7,043 research outputs found
A 'moment-conserving' reformulation of GW theory
We show how to construct an effective Hamiltonian whose dimension scales
linearly with system size, and whose eigenvalues systematically approximate the
excitation energies of theory. This is achieved by rigorously expanding
the self-energy in order to exactly conserve a desired number of
frequency-independent moments of the self-energy dynamics. Recasting in
this way admits a low-scaling approach to build this
Hamiltonian, with a proposal to reduce this further to . This
relies on exposing a novel recursive framework for the density response moments
of the random phase approximation (RPA), where the efficient calculation of its
starting point mirrors the low-scaling approaches to compute RPA correlation
energies. The frequency integration of which distinguishes so many
different variants can be performed directly and cheaply in this moment
representation. Furthermore, the solution to the Dyson equation can be
performed exactly, avoiding analytic continuation, diagonal approximations or
iterative solutions to the quasiparticle equation, with the full-frequency
spectrum of all solutions obtained in a complete diagonalization of this
effective static Hamiltonian. We show how this approach converges rapidly with
respect to the order of the conserved self-energy moments, and is applied
across the benchmark dataset to obtain accurate spectra in
comparison to traditional implementations. We also show the ability to
systematically converge all-electron full-frequency spectra and high-energy
features beyond frontier excitations, as well as avoiding discontinuities in
the spectrum which afflict many other approaches
Serving on corporate boards plays a vital role in the career success of executives
They're more likely to be promoted as first-time CEOs and to get a raise, even without a promotion, write Steven Boivie, Scott D. Graffin, Abbie G. Oliver and Michael C. Wither
Cosmological Parameter Estimation Using 21 cm Radiation from the Epoch of Reionization
A number of radio interferometers are currently being planned or constructed
to observe 21 cm emission from reionization. Not only will such measurements
provide a detailed view of that epoch, but, since the 21 cm emission also
traces the distribution of matter in the Universe, this signal can be used to
constrain cosmological parameters at 6 < z < 20. The sensitivity of an
interferometer to the cosmological information in the signal may depend on how
precisely the angular dependence of the 21 cm 3-D power spectrum can be
measured. Utilizing an analytic model for reionization, we quantify all the
effects that break the spherical symmetry of the 3-D 21 cm power spectrum and
produce physically motivated predictions for this power spectrum. We find that
upcoming observatories will be sensitive to the 21 cm signal over a wide range
of scales, from larger than 100 to as small as 1 comoving Mpc. We consider
three methods to measure cosmological parameters from the signal: (1) direct
fitting of the density power spectrum to the signal, (2) using only the
velocity field fluctuations in the signal, (3) looking at the signal at large
enough scales such that all fluctuations trace the density field. With the
foremost method, the first generation of 21 cm observations should moderately
improve existing constraints on cosmological parameters for certain
low-redshift reionization scenarios, and a two year observation with the second
generation interferometer MWA5000 can improve constraints on Omega_w, Omega_m
h^2, Omega_b h^2, Omega_nu, n_s, and alpha_s. If the Universe is substantially
ionized by z = 12 or if spin temperature fluctuations are important, we show
that it will be difficult to place competitive constraints on cosmological
parameters with any of the considered methods.Comment: 20 pages, 12 figures, accepted by Ap
The SCUBA 8-mJy survey - II: Multiwavelength analysis of bright sub-mm sources
We present the results of a multi-wavelength study of the 19 most significant
sub-mm sources detected in the SCUBA 8-mJy survey. As described in Scott et al.
(2001), this survey covers ~260 arcmin^2 using the sub-millimetre camera SCUBA,
to a limiting source detection limit S(850um) ~ 8 mJy. One advantage of this
relatively bright flux-density limit is that accurate astrometric positions are
potentially achievable for every source using existing radio and/or mm-wave
interferometers. However, an associated advantage is that SED-based redshift
constraints should be more powerful than in fainter sub-mm surveys. Here we
therefore exploit the parallel SCUBA 450um data, in combination with existing
radio and ISO data at longer and shorter wavelengths to set constraints on the
redshift of each source. We also analyse new and existing optical and
near-infrared imaging of our SCUBA survey fields to select potential
identifications consistent with these constraints. Our derived SED-based
redshift constraints, and the lack of statistically significant associations
with even moderately bright galaxies allow us to conclude that all 19 sources
lie at z > 1, and at least half of them apparently lie at z > 2.Comment: 14 pages (including 30 figures and 4 tables) accepted by MNRAS.
