Evolution of Compact Groups of Galaxies I. Merging Rates

We discuss the merging rates in compact groups of 5 identical elliptical galaxies. All groups have the same mass and binding energy. We consider both cases with individual halos and cases where the halo is common to all galaxies and enveloping the whole group. In the latter situation the merging rate is slower if the halo is more massive. The mass of individual halos has little influence on the merging rates, due to the fact that all galaxies in our simulations have the same mass, and so the more extended ones have a smaller velocity dispersion. Groups with individual halos merge faster than groups with common halos if the configuration is centrally concentrated, like a King distribution of index 10. On the other hand for less concentrated configurations the merging is initially faster for individual halo cases, and slower after part of the group has merged. In cases with common halo, centrally concentrated configurations merge faster for high halo-to-total mass ratios and slower for low halo-to-total mass ratios. Groups whose virial ratio is initially less than one merge faster, while groups that have initially cylindrical rotation merge slower than groups starting in virial equilibrium. In order to test how long a virialised group can survive before merging we followed the evolution of a group with a high halo-to-total mass ratio and a density distribution with very little central concentration. We find that the first merging occurred only after a large number of crossing times, which with areasonable calibration should be larger than a Hubble time. Hence, at least for appropriate initial conditions, the longevity of compact groups is not necessarily a problem, which is an alternative explanation to why we observe so many compact groups despite the fact that their lifetimes seem short.Comment: 15 pages Latex, with 12 figures included, requires mn.sty, accepted for publication in MNRA

A general framework for online audio source separation

We consider the problem of online audio source separation. Existing algorithms adopt either a sliding block approach or a stochastic gradient approach, which is faster but less accurate. Also, they rely either on spatial cues or on spectral cues and cannot separate certain mixtures. In this paper, we design a general online audio source separation framework that combines both approaches and both types of cues. The model parameters are estimated in the Maximum Likelihood (ML) sense using a Generalised Expectation Maximisation (GEM) algorithm with multiplicative updates. The separation performance is evaluated as a function of the block size and the step size and compared to that of an offline algorithm.Comment: International conference on Latente Variable Analysis and Signal Separation (2012

The dynamics of spiral arms in pure stellar disks

It has been believed that spirals in pure stellar disks, especially the ones spontaneously formed, decay in several galactic rotations due to the increase of stellar velocity dispersions. Therefore, some cooling mechanism, for example dissipational effects of the interstellar medium, was assumed to be necessary to keep the spiral arms. Here we show that stellar disks can maintain spiral features for several tens of rotations without the help of cooling, using a series of high-resolution three-dimensional $N$-body simulations of pure stellar disks. We found that if the number of particles is sufficiently large, e.g., $3\times 10^6$, multi-arm spirals developed in an isolated disk can survive for more than 10 Gyrs. We confirmed that there is a self-regulating mechanism that maintains the amplitude of the spiral arms. Spiral arms increase Toomre's $Q$ of the disk, and the heating rate correlates with the squared amplitude of the spirals. Since the amplitude itself is limited by the value of $Q$, this makes the dynamical heating less effective in the later phase of evolution. A simple analytical argument suggests that the heating is caused by gravitational scattering of stars by spiral arms, and that the self-regulating mechanism in pure-stellar disks can effectively maintain spiral arms on a cosmological timescale. In the case of a smaller number of particles, e.g., $3\times 10^5$, spiral arms grow faster in the beginning of the simulation (while $Q$ is small) and they cause a rapid increase of $Q$. As a result, the spiral arms become faint in several Gyrs.Comment: 18 pages, 19 figures, accepted for Ap

All Maximal Independent Sets and Dynamic Dominance for Sparse Graphs

We describe algorithms, based on Avis and Fukuda's reverse search paradigm, for listing all maximal independent sets in a sparse graph in polynomial time and delay per output. For bounded degree graphs, our algorithms take constant time per set generated; for minor-closed graph families, the time is O(n) per set, and for more general sparse graph families we achieve subquadratic time per set. We also describe new data structures for maintaining a dynamic vertex set S in a sparse or minor-closed graph family, and querying the number of vertices not dominated by S; for minor-closed graph families the time per update is constant, while it is sublinear for any sparse graph family. We can also maintain a dynamic vertex set in an arbitrary m-edge graph and test the independence of the maintained set in time O(sqrt m) per update. We use the domination data structures as part of our enumeration algorithms.Comment: 10 page

