1,265 research outputs found
The resolution bias: low resolution feedback simulations are better at destroying galaxies
Feedback from super-massive black holes (SMBHs) is thought to play a key role
in regulating the growth of host galaxies. Cosmological and galaxy formation
simulations using smoothed particle hydrodynamics (SPH), which usually use a
fixed mass for SPH particles, often employ the same sub-grid Active galactic
nuclei (AGN) feedback prescription across a range of resolutions. It is thus
important to ask how the impact of the simulated AGN feedback on a galaxy
changes when only the numerical resolution (the SPH particle mass) changes. We
present a suite of simulations modelling the interaction of an AGN outflow with
the ambient turbulent and clumpy interstellar medium (ISM) in the inner part of
the host galaxy at a range of mass resolutions. We find that, with other things
being equal, degrading the resolution leads to feedback becoming more efficient
at clearing out all gas in its path. For the simulations presented here, the
difference in the mass of the gas ejected by AGN feedback varies by more than a
factor of ten between our highest and lowest resolution simulations. This
happens because feedback-resistant high density clumps are washed out at low
effective resolutions. We also find that changes in numerical resolution lead
to undesirable artifacts in how the AGN feedback affects the AGN immediate
environment.Comment: 15 pages, 12 figures, accepted for publication in MNRA
AGN jet feedback on a moving mesh: lobe energetics and X-ray properties in a realistic cluster environment
Jet feedback from active galactic nuclei (AGN) harboured by brightest cluster
galaxies is expected to play a fundamental role in regulating cooling in the
intracluster medium (ICM). While observations and theory suggest energy within
jet lobes balances ICM radiative losses, the modus operandi of energy
communication with the ICM remains unclear. We present simulations of very
high-resolution AGN-driven jets launching in a live, cosmological galaxy
cluster, within the moving mesh-code Arepo. As the jet propagates through the
ICM the majority of its energy, which is initially in the kinetic form,
thermalises quickly through internal shocks and inflates lobes of very hot gas.
The jets effectively heat the cluster core, with work and mixing being
the main channels of energy transfer from the lobes to the ICM, while
turbulence and strong shocks are sub-dominant. We additionally present detailed
mock X-ray maps at different stages of evolution, revealing clear cavities
surrounded by X-ray bright rims, with lobes being detectable for up to
yrs even when magnetic draping is ineffective. We find bulk motions in the
cluster can significantly affect lobe propagation, offsetting them from the jet
direction and imparting bulk velocities that can dominate over the
buoyantly-rising motion.Comment: 5 pages, 3 figures, submitted to MNRAS Letter
Recent Progress in Modeling the Macro- and Micro-Physics of Radio Jet Feedback in Galaxy Clusters
Radio jets and the lobes they inflate are common in cool-core clusters and
are known to play a critical role in regulating the heating and cooling of the
intracluster medium (ICM). This is an inherently multi-scale problem, and much
effort has been made to understand the processes governing the inflation of
lobes and their impact on the cluster, as well as the impact of the environment
on the jet-ICM interaction, on both macro- and microphysical scales.
Developments of new numerical techniques and improving computational resources
have seen simulations of jet feedback in galaxy clusters become ever more
sophisticated. This ranges from modelling ICM plasma physics processes such as
the effects of magnetic fields, cosmic rays and viscosity to including jet
feedback in cosmologically evolved cluster environments in which the ICM
thermal and dynamic properties are shaped by large-scale structure formation.
In this review, we discuss the progress made over the last ~decade in capturing
both the macro- and microphysical processes in numerical simulations,
highlighting both the current state of the field as well as open questions and
potential ways in which these questions can be addressed in the future.Comment: 49 pages, 7 figures, Review Article accepted for publication in
Galaxies Special Issue "New Perspectives on Radio Galaxy Dynamics
Structure-Preserving Matrix Methods for Computations on Univariate and Bivariate Bernstein Polynomials
Curve and surface intersection finding is a fundamental problem in computer-aided geometric design (CAGD). This practical problem motivates the undertaken study into methods for computing the square-free factorisation of univariate and bivariate polynomials in
Bernstein form. It will be shown how these two problems are intrinsically linked and
how finding univariate polynomial roots and bivariate polynomial factors is equivalent to finding curve and surface intersection points.
The multiplicities of a polynomial’s factors are maintained through the use of a square free factorisation algorithm and this is analogous to the maintenance of smooth intersections between curves and surfaces, an important property in curve and surface design. Several aspects of the univariate and bivariate polynomial factorisation problem will be considered.
This thesis examines the structure of the greatest common divisor (GCD) problem within the context of the square-free factorisation problem. It is shown that an accurate approximation of the GCD can be computed from inexact polynomials even in the presence of significant levels of noise. Polynomial GCD computations are ill-posed, in that noise in the coefficients of two polynomials which have a common factor typically causes the polynomials to become coprime. Therefore, a method for determining the approximate greatest common divisor (AGCD) is developed, where the AGCD is defined to have the same degree as the GCD and its coefficients are sufficiently close to those of the exact GCD. The algorithms proposed assume no prior knowledge of the level of noise added to the exact polynomials, differentiating this method from others which require derived threshold values in the GCD computation.
