475 research outputs found
Line-of-sight velocity distributions of elliptical galaxies from collisionless mergers
We analyse the skewness of the line-of-sight velocity distributions in model
elliptical galaxies built through collisionless galaxy mergers. We build the
models using large N-body simulations of mergers between either two spiral or
two elliptical galaxies. Our aim is to investigate whether the observed ranges
of skewness coefficient (h3) and the rotational support (V/sigma), as well as
the anticorrelation between h3 and V, may be reproduced through collisionless
mergers. Previous attempts using N-body simulations failed to reach V/sigma ~
1-2 and corresponding high h3 values, which suggested that gas dynamics and
ensuing star formation might be needed in order to explain the skewness
properties of ellipticals through mergers. Here we show that high V/sigma and
high h3 are reproduced in collisionless spiral-spiral mergers whenever a
central bulge allows the discs to retain some of their original angular
momentum during the merger. We also show that elliptical-elliptical mergers,
unless merging from a high-angular momentum orbit, reproduce the strong
skewness observed in non-rotating, giant, boxy ellipticals. The behaviour of
the h3 coefficient therefore associates rapidly-rotating disky ellipticals to
disc-disc mergers, and associates boxy, slowly-rotating giant ellipticals to
elliptical-elliptical mergers, a framework generally consistent with the
expectations of hierarchical galaxy formation.Comment: 5 pages, 4 figures, MNRAS Letters, in pres
Intermittent energy dissipation by turbulent reconnection
Magnetic reconnection—the process responsible for many explosive phenomena in both nature and laboratory—is efficient at dissipating magnetic energy into particle energy. To date, exactly how this dissipation happens remains unclear, owing to the scarcity of multipoint measurements of the “diffusion region” at the sub-ion scale. Here we report such a measurement by Cluster—four spacecraft with separation of 1/5 ion scale. We discover numerous current filaments and magnetic nulls inside the diffusion region of magnetic reconnection, with the strongest currents appearing at spiral nulls (O-lines) and the separatrices. Inside each current filament, kinetic-scale turbulence is significantly increased and the energy dissipation, E′ ⋅ j, is 100 times larger than the typical value. At the jet reversal point, where radial nulls (X-lines) are detected, the current, turbulence, and energy dissipations are surprisingly small. All these features clearly demonstrate that energy dissipation in magnetic reconnection occurs at O-lines but not X-lines
Nonasymptotic Convergence Rates for Cooperative Learning Over Time-Varying Directed Graphs
We study the problem of distributed hypothesis testing with a network of
agents where some agents repeatedly gain access to information about the
correct hypothesis. The group objective is to globally agree on a joint
hypothesis that best describes the observed data at all the nodes. We assume
that the agents can interact with their neighbors in an unknown sequence of
time-varying directed graphs. Following the pioneering work of Jadbabaie,
Molavi, Sandroni, and Tahbaz-Salehi, we propose local learning dynamics which
combine Bayesian updates at each node with a local aggregation rule of private
agent signals. We show that these learning dynamics drive all agents to the set
of hypotheses which best explain the data collected at all nodes as long as the
sequence of interconnection graphs is uniformly strongly connected. Our main
result establishes a non-asymptotic, explicit, geometric convergence rate for
the learning dynamic
Theoretical modeling of propagation of magneto-acoustic waves in magnetic regions below sunspots
We use 2D numerical simulations and eikonal approximation, to study
properties of MHD waves traveling below the solar surface through the magnetic
structure of sunspots. We consider a series of magnetostatic models of sunspots
of different magnetic field strengths, from 10 Mm below the photosphere to the
low chromosphere. The purpose of these studies is to quantify the effect of the
magnetic field on local helioseismology measurements by modeling waves excited
by sub-photospheric sources. Time-distance propagation diagrams and wave travel
times are calculated for models of various field strength and compared to the
non-magnetic case. The results clearly indicate that the observed time-distance
helioseismology signals in sunspot regions correspond to fast MHD waves. The
slow MHD waves form a distinctly different pattern in the time-distance
diagram, which has not been detected in observations. The numerical results are
in good agreement with the solution in the short-wavelength (eikonal)
approximation, providing its validation. The frequency dependence of the travel
times is in a good qualitative agreement with observations.Comment: accepted by Ap
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