On the filamentary environment of galaxies

The correlation between the large-scale distribution of galaxies and their spectroscopic properties at z=1.5 is investigated using the Horizon MareNostrum cosmological run. We have extracted a large sample of 10^5 galaxies from this large hydrodynamical simulation featuring standard galaxy formation physics. Spectral synthesis is applied to these single stellar populations to generate spectra and colours for all galaxies. We use the skeleton as a tracer of the cosmic web and study how our galaxy catalogue depends on the distance to the skeleton. We show that galaxies closer to the skeleton tend to be redder, but that the effect is mostly due to the proximity of large haloes at the nodes of the skeleton, rather than the filaments themselves. This effects translate into a bimodality in the colour distribution of our sample. The origin of this bimodality is investigated and seems to follow from the ram pressure stripping of satellite galaxies within the more massive clusters of the simulation. The virtual catalogues (spectroscopical properties of the MareNostrum galaxies at various redshifts) are available online at http://www.iap.fr/users/pichon/MareNostrum/cataloguesComment: 18 pages, 27 figures, accepted for publication in MNRA

A concurrency semantics for relaxed atomics that permits optimisation and avoids thin-air executions

Copyright is held by the owner/author(s). Despite much research on concurrent programming languages, especially for Java and C/C++, we still do not have a satisfactory definition of their semantics, one that admits all common optimisations without also admitting undesired behaviour. Especially problematic are the "thin-Air" examples involving high-performance concurrent accesses, such as C/C++11 relaxed atomics. The C/C++11 model is in a per-candidate-execution style, and previous work has identified a tension between that and the fact that compiler optimisations do not operate over single candidate executions in isolation; rather, they operate over syntactic representations that represent all executions. In this paper we propose a novel approach that circumvents this difficulty. We define a concurrency semantics for a core calculus, including relaxed-Atomic and non-Atomic accesses, and locks, that admits a wide range of optimisation while still forbidding the classic thin-Air examples. It also addresses other problems relating to undefined behaviour. The basic idea is to use an event-structure representation of the current state of each thread, capturing all of its potential executions, and to permit interleaving of execution and transformation steps over that to reflect optimisation (possibly dynamic) of the code. These are combined with a non-multi-copy-Atomic storage subsystem, to reflect common hardware behaviour. The semantics is defined in a mechanised and executable form, and designed to be implementable above current relaxed hardware and strong enough to support the programming idioms that C/C++11 does for this fragment. It offers a potential way forward for concurrent programming language semantics, beyond the current C/C++11 and Java models.This work was partly funded by the EPSRC Programme Grant REMS: Rigorous Engineering for Mainstream Systems, EP/K008528/

Propagators in Lagrangian space

It has been found recently that propagators, e.g. the cross-correlation spectra of the cosmic fields with the initial density field, decay exponentially at large-k in an Eulerian description of the dynamics. We explore here similar quantities defined for a Lagrangian space description. We find that propagators in Lagrangian space do not exhibit the same properties: they are found not to be monotonic functions of time, and to track back the linear growth rate at late time (but with a renormalized amplitude). These results have been obtained with a novel method which we describe alongside. It allows the formal resummation of the same set of diagrams as those that led to the known results in Eulerian space. We provide a tentative explanation for the marked differences seen between the Eulerian and the Lagrangian cases, and we point out the role played by the vorticity degrees of freedom that are specific to the Lagrangian formalism. This provides us with new insights into the late-time behavior of the propagators.Comment: 14 pages, 5 figure

Acclimatation d'une biomasse anaérobie à un effluent de fabrication d'une pâte à papier à concentration élevée en composés soufrés

