Cosmological Simulations on a Grid of Computers

The work presented in this paper aims at restricting the input parameter values of the semi-analytical model used in GALICS and MOMAF, so as to derive which parameters influence the most the results, e.g., star formation, feedback and halo recycling efficiencies, etc. Our approach is to proceed empirically: we run lots of simulations and derive the correct ranges of values. The computation time needed is so large, that we need to run on a grid of computers. Hence, we model GALICS and MOMAF execution time and output files size, and run the simulation using a grid middleware: DIET. All the complexity of accessing resources, scheduling simulations and managing data is harnessed by DIET and hidden behind a web portal accessible to the users.Comment: Accepted and Published in AIP Conference Proceedings 1241, 2010, pages 816-82

Verifying service continuity in a satellite reconfiguration procedure: application to a satellite

The paper discusses the use of the TURTLE UML profile to model and verify service continuity during dynamic reconfiguration of embedded software, and space-based telecommunication software in particular. TURTLE extends UML class diagrams with composition operators, and activity diagrams with temporal operators. Translating TURTLE to the formal description technique RT-LOTOS gives the profile a formal semantics and makes it possible to reuse verification techniques implemented by the RTL, the RT-LOTOS toolkit developed at LAAS-CNRS. The paper proposes a modeling and formal validation methodology based on TURTLE and RTL, and discusses its application to a payload software application in charge of an embedded packet switch. The paper demonstrates the benefits of using TURTLE to prove service continuity for dynamic reconfiguration of embedded software

Distributed Hybrid Simulation of the Internet of Things and Smart Territories

This paper deals with the use of hybrid simulation to build and compose heterogeneous simulation scenarios that can be proficiently exploited to model and represent the Internet of Things (IoT). Hybrid simulation is a methodology that combines multiple modalities of modeling/simulation. Complex scenarios are decomposed into simpler ones, each one being simulated through a specific simulation strategy. All these simulation building blocks are then synchronized and coordinated. This simulation methodology is an ideal one to represent IoT setups, which are usually very demanding, due to the heterogeneity of possible scenarios arising from the massive deployment of an enormous amount of sensors and devices. We present a use case concerned with the distributed simulation of smart territories, a novel view of decentralized geographical spaces that, thanks to the use of IoT, builds ICT services to manage resources in a way that is sustainable and not harmful to the environment. Three different simulation models are combined together, namely, an adaptive agent-based parallel and distributed simulator, an OMNeT++ based discrete event simulator and a script-language simulator based on MATLAB. Results from a performance analysis confirm the viability of using hybrid simulation to model complex IoT scenarios.Comment: arXiv admin note: substantial text overlap with arXiv:1605.0487

Great Circle tidal streams: evidence for a nearly spherical massive dark halo around the Milky Way

An all high-latitude sky survey for cool carbon giant stars in the Galactic halo has revealed 75 such stars, of which the majority are new detections. Of these, more than half are clustered on a Great Circle on the sky which intersects the center of Sagittarius dwarf galaxy (Sgr) and is parallel to its proper motion vector, while many of the remainder are outlying Magellanic Cloud C-stars. A pole-count analysis of the carbon star distribution clearly indicates that the Great Circle stream we have isolated is statistically significant, being a 5-6 sigma over-density. These two arguments strongly support our conclusion that a large fraction of the Halo carbon stars originated in Sgr. The stream orbits the Galaxy between the present location of Sgr, 16 kpc from the Galactic center, and the most distant stream carbon star, at ~60 kpc. It follows neither a polar nor a Galactic plane orbit, so that a large range in both Galactic R and z distances are probed. That the stream is observed as a Great Circle indicates that the Galaxy does not exert a significant torque upon the stream, so the Galactic potential must be nearly spherical in the regions probed by the stream. We present N-body experiments simulating this disruption process as a function of the distribution of mass in the Galactic halo. A likelihood analysis shows that, in the Galactocentric distance range 16 kpc < R < 60 kpc, the dark halo is most likely almost spherical. We rule out, at high confidence levels, the possibility that the Halo is significantly oblate, with isodensity contours of aspect q_m < 0.7. This result is quite unexpected and contests currently popular galaxy formation models. (Abridged)Comment: 26 pages, 13 figures (6 in color, 8 chunky due to PS compression), minor revisions, accepted by Ap

Design Concept for a Failover Mechanism in Distributed SDN Controllers

Software defined networking allows the separation of the control plane and data plane in networking. It provides scalability, programmability, and centralized control. It will use these traits to reach ubiquitous connectivity. Like all concepts software defined networking does not offer these advantages without a cost. By utilizing a centralized controller, a single point of failure is created. To address this issue, this paper proposes a distributed controller failover. This failover will provide a mechanism for recovery when controllers are not located in the same location. This failover mechanism is based on number of hops from orphan nodes to the controller in addition to the link connection. This mechanism was simulated in Long Term Evolution telecommunications architecture

The Nature and Origin of Low-Redshift O VI Absorbers

The O VI ion observed in quasar absorption line spectra is the most accessible tracer of the cosmic metal distribution in the low redshift (z<0.5) intergalactic medium (IGM). We explore the nature and origin of O VI absorbers using cosmological hydrodynamic simulations including galactic outflows. We consider the effects of ionization background variations, non-equilibrium ionization and cooling, uniform metallicity, and small-scale (sub-resolution) turbulence. Our main results are 1) IGM O VI is predominantly photo-ionized with T= 10^(4.2+/-0.2) K. A key reason for this is that O VI absorbers preferentially trace over-enriched regions of the IGM at a given density, which enhances metal-line cooling such that absorbers can cool within a Hubble time. As such, O VI is not a good tracer of the WHIM. 2) The predicted O VI properties fit observables only if sub-resolution turbulence is added. The required turbulence increases with O VI absorber strength such that stronger absorbers arise from more recent outflows with turbulence dissipating on the order of a Hubble time. The amount of turbulence is consistent with other examples of turbulence observed in the IGM and galactic halos. 3) Metals traced by O VI and H I do not trace exactly the same baryons, but reside in the same large-scale structure. Observed alignment statistics are reproduced in our simulations. 4) Photo-ionized O VI traces gas in a variety of environments, and is not directly associated with the nearest galaxy, though is typically nearest to ~0.1L* galaxies. Weaker O VI components trace some of the oldest cosmic metals. 5) Very strong absorbers are more likely to be collisionally ionized, tracing more recent enrichment (<2 Gyr) within or near galactic halos.Comment: 33 pages, 18 figures, accepted to MNRAS. Two new figures adde