69 research outputs found
Solar light-driven photocatalytic hydrogen evolution over ZnIn2S4 loaded with transition-metal sulfides
A series of Pt-loaded MS/ZnIn2S4 (MS = transition-metal sulfide: Ag2S, SnS, CoS, CuS, NiS, and MnS) photocatalysts was investigated to show various photocatalytic activities depending on different transition-metal sulfides. Thereinto, CoS, NiS, or MnS-loading lowered down the photocatalytic activity of ZnIn2S4, while Ag2S, SnS, or CuS loading enhanced the photocatalytic activity. After loading 1.0 wt.% CuS together with 1.0 wt.% Pt on ZnIn2S4, the activity for H2 evolution was increased by up to 1.6 times, compared to the ZnIn2S4 only loaded with 1.0 wt.% Pt. Here, transition-metal sulfides such as CuS, together with Pt, acted as the dual co-catalysts for the improved photocatalytic performance. This study indicated that the application of transition-metal sulfides as effective co-catalysts opened up a new way to design and prepare high-efficiency and low-cost photocatalysts for solar-hydrogen conversion
Batch solution of small PDEs with the OPS DSL
In this paper we discuss the challenges and optimisations opportunities when solving a large number of small, equally sized discretised PDEs on regular grids. We present an extension of the OPS (Oxford Parallel library for Structured meshes) embedded Domain Specific Language, and show how support can be added for solving multiple systems, and how OPS makes it easy to deploy a variety of transformations and optimisations. The new capabilities in OPS allow to automatically apply data structure transformations, as well as execution schedule transformations to deliver high performance on a variety of hardware platforms. We evaluate our work on an industrially representative finance simulation on Intel CPUs, as well as NVIDIA GPUs
Encounters between spherical galaxies II: systems with dark halo
We performe N-body simulations of encounters between spherical systems
surrounded by a spherical halo. Following a preceding paper with a similar aim,
the initial systems include a spherical Jaffe model for the luminous matter and
a Hernquist model for the halo. The merger remnants from this sample are mainly
slowly rotating, prolate spheroids with a radially anisotropic velocity
distribution. The results are compared with real-life ellipticals and with the
models without halo in paper I. We argue that elliptical galaxies with evidence
of dark matter could be formed in the field via a merger of spheroids
surrounded by a dark matter halo, while ellipticals with no evidence of dark
matter might be formed via a merger of two spheroids in a cluster.Comment: 12 pages, 15 figures, accepted for publication in MNRAS. High res.
version avaible at http://www.iac.es/folleto/research/Publi02/preprints05.ht
Modelling galactic spectra: I - A dynamical model for NGC3258
In this paper we present a method to analyse absorption line spectra of a
galaxy designed to determine the stellar dynamics and the stellar populations
by a direct fit to the spectra. This paper is the first one to report on the
application of the method to data. The modelling results in the knowledge of
distribution functions that are sums of basis functions. The practical
implementation of the method is discussed and a new type of basis functions is
introduced.
With this method, a dynamical model for NGC 3258 is constructed. This galaxy
can be successfully modelled with a potential containing 30% dark matter within
1r_e with a mass of 1.6x10^11 M_o. The total mass within 2r_e is estimated as
5x10^11 M_o, containing 63% dark matter. The model is isotropic in the centre,
is radially anisotropic between 0.2 and 2 kpc (0.88 r_e) and becomes
tangentially anisotropic further on. The photometry reveals the presence of a
dust disk near the centre
The degeneracy between star-formation parameters in dwarf galaxy simulations and the Mstar-Mhalo relation
We present results based on a set of N-Body/SPH simulations of isolated dwarf
galaxies. The simulations take into account star formation, stellar feedback,
radiative cooling and metal enrichment. The dark matter halo initially has a
cusped profile, but, at least in these simulations, starting from idealised,
spherically symmetric initial conditions, a natural conversion to a core is
observed due to gas dynamics and stellar feedback.