A rigorous formulation of the cosmological Newtonian limit without averaging

We prove the existence of a large class of one-parameter families of cosmological solutions to the Einstein-Euler equations that have a Newtonian limit. This class includes solutions that represent a finite, but otherwise arbitrary, number of compact fluid bodies. These solutions provide exact cosmological models that admit Newtonian limits but, are not, either implicitly or explicitly, averaged

N-body simulations of galaxies and groups of galaxies with the Marseille GRAPE systems

I review the Marseille GRAPE systems and the N-body simulations done with them. I first describe briefly the available hardware and software, their possibilities and their limitations. I then describe work done on interacting galaxies and groups of galaxies. This includes simulations of the formation of ring galaxies, simulations of bar destruction by massive compact satellites, of merging in compact groups and of the formation of brightest members in clusters of galaxies.Comment: 13 pages, 5 figures, to be published in "Non-linear Dynamics and Chaos in Astrophysics", eds. J.R. Buchler, S. Gottesman, J. Hunter and H. Kandrup, Annals of the New York Academy of Science

Phase separation in the crust of accreting neutron stars

Nucleosynthesis, on the surface of accreting neutron stars, produces a range of chemical elements. We perform molecular dynamics simulations of crystallization to see how this complex composition forms new neutron star crust. We find chemical separation, with the liquid ocean phase greatly enriched in low atomic number elements compared to the solid crust. This phase separation should change many crust properties such as the thermal conductivity and shear modulus. The concentration of carbon, if present, is enriched in the ocean. This may allow unstable thermonuclear burning of the carbon and help explain the ignition of the very energetic explosions known as superbursts.Comment: 8 pages, 6 figures, minor changes, Physical Review E in pres

Multiadaptive Galerkin Methods for ODEs III: A Priori Error Estimates

The multiadaptive continuous/discontinuous Galerkin methods mcG(q) and mdG(q) for the numerical solution of initial value problems for ordinary differential equations are based on piecewise polynomial approximation of degree q on partitions in time with time steps which may vary for different components of the computed solution. In this paper, we prove general order a priori error estimates for the mcG(q) and mdG(q) methods. To prove the error estimates, we represent the error in terms of a discrete dual solution and the residual of an interpolant of the exact solution. The estimates then follow from interpolation estimates, together with stability estimates for the discrete dual solution

Recoverin Regulates Light-dependent Phosphodiesterase Activity in Retinal Rods

The Ca2+-binding protein recoverin may regulate visual transduction in retinal rods and cones, but its functional role and mechanism of action remain controversial. We compared the photoresponses of rods from control mice and from mice in which the recoverin gene was knocked out. Our analysis indicates that Ca2+-recoverin prolongs the dark-adapted flash response and increases the rod's sensitivity to dim steady light. Knockout rods had faster Ca2+ dynamics, indicating that recoverin is a significant Ca2+ buffer in the outer segment, but incorporation of exogenous buffer did not restore wild-type behavior. We infer that Ca2+-recoverin potentiates light-triggered phosphodiesterase activity, probably by effectively prolonging the catalytic activity of photoexcited rhodopsin

On the Spin History of the X-ray Pulsar in Kes 73: Further Evidence For an Utramagnetized Neutron Star

In previous papers, we presented the discovery of a 12-s X-ray pulsar in the supernova remnant Kes 73, providing the first direct evidence for an ultramagnetized neutron star, a magnetar, with an equivalent dipole field of nearly twenty times the quantum critical magnetic field. Our conclusions were based on two epochs of measurement of the spin, along with an age estimate of the host supernova remnant. Herein, we present a spin chronology of the pulsar using additional GINGA, ASCA, XTE, & SAX datasets spanning over a decade. Timing and spectral analysis confirms our initial results and severely limit an accretion origin for the observed flux. Over the 10 year baseline, the pulsar is found to undergo a rapid, constant spindown, while maintaining a steady flux and an invariant pulse profile. Within the measurement uncertainties, no systematic departures from a linear spin-down are found - departures as in the case of glitches or simply stochastic fluctuations in the pulse times-of-arrival (e.g. red timing noise). We suggest that this pulsar is akin to the soft gamma-ray repeaters, however, it is remarkably stable and has yet to display similar outbursts; future gamma-ray activity from this object is likely.Comment: 6 pages with 3 embedded figures, LaTex, emulateapj.sty. Submitted to the ApJ Letter