2,303 research outputs found
Population III star formation in a Lambda CDM universe, II: Effects of a photodissociating background
We examine aspects of primordial star formation in the presence of a
molecular hydrogen-dissociating ultraviolet background. We compare a set of AMR
hydrodynamic cosmological simulations using a single cosmological realization
but with a range of ultraviolet background strengths in the Lyman-Werner band.
This allows us to study the effects of Lyman-Werner radiation on suppressing H2
cooling at low densities as well as the high-density evolution of the
collapsing core in a self-consistent cosmological framework. We find that the
addition of a photodissociating background results in a delay of the collapse
of high density gas at the center of the most massive halo in the simulation
and, as a result, an increase in the virial mass of this halo at the onset of
baryon collapse. We find that, contrary to previous results, Population III
star formation is not suppressed for J, but occurs even with
backgrounds as high as J. We find that H2 cooling leads to collapse
despite the depressed core molecular hydrogen fractions due to the elevated H2
cooling rates at K. We observe a relationship between the
strength of the photodissociating background and the rate of accretion onto the
evolving protostellar cloud core, with higher LW background fluxes resulting in
higher accretion rates. Finally, we find that the collapsing halo cores in our
simulations do not fragment at densities below cm
regardless of the strength of the LW background, suggesting that Population III
stars forming in halos with T K may still form in isolation.Comment: 46 pages, 14 figures (9 color). Accepted by the Astrophysical
Journal, some minor revision
From Afghanistan 1979 to Afghanistan 2001: How Three Current High School History Textbooks Frame the Origins of the âWar on Terrorâ Historical Analysis and Interviews with William Blum, Noam Chomsky and James Loewen
This work examines how three current eleventh grade U.S. high school history textbooks report on the origins of the âwar on terror.â The researcher chose one textbook from each of the three leading publishing houses that supply the high school market: Holt McDougal, Prentice Hall, and McGraw-Hill. The scope of the researcherâs inquiry covers the 1979 Soviet invasion of Afghanistan to the U.S. invasion in 2001. The nomenclature of the âwar on terror,â Reaganâs âwar on terror,â U.S. support for the Afghan Mujahideen, the Persian Gulf War, and the decision to invade Afghanistan are examined. Interviews with authors William Blum, James Loewen, and Noam Chomsky are also reported. Findings indicate factual errors regarding historical events, as well as numerous omissions of historical context. Such errors and omissions may lead the high school reader to form inaccurate conclusions regarding current U.S. foreign policy regarding terrorism. Therefore, it is the responsibility of the instructor to devote considerable research to this topic so to supplement the inadequacies of the textbook
The Fate of the First Galaxies. I. Self-Consistent Cosmological Simulations with Radiative Transfer
In cold dark matter (CDM) cosmogonies, low-mass objects play an important
role in the evolution of the universe. Not only are they the first luminous
objects to shed light in a previously dark universe, but, if their formation is
not inhibited by their own feedback, they dominate the galaxy mass function
until redshift z \sim 5. In this paper we present and discuss the
implementation of a 3D cosmological code that includes most of the needed
physics to simulate the formation and evolution of the first galaxies with a
self-consistent treatment of radiative feedback. The simulation includes
continuum radiative transfer using the ``Optically Thin Variable Eddington
Tensor'' (OTVET) approximation and line-radiative transfer in the H_2
Lyman-Werner bands of the background radiation. We include detailed chemistry
for H_2 formation/destruction, molecular and atomic cooling/heating processes,
ionization by secondary electrons, and heating by Ly\alpha resonant scattering.
We find that the first galaxies ("small-halos") are characterized by a
bursting star formation, self-regulated by a feedback process that acts on
cosmological scales. Their formation is not suppressed by feedback processes;
therefore, their impact on cosmic evolution cannot be neglected. The main focus
of this paper is on the methodology of the simulations, and we only briefly
introduce some of the results. An extensive discussion of the results and the
nature of the feedback mechanism are the focus of a companion paper.Comment: Accepted for publication on ApJ, 33 pages, including 14 figures and 2
tables. Movies and a higher quality version of the paper (figures) are
available at: http://casa.colorado.edu/~ricotti/MOVIES.htm
Ultra-low power generation of twin photons in a compact silicon ring resonator
We demonstrate efficient generation of correlated photon pairs by spontaneous
four wave mixing in a 5 \mu m radius silicon ring resonator in the telecom band
around 1550 nm. By optically pumping our device with a 200 \mu W continuous
wave laser, we obtain a pair generation rate of 0.2 MHz and demonstrate photon
time correlations with a coincidence-to-accidental ratio as high as 250. The
results are in good agreement with theoretical predictions and show the
potential of silicon micro-ring resonators as room temperature sources for
integrated quantum optics applications.Comment: 8 pages, 3 figure
Galactic Wind Signatures around High Redshift Galaxies
We carry out cosmological chemodynamical simulations with different strengths
of supernova (SN) feedback and study how galactic winds from star-forming
galaxies affect the features of hydrogen (HI) and metal (CIV and OVI)
absorption systems in the intergalactic medium at high redshift. We find that
the outflows tend to escape to low density regions, and hardly affect the dense
filaments visible in HI absorption. As a result, the strength of HI absorption