L'effluent de fabrication à partir de bois de résineux d'une Pâte Chimico-Thermo-Mécanique (CTMP) contenant des dérivés soufrés organiques et des sulfates, est très difficilement épuré par voie anaérobie. La présente étude montre qu'il est indispensable d'acclimater la biomasse à de fortes concentrations en soufre dans l'effluent. Ceci est réalisé en alimentant un réacteur à biomasse fixée (pilote de laboratoire), en fonctionnement continu, par des mélanges d'un effluent de distillerie et de l'effluent CTMP, dans lesquels la proportion du second est augmentée par paliers successifs jusqu'à 100 %.On peut alors finalement obtenir une épuration convenable de l'effluent CTMP à des teneurs en soufre jusqu'à 500 mg/l (pourvu que le rapport DCO/S soit supérieur à 10 environ).Anaerobic treatment of wastewater from chemico-thermomechanical pulping (CTMP) of softwoods was inhibited by sulphur containing compounds. Distillery skips are characterized by a high sulphur content and a more appropriate COD/sulphur ratio than CTMP wastewater. Using CTMP wastewater together with distiliery stops during initial starting up resulted in successful acclimatation of microorganisms to significant sulphate contents.Provided the COD/Sulphur was aboue 10 reasonably good treatment efficiency was obtained with CTMP wastewater containing to 500 mg total sulphur per liter

Non Gaussian extrema counts for CMB maps

In the context of the geometrical analysis of weakly non Gaussian CMB maps, the 2D differential extrema counts as functions of the excursion set threshold is derived from the full moments expansion of the joint probability distribution of an isotropic random field, its gradient and invariants of the Hessian. Analytic expressions for these counts are given to second order in the non Gaussian correction, while a Monte Carlo method to compute them to arbitrary order is presented. Matching count statistics to these estimators is illustrated on fiducial non-Gaussian "Planck" data.Comment: 4 pages, 1 figur

Massive spheroids can form in single minor mergers

Accepted for publication in MNRAS, 12 pages, 6 figuresUnderstanding how rotationally supported discs transform into dispersion-dominated spheroids is central to our comprehension of galaxy evolution. Morphological transformation is largely merger-driven. While major mergers can efficiently create spheroids, recent work has highlighted the significant role of other processes, like minor mergers, in driving morphological change. Given their rich merger histories, spheroids typically exhibit large fractions of ‘ex situ’ stellar mass, i.e. mass that is accreted, via mergers, from external objects. This is particularly true for the most massive galaxies, whose stellar masses typically cannot be attained without a large number of mergers. Here, we explore an unusual population of extremely massive (M ∗ > 10 11M) spheroids, in the Horizon-AGN simulation, which exhibit anomalously low ex situ mass fractions, indicating that they form without recourse to significant merging. These systems form in a single minor-merger event (with typical merger mass ratios of 0.11–0.33), with a specific orbital configuration, where the satellite orbit is virtually co-planar with the disc of the massive galaxy. The merger triggers a catastrophic change in morphology, over only a few hundred Myr, coupled with strong in situ star formation. While this channel produces a minority (∼5 per cent) of such galaxies, our study demonstrates that the formation of at least some of the most massive spheroids need not involve major mergers – or any significant merging at all – contrary to what is classically believed.Peer reviewedFinal Accepted Versio

Probing magnetic fields with multi-frequency polarized synchrotron emission

We investigate the problem of probing the local spatial structure of the magnetic field of the interstellar medium using multi-frequency polarized maps of the synchrotron emission at radio wavelengths. We focus in this paper on the three-dimensional reconstruction of the largest scales of the magnetic field, relying on the internal depolarization (due to differential Faraday rotation) of the emitting medium as a function of electromagnetic frequency. We argue that multi-band spectroscopy in the radio wavelengths, developed in the context of high-redshift extragalactic HI lines, can be a very useful probe of the 3D magnetic field structure of our Galaxy when combined with a Maximum A Posteriori reconstruction technique. When starting from a fair approximation of the magnetic field, we are able to recover the true one by using a linearized version of the corresponding inverse problem. The spectral analysis of this problem allows us to specify the best sampling strategy in electromagnetic frequency and predicts a spatially anisotropic distribution of posterior errors. The reconstruction method is illustrated for reference fields extracted from realistic magneto-hydrodynamical simulations