12,379 research outputs found
Off the Beaten Path: A New Approach to Realistically Model The Orbital Decay of Supermassive Black Holes in Galaxy Formation Simulations
We introduce a force correction term to better model the dynamical friction
(DF) experienced by a supermassive black hole (SMBH) as it orbits within its
host galaxy. This new approach accurately follows the orbital decay of a SMBH
and drastically improves over commonly used advection methods. The force
correction introduced here naturally scales with the force resolution of the
simulation and converges as resolution is increased. In controlled experiments
we show how the orbital decay of the SMBH closely follows analytical
predictions when particle masses are significantly smaller than that of the
SMBH. In a cosmological simulation of the assembly of a small galaxy, we show
how our method allows for realistic black hole orbits. This approach overcomes
the limitations of the advection scheme, where black holes are rapidly and
artificially pushed toward the halo center and then forced to merge, regardless
of their orbits. We find that SMBHs from merging dwarf galaxies can spend
significant time away from the center of the remnant galaxy. Improving the
modeling of SMBH orbital decay will help in making robust predictions of the
growth, detectability, and merger rates of SMBHs, especially at low galaxy
masses or at high redshift.Comment: 8 pages, 4 figure, Accepted by MNRA
Steady state sedimentation of ultrasoft colloids
The structural and dynamical properties of ultra-soft colloids - star
polymers - exposed to a uniform external force field are analyzed applying the
multiparticle collision dynamics approach, a hybrid coarse-grain mesoscale
simulation approach, which captures thermal fluctuations and long-range
hydrodynamic interactions. In the weak field limit, the structure of the star
polymer is nearly unchanged, however in an intermediate regime, the radius of
gyration decreases, in particular transverse to the sedimentation direction. In
the limit of a strong field, the radius of gyration increases with field
strength. Correspondingly, the sedimentation coefficient increases with
increasing field strength, passes through a maximum and decreases again at high
field strengths. The maximum value depends on the functionality of the star
polymer. High field strengths lead to symmetry breaking with trailing, strongly
stretched polymer arms and a compact star polymer body. In the weak field
linear response regime, the sedimentation coefficient follows the scaling
relation of a star polymer in terms of functionality and arm length
Non-classical computing: feasible versus infeasible
Physics sets certain limits on what is and is not computable. These limits are very far from having been reached by current technologies. Whilst proposals for hypercomputation are almost certainly infeasible, there are a number of non classical approaches that do hold considerable promise. There are a range of possible architectures that could be implemented on silicon that are distinctly different from the von Neumann model. Beyond this, quantum simulators, which are the quantum equivalent of analogue computers, may be constructable in the near future
Configurational Entropy and Diffusivity of Supercooled Water
We calculate the configurational entropy S_conf for the SPC/E model of water
for state points covering a large region of the (T,rho) plane. We find that (i)
the (T,rho) dependence of S_conf correlates with the diffusion constant and
(ii) that the line of maxima in S_conf tracks the line of density maxima. Our
simulation data indicate that the dynamics are strongly influenced by S_conf
even above the mode-coupling temperature T_MCT(rho).Comment: Significant update of reference
Modeling of solvent flow effects in enzyme catalysis under physiological conditions
A stochastic model for the dynamics of enzymatic catalysis in explicit,
effective solvents under physiological conditions is presented.
Analytically-computed first passage time densities of a diffusing particle in a
spherical shell with absorbing boundaries are combined with densities obtained
from explicit simulation to obtain the overall probability density for the
total reaction cycle time of the enzymatic system. The method is used to
investigate the catalytic transfer of a phosphoryl group in a phosphoglycerate
kinase-ADP-bis phosphoglycerate system, one of the steps of glycolysis. The
direct simulation of the enzyme-substrate binding and reaction is carried out
using an elastic network model for the protein, and the solvent motions are
described by multiparticle collision dynamics, which incorporates hydrodynamic
flow effects. Systems where solvent-enzyme coupling occurs through explicit
intermolecular interactions, as well as systems where this coupling is taken
into account by including the protein and substrate in the multiparticle
collision step, are investigated and compared with simulations where
hydrodynamic coupling is absent. It is demonstrated that the flow of solvent
particles around the enzyme facilitates the large-scale hinge motion of the
enzyme with bound substrates, and has a significant impact on the shape of the
probability densities and average time scales of substrate binding for
substrates near the enzyme, the closure of the enzyme after binding, and the
overall time of completion of the cycle.Comment: 15 pages in double column forma
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