24 research outputs found

### 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 Ly$\alpha$ 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