The methods of polynomial factorisation devised in this thesis utilise the Sylvester matrix and a sequence of subresultant matrices for the GCD finding component. The classical definition of the Sylvester matrix is extended to compute the GCD of two and three bivariate polynomials defined in Bernstein form, and a new method of GCD computation is devised specifically for bivariate polynomials in Bernstein form which have been defined over a rectangular domain. These extensions are necessary for the computation of the factorisation of bivariate polynomials defined in the Bernstein form
High diversity of microplankton surrounds deep-water coral reef in the Norwegian Sea
Coral reefs that exist in the depths of the oceans are surrounded by Eukarya, Archaea and bacterial communities that may play an important role in the nutrition and health of the reef. The first interdomain community structure of planktonic organisms in seawater from a deep-water coral reef is described. Community profiling and analysis of ribosomal RNA gene sequences from a coral reef system at 350 m depth in the Norwegian Sea revealed a rich diversity of Eukarya and Bacteria and a moderate diversity of Archaea. Most sequences affiliated with marine microplankton from deep-sea to cold-surface regions, with many sequences being similar to those described in studies of mesopelagic and oxygen minimum zones. Dominant phylotypes belonged to the Alveolata (group I, II, dinoflagellates), Stramenopiles (silicoflagellates), Alphaproteobacteria (Pelagibacter ubique), Gammaproteobacteria (ARCTIC96BD-19), Bacteroidetes (Flavobacteria) and mesophilic Crenarchaeota (Nitrosopumilus maritimus). Several rare and novel members of the community fell into distinct phylogenetic groups. The inferred function of dominant community members suggested autotrophs that utilise light, ammonium or sulphide, and lifestyles based on host associations. The high diversity reflected a microplankton community structure, which is significantly different from that of microplankton collected at the same depth at a pelagic station away from reefs
Semi-Lagrangian Scheme with Arakawa Splitting for Gyro-kinetic Equations
The gyro-kinetic model is an approximation of the Vlasov-Maxwell system in a
strongly magnetized magnetic field. We propose a new algorithm for solving it
combining the Semi-Lagrangian (SL) method and the Arakawa (AKW) scheme with a
time-integrator. Both methods are successfully used in practice for different
kinds of applications, in our case, we combine them by first decomposing the
problem into a fast (parallel) and a slow (perpendicular) dynamical system. The
SL approach and the AKW scheme will be used to solve respectively the fast and
the slow subsystems. Compared to the scheme in [1], where the entire model is
solved using only the SL method, our goal is to replace the method used in the
slow subsystem by the AKW scheme, in order to improve the conservation of the
physical constants
Learning technology and organisations: transformational impact? [Editorial]
This collection of papers contributes to a wide and ongoing strand of interest in alternative learning technologies: how are digitally-enabled ways of working transforming organisations? In 'transformation' we look for radical change, rather than just doing the same at a different scale. And in organisations we centre on educational institutions – across the full range of schooling, training, further and higher education – while recognising that conventional boundaries are increasingly broken as different kinds of organisations invest in digitally-enabled learning
Chirale Lactole, IV. Selektivitäten bei Acetalisierungsreaktionen enantiomerenreiner Lactole am Beispiel von Octahydro-8,9,9-trimethyl-5,8-methano-2H-1-benzopyran-2-ol
Eine Synthese für das Lactol 1 wird beschrieben und seine Eignung als Reagens zur Racemattrennung an einigen Beispielen gezeigt. Aufgrund einer Röntgenstruktur und konformationsanalytischer Überlegungen wird eine allgemeine Regel erarbeitet, nach welcher die Absolutkonfiguration acyclischer Alkyl-aryl-carbinole aus dem Verlauf der Acetalisierungsreaktion eines enantiomerenreinen Lactols mit dem racemischen Alkohol vorhergesagt werden kann
AGN jet feedback on a moving mesh: lobe energetics and X-ray properties in a realistic cluster environment
Jet feedback from active galactic nuclei (AGN) harboured by brightest cluster galaxies is expected to play a fundamental role in regulating cooling in the intracluster medium (ICM). While observations and theory suggest energy within jet lobes balances ICM radiative losses, the modus operandi of energy communication with the ICM remains unclear. We present simulations of very high resolution AGN-driven jets launching in a live, cosmological galaxy cluster, within the moving mesh code arepo. As the jet propagates through the ICM the majority of its energy, which is initially in the kinetic form, thermalizes quickly through internal shocks and inflates lobes of very hot gas. The jets effectively heat the cluster core, with PdV work and weather-aided mixing being the main channels of energy transfer from the lobes to the ICM, while strong shocks and turbulence are subdominant. We additionally present detailed mock X-ray maps at different stages of evolution, revealing clear cavities surrounded by X-ray bright rims, with lobes being detectable for up to ∼108 yr even when magnetic draping is ineffective. We find bulk motions in the cluster can significantly affect lobe propagation, offsetting them from the jet direction and imparting bulk velocities that can dominate over the buoyantly rising motion
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