Figure 1 and 2 and a higher quality version of the full paper are available
at http://astro.ic.ac.uk/~mfox
The coincidence and angular clustering of Chandra and SCUBA sources
NRC publication: N
The rarity of depth refugia from coral bleaching heat stress in the Western and Central Pacific Islands
Some researchers have suggested that corals living in deeper reefs may escape heat stress experienced by shallow corals. We evaluated the potential of deep coral reef refugia from bleaching stress by leveraging a long record of satellite-derived sea surface temperature data with a temporal, spatial, and depth precision of in situ temperature records. We calculated an in situ stress metric using a depth bias-adjusted threshold for 457 coral reef sites among 49 islands in the western and central Pacific Ocean over the period 2001–2017. Analysis of 1,453 heating events found no meaningful depth refuge from heat stress down to 38 m, and no significant association between depth and subsurface heat stress. Further, the surface metric underestimated subsurface stress by an average of 39.3%, across all depths. Combining satellite and in situ temperature data can provide bleaching-relevant heat stress results to avoid misrepresentation of heat stress exposure at shallow reefs
HerMES: the rest-frame UV emission and a lensing model for the z= 6.34 luminous dusty starburst galaxy HFLS3
We discuss the rest-frame ultraviolet emission from the starbursting galaxy HFLS3 at a redshift of 6.34. The galaxy was discovered in Herschel/SPIRE data due to its red color in the submillimeter wavelengths from 250 to 500 μm. Keck/NIRC2 K s -band adaptive optics imaging data showed two potential near-IR counterparts near HFLS3. Previously, the northern galaxy was taken to be in the foreground at z = 2.1, while the southern galaxy was assumed to be HFLS3's near-IR counterpart. The recently acquired Hubble/WFC3 and Advanced Camera for Surveys (ACS) imaging data show conclusively that both optically bright galaxies are in the foreground at z < 6. A new lensing model based on the Hubble imaging data and the millimeter-wave continuum emission yields a magnification factor of 2.2 ± 0.3, with a 95% confidence upper limit on the magnification of 3.5. When corrected for lensing, the instantaneous star formation rate is 1320 M ☉ yr–1, with the 95% confidence lower limit around 830 M ☉ yr–1. The dust and stellar masses of HFLS3 from the same spectral energy distribution (SED) models are at the level of 3 × 108 M ☉ and ~5 × 1010 M ☉, respectively, with large systematic uncertainties on assumptions related to the SED model. With Hubble/WFC3 images, we also find diffuse near-IR emission about 0.5 arcsec (~3 kpc) to the southwest of HFLS3 that remains undetected in the ACS imaging data. The emission has a photometric redshift consistent with either z ~ 6 or a dusty galaxy template at z ~ 2
A national appraisal of haemodialysis vascular access provision in Scotland
Purpose:
Published registry data demonstrate longstanding variation in the utilisation of different vascular access (VA) modalities between Scottish renal units; this may reflect different clinical processes between centres. A comprehensive appraisal was undertaken to understand the processes underpinning VA creation and maintenance across Scotland.
Methods:
A mixed methods approach was utilised. Fifty-two semi-structured interviews were conducted with patients and clinicians in all ten, adult and paediatric, Scottish renal units. Interview transcripts were subjected to thematic analysis. Clinical activity data were prospectively collected for six weeks, and correlated with registry data.
Results:
VA accounts for a large clinical workload. There was significant inter-centre variation in the utilisation of different VA modalities, and patients described frustrating, dissatisfying experiences. VA creation and maintenance pathways functioned best when nephrologists, surgeons and radiologists were co-located on the same campus with close multi-disciplinary working, protected clinical time, and proactive VA maintenance. No unit routinely measured or discussed procedure outcomes or strategic aspects of their service.
Conclusions:
Varied clinical outcomes reflected varied clinical processes. Optimised clinical pathways, staff education and measurement of clinical outcomes may improve VA service quality and facilitate safer, more effective, patient-centred care
- …