Properties of Galactic Outflows: Measurements of the Feedback from Star Formation

Properties of starburst-driven outflows in dwarf galaxies are compared to those in more massive galaxies. Over a factor of roughly 10 in galactic rotation speed, supershells are shown to lift warm ionized gas out of the disk at rates up to several times the star formation rate. The amount of mass escaping the galactic potential, in contrast to the disk, does depend on the galactic mass. The temperature of the hottest extended \x emission shows little variation around $\sim 10^{6.7}$ K, and this gas has enough energy to escape from the galaxies with rotation speed less than approximately 130 km/s.Comment: 11 pages + 3 figues. Accepted for publication in the Astrophysical Journa

Double percolation effects and fractal behavior in magnetic/superconducting hybrids

Perpendicular magnetic anisotropy ferromagnetic/ superconducting (FM/SC) bilayers with a labyrinth domain structure are used to study nucleation of superconductivity on a fractal network, tunable through magnetic history. As clusters of reversed domains appear in the FM layer, the SC film shows a percolative behavior that depends on two independent processes: the arrangement of initial reversed domains and the fractal geometry of expanding clusters. For a full labyrinth structure, the behavior of the upper critical field is typical of confined superconductivity on a fractal network.Comment: 15 pages, 5 figure

The Origin and Kinematics of Cold Gas in Galactic Winds: Insight from Numerical Simulations

We study the origin of Na I absorbing gas in ultraluminous infrared galaxies motivated by the recent observations by Martin of extremely superthermal linewidths in this cool gas. We model the effects of repeated supernova explosions driving supershells in the central regions of molecular disks with M_d=10^10 M_\sun, using cylindrically symmetric gas dynamical simulations run with ZEUS-3D. The shocked swept-up shells quickly cool and fragment by Rayleigh-Taylor instability as they accelerate out of the dense, stratified disks. The numerical resolution of the cooling and compression at the shock fronts determines the peak shell density, and so the speed of Rayleigh-Taylor fragmentation. We identify cooled shells and shell fragments as Na I absorbing gas and study its kinematics. We find that simulations with a numerical resolution of \le 0.2 pc produce multiple Rayleigh-Taylor fragmented shells in a given line of sight. We suggest that the observed wide Na I absorption lines, = 320 \pm 120 km s^-1 are produced by these multiple fragmented shells traveling at different velocities. We also suggest that some shell fragments can be accelerated above the observed average terminal velocity of 750 km s^-1 by the same energy-driven wind with an instantaneous starburst of \sim 10^9 M_\sun. The bulk of mass is traveling with the observed average shell velocity 330 \pm 100 km s^-1. Our results show that an energy-driven bubble causing Rayleigh-Taylor instabilities can explain the kinematics of cool gas seen in the Na I observations without invoking additional physics relying primarily on momentum conservation, such as entrainment of gas by Kelvin-Helmholtz instabilities, ram pressure driving of cold clouds by a hot wind, or radiation pressure acting on dust. (abridged)Comment: 65 pages, 22 figures, accepted by Astrophys. J. Changes during refereeing focused on context and comparison to observation

Ianus: an Adpative FPGA Computer

Dedicated machines designed for specific computational algorithms can outperform conventional computers by several orders of magnitude. In this note we describe {\it Ianus}, a new generation FPGA based machine and its basic features: hardware integration and wide reprogrammability. Our goal is to build a machine that can fully exploit the performance potential of new generation FPGA devices. We also plan a software platform which simplifies its programming, in order to extend its intended range of application to a wide class of interesting and computationally demanding problems. The decision to develop a dedicated processor is a complex one, involving careful assessment of its performance lead, during its expected lifetime, over traditional computers, taking into account their performance increase, as predicted by Moore's law. We discuss this point in detail

Bohmian transmission and reflection dwell times without trajectory sampling

Within the framework of Bohmian mechanics dwell times find a straightforward formulation. The computation of associated probabilities and distributions however needs the explicit knowledge of a relevant sample of trajectories and therefore implies formidable numerical effort. Here a trajectory free formulation for the average transmission and reflection dwell times within static spatial intervals [a,b] is given for one-dimensional scattering problems. This formulation reduces the computation time to less than 5% of the computation time by means of trajectory sampling.Comment: 14 pages, 7 figures; v2: published version, significantly revised and shortened (former sections 2 and 3 omitted, appendix A added, simplified mathematics

The ISLANDS project I: Andromeda XVI, An Extremely Low Mass Galaxy not Quenched by Reionization

Based on data aquired in 13 orbits of HST time, we present a detailed evolutionary history of the M31 dSph satellite Andromeda XVI, including its life-time star formation history, the spatial distribution of its stellar populations, and the properties of its variable stars. And XVI is characterized by prolonged star formation activity from the oldest epochs until star formation was quenched ~6 Gyr ago, and, notably, only half of the mass in stars of And XVI was in place 10 Gyr ago. And XVI appears to be a low mass galaxy for which the early quenching by either reionization or starburst feedback seems highly unlikely, and thus, is most likely due to an environmental effect (e.g., an interaction), possibly connected to a late infall in the densest regions of the Local Group. Studying the star formation history as a function of galactocentric radius, we detect a mild gradient in the star formation history: the star formation activity between 6 and 8 Gyr ago is significantly stronger in the central regions than in the external regions, although the quenching age appears to be the same, within 1 Gyr. We also report the discovery of 9 RR Lyrae stars, 8 of which belong to And XVI. The RR Lyrae stars allow a new estimate of the distance, (m-M)0= 23.72+/-0.09 mag, which is marginally larger than previous estimates based on the tip of the red giant branch.Comment: Accepted for publication on Ap

Critical properties of the four-state Commutative Random Permutation Glassy Potts model in three and four dimensions

We investigate the critical properties of the four-state commutative random permutation glassy Potts model in three and four dimensions by means of Monte Carlo simulation and of a finite size scaling analysis. Thanks to the use of a field programmable gate array we have been able to thermalize a large number of samples of systems with large volume. This has allowed us to observe a spin-glass ordered phase in d=4 and to study the critical properties of the transition. In d=3, our results are consistent with the presence of a Kosterlitz-Thouless transition, but we cannot exclude transient effects due to a value of the lower critical dimension slightly below 3.Comment: 9 pages, 8 Postscript figure

Inflation on the Brane with Vanishing Gravity

Many existing models of brane inflation suffer from a steep irreducible gravitational potential between the branes that causes inflation to end too early. Inspired by the fact that point masses in 2+1 D exert no gravitational force, we propose a novel unwarped and non-supersymmetric setup for inflation, consisting of 3-branes in two extra dimensions compactified on a sphere. The size of the sphere is stabilized by a combination of a bulk cosmological constant and a magnetic flux. Computing the 4D effective potential between probe branes in this background, we find a non-zero contribution only from exchange of level-1 KK modes of the graviton and radion. Identifying antipodal points on the 2-sphere projects out these modes, eliminating entirely the troublesome gravitational contribution to the inflationary potential.Comment: 19 pages, 11 figures, JHEP forma

The Spin Glass Phase in the Four-State, Three-Dimensional Potts Model

We perform numerical simulations, including parallel tempering, on the Potts glass model with binary random quenched couplings using the JANUS application-oriented computer. We find and characterize a glassy transition, estimating the location of the transition and the value of the critical exponents. We show that there is no ferromagnetic transition in a large temperature range around the glassy critical temperature. We also compare our results with those obtained recently on the "random permutation" Potts glass.Comment: 7 pages and 3 figures. Corrected minor typo