1,743 research outputs found
Lyman-alpha forest-CMB cross-correlation and the search for the ionized baryons at high redshift
The intergalactic neutral hydrogen which is responsible for the Lyman alpha
forest of quasar absorption is a tracer of much larger amounts of ionised
hydrogen. The ionised component has yet to be detected directly, but is
expected to scatter CMB photons via the Sunyaev-Zel'dovich (SZ) effect. We use
hydrodynamic simulations of a LambdaCDM universe to create mock quasar spectra
and CMB sky maps. We find that the high-z Lya forest gas causes temperature
fluctuations of the order of 1 muK rms in the CMB on arcmin scales. The kinetic
and thermal SZ effects have a similar magnitude at z=3, with the thermal effect
becoming relatively weaker as expected at higher z. The CMB signal associated
with lines of sight having HI column densities > 10^18 cm^-2 is only marginally
stronger than that for lower column densities. The strong dependence of rms
temperature fluctuation on mean Lya absorbed flux, however, suggests that the
CMB signal effectively arises in lower density material. We investigate the use
of the cross-correlation of the Lya forest and the microwave background to
detect the SZ effect at redshifts 2-4. In so doing we are able to set direct
limits on the density of diffuse ionised intergalactic baryons. We carry out a
preliminary comparison at a mean redshift z=3 of 3488 quasar spectra from SDSS
Data Release 3 and the WMAP first year data. Assuming that the baryons are
clustered as in a LambdaCDM cosmology, and have the same mean temperature, the
cross-correlation yields a weak limit on the cosmic density of ionised baryons
Omega_(b,I), which is Omega_(b,I) < 0.8 at 95% confidence. With data from
upcoming CMB telescopes, we anticipate that a direct detection of the high
redshift ionised IGM will soon be possible, providing an important consistency
check on cosmological models.Comment: 14 pages, 10 figures, submitted to MNRA
Ionizing radiation fluctuations and large-scale structure in the Lyman-alpha forest
We investigate the large-scale inhomogeneities of the hydrogen ionizing
radiation field in the Universe at redshift z=3. Using a raytracing algorithm,
we simulate a model in which quasars are the dominant sources of radiation. We
make use of large scale N-body simulations of a LambdaCDM universe, and include
such effects as finite quasar lifetimes and output on the lightcone, which
affects the shape of quasar light echoes. We create Lya forest spectra that
would be generated in the presence of such a fluctuating radiation field,
finding that the power spectrum of the Lya forest can be suppressed by as much
as 15 % for modes with k=0.05-1 Mpc/h. This relatively small effect may have
consequences for high precision measurements of the Lya power spectrum on
larger scales than have yet been published. We also investigate another
radiation field probe, the cross-correlation of quasar positions and the Lya
forest. For both quasar lifetimes which we simulate (10^7 yr and 10^8 yr), we
expect to see a strong decrease in the Lya absorption close to other quasars
(the ``foreground'' proximity effect). We then use data from the Sloan Digital
Sky Survey First Data Release to make an observational determination of this
statistic. We find no sign of our predicted lack of absorption, but instead
increased absorption close to quasars. If the bursts of radiation from quasars
last on average < 10^6 yr, then we would not expect to be able to see the
foreground effect. However, the strength of the absorption itself seems to be
indicative of rare objects, and hence much longer total times of emission per
quasar. Variability of quasars in bursts with timescales > 10^4yr and < 10^6 yr
could reconcile these two facts.Comment: Submitted to ApJ, 21 pages, 17 postscript figures, emulateapj.st
Constraining quasar host halo masses with the strength of nearby Lyman-alpha forest absorption
Using cosmological hydrodynamic simulations we measure the mean transmitted
flux in the Lyman alpha forest for quasar sightlines that pass near a
foreground quasar. We find that the trend of absorption with pixel-quasar
separation distance can be fitted using a simple power law form including the
usual correlation function parameters r_{0} and \gamma so that ( = \sum
exp(-tau_eff*(1+(r/r_{0})^(-\gamma)))). From the simulations we find the
relation between r_{0} and quasar mass and formulate this as a way to estimate
quasar host dark matter halo masses, quantifying uncertainties due to
