814 research outputs found
Fast Large Volume Simulations of the 21 cm Signal from the Reionization and pre-Reionization Epochs
While limited to low spatial resolution, the next generation low-frequency
radio interferometers that target 21 cm observations during the era of
reionization and prior will have instantaneous fields-of-view that are many
tens of square degrees on the sky. Predictions related to various statistical
measurements of the 21 cm brightness temperature must then be pursued with
numerical simulations of reionization with correspondingly large volume box
sizes, of order 1000 Mpc on one side. We pursue a semi-numerical scheme to
simulate the 21 cm signal during and prior to Reionization by extending a
hybrid approach where simulations are performed by first laying down the linear
dark matter density field, accounting for the non-linear evolution of the
density field based on second-order linear perturbation theory as specified by
the Zel'dovich approximation, and then specifying the location and mass of
collapsed dark matter halos using the excursion-set formalism. The location of
ionizing sources and the time evolving distribution of ionization field is also
specified using an excursion-set algorithm. We account for the brightness
temperature evolution through the coupling between spin and gas temperature due
to collisions, radiative coupling in the presence of Lyman-alpha photons and
heating of the intergalactic medium, such as due to a background of X-ray
photons. The hybrid simulation method we present is capable of producing the
required large volume simulations with adequate resolution in a reasonable time
so a large number of realizations can be obtained with variations in
assumptions related to astrophysics and background cosmology that govern the 21
cm signal.Comment: 14 pages and 15 figures. New version to match accepted version for
MNRAS. Code available in: http://www.SimFast21.or
Semi-numeric simulations of helium reionization and the fluctuating radiation background
Recent He II Lyman-alpha forest observations from 2.0 2.7. These results point to a fluctuating He-ionizing background, which may be due to the end of helium reionization of this era. We present a fast, semi-numeric procedure to approximate detailed cosmological simulations. We compute the distribution of dark matter halos, ionization state of helium, and density field at z = 3 in broad agreement with recent simulations. Given our speed and flexibility, we investigate a range of ionizing source and active quasar prescriptions. Spanning a large area of parameter space, we find order-of-magnitude fluctuations in the He II ionization rate in the post-reionization regime. During reionization, the fluctuations are even stronger and develop a bimodal distribution, in contrast to semi-analytic models and the hydrogen equivalent. These distributions indicate a low-level ionizing background even at significant He II fractions
Modification of the 21-cm power spectrum by X-rays during the epoch of reionisation
We incorporate a contribution to reionization from X-rays within analytic and
semi-numerical simulations of the 21-cm signal arising from neutral hydrogen
during the epoch of reionization. We explore the impact that X-ray ionizations
have on the power spectrum (PS) of 21-cm fluctuations by varying both the
average X-ray MFP and the fractional contribution of X-rays to reionization. In
general, prior to the epoch when the intergalactic medium is dominated by
ionized regions (H {\sevensize II} regions), X-ray-induced ionization enhances
fluctuations on spatial scales smaller than the X-ray MFP, provided that X-ray
heating does not strongly supress galaxy formation. Conversely, at later times
when \H2 regions dominate, small-scale fluctuations in the 21-cm signal are
suppressed by X-ray ionization. Our modelling also shows that the modification
of the 21-cm signal due to the presence of X-rays is sensitive to the relative
scales of the X-ray MFP, and the characteristic size of \H2 regions. We
therefore find that X-rays imprint an epoch and scale-dependent signature on
the 21-cm PS, whose prominence depends on fractional X-ray contribution. The
degree of X-ray heating of the IGM also determines the extent to which these
features can be discerned. We show that the MWA will have sufficient
sensitivity to detect this modification of the PS, so long as the X-ray photon
MFP falls within the range of scales over which the array is most sensitive
( Mpc). In cases in which this MFP takes a much smaller value,
an array with larger collecting area would be required.Comment: 15 pages, 6 figures, Accepted for publication in MNRAS X-ray heating
contribution now adde
The Impact of HI in Galaxies on 21-cm Intensity Fluctuations During the Reionisation Epoch
We investigate the impact of neutral hydrogen (HI) in galaxies on the
statistics of 21-cm fluctuations using analytic and semi-numerical modelling.
Following the reionisation of hydrogen the HI content of the Universe is
dominated by damped absorption systems (DLAs), with a cosmic density in HI that
is observed to be constant at a level equal to ~2% of the cosmic baryon density
from z~1 to z~5. We show that extrapolation of this constant fraction into the
reionisation epoch results in a reduction of 10-20% in the amplitude of 21-cm
fluctuations over a range of spatial scales. The assumption of a different
percentage during the reionisation era results in a proportional change in the
21-cm fluctuation amplitude. We find that consideration of HI in galaxies/DLAs
reduces the prominence of the HII region induced shoulder in the 21-cm power
spectrum (PS), and hence modifies the scale dependence of 21-cm fluctuations.
