10,940 research outputs found
Lyman Alpha Radiative Transfer in a Multi-Phase Medium
Hydrogen Ly-alpha is our primary emission-line window into high redshift
galaxies. Surprisingly, despite an extensive literature, Ly-alpha radiative
transfer in the most realistic case of a dusty, multi-phase medium has not
received detailed theoretical attention. We investigate resonant scattering
through an ensemble of dusty, moving, optically thick gas clumps. We treat each
clump as a scattering particle and use Monte Carlo simulations of surface
scattering to quantify continuum and Ly-alpha surface scattering angles,
absorption probabilities, and frequency redistribution, as a function of the
gas dust content. This atomistic approach speeds up the simulations by many
orders of magnitude, making possible calculations which are otherwise
intractable. With these surface scattering results, we develop an analytic
framework for estimating escape fractions and line widths as a function of gas
geometry, motion, and dust content. We show that the key geometric parameter is
the average number of surface scatters for escape in the absence of absorption.
We consider two interesting applications: (i) Equivalent widths. Ly-alpha can
preferentially escape from a dusty multi-phase ISM if most of the dust lies in
cold neutral clouds, possibly explaining anomalously high EWs seen in many high
redshift/submm sources. (ii) Multi-phase galactic outflows. We show the
characteristic profile is asymmetric with a broad red tail, and relate the
profile features to the outflow speed and gas geometry. Many future
applications are envisaged. [Abridged]Comment: Submitted to MNRAS. 27 pages, 25 figure
Fast CMB Power Spectrum Estimation of Temperature and Polarisation with Gabor Transforms
We extend the analysis of Gabor transforms on a Cosmic Microwave Background
(CMB) temperature map (Hansen, Gorski and Hivon 2002) to polarisation. We study
the temperature and polarisation power spectra on the cut sky, the so-called
pseudo power spectra. The transformation kernels relating the full-sky
polarisation power spectra and the polarisation pseudo power spectra are found
to be similar to the kernel for the temperature power spectrum. This fact is
used to construct a fast power spectrum estimation algorithm using the pseudo
power spectrum of temperature and polarisation as data vectors in a maximum
likelihood approach. Using the pseudo power spectra as input to the likelihood
analysis solves the problem of having to invert huge matrices which makes the
standard likelihood approach infeasible.Comment: 32 pages, 25 figures, submitted to MNRA
The galaxy halo formation in the absence of violent relaxation and a universal density profile of the halo center
While N-body simulations testify for a cuspy profile of the central region of
the dark matter haloes, observations favor a shallow, cored density profile of
the central region of, at least, some spiral galaxies and dwarf spheroidals. We
show that a central profile, very close to the observed one, inevitably forms
in the center of dark matter haloes if we make a supposition about a moderate
energy relaxation of the system during the halo formation. If we assume the
energy exchange between dark matter particles during the halo collapse to be
not too intensive, the profile is universal: it depends almost not at all on
the properties of the initial perturbation and is very akin, but not identical,
to the Einasto profile with small Einasto index . We estimate the
size of the 'central core' of the distribution, i.e., the extent of the very
central region with a respectively gentle profile, and show that the cusp
formation is unlikely, even if the dark matter is cold. The obtained profile is
in a good agreement with observational data for, at least, some types of
galaxies, but clearly disagrees with N-body simulations.Comment: 8 pages, 4 figure
Gauge invariant grid discretization of Schr\"odinger equation
Using the Wilson formulation of lattice gauge theories, a gauge invariant
grid discretization of a one-particle Hamiltonian in the presence of an
external electromagnetic field is proposed. This Hamiltonian is compared both
with that obtained by a straightforward discretization of the continuous
Hamiltonian by means of balanced difference methods, and with a tight-binding
Hamiltonian. The proposed Hamiltonian and the balanced difference one are used
to compute the energy spectrum of a charged particle in a two-dimensional
parabolic potential in the presence of a perpendicular, constant magnetic
field. With this example we point out how a "naive" discretization gives rise
to an explicit breaking of the gauge invariance and to large errors in the
computed eigenvalues and corresponding probability densities; in particular,
the error on the eigenfunctions may lead to very poor estimates of the mean
values of some relevant physical quantities on the corresponding states. On the
contrary, the proposed discretized Hamiltonian allows a reliable computation of
both the energy spectrum and the probability densities.Comment: 7 pages, 4 figures, discussion about tight-binding Hamiltonians adde
Myths and Truths Concerning Estimation of Power Spectra
It is widely believed that maximum likelihood estimators must be used to
provide optimal estimates of power spectra. Since such estimators require
require of order N_d^3 operations they are computationally prohibitive for N_d
greater than a few tens of thousands. Because of this, a large and
inhomogeneous literature exists on approximate methods of power spectrum
estimation. These range from manifestly sub-optimal, but computationally fast
methods, to near optimal but computationally expensive methods. Furthermore,
much of this literature concentrates on the power spectrum estimates rather
than the equally important problem of deriving an accurate covariance matrix.
