407 research outputs found
Metal enrichment in galactic winds
Observations give evidences of the presence of metals in the intergalactic
medium (IGM). The stars responsible for transforming hydrogen and helium into
more complex atoms do not form outside the galaxies in the standard scenario of
galaxy formation. Supernovae-driven winds and their associated feedback was
proposed as a possible solution to explain such enrichment of the IGM. It
turned out that a proper modelling of supernovae explosions within a turbulent
interstellar medium (ISM) is a difficult task. Recent advances have been
obtained using a multiphase approach to solve for the thermal state of the ISM,
plus some additional recipes to account for the kinetic effect of supernovae on
the galactic gas. We briefly describe here our implementation of supernovae
feedback within the RAMSES code, and apply it to the formation and evolution of
isolated galaxies of various masses and angular momenta. We have explored under
what conditions a galactic wind can develop, if one considers only a quiescent
mode of star formation. We have also characterized the distribution and
evolution of metallicity in the gas outflow spreading in the IGM.Comment: 6 pages, 6 figures, To appear in the proceedings of the
CRAL-Conference Series I "Chemodynamics: from first stars to local galaxies",
Lyon 10-14 July 2006, France, Eds. Emsellem, Wozniak, Massacrier, Gonzalez,
Devriendt, Champavert, EAS Publications Serie
AGN feedback using AMR cosmological simulations
Feedback processes are thought to solve some of the long-standing issues of
the numerical modelling of galaxy formation: over-cooling, low angular
momentum, massive blue galaxies, extra-galactic enrichment, etc. The accretion
of gas onto super-massive black holes in the centre of massive galaxies can
release tremendous amounts of energy to the surrounding medium. We show, with
cosmological Adaptive Mesh Refinement simulations, how the growth of black
holes is regulated by the feedback from Active Galactic Nuclei using a new dual
jet/heating mechanism. We discuss how this large amount of feedback is able to
modify the cold baryon content of galaxies, and perturb the properties of the
hot plasma in their vicinity.Comment: 4 pages, 2 figures, contribution to the Astronomical Society of the
Pacific Conference Series for the Cefal\`u meeting "Advances in computational
astrophysics: methods, tools and outcomes
Robust parameter estimation of density functions under fuzzy interval observations
International audienceThis study deals with the derivation of a probabilistic parametric model from interval data using the maximum likelihood principle. In contrast with classical techniques such as the EM algorithm, that define a precise likelihood function by computing the probability of observations viewed as a collection of non-elementary events, our approach presupposes that each imprecise observation underlies a precise one, and that the uncertainty that pervades its observation is epistemic, rather than representing noise. We define an interval-valued likelihood function and apply robust optimisation methods to find a safe plausible estimate of the statistical parameters. The approach is extended to fuzzy data by optimizing the average of lower likelikoods over a collection of data sets obtained from cuts of the fuzzy intervals, as a trade off between optimistic and pessimistic interpretations of fuzzy data. The principles of this method are compared with those of other existing approaches to handle incompleteness of observations, especially the EM technique
Magnetised winds in dwarf galaxies
The origin and the magnitude of the inter-galactic magnetic field is of
primordial importance in the global picture of magnetic field evolution, as it
is considered to be the missing link between galactic magnetic fields and
cluster magnetic fields on much larger scales. We are testing whether dwarf
galaxies are good candidates to explain the enrichment of the IGM: after their
discs form and trigger galactic dynamos, supernova feedback will launch strong
winds, expelling magnetic field lines in the IGM. We have performed MHD
simulations of an isolated dwarf galaxy, forming self-consistently inside a
cooling halo. Using the RAMSES code, we have for the first time simulated the
formation of a magnetised supernova-driven galactic outflow. This simulation is
an important step towards a more realistic modelling using fully cosmological
simulations. Our simulations reproduce well the observed properties of magnetic
fields in spiral galaxies. The formation and the evolution of our simulated
disc lead to a strong magnetic field amplification: the magnetic field in the
final wind bubble is one order of magnitude larger than the initial value. The
magnetic field in the disc, essentially toroidal, is growing linearly with time