A degeneracy between the efficiency with which the interstellar medium
absorbs energy feedback from supernovae and stellar winds on the one hand, and
the density threshold for star formation on the other, is found. We performed a
parameter survey to determine, with the aid of the observed kinematic and
photometric scaling relations, which combinations of these two parameters
produce simulated galaxies that are in agreement with the observations.
With the implemented physics we are unable to reproduce the relation between
the stellar mass and the halo mass as determined by Guo et al. (2010), however
we do reproduce the slope of this relation.Comment: Accepted for publication in MNRAS | 12 pages, 8 figure
Measuring the non-thermal pressure in early type galaxy atmospheres: A comparison of X-ray and optical potential profiles in M87 and NGC1399
We compare the gravitational potential profiles of the elliptical galaxies
NGC 4486 (M87) and NGC 1399 (the central galaxy in the Fornax cluster) derived
from X-ray and optical data. This comparison suggests that the combined
contribution of cosmic rays, magnetic fields and micro-turbulence to the
pressure is ~10% of the gas thermal pressure in the cores of NGC 1399 and M87,
although the uncertainties in our model assumptions (e.g., spherical symmetry)
are sufficiently large that the contribution could be consistent with zero. In
the absence of any other form of non-thermal pressure support, these upper
bounds translate into upper limits on the magnetic field of ~10-20 muG at a
distance of 1'-2' from the centers of NGC1399 and M87. We show that these
results are consistent with the current paradigm of cool cluster cores, based
on the assumption that AGN activity regulates the thermal state of the gas by
injecting energy into the intra-cluster medium. The limit of ~10-20% on the
energy density in the form of relativistic protons applies not only to the
current state of the gas, but essentially to the entire history of the
intra-cluster medium, provided that cosmic ray protons evolve adiabatically and
that their spatial diffusion is suppressed.Comment: Accepted for MNRAS. 19 pages; 14 figures; expanded version in
response to comments from the refere
AIMES: advanced computation and I/O methods for earth-system simulations
Dealing with extreme scale Earth-system models is challenging from the computer science perspective, as the required computing power and storage capacity are steadily increasing.
Scientists perform runs with growing resolution or aggregate results from many similar smaller-scale runs with slightly different initial conditions (the so-called ensemble runs).
In the fifth Coupled Model Intercomparison Project (CMIP5), the produced datasets require more than three Petabytes of storage and the compute and storage requirements are increasing significantly for CMIP6.
Climate scientists across the globe are developing next-generation models based on improved numerical formulation leading to grids that are discretized in alternative forms such as an icosahedral (geodesic) grid.
The developers of these models face similar problems in scaling, maintaining and optimizing code.
Performance portability and the maintainability of code are key concerns of scientists as, compared to industry projects, model code is continuously revised and extended to incorporate further levels of detail.
This leads to a rapidly growing code base that is rarely refactored.
However, code modernization is important to maintain productivity of the scientist working
with the code and for utilizing performance provided by modern and future architectures.
The need for performance optimization is motivated by the evolution of the parallel architecture landscape from
homogeneous flat machines to heterogeneous combinations of processors with deep memory hierarchy.
Notably, the rise of many-core, throughput-oriented accelerators, such as GPUs, requires non-trivial code changes at minimum and, even worse, may necessitate a substantial rewrite of the existing codebase.
At the same time, the code complexity increases the difficulty for computer scientists and vendors to understand and optimize the code for a given system.
Storing the products of climate predictions requires a large storage and archival system which is expensive.
Often, scientists restrict the number of scientific variables and write interval to keep the costs
balanced.
Compression algorithms can reduce the costs significantly but can also increase the scientific yield of simulation runs.
In the AIMES project, we addressed the key issues of programmability, computational efficiency and I/O limitations that are common in next-generation icosahedral earth-system models.