In this paper, I consider the problem of estimating the power spectrum of
cosmic microwave background (CMB) anisotropies from large data sets. Various
analytic results on power spectrum estimators are derived, or collated from the
literature, and tested against numerical simulations. An unbiased hybrid
estimator is proposed that combines a maximum likelihood estimator at low
multipoles and pseudo-C_\ell estimates at high multipoles. The hybrid estimator
is computationally fast, nearly optimal over the full range of multipoles, and
returns an accurate and nearly diagonal covariance matrix for realistic
experimental configurations (provided certain conditions on the noise
properties of the experiment are satisfied). It is argued that, in practice,
computationally expensive methods that approximate the N_d^3 maximum likelihood
solution are unlikely to improve on the hybrid estimator, and may actually
perform worse. The results presented here can be generalised to CMB
polarization and to power spectrum estimation using other types of data, such
as galaxy clustering and weak gravitational lensing.Comment: 27 pages, 15 figures, MNRAS in press. Resubmission matches accepted
versio
Fast, exact CMB power spectrum estimation for a certain class of observational strategies
We describe a class of observational strategies for probing the anisotropies
in the cosmic microwave background (CMB) where the instrument scans on rings
which can be combined into an n-torus, the {\em ring torus}. This class has the
remarkable property that it allows exact maximum likelihood power spectrum
estimation in of order operations (if the size of the data set is )
under circumstances which would previously have made this analysis intractable:
correlated receiver noise, arbitrary asymmetric beam shapes and far side lobes,
non-uniform distribution of integration time on the sky and partial sky
coverage. This ease of computation gives us an important theoretical tool for
understanding the impact of instrumental effects on CMB observables and hence
for the design and analysis of the CMB observations of the future. There are
members of this class which closely approximate the MAP and Planck satellite
missions. We present a numerical example where we apply our ring torus methods
to a simulated data set from a CMB mission covering a 20 degree patch on the
sky to compute the maximum likelihood estimate of the power spectrum
with unprecedented efficiency.Comment: RevTeX, 14 pages, 5 figures. A full resolution version of Figure 1
and additional materials are at http://feynman.princeton.edu/~bwandelt/RT
Suppression of HD-cooling in protogalactic gas clouds by Lyman-Werner radiation
It has been shown that HD molecules can form efficiently in metal-free gas
collapsing into massive protogalactic halos at high redshift. The resulting
radiative cooling by HD can lower the gas temperature to that of the cosmic
microwave background, T_CMB=2.7(1+z)K, significantly below the temperature of a
few 100 K achievable via H_2-cooling alone, and thus reduce the masses of the
first generation of stars. Here we consider the suppression of HD-cooling by UV
irradiation in the Lyman-Werner (LW) bands. We include photo-dissociation of
both H_2 and HD, and explicitly compute the self-shielding and shielding of
both molecules by neutral hydrogen as well as the shielding of HD by H_2. We
use a simplified dynamical collapse model, and follow the chemical and thermal
evolution of the gas, in the presence of a UV background. We find that a LW
flux of J_crit = 1e-22 erg/cm^2/sr/s/Hz is able to suppress HD cooling and thus
prevent collapsing primordial gas from reaching temperatures below 100 K. The
main reason for the lack of HD cooling for J>J_crit is the partial
photo-dissociation of H_2, which prevents the gas from reaching sufficiently
low temperatures (T<150K) for HD to become the dominant coolant; direct HD
photo-dissociation is unimportant except for a narrow range of fluxes and
column densities. Since the prevention of HD-cooling requires only partial H_2