cosmological and IGM parameters, and redshift errors. With this method, we
examine data for ~3000 quasars from the Sloan Digital Sky Survey (SDSS) Data
Release 3, assuming that the effect of ionizing radiation from quasars (the
so-called transverse proximity effect) is unimportant (no evidence for it is
seen in the data.) We find that the best fit host halo mass for SDSS quasars
with mean redshift z=3 and absolute G band magnitude -27.5 is log10(M/M_sun) =
12.48^{+0.53}_{-0.89}. We also use the Lyman-Break Galaxy (LBG) and Lyman alpha
forest data of Adelberger et al in a similar fashion to constrain the halo mass
of LBGs to be log10(M/M_sun) = 11.13^{+0.39}_{-0.55}, a factor of ~20 lower
than the bright quasars. In addition, we study the redshift distortions of the
Lyman alpha forest around quasars, using the simulations. We use the quadrupole
to monopole ratio of the quasar-Lyman alpha forest correlation function as a
measure of the squashing effect. We find that this does not have a measurable
dependence on halo mass, but may be useful for constraining cosmic geometry.Comment: 10 pages, 11 figures, submitted to MNRA
Rethinking globalised resistance : feminist activism and critical theorising in international relations
This article argues that a feminist approach to the 'politics of resistance' offers a number of important empirical insights which, in turn, open up lines of theoretical inquiry which critical theorists in IR would do well to explore. Concretely, we draw on our ongoing research into feminist 'anti-globalisation' activism to rethink the nature of the subject of the politics of resistance, the conditions under which resistance emerges and how resistance is enacted and expressed. We begin by discussing the relationship of feminism to critical IR theory as a way of situating and explaining the focus and approach of our research project. We then summarise our key empirical arguments regarding the emergence, structure, beliefs, identities and practices of feminist 'anti-globalisation' activism before exploring the implications of these for a renewed critical theoretical agenda in IR
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Hydrogen Atom Transfer-Driven Enantioselective Minisci Reaction of Amides.
Minisci-type reactions constitute one of the most powerful methods for building up complexity around basic heteroarenes. The most desirable variants involve formal oxidative coupling of a C-H bond on each partner, leading back to the simplest possible starting materials. We herein disclose a method that enables such a coupling of linear amides and heteroarenes with full control of enantioselectivity at the newly formed stereocenter as well as site selectivity on both the heteroarene and the amide. This is achieved by the use of a chiral phosphoric acid catalyst in conjunction with diacetyl as a combined hydrogen atom transfer reagent and oxidant. Diacetyl is directly photoexcitable, and thus, no extraneous photocatalyst is required: an added feature that contributes to the simplicity and practicality of the protocol.EPSRC, GlaxoSmithKline, ER
Binary nanocrystalline alloys with strong glass forming interfacial regions: Complexion stability, segregation competition, and diffusion pathways
Stabilization of grain structure is important for nanocrystalline alloys, and
grain boundary segregation is a common approach to restrict coarsening. Doping
can alter grain boundary structure, with high temperature states such as
amorphous complexions being particularly promising for stabilization. Dopant
enrichment at grain boundaries may also result in precipitate formation, giving
rise to dopant partitioning between these two types of features. The present
study elucidates the effect of dopant choice on the retention of amorphous
complexions and the stabilization of grain size due to various forms of
interfacial segregation in three binary nanocrystalline Al-rich systems, Al-Mg,
Al-Ni, and Al-Y as investigated in detail using transmission electron
microscopy. Amorphous complexions were retained in Al-Y even for very slow
cooling conditions, suggesting that Y is the most efficient complexion
stabilizer. Moreover, this system exhibited the highest number density of
nanorod precipitates, reinforcing a recently observed correlation between
amorphous complexions and grain boundary precipitation events. The dopant
concentration at the grain boundaries in Al-Y is lower than in the other two
systems, although enrichment compared to the matrix is similar, while secondary
segregation to nanorod precipitate edges is much stronger in Al-Y than in Al-Mg
