Cheating and the evolutionary stability of mutualisms

Interspecific mutualisms have been playing a central role in the functioning of all ecosystems since the early history of life. Yet the theory of coevolution of mutualists is virtually nonexistent, by contrast with well-developed coevolutionary theories of competition, predator–prey and host–parasite interactions. This has prevented resolution of a basic puzzle posed by mutualisms: their persistence in spite of apparent evolutionary instability. The selective advantage of 'cheating', that is, reaping mutualistic benefits while providing fewer commodities to the partner species, is commonly believed to erode a mutualistic interaction, leading to its dissolution or reciprocal extinction. However, recent empirical findings indicate that stable associations of mutualists and cheaters have existed over long evolutionary periods. Here, we show that asymmetrical competition within species for the commodities offered by mutualistic partners provides a simple and testable ecological mechanism that can account for the long-term persistence of mutualisms. Cheating, in effect, establishes a background against which better mutualists can display any competitive superiority. This can lead to the coexistence and divergence of mutualist and cheater phenotypes, as well as to the coexistence of ecologically similar, but unrelated mutualists and cheaters

Magnetic Flux Expulsion in the Powerful Superbubble Explosions and the Alpha-Omega Dynamo

The possibility of the magnetic flux expulsion from the Galaxy in the superbubble (SB) explosions, important for the Alpha-Omega dynamo, is considered. Special emphasis is put on the investigation of the downsliding of the matter from the top of the shell formed by the SB explosion which is able to influence the kinematics of the shell. It is shown that either Galactic gravity or the development of the Rayleigh-Taylor instabilities in the shell limit the SB expansion, thus, making impossible magnetic flux expulsion. The effect of the cosmic rays in the shell on the sliding is considered and it is shown that it is negligible compared to Galactic gravity. Thus, the question of possible mechanism of flux expulsion in the Alpha-Omega dynamo remains open.Comment: MNRAS, in press, 11 pages, 9 figure

Biermann Mechanism in Primordial Supernova Remnant and Seed Magnetic Fields

We study generation of magnetic fields by the Biermann mechanism in the pair-instability supernovae explosions of first stars. The Biermann mechanism produces magnetic fields in the shocked region between the bubble and interstellar medium (ISM), even if magnetic fields are absent initially. We perform a series of two-dimensional magnetohydrodynamic simulations with the Biermann term and estimate the amplitude and total energy of the produced magnetic fields. We find that magnetic fields with amplitude $10^{-14}-10^{-17}$ G are generated inside the bubble, though the amount of magnetic fields generated depend on specific values of initial conditions. This corresponds to magnetic fields of $10^{28}-10^{31}$ erg per each supernova remnant, which is strong enough to be the seed magnetic field for galactic and/or interstellar dynamo.Comment: 12 pages, 3 figure

Shocked Quartz in Polymict Impact Breccia from the Upper Cretaceous Yallalie Impact Structure in Western Australia

Yallalie is a ~12 km diameter circular structure located ~200 km north of Perth, Australia. Previous studies have proposed that the buried structure is a complex impact crater based on geophysical data. Allochthonous breccia exposed near the structure has previously been interpreted as proximal impact ejecta; however, no diagnostic indicators of shock metamorphism have been found. Here we report multiple (27) shocked quartz grains containing planar fractures (PFs) and planar deformation features (PDFs) in the breccia. The PFs occur in up to five sets per grain, while the PDFs occur in up to four sets per grain. Universal stage measurements of all 27 shocked quartz grains confirms that the planar microstructures occur in known crystallographic orientations in quartz corresponding to shock compression from 5 to 20 GPa. Proximity to the buried structure (~4 km) and occurrence of shocked quartz indicates that the breccia represents either primary or reworked ejecta. Ejecta distribution simulated using iSALE hydrocode predicts the same distribution of shock levels at the site as those found in the breccia, which supports a primary ejecta interpretation, although local reworking cannot be excluded. The Yallalie impact event is stratigraphically constrained to have occurred in the interval from 89.8 to 83.6 Ma based on the occurrence of Coniacian clasts in the breccia and undisturbed overlying Santonian to Campanian sedimentary rocks. Yallalie is thus the first confirmed Upper Cretaceous impact structure in Australia

The 511 keV emission from positron annihilation in the Galaxy

The first gamma-ray line originating from outside the solar system that was ever detected is the 511 keV emission from positron annihilation in the Galaxy. Despite 30 years of intense theoretical and observational investigation, the main sources of positrons have not been identified up to