27 research outputs found
Did Massive Primordial Stars Preenrich the Lyman Alpha Forest?
We examine the dynamical evolution and statistical properties of the
supernova ejecta of massive primordial stars in a cosmological framework to
determine whether this first population of stars could have enriched the
universe to the levels and dispersions seen by the most recent observations of
the Lyman-Alpha forest. We evolve a lambda CDM model in a 1 Mpc^3 volume to a
redshift of z = 15 and add ``bubbles'' of metal corresponding to the supernova
ejecta of the first generation of massive stars in all dark matter halos with
masses greater than 5 times 10^5 solar masses. These initial conditions are
then evolved to z = 3 and the distribution and levels of metals are compared to
observations. In the absence of further star formation the primordial metal is
initially contained in halos and filaments. Photoevaporation of metal-enriched
gas due to the metagalactic ultraviolet background radiation at the epoch of
reionization (z ~ 6) causes a sharp increase of the metal volume filling
factor. At z = 3, ~ 2.5% of the simulation volume (approx. 20% of the total gas
mass) is filled with gas enriched above a metallicity of 10^-4 Z_solar, and
less than 0.6% of the volume is enriched above a metallicity of 10^-3 Z_solar.
This suggests that, even with the most optimistic prescription for placement of
primordial supernova and the amount of metals produced by each supernova, this
population of stars cannot entirely be responsible for the enrichment of the
Lyman- forest to the levels and dispersions seen by current
observations unless we have severely underestimated the duration of the Pop III
epoch. However, comparison to observations show that Pop III supernovae can be
significant contributors to the very low overdensity Lyman-Alpha forest.Comment: 4 pages, 3 figures (color). Accepted to ApJ Letters. Replaced version
has some correction
A Statistical Analysis of Intergalactic Medium Transmission Approaching Reionization
We use hydrodynamic cosmological simulations in a 9.6 Mpc box to explore the
evolution of the intergalactic medium (IGM) transmissivity from z=2 through the
epoch of reionization. Reionization is achieved through an ultraviolet
background (UVB) that includes evolving stellar and QSO source populations. We
construct and analyze noiseless synthetic HI Lya absorption spectra along lines
of sight through our continuously evolving box and find a smooth evolution of
the effective optical depth under a power law up to the epoch of reionization.
Crossing into the epoch of reionization, both the mean transmitted flux (MTF)
and variance to the mean transmitted flux sharply deviate from a smooth
evolution and hence such observables have large margins of error. Despite the
statistical uncertainty in inferring the reionization profile from spectra, the
end of an opacity phase transition of the IGM correlates well with the redshift
when both the mean and variance of the transmitted flux rapidly deviate from
the post-reionization profile. However, an unobtainable number of lines of
sight is needed to allow an estimate of the MTF with less than 10% relative
margin of error. In addition to optical transmission, we compare the predicted
dark gap length distribution with observations. We show that this statistic is
sensitive to spectral resolution at reionization redshifts, but overall in
agreement with results by Songaila and Cowie (2002). Finally, we derive a
positive correlation between the mean optical depth within a gap and the size
of the gap, in attempt to relate "transmission statistics" to "dark gap
statistics" in high redshift studies of the IGM.Comment: 53 pages, 24 figure
Cosmological parameters sigma_8, the baryon density, and the UV background intensity from a calibrated measurement of H I Lyman-alpha absorption at z = 1.9
We identify a concordant model for the intergalactic medium (IGM) at redshift
z=1.9 that uses popular values for cosmological and astrophysical parameters
and accounts for all baryons with an uncertainty of 6%. We have measured the
amount of absorption, DA, in the Ly-alpha forest at redshift 1.9 in spectra of
77 QSO from the Kast spectrograph. We calibrated the continuum fits with
realistic artificial spectra, and we found that averaged over all 77 QSOs the
mean continuum level is within 1-2% of the correct value. Absorption from all
lines in the Ly-alpha forest at z=1.9 removes DA=15.1 +/- 0.7% of the flux
between 1070 and 1170 (rest) Angstroms. This is the first measurement using
many QSOs at this z, and the first calibrated measurement at any redshift.