The project focused on the separation of concerns between domain scientist, computational scientists, and computer scientists
Dark matter in early-type galaxies: dynamical modelling of IC1459, IC3370, NGC3379 and NGC4105
We analyse long-slit spectra of four early-type galaxies which extend from ~1
to ~3 effective radii: IC1459, IC3370, NGC3379 and NGC4105. We have extracted
the full line-of-sight velocity distribution (in the case of NGC3379 we also
used data from the literature) which we model using the two-integral approach.
Using two-integral modelling we find no strong evidence for dark haloes, but
the fits suggest that three-integral modelling is necessary. We also find that
the inferred constant mass-to-light ratio in all four cases is typical for
early-type galaxies. Finally, we also discuss the constraints on the
mass-to-light ratio which can be obtained using X-ray haloes in the case of
IC1459, NGC3379 and NGC4105 and compare the estimated values with the
predictions from the dynamical modelling.Comment: 42 pages, 18 figures, accepted for publication in MNRA
Comparison of an approximately isothermal gravitational potentials of elliptical galaxies based on X-ray and optical data
We analyze six X-ray bright elliptical galaxies, observed with Chandra and
XMM-Newton, and approximate their gravitational potentials by isothermal
spheres phi(r)=v_c^2 ln(r) over a range of radii from ~0.5 to ~25 kpc. We then
compare the circular speed v_c derived from X-ray data with the estimators
available from optical data. In particular we discuss two simple and robust
procedures for evaluating the circular speed of the galaxy using the observed
optical surface brightness and the line-of-sight velocity dispersion profiles.
The best fitting relation between the circular speeds derived from optical
observations of stars and X-ray observations of hot gas is v_{c,opt}~ \eta *
v_{c,X}, where \eta=1.10-1.15 (depending on the method), suggesting, albeit
with large statistical and systematic uncertainties, that non-thermal pressure
on average contributes ~20-30% of the gas thermal pressure.Comment: 24 pages, 15 figures; Accepted for publication in MNRA
The non-evolving internal structure of early-type galaxies: the case study SDSS J0728+3835 at z = 0.206
We study the internal dynamical structure of the early-type lens galaxy SDSS
J0728+3835 at z = 0.206. The analysis is based on two-dimensional kinematic
maps extending out to 1.7 effective radii obtained from Keck spectroscopy, on
lensing geometry and on stellar mass estimates obtained from multiband Hubble
Space Telescope imaging. The data are modelled under the assumptions of axial
symmetry supported by a two-integral distribution function (DF), by applying
the combined gravitational lensing and stellar dynamics code CAULDRON, and
yielding high-quality constraints for an early-type galaxy at cosmological
redshifts. Modelling the total density profile as a power-law of the form
rho_tot ~ 1/r^{gamma}, we find that it is nearly isothermal (logarithmic slope
gamma = 2.08^{+0.04}_{-0.02}), and quite flattened (axial ratio q =
0.60^{+0.08}_{-0.03}). The galaxy is mildly anisotropic (delta = 0.08 +/- 0.02)
and shows a fair amount of rotational support, in particular towards the outer
regions. We determine a dark matter fraction lower limit of 28 per cent within
the effective radius. The stellar contribution to the total mass distribution
is close to maximal for a Chabrier initial mass function (IMF), whereas for a
Salpeter IMF the stellar mass exceeds the total mass within the galaxy inner
regions. We find that the combination of a NFW dark matter halo with the
maximally rescaled luminous profile provides a remarkably good fit to the total
mass distribution over a broad radial range. Our results confirm and expand the
findings of the SLACS survey for early-type galaxies of comparable velocity
dispersion (sigma_SDSS = 214 +/- 11 km/s). The internal structure of J0728 is
consistent with that of local early-type galaxies of comparable velocity
dispersion as measured by the SAURON project, suggesting lack of evolution in
the past two billion years.Comment: 13 pages, 10 figures. MNRAS in press. Revised to match accepted
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