photo-dissociation, the critical flux J_crit is modest, and is below the UV
background required to reionize the universe at redshift z=10-20. We conclude
that HD-cooling can reduce the masses of typical stars only in rare halos
forming well before the epoch of reionization.Comment: 14 pages with 9 figures, submitted to MNRA
Fossil HII Regions: Self-Limiting Star Formation at High Redshift
Recent results by the WMAP satellite suggest that the intergalactic medium
was significantly reionized at redshifts as high as z~17. At this early epoch,
the first ionizing sources likely appeared in the shallow potential wells of
mini-halos with virial temperatures T < 10^4 K. Once such an ionizing source
turns off, its surrounding HII region Compton cools and recombines.
Nonetheless, we show that the ``fossil'' HII regions left behind remain at high
adiabats, prohibiting gas accretion and cooling in subsequent generations of
mini-halos. Thus, early star formation is self-limiting. We quantify this
effect to show that star formation in mini-halos cannot account for the bulk of
the electron scattering opacity measured by WMAP, which must be due to more
massive objects. We argue that gas entropy, rather than IGM metallicity,
regulates the evolution of the global ionizing emissivity, and impedes full
reionization until lower redshifts. We discuss several important consequences
of this early entropy floor for reionization. It reduces gas clumping,
curtailing the required photon budget for reionization. An entropy floor also
prevents H2 formation and cooling, due to reduced gas densities: it greatly
enhances feedback from UV photodissociation of H2. An early X-ray background
would also furnish an entropy floor to the entire IGM; thus, X-rays impede
rather than enhance H2 formation. Future 21cm observations may probe the
topology of fossil HII regions.Comment: Submitted to MNRA
Regulation of caspase-3 processing by cIAP2 controls the switch between pro-inflammatory activation and cell death in microglia.
Cell Death and Disease is an open-access journal published by Nature Publishing Group. This work is licensed under a Creative Commons Attribution 4.0 International Licence. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in the credit line; if the material is not included under the Creative Commons licence, users will need to obtain permission from the licence holder to reproduce the material.The activation of microglia, resident immune cells of the central nervous system, and inflammation-mediated neurotoxicity are typical features of neurodegenerative diseases, for example, Alzheimer's and Parkinson's diseases. An unexpected role of caspase-3, commonly known to have executioner role for apoptosis, was uncovered in the microglia activation process. A central question emerging from this finding is what prevents caspase-3 during the microglia activation from killing those cells? Caspase-3 activation occurs as a two-step process, where the zymogen is first cleaved by upstream caspases, such as caspase-8, to form intermediate, yet still active, p19/p12 complex; thereafter, autocatalytic processing generates the fully mature p17/p12 form of the enzyme. Here, we show that the induction of cellular inhibitor of apoptosis protein 2 (cIAP2) expression upon microglia activation prevents the conversion of caspase-3 p19 subunit to p17 subunit and is responsible for restraining caspase-3 in terms of activity and subcellular localization. We demonstrate that counteracting the repressive effect of cIAP2 on caspase-3 activation, using small interfering RNA targeting cIAP2 or a SMAC mimetic such as the BV6 compound, reduced the pro-inflammatory activation of microglia cells and promoted their death. We propose that the different caspase-3 functions in microglia, and potentially other cell types, reside in the active caspase-3 complexes formed. These results also could indicate cIAP2 as a possible therapeutic target to modulate microglia pro-inflammatory activation and associated neurotoxicity observed in neurodegenerative disorders
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