We also estimate the 21cm-galaxy cross PS, and show that the cross PS changes
sign on scales corresponding to the HII regions. From consideration of the
sensitivity for forthcoming low-frequency arrays we find that the effects of HI
in galaxies/DLAs on the statistics of 21-cm fluctuations will be significant
with respect to the precision of a PS or cross PS measurement. In addition,
since overdense regions are reionised first we demonstrate that the
cross-correlation between galaxies and 21-cm emission changes sign at the end
of the reionisation era, providing an alternative avenue to pinpoint the end of
reionisation. The sum of our analysis indicates that the HI content of the
galaxies that reionise the universe will need to be considered in detailed
modelling of the 21-cm intensity PS in order to correctly interpret
measurements from forthcoming low-frequency arrays.Comment: 11 pages, 6 figures. Submitted to MNRA
Ultraviolet Line Emission from Metals in the Low-Redshift Intergalactic Medium
We use a high-resolution cosmological simulation that includes hydrodynamics,
multiphase star formation, and galactic winds to predict the distribution of
metal line emission at z~0 from the intergalactic medium (IGM). We focus on two
ultraviolet doublet transitions, OVI 1032,1038 and CIV 1548,1551. Emission from
filaments with moderate overdensities is orders of magnitude smaller than the
background, but isolated emission from enriched, dense regions with
T~10^5-10^5.5 K and characteristic sizes of 50-100 kpc can be detected above
the background. We show that the emission from these regions is substantially
greater when we use the metallicities predicted by the simulation (which
includes enrichment through galactic winds) than when we assume a uniform IGM
metallicity. Luminous regions correspond to volumes that have recently been
influenced by galactic winds. We also show that the line emission is clustered
on scales ~1 h^-1 Mpc. We argue that although these transitions are not
effective tracers of the warm-hot intergalactic medium, they do provide a route
to study the chemical enrichment of the IGM and the physics of galactic winds.Comment: replaced by version to appear in ApJ (conclusions unchanged, one new
figure), 16 pages (emulateapj), 11 figures, version with higher resolution
figures available at
http://www.tapir.caltech.edu/~sfurlane/metals/coverpage.htm
How Universal is the Gunn-Peterson Trough at z~6?: A Closer Look at the Quasar SDSS J1148+5251
Detectable flux is visible in the Ly-alpha and Ly-beta troughs of the highest
redshift () quasar found to date, SDSS J1148+5251. This has previously
been interpreted as continuum contamination from an interloper galaxy at
z=4.94. We examine the Ly-gamma trough of SDSS J1148+5251 and show that this
interpretation is untenable: the spectrum does not show the continuum break in
a z=4.94 galaxy expected from absorption by the intervening Ly-alpha forest.
Therefore, flux must be leaking through at least one of the troughs from the
quasar itself. Contrary to previous claims, the flux ratios in the Ly-alpha and
Ly-beta troughs are consistent with pure transmission. From the Ly-gamma
trough, we place an upper bound on the effective Ly-alpha optical depth at
z~6.2 of tau < 14.3 (2 sigma). This implies a highly ionized IGM along this
line of sight and significant cosmic variance in the transition toward complete
Gunn-Peterson absorption. Detailed study of the observed transmission features
will shed light on this era.Comment: Submitted to ApJ Letter
Constraining the Collisional Nature of the Dark Matter Through Observations of Gravitational Wakes
We propose to use gravitational wakes as a direct observational probe of the
collisional nature of the dark matter. We calculate analytically the structure
of a wake generated by the motion of a galaxy in the core of an X-ray cluster
for dark matter in the highly-collisional and collisionless limits. We show
that the difference between these limits can be recovered from detailed X-ray
or weak lensing observations. We also discuss the sizes of sub-halos in these
limits. Preliminary X-ray data on the motion of NGC 1404 through the Fornax
group disfavors fluid-like dark matter but does not exclude scenarios in which
the dark matter is weakly collisional.Comment: 29 pages, 3 figures, submitted to Ap
Searching for the earliest galaxies in the 21 cm forest
We use a model developed by Xu et al. (2010) to compute the 21 cm line
absorption signatures imprinted by star-forming dwarf galaxies (DGs) and
starless minihalos (MHs). The method, based on a statistical comparison of the
equivalent width (W_\nu) distribution and flux correlation function, allows us
to derive a simple selection criteria for candidate DGs at very high (z >= 8)
redshift. We find that ~ 18% of the total number of DGs along a line of sight
to a target radio source (GRB or quasar) can be identified by the condition
W_\nu < 0; these objects correspond to the high-mass tail of the DG
distribution at high redshift, and are embedded in large HII regions. The
criterion W_\nu > 0.37 kHz instead selects ~ 11% of MHs. Selected candidate DGs
could later be re-observed in the near-IR by the JWST with high efficiency,
thus providing a direct probe of the most likely reionization sources.Comment: 8 pages, 3 figures. Accepted for publication in Science in China
Series
Application of experimental design methodologies in the enantioseparation of pharmaceuticals by capillary electrophoresis: a review
Chirality is one of the major issues in pharmaceutical research and industry. Capillary electrophoresis (CE) is an interesting alternative to the more frequently used chromatographic techniques in the enantioseparation of pharmaceuticals, and is used for the determination of enantiomeric ratio, enantiomeric purity, and in pharmacokinetic studies. Traditionally, optimization of CE methods is performed using a univariate one factor at a time (OFAT) approach; however, this strategy does not allow for the evaluation of interactions between experimental factors, which may result in ineffective method development and optimization. In the last two decades, Design of Experiments (DoE) has been frequently employed to better understand the multidimensional effects and interactions of the input factors on the output responses of analytical CE methods. DoE can be divided into two types: screening and optimization designs. Furthermore, using Quality by Design (QbD) methodology to develop CE-based enantioselective techniques is becoming increasingly popular. The review presents the current use of DoE methodologies in CE-based enantioresolution method development and provides an overview of DoE applications in the optimization and validation of CE enantioselective procedures in the last 25 years. Moreover, a critical perspective on how different DoE strategies can aid in the optimization of enantioseparation procedures is presented
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