and Al-Ni. Y is generally observed to be an efficient doping additive, as it
stabilizes amorphous features and nanorod precipitates, and leaves very few
atoms trapped in the matrix. As a result, all grains in Al-Y remained nanosized
whereas abnormal grain growth occurred in the Al-Mg and Al-Ni alloys. The
present study demonstrates nanocrystalline stability via simple alloy
formulations and fewer dopant elements, which further encourage the usage of
bulk nanostructured materials
Requirements for Developing the Model of Spatial Orientation into an Applied Cockpit Warning System
Refinements have been made to a model of spatial disorientation (SD) to improve simulation of acceleration stimuli and visual-vestibular interactions. The improved model has been applied to aviation mishaps. The model is considered a technological countermeasure for SD because it is implemented as prototype software to aid the identification of mishap contributors in a way that should benefit didactic training. There is a more direct way this countermeasure can prevent SD, which is by adapting it for use as part of a cockpit warning system. The idea is to expand the model from one which explains mishaps post-hoc into one that warns pilots proactively whenever they are most likely to experience SD. This report introduces the general requirements that must be met to successfully expand the model for use as a proactive cockpit warning system, as well as the key criteria for determining whether it is effective
Design and Performance of the Advanced-Light-Source Double-crystal Monochromator
A new âCowan typeâ double-crystal monochromator, based on the boomerang design used at National Synchrotron Light Source (NSLS) beamline X-24A, has been developed for beamline 9.3.1 at the Advanced Light Source (ALS), a windowless ultrahigh vacuum beamline covering the 1-6 keV photon-energy range. Beamline 9.3.1 is designed to simultaneously achieve the goals of high energy resolution, high flux, and high brightness at the sample. The mechanical design of the monochromator has been simplified, and recent developments in technology have been included. Measured mechanical precision of the monochromator shows significant improvement over existing designs. In tests with x-rays at NSLS beamline X-23É
2, maximum deviations in the intensity of monochromatic light were just 7% during scans of several hundred eV in the vicinity of the Cr K edge (6 keV) with the monochromator operating without intensity feedback. Such precision is essential because of the high brightness of the ALS radiation and the overall length of beamline 9.3.1 (26 m)
Intermetallic particle heterogeneity controls shear localization in high-strength nanostructured Al alloys
The mechanical behavior of two nanocrystalline Al alloys, Al-Mg-Y and
Al-Fe-Y, is investigated with in-situ micropillar compression testing. Both
alloys were strengthened by a hierarchical microstructure including grain
boundary segregation, nanometer-thick amorphous complexions, carbide nanorod
precipitates with sizes of a few nanometers, and submicron-scale intermetallic
particles. The maximum yield strength of the Al-Mg-Y system is measured to be
950 MPa, exceeding that of the Al-Fe-Y system (680 MPa), primarily due to a
combination of more carbide nanorods and more amorphous complexions. Both
alloys exhibited yield strengths much higher than those of commercial Al
alloys, and therefore have great potential for structural applications.
However, some micropillar specimens were observed to plastically soften through
shear banding. Post-mortem investigation revealed that intermetallic-free
deformation pathways of a few micrometers in length were responsible for this
failure. Further characterization showed significant grain growth within the
shear band. The coarsened grains maintained the same orientation with each
other, pointing to grain boundary mechanisms for plastic flow, specifically
grain rotation and/or grain boundary migration. The presence of intermetallic
particles makes it difficult for both matrix and intermetallic grains to rotate
into the same orientation due to the different lattice parameters and slip
systems. Therefore, we are able to conclude that a uniform distribution of
intermetallic particles with an average spacing less than the percolation
length of shear localization can effectively prevent the maturation of shear
bands, offering a design strategy for high-strength nanocrystalline Al alloys
with both high strength and stable plastic flow
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