Intergalactic Transmission and its Impact on the Ly{\alpha} Line

We study the intergalactic transmission of radiation in the vicinity of the Ly{\alpha} wavelength. Simulating sightlines through the intergalactic medium (IGM) in detailed cosmological hydrosimulations, the impact of the IGM on the shape of the line profile from Ly{\alpha} emitting galaxies at redshifts 2.5 to 6.5 is investigated. In particular we show that taking into account the correlation of the density and velocity fields of the IGM with the galaxies, the blue part of the spectrum may be appreciably reduced, even at relatively low redshifts. This may in some cases provide an alternative to the often-invoked outflow scenario, although it is concluded that this model is still a plausible explanation of the many asymmetric Ly{\alpha} profiles observed. Applying the calculated wavelength dependent transmission to simulated spectra from Ly{\alpha} emitting galaxies, we derive the fraction of photons that are lost in the IGM, in addition to what is absorbed internally in the galaxies due to dust. Moreover, by comparing the calculated transmission of radiation blueward of the Ly{\alpha} line, the total optical depth to Thomson scattering of cosmic microwave background, with corresponding observations, we are able to constrain the epoch when the Universe was reionized to z <~ 8.5.Comment: Substantially extended, ~30 references added, 1.5 page extra (article style) in particular on the impact of the IGM at z~5.8 and z~6.5, 2 extra figures, unnecessary fluff cut out, accepted for publication in Ap

Modeling Lyman continuum emission from young galaxies

Based on cosmological simulations, we model Lyman continuum emission from a sample of 11 high-redshift star forming galaxies spanning a mass range of a factor 20. Each of the 11 galaxies has been simulated both with a Salpeter and a Kroupa initial mass function (IMF). We find that the Lyman continuum (LyC) luminosity of an average star forming galaxy in our sample declines from z=3.6 to 2.4 due to the steady gas infall and higher gas clumping at lower redshifts, increasingly hampering the escape of ionizing radiation. The galaxy-to-galaxy variation of apparent LyC emission at a fixed redshift is caused in approximately equal parts by the intrinsic variations in the LyC emission and by orientation effects. The combined scatter of an order of magnitude can explain the variance in the far-UV spectra of high-redshift galaxies detected by Shapley et al. (2006). Our results imply that the cosmic galactic ionizing UV luminosity would be monotonically decreasing from z=3.6 to 2.4, curiously anti-correlated with the star formation rate in the smaller galaxies, which on average rises during this redshift interval.Comment: 8 pages, 12 figures, ApJ, in pres

Escape of ionizing radiation from star forming regions in young galaxies

Using results from high-resolution galaxy formation simulations in a standard Lambda-CDM cosmology and a fully conservative multi-resolution radiative transfer code around point sources, we compute the energy-dependent escape fraction of ionizing photons from a large number of star forming regions in two galaxies at five different redshifts from z=3.8 to 2.39. All escape fractions show a monotonic decline with time, from (at the Lyman-limit) ~6-10% at z=3.6 to ~1-2% at z=2.39, due to higher gas clumping at lower redshifts. It appears that increased feedback can lead to higher f_esc at z>3.4 via evacuation of gas from the vicinity of star forming regions and to lower f_esc at z<2.39 through accumulation of swept-up shells in denser environments. Our results agree well with the observational findings of \citet{inoue..06} on redshift evolution of f_esc in the redshift interval z=2-3.6.Comment: four pages, four figures, submitted to ApJ

The Long Term: Six-dimensional Core-collapse Supernova Models

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: 23 pages. Proceedings of Open Issues in Understanding Core Collapse Supernovae, National Institute for Nuclear Theory, University of Washington, 22-24 June 2004, World Scientific, in pres

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

Adaptive Mesh Refinement Simulations of the Ionization Structure and Kinematics of Damped Lyα\alpha Systems with Self-consistent Radiative Transfer