as a consequence of differential rotation. We discuss the consequence of this
simple mechanism on the cosmic evolution of the magnetic field: we propose a
new scenario for the evolution of the magnetic field, with dwarf galaxies
playing a key role in amplifying and ejecting magnetic energy in the IGM,
resulting in what we call a "Cosmic Dynamo" that could contribute to the rather
high field strengths observed in galaxies and clusters today.Comment: 20 pages, 12 figures. Accepted for publication in A&
Snap, Crackle, Pop: sub-grid supernova feedback in AMR simulations of disk galaxies
We compare 5 sub-grid models for supernova (SN) feedback in adaptive mesh
refinement (AMR) simulations of isolated dwarf and L-star disk galaxies with
20-40 pc resolution. The models are thermal dump, stochastic thermal,
'mechanical' (injecting energy or momentum depending on the resolution),
kinetic, and delayed cooling feedback. We focus on the ability of each model to
suppress star formation and generate outflows. Our highest-resolution runs
marginally resolve the adiabatic phase of the feedback events, which correspond
to 40 SN explosions, and the first three models yield nearly identical results,
possibly indicating that kinetic and delayed cooling feedback converge to wrong
results. At lower resolution all models differ, with thermal dump feedback
becoming inefficient. Thermal dump, stochastic, and mechanical feedback
generate multiphase outflows with mass loading factors , which is
much lower than observed. For the case of stochastic feedback we compare to
published SPH simulations, and find much lower outflow rates. Kinetic feedback
yields fast, hot outflows with , but only if the wind is in effect
hydrodynamically decoupled from the disk by using a large bubble radius.
Delayed cooling generates cold, dense and slow winds with , but large
amounts of gas occupy regions of temperature-density space with short cooling
times. We conclude that either our resolution is too low to warrant physically
motivated models for SN feedback, that feedback mechanisms other than SNe are
important, or that other aspects of galaxy evolution, such as star formation,
require better treatment.Comment: 22 pages, 15 figures. Accepted for publication in MNRAS with minor
revision
Black hole evolution: I. Supernova-regulated black hole growth
The growth of a supermassive black hole (BH) is determined by how much gas
the host galaxy is able to feed it, which in turn is controlled by the cosmic
environment, through galaxy mergers and accretion of cosmic flows that time how
galaxies obtain their gas, but also by internal processes in the galaxy, such
as star formation and feedback from stars and the BH itself. In this paper, we
study the growth of a 10^12 Msun halo at z=2, which is the progenitor of al
group of galaxies at z=0, and of its central BH by means of a high-resolution
zoomed cosmological simulation, the Seth simulation. We study the evolution of
the BH driven by the accretion of cold gas in the galaxy, and explore the
efficiency of the feedback from supernovae (SNe). For a relatively inefficient
energy input from SNe, the BH grows at the Eddington rate from early times, and
reaches self-regulation once it is massive enough. We find that at early cosmic
times z>3.5, efficient feedback from SNe forbids the formation of a settled
disc as well as the accumulation of dense cold gas in the vicinity of the BH
and starves the central compact object. As the galaxy and its halo accumulate
mass, they become able to confine the nuclear inflows provided by major mergers
and the BH grows at a sustained near-to-Eddington accretion rate. We argue that
this mechanism should be ubiquitous amongst low-mass galaxies, corresponding to
galaxies with a stellar mass below <10^9 Msun in our simulations.Comment: 19 pages, 15 figures, MNRA
Seeking stability of supply chain management decisions under uncertain criteria
The leading theme of MOSIMâ12 is "Performance, Interoperability and Safety for sustainable development"International audienceThis paper tackles the question of the anticipation of the supply chain partner's decisional behaviour under uncertain criteria. In other words, we propose a model to support sequential decisions under uncertainty where the decision maker has to make hypothesis about the decision criteria. For example, Hurwicz criterion weights extreme optimism and pessimism positions and a classic criticism of this criterion consisting in the difficulty of the weight assessment and the involving decision instability. To achieve this, we present a method based on fuzzy representation of weight vision. Finally, the model allows sequential decision of a Decision Tree to be compute thanks a pignistic probabilities treatment of the fuzzy representation of the decision maker optimism-pessimism index. This approach is illustrated through an industrial case study