near galaxies is not reduced by galactic winds, but even slightly increases. We
also find that a lack of HI absorption for lines of sight (LOS) close to
galaxies, as found by Adelberger et al., can be created by hot gas around the
galaxies induced by accretion shock heating. In contrast to HI, metal
absorption systems are sensitive to the presence of winds. The models without
feedback can produce the strong CIV and OVI absorption lines in LOS within 50
kpc from galaxies, while strong SN feedback is capable of creating strong CIV
and OVI lines out to about twice that distance. We also analyze the mean
transmissivity of HI, CIV, and OVI within 1 h Mpc from star-forming
galaxies. The probability distribution of the transmissivity of HI is
independent of the strength of SN feedback, but strong feedback produces LOS
with lower transmissivity of metal lines. Additionally, strong feedback can
produce strong OVI lines even in cases where HI absorption is weak. We conclude
that OVI is probably the best tracer for galactic winds at high redshift.Comment: 16 pages, 16 figures, ApJ in press. Higher resolution version
available at http://www.ociw.edu/~dkawata/research/papers.htm
An integrated source of spectrally filtered correlated photons for large scale quantum photonic systems
We demonstrate the generation of quantum-correlated photon-pairs combined
with the spectral filtering of the pump field by more than 95dB using Bragg
reflectors and electrically tunable ring resonators. Moreover, we perform
demultiplexing and routing of signal and idler photons after transferring them
via a fiber to a second identical chip. Non-classical two-photon temporal
correlations with a coincidence-to-accidental ratio of 50 are measured without
further off-chip filtering. Our system, fabricated with high yield and
reproducibility in a CMOS process, paves the way toward truly large-scale
quantum photonic circuits by allowing sources and detectors of single photons
to be integrated on the same chip.Comment: 4 figure
Nonlinear characterisation of a silicon integrated Bragg waveguide filter
Bragg waveguides are promising optical filters for pump suppression in
spontaneous Four-Wave Mixing (FWM) photon sources. In this work, we investigate
the generation of unwanted photon pairs in the filter itself. We do this by
taking advantage of the relation between spontaneous and classical FWM, which
allows for the precise characterisation of the nonlinear response of the
device. The pair generation rate estimated from the classical measurement is
compared with the theoretical value calculated by means of a full quantum model
of the filter, which also allows to investigate the spectral properties of the
generated pairs. We find a good agreement between theory and experiment,
confirming that stimulated FWM is a valuable approach to characterise the
nonlinear response of an integrated filter, and that the pairs generated in a
Bragg waveguide are not a serious issue for the operation of a fully integrated
nonclassical source
Racial Differences in Neutrophil Response.
PosterBacterial lipopolysaccharide (LPS), or endotoxin, is a mediator of inflammation. Repeated translocation of endotoxin from oral and intestinal bacteria into the bloodstream has been associated with low-grade systemic inflammation which in-turn increases the risk for systemic disease. A recent IUSD study linked experimental gingivitis to low-grade endotoxemia in both African-Americans and Caucasians. Interestingly, the study also reported differences in neutrophil numbers and oxidative burst activity between the two races. The aim of this preliminary study was to assess the in vitro neutrophil response to low dose LPS priming and subsequent activation with formyl-methionyl-leucyl-phenylalanine (fMLP) by Caucasian (C, n=6) and African American (AA, n=6) males 18 â 40 years of age. Following 6% polysucrose sedimentation of whole blood to reduce red blood cell contamination, fresh neutrophils were isolated by centrifugation over HistopaqueÂź separation media. Neutrophils were resuspended in RPMI medium supplemented with 5% autologous serum, primed with 1 ng/ml LPS for 30 or 60 min and then activated with fMLP. Subsequently, cell-free culture media were collected, aliquoted and stored frozen until tested by ELISA for levels of myeloperoxidase (MPO), bactericidal permeability increasing protein (BPI) and acyloxyacyl hydrolase (AOAH) as markers of neutrophil activation. Activated neutrophils from C subjects released significantly higher levels of BPI compared to AA subjects (p=0.0077 & 0.0197, 30 and 60 minute prime, respectively). Similarly, the mean MPO levels in culture supernatants were higher for C males although the differences were not significant. AOAH was undetectable in the cell culture supernatants. In conclusion, neutrophils from C males displayed a stronger response (BPI and MPO) to LPS than neutrophils from AA male subjects suggesting a biological basis for the reported racial disparity in neutrophil response. (Supported by the IUPUI Office of the Vice Chancellor for Research
Energy correlations of photon pairs generated by a silicon microring resonator probed by Stimulated Four Wave Mixing
Compact silicon integrated devices, such as micro-ring resonators, have
recently been demonstrated as efficient sources of quantum correlated photon
pairs. The mass production of integrated devices demands the implementation of
fast and reliable techniques to monitor the device performances. In the case of
time-energy correlations, this is particularly challenging, as it requires high
spectral resolution that is not currently achievable in coincidence
measurements. Here we reconstruct the joint spectral density of photons pairs
generated by spontaneous four-wave mixing in a silicon ring resonator by
studying the corresponding stimulated process, namely stimulated four wave
mixing. We show that this approach, featuring high spectral resolution and
short measurement times, allows one to discriminate between nearly-uncorrelated
and highly-correlated photon pairs.Comment: 7 pages, 4 figure
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