33 research outputs found
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 in protogalaxies with at
redshifts , 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 stars, while this
efficiency is lower for stars with . For more massive stars
(), the flux of ionizing photons is strong enough to
considerably reduce the gas density in the ionization zone, and the typical
lifetimes of stars ( Myr) are insufficient for the growth of
instabilities. The gas in a protogalaxy with with a 200
central star is completely ionized by the end of the star's lifetime;
in the case of a 120 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 , 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