We use high resolution Eulerian hydrodynamics simulations to study kinematic properties of the low ionization species in damped Ly-alpha systems at redshift z=3. Our adaptive mesh refinement simulations include most key ingredients relevant for modeling neutral gas in high-column density absorbers: hydrodynamics, gravitational collapse, continuum radiative transfer and gas chemistry, but no star formation. We model high-resolution Keck spectra with unsaturated low ion transitions in two Si II lines (1526 and 1808 A), and compare simulated line profiles to the data from the SDSS DLA survey. We find that with increasing grid resolution the models show a trend in convergence towards the observed distribution of HI column densities. While in our highest resolution model we recover the cumulative number of DLAs per unit absorption distance, none of our models predicts DLA velocity widths as high as indicated by the data, suggesting that feedback from star formation might be important. At z=3 a non-negligible fraction of DLAs with column densities below 10^21 cm^-2 is caused by tidal tails due to galaxy-galaxy interactions in more massive halo environments. Lower column density absorbers with N_HI < 10^21.4 cm^-2 are sensitive to changes in the UV background resulting in a 10% reduction of the cumulative number of DLAs for twice the quasar background relative to the fiducial value. We find that the mass cut-off below which a large fraction of dwarf galaxies cannot retain gas after reionization is 7*10^7 msun, lower than the previous estimates. Finally, we show that models with self-shielding commonly used in the literature produce significantly lower DLA velocity widths than the full radiative transfer runs.Comment: 13 pages, 11 figures, updated version, accepted to Ap

Chasing Lyman alpha-emitting galaxies at z = 8.8

With a total integration time of 168 hours and a narrowband (NB) filter tuned to Lyman alpha at z = 8.8, the UltraVISTA survey has set out to find some of the most distant galaxies, on the verge of the Epoch of Reionization. Previous calculations of the expected number of detected Lya-emitting galaxies (LAEs) at this redshift did not explicitly take into account the radiative transfer (RT) of Lya. In this work we combine a theoretical model for the halo mass function with numerical results from high-res cosmological hydrosimulations with LyC+Lya RT, assessing the visibility of LAEs residing in these halos. Uncertainties such as cosmic variance and the anisotropic escape of Lya are taken into account, and it is predicted that once the survey has finished, the probabilities of detecting none, one, or more than one are ~90%, ~10%, and ~1%; a significantly smaller success rate compared to earlier predictions, due to the combined effect of a highly neutral IGM scattering Lya to such large distances from the galaxy that they fall outside the observational aperture, and to the actual depth of the survey being less than predicted. Because the IGM affects NB and broadband (BB) magnitudes differently, we argue for a relaxed color selection criterion of NB - BB ~ +0.85. But since the flux is continuum-dominated, even if a galaxy is detectable in the NB its probability of being selected as a NB excess object is <~35%. Various properties of galaxies at this redshift are predicted, e.g. UV and Lya LFs, M*-Mh relation, spectral shape, optimal aperture, and the anisotropic escape of Lya through both a dusty ISM and a partly neutral IGM. Finally, we describe and publish a fast numerical code for adding numbers with asymmetric uncertainties ("x_{-sigma_1}^{+sigma_2}") proving to be significantly better than the standard, but wrong, way of adding upper and lower uncertainties in quadrature separately.Comment: Submitted to A&A, comments are welcom

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

Calculating the inhomogeneous reionization of the universe

A numerical scheme for the solution of the three-dimensional, frequency- and time-dependent radiative transfer equation with variable optical depth is developed for modelling the reionization of the Universe. Until now, the main difficulty in simulating the inhomogeneous reionization has been the treatment of cosmological radiative transfer. The proposed approach is drastically different from previous studies, which either resorted to a very simplified, parametric treatment of radiative transfer, or relied on one-dimensional models. The algorithm presented here is based on explicit multidimensional advection of wavefronts at the speed of light, combined with the implicit solution of the local chemical rate equations separately at each point. I implement the ray-tracing version of this algorithm on a desktop workstation and check its performance on a wide variety of test problems, showing that explicit advection at the speed of light is an attractive choice for simulation of astrophysical ionization fronts, particularly when one is interested in covering a wide range of optical depths within a 3D clumpy medium. This scheme is then applied to the calculation of time-dependent, multi-frequency radiative transfer during the epoch of first object formation in the Universe. In a series of models, the 2.5 Mpc (comoving) simulation volume is evolved between the redshifts of z = 15 and z = 10 for different scenarios of star formation and quasar activity. The highest numerical resolution employed is 64³ (spatial) x 10² (angular) x 3 (frequency), and at each point in space I calculate various stages of hydrogen and helium ionization accounting for nine chemical species altogether. It is shown that at higher numerical resolution these models of inhomogeneous reionization can be used to predict the observational signatures of the earliest astrophysical objects in the Universe. At present, the calculations are accurate enough to resolve primordial objects to the scale typical of globular clusters, 1O[superscript 6] M⊙.Science, Faculty ofPhysics and Astronomy, Department ofGraduat