2,169 research outputs found
Turbulence in the intracluster medium: simulations, observables & thermodynamics
We conduct two kinds of homogeneous isotropic turbulence simulations relevant
for the intracluster medium (ICM): (i) pure turbulence runs without radiative
cooling; (ii) turbulent heatingradiative cooling runs with global thermal
balance. For pure turbulence runs in the subsonic regime, the rms density and
surface brightness (SB) fluctuations vary as the square of the rms Mach number
(). However, with thermal balance, the density and SB
fluctuations are much larger. These scalings have implications
for translating SB fluctuations into a turbulent velocity, particularly for
cool cores. For thermal balance runs with large (cluster core) scale driving,
both the hot and cold phases of the gas are supersonic. For small scale (one
order of magnitude smaller than the cluster core) driving, multiphase gas forms
on a much longer timescale but is smaller. Both
small and large scale driving runs have velocities larger than the Hitomi
results from the Perseus cluster. Thus turbulent heating as the dominant
heating source in cool cluster cores is ruled out if multiphase gas is assumed
to condense out from the ICM. Next we perform thermal balance runs in which we
partition the input energy into thermal and turbulent parts and tune their
relative magnitudes. The contribution of turbulent heating has to be in order for turbulence velocities to match Hitomi observations. If the
dominant source of multiphase gas is not cooling from the ICM (but say uplift
from the central galaxy), the importance of turbulent heating cannot be
excluded.Comment: MNRAS accepted version; for movies see:
http://www.mso.anu.edu.au/~rajsekha/BT_movies.htm
Stellar Populations and Surface Brightness Fluctuations: New Observations and Models
We examine the use of surface brightness fluctuations (SBF) for both stellar
population and distance studies. New V-band SBF data are reported for five
Fornax cluster galaxies and combined with literature data to define a new
V-band SBF distance indicator. We use new stellar population models, based on
the latest Padua isochrones transformed empirically to the observational plane,
to predict SBF magnitudes and integrated colours for a wide range of population
ages and metallicities. We examine the sensitivity of the predictions to
changes in the isochrones, transformations, and IMF. The new models reproduce
the SBF data for globular clusters fairly well, especially if higher
metallicity globulars are younger. The models also give a good match to the
"fluctuation colors" of elliptical galaxies. In order to obtain theoretical
calibrations of the SBF distance indicators, we combine our single-burst models
into composite population models. These models reproduce the observed behavior
of the SBF magnitudes as a function of stellar population parameters, including
the steep colour dependence found for HST/WFPC2 F814W SBF data. Because the
theoretical SBF calibrations are fairly sensitive to uncertain details of
stellar evolution, the empirical calibrations are more secure. However, the
sensitivity of SBF to these finer details potentially makes it a powerful
constraint for stellar evolution and population synthesis. [abbridged]Comment: 24 pages with 17 embedded figures. MNRAS, in pres
A review on the complementarity of renewable energy sources: concept, metrics, application and future research directions
It is expected, and regionally observed, that energy demand will soon be
covered by a widespread deployment of renewable energy sources. However, the
weather and climate driven energy sources are characterized by a significant
spatial and temporal variability. One of the commonly mentioned solutions to
overcome the mismatch between demand and supply provided by renewable
generation is a hybridization of two or more energy sources in a single power
station (like wind-solar, solar-hydro or solar-wind-hydro). The operation of
hybrid energy sources is based on the complementary nature of renewable
sources. Considering the growing importance of such systems and increasing
number of research activities in this area this paper presents a comprehensive
review of studies which investigated, analyzed, quantified and utilized the
effect of temporal, spatial and spatio-temporal complementarity between
renewable energy sources. The review starts with a brief overview of available
research papers, formulates detailed definition of major concepts, summarizes
current research directions and ends with prospective future research
activities. The review provides a chronological and spatial information with
regard to the studies on the complementarity concept.Comment: 34 pages 7 figures 3 table
Fully Frustrated Ising System on a 3D Simple Cubic Lattice: Revisited
Using extensive Monte Carlo simulations, we clarify the critical behaviour of
the 3 dimensional simple cubic Ising Fully Frustrated system. We find two
transition temperatures and two long range ordered phases. Within the present
numerical accuracy, the transition at higher temperature is found to be second
order and we have extracted the standard critical exponent using finite size
scaling method. On the other hand, the transition at lower temperature is found
to be first order. It is argued that entropy plays a major role on determining
the low temperature state.Comment: 14 pages 14 figures iop style include
The Self-Organisation of Innovation Networks
This paper explores the self-organising principles of horizontally-integrated innovation networks. It isshown that such networks can self-organising in environments where the co-ordination and production ofnew knowledge is itself a complex, dynamic and highly non-linear processes. The paper argues thedevelopment of a self-organisation perspective of innovation networks has two advantages. First, itprovides a general framework of dynamic systems in which different strands of a highly fragmentedliterature can be drawn together. Second, formal self-organisation modelling techniques can provideinteresting new insights into the micro-macro processes driving dynamic innovation systems.Section 1 of the paper identifies the four key principles of self-organisation: local interaction, non-linearity,thermodynamic openness and emergence. Section 2 discusses important complementarities between self-organisationtheory and the ‘new’ theory of innovation, with the latter’s emphasis of the systemic nature ofknowledge production within innovation networks containing multiple private and public institutions thatare connected in highly complicated and non-linear ways. This paves the way for a formal model of self-organisinginnovation networks presented in section 3. Section 4 discusses the main properties of theoutputs generated by the model and its novel insights, section 5 summarising and considering the potentialadvantages for current and future research offered by the self-organisation approach.research and development ;
Modeling the HeII Transverse Proximity Effect: Constraints on Quasar Lifetime and Obscuration
The HeII transverse proximity effect - enhanced HeII Ly{\alpha} transmission
in a background sightline caused by the ionizing radiation of a foreground
quasar - offers a unique opportunity to probe the emission properties of
quasars, in particular the emission geometry (obscuration, beaming) and the
quasar lifetime. Building on the foreground quasar survey published in
Schmidt+2017, we present a detailed model of the HeII transverse proximity
effect, specifically designed to include light travel time effects, finite
quasar ages, and quasar obscuration. We post-process outputs from a
cosmological hydrodynamical simulation with a fluctuating HeII UV background
model, plus the added effect of the radiation from a single bright foreground
quasar. We vary the age and obscured sky fractions
of the foreground quasar, and explore the resulting
effect on the HeII transverse proximity effect signal. Fluctuations in IGM
density and the UV background, as well as the unknown orientation of the
foreground quasar, result in a large variance of the HeII Ly{\alpha}
transmission along the background sightline. We develop a fully Bayesian
statistical formalism to compare far UV HeII Ly{\alpha} transmission spectra of
the background quasars to our models, and extract joint constraints on
and for the six Schmidt+2017 foreground
quasars with the highest implied HeII photoionization rates. Our analysis
suggests a bimodal distribution of quasar emission properties, whereby one
foreground quasar, associated with a strong HeII transmission spike, is
relatively old and unobscured ,
whereas three others are either younger than or highly
obscured .Comment: 19 pages, 6 figures, submitted to Ap
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