Metal lines absorb 2.3 +/- 0.5%, and LLS absorb 1.0 +/- 0.4% leaving 11.8 +/-
1.0% from the lower density bulk of the IGM. Averaging over Delta z=0.1 or 154
Mpc, the dispersion is 6.1 +/- 0.3% including LLS and metal lines, or 3.9
(+0.5, -0.7)% for the lower density IGM alone, consistent with the usual
description of large scale structure. LLS and metal lines are major
contributors to the variation in the mean flux, and they make the flux field
significantly non-Gaussian. We find that a hydrodynamic simulation on a 1024
cubed grid in a 75.7 Mpc box reproduces the observed DA from the low density
IGM with parameters values H_o=71 km/s/Mpc, Omega_Lambda=0.73, Omega_m=0.27,
Omega_b=0.044, sigma_8=0.9 and a UV background that has an ionization rate that
is 1.08 +/- 0.08 times the prediction by Madau, Haardt & Rees (1999).Comment: Submitted to Ap
Creating a content delivery network for general science on the internet backbone using XCaches
A general problem faced by computing on the grid for opportunistic users is
that delivering cycles is simpler than delivering data to those cycles. In this
project we show how we integrated XRootD caches placed on the internet backbone
to implement a content delivery network for general science workflows. We will
show that for some workflows on different science domains like high energy
physics, gravitational waves, and others the combination of data reuse from the
workflows together with the use of caches increases CPU efficiency while
decreasing network bandwidth use
The Lyman-alpha forest at redshifts 0.1 -- 1.6: good agreement between a large hydrodynamic simulation and HST spectra
We give a comprehensive statistical description of the Lyman-alpha absorption
from the intergalactic medium in a hydrodynamic simulation at redshifts
0.1-1.6, the range of redshifts covered by HST spectra of QSOs. We use the ENZO
code to make a 76 comoving Mpc cube simulation using 75 kpc cells, for a Hubble
constant of 71 km/s/Mpc. The best prior work, by \citet{dave99},used an SPH
simulation in a 15.6 Mpc box with an effective resolution of 245 kpc and
slightly different cosmological parameters. At redshifts z=2 this simulation is
different from data. \citet{tytler07b} found that the simulated spectra at z=2
have too little power on large scales, Lyman-alpha lines are too wide, there is
a lack high column density lines, and there is a lack of pixels with low flux.
Here we present statistics at z<1.6, including the flux distribution, the mean
flux, the effective opacity, and the power and correlation of the flux. We also
give statistics of the lyman alpha lines including the line width distribution,
the column density distribution, the number of lines per unit equivalent width
and redshift, and the correlation between the line width and column density. We
find that the mean amount of absorption in the simulated spectra changes
smoothly with redshift with DA(z)=0.01(1+z)^{2.25}. Both the trend and absolute
values are close to measurements of HST spectra by \citet{kirkman07a}. The
column density and line width distributions are also close to those measured
from HST spectra by \citet{janknecht06a}, except for the mode of the line width
distribution which is smaller in the HST spectra. Although some differences
that we saw at z=2 are too subtle to be seen in existing HST spectra, overall,
the simulation gives an good description of HST spectra at 0.1<z<1.6
The Effect of Large-Scale Power on Simulated Spectra of the Lya forest
We study the effects of box size on ENZO simulations of the intergalactic
medium (IGM) at z = 2. We follow statistics of the cold dark matter (CDM) and
the Lya absorption. We find that the larger boxes have fewer pixels with
significant absorption (flux < 0.96) and more pixels in longer stretches with
little or no absorption, and they have wider Lya lines. We trace these effect
back to the additional power in larger boxes from longer wavelength modes. The
IGM in our larger boxes is hotter, from increased pressure heating due to
faster hydrodynamical infall. When we increase the photoheating in smaller
boxes to compensate, their Lya statistics change to mimic those of a box of
twice the size. Statistics converge towards their value in the largest (76.8
Mpc) box, except for the most common value of the CDM density which continues
to rise. When we compare to errors with data, we find that our 76.8 Mpc box is
larger than we need for the mean flux, barely large enough for the column
density distribution and the power spectrum of the flux, and too small for the
line widths. This box with 75 kpc cells has approximately the same mean flux as
QSO spectra, but the Lya lines are too wide by 2.6 km/s, there are too few
lines with log H I column densities > 10^17 cm^-2, and the power of the flux is
too low by 20 - 50%, from small to large scales. Four times smaller cell size
does not resolve these differences, nor do simple changes to the ultraviolet
background that drives the H and He II ionization. It is hard to see how
simulations using popular cosmological and astrophysical parameters can match
Lyman-alpha forest data at z=2
H I gas in higher density regions of the intergalactic medium
Using H I absorption alone, we attempt to separate H I absorption lines in
quasar spectra into two categories; HDLs (Higher Density Lines) and LDLs (Lower
Density Lines), and we discuss the difference in their physical properties. We
deblend and fit all H I lines with Voigt profiles, and make an unbiased sample
of H I lines covering a wide column density range (12 < log N_HI < 19 cm^-2).
To reduce the influence of line blending, we simultaneously fit several Lyman
series lines. As a result of a two-point correlation analysis, we found that
higher column density H I lines are clustering at v < 200 km/s, while
lower ones at v < 100 km/s. We define HDLs as H I lines with 15 < log
N_HI
15 cm^-2, and LDLs as others with 12 < log N_HI < 15 cm^-2. We found that the
HDLs have smaller minimum b-values for a given column density than the LDLs.
This difference is successfully reproduced by our Hydrodynamic simulation. The
LDLs seem to be cool or shock-heated diffuse IGM gas, while the HDLs are likely
to be cooler dense gas near to galaxies.Comment: 22 pages, 7 figures, accepted for publication in the Astronomical
Journa
Background Monte Carlo Samples for a Future Hadron Collider
A description of Standard Model background Monte Carlo samples produced for
studies related to future hadron colliders