How active galactic nucleus feedback and metal cooling shape cluster entropy profiles
Observed clusters of galaxies essentially come in two flavours: non-cool-core clusters characterized by an isothermal temperature profile and a central entropy floor, and cool-core clusters where temperature and entropy in the central region are increasing with radius. Using cosmological resimulations of a galaxy cluster, we study the evolution of its intracluster medium (ICM) gas properties, and through them we assess the effect of different (subgrid) modelling of the physical processes at play, namely gas cooling, star formation, feedback from supernovae and active galactic nuclei (AGNs). More specifically, we show that AGN feedback plays a major role in the pre-heating of the protocluster as it prevents a high concentration of mass from collecting in the centre of the future galaxy cluster at early times. However, AGN activity during the cluster's later evolution is also required to regulate the mass flow into its core and prevent runaway star formation in the central galaxy. Whereas the energy deposited by supernovae alone is insufficient to prevent an overcooling catastrophe, supernovae are responsible for spreading a large amount of metals at high redshift, enhancing the cooling efficiency of the ICM gas. As the AGN energy release depends on the accretion rate of gas on to its central black hole engine, the AGNs respond to this supernova-enhanced gas accretion by injecting more energy into the surrounding gas, and as a result increase the amount of early pre-heating. We demonstrate that the interaction between an AGN jet and the ICM gas that regulates the growth of the AGN's black hole can naturally produce cool-core clusters if we neglect metals. However, as soon as metals are allowed to contribute to the radiative cooling, only the non-cool-core solution is produce
Decision support with ill-known criteria in the collaborative supply chain context
International audienceIn the field of Supply Chain Risk Management, the attitude of managers toward risk affect the tactical decision-making process in collaborative supply chains under an uncertain environment, concerning especially capacity levels, lot-sizing rules, purchasing strategies, production scheduling,âŠ, etc. The issue can be formulated as a sequential decision problem under uncertainty where the customer decisions affect the decisions made by the supplier. In this paper we deal with two kinds of uncertainties. The first one is the uncertainty on the indicators of performance (which are not comparable) used by the decision maker to choose a solution (for example: service quality or inventory cost). Hence, we propose an approach based on subjective probability to evaluate the probability that a decision is optimal for the first actor and the probability that it is optimal for both. From these two evaluations, we propose a ranking function to help the first actor to take into account the second one when selecting a decision. The second kind of uncertainty pertains to the demand. A classical criterion under total uncertainty is Hurwicz criterion where a weight expresses a degree of pessimism. Nevertheless, the degree of pessimism is itself ill-known. Thus, it becomes difficult to take into account the behavior of the actors. Hence, we propose an approach based on possibility theory and the so-called pignistic transform, which computes a subjective probability distribution over the criteria. Then, we apply the method used for uncertain criterion. This approach is illustrated through an example and an industrial case study
Galactic star formation in parsec-scale resolution simulations
The interstellar medium (ISM) in galaxies is multiphase and cloudy, with
stars forming in the very dense, cold gas found in Giant Molecular Clouds
(GMCs). Simulating the evolution of an entire galaxy, however, is a
computational problem which covers many orders of magnitude, so many
simulations cannot reach densities high enough or temperatures low enough to
resolve this multiphase nature. Therefore, the formation of GMCs is not
captured and the resulting gas distribution is smooth, contrary to
observations. We investigate how star formation (SF) proceeds in simulated
galaxies when we obtain parsec-scale resolution and more successfully capture
the multiphase ISM. Both major mergers and the accretion of cold gas via
filaments are dominant contributors to a galaxy's total stellar budget and we
examine SF at high resolution in both of these contexts.Comment: 4 pages, 4 figures. To appear in the proceedings for IAU Symposium
270: Computational Star Formation (eds. Alves, Elmegreen, Girart, Trimble
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