilities for Newtonian self-gravity, Newtonian and special relativistic magnetohydrodynamics (with `realistic' equation of state), and special relativistic energy- and angle-dependent neutrino transport--including full treatment of the energy and angle dependence of scattering and pair interactions.Comment: 5 pages. Proceedings of SciDAC 2005, Scientific Discovery through Advanced Computing, San Francisco, CA, 26-30 June 200

Instabilities in the Ionization Zones Around the First Stars

We consider the evolution of the ionization zone around Population III stars with $M_*\sim 25-200 M_\odot$ in protogalaxies with $M\sim 10^7 M_\odot$ at redshifts $z = 12$, assuming that the dark matter profile is a modified isothermal sphere. We study the conditions for the growth of instabilities in the ionization zones. The Rayleigh-Taylor and thermal instabilities develop efficiently in the ionization zones around 25-40 $M_\odot$ stars, while this efficiency is lower for stars with $\sim 120 M_\odot$. For more massive stars ($\sim 200 M_\odot$), the flux of ionizing photons is strong enough to considerably reduce the gas density in the ionization zone, and the typical lifetimes of stars ($\sim 2$ Myr) are insufficient for the growth of instabilities. The gas in a protogalaxy with $M\sim 10^7 M_\odot$ with a 200 $M_\odot$ central star is completely ionized by the end of the star's lifetime; in the case of a 120 $M_\odot$ central star, only one-third of the total mass of gas is ionized. Thus, ionizing photons from stars with M_*\simlt 120 M_\odot cannot leave protogalaxies with M\simgt 10^7 M_\odot. If the masses of the central stars are 25 and 40 $M_\odot$, the gas in protogalaxies of this mass remains essentially neutral. We discuss the consequences of the evolution of the ionization zones for the propagation of the envelope after the supernova explosions of the stars and the efficiency of enrichment of the intergalactic medium in heavy elements.Comment: 11 pages, 4 figure

Cosmological Radiative Transfer Codes Comparison Project I: The Static Density Field Tests

Radiative transfer simulations are now at the forefront of numerical astrophysics. They are becoming crucial for an increasing number of astrophysical and cosmological problems; at the same time their computational cost has come to the reach of currently available computational power. Further progress is retarded by the considerable number of different algorithms (including various flavours of ray-tracing and moment schemes) developed, which makes the selection of the most suitable technique for a given problem a non-trivial task. Assessing the validity ranges, accuracy and performances of these schemes is the main aim of this paper, for which we have compared 11 independent RT codes on 5 test problems: (0) basic physics, (1) isothermal H II region expansion and (2) H II region expansion with evolving temperature, (3) I-front trapping and shadowing by a dense clump, (4) multiple sources in a cosmological density field. The outputs of these tests have been compared and differences analyzed. The agreement between the various codes is satisfactory although not perfect. The main source of discrepancy appears to reside in the multi-frequency treatment approach, resulting in different thicknesses of the ionized-neutral transition regions and different temperature structure. The present results and tests represent the most complete benchmark available for the development of new codes and improvement of existing ones. To this aim all test inputs and outputs are made publicly available in digital form.Comment: 32 pages, 39 figures (all color), comments welcom

Cosmic microwave background constraints on the duration and timing of reionization from the South Pole Telescope

The epoch of reionization is a milestone of cosmological structure formation, marking the birth of the first objects massive enough to yield large numbers of ionizing photons. The mechanism and timescale of reionization remain largely unknown. Measurements of the CMB Doppler effect from ionizing bubbles embedded in large-scale velocity streams (the patchy kinetic Sunyaev-Zel'dovich effect) can constrain the duration of reionization. When combined with large-scale CMB polarization measurements, the evolution of the ionized fraction can be inferred. Using new multi-frequency data from the South Pole Telescope (SPT), we show that the ionized fraction evolved relatively rapidly. For our basic foreground model, we find the kinetic Sunyaev-Zel'dovich power sourced by reionization at l=3000 to be <= 2.1 micro K^2 at 95% CL. Using reionization simulations, we translate this to a limit on the duration of reionization of Delta z <= 4.4 (95% CL). We find that this constraint depends on assumptions about the angular correlation between the thermal Sunyaev-Zel'dovich power and the cosmic infrared background (CIB). Introducing the degree of correlation as a free parameter, we find that the limits on kSZ power weaken to <= 4.9 micro K^2, implying Delta z <= 7.9 (95% CL). We combine the SPT constraint on the duration of reionization with the WMAP7 measurement of the integrated optical depth to probe the cosmic ionization history. We find that reionization ended with 95% CL at z > 7.2 under the assumption of no tSZ-CIB correlation, and z>5.8 when correlations are allowed. Improved constraints from the full SPT data set in conjunction with upcoming Herschel and Planck data should detect extended reionization at >95% CL provided Delta z >= 4. (abbreviated)Comment: 16 pages, 13 figures, version accepted by ApJ, improved forecast of Herschel-SPT reionization constraint

Ionizing Radiation from z = 4-10 Galaxies

We compute the escape of ionizing radiation from galaxies in the redshift interval z=4-10, i.e., during and after the epoch of reionization, using a high-resolution set of galaxies, formed in fully cosmological simulations. The simulations invoke early, energetic feedback, and the galaxies evolve into a realistic population at z=0. Our galaxies cover nearly four orders of magnitude in masses (10^{7.8}-10^{11.5}\msun) and more than five orders in star formation rates (10^{-3.5}-10^{1.7}\msun\yr^{-1}), and we include an approximate treatment of dust absorption. We show that the source-averaged Lyman-limit escape fraction at z=10.4 is close to 80% declining monotonically with time as more massive objects build up at lower redshifts. Although the amount of dust absorption is uncertain to 1-1.5 dex, it is tightly correlated with metallicity; we find that dust is unlikely to significantly impact the observed UV output. These results support reionization by stellar radiation from low-luminosity dwarf galaxies and are also compatible with Lyman continuum observations and theoretical predictions at z\sim3-4.Comment: ApJ accepted, 9 figures, printing in colour recommended, comments welcome; replaced version: 2 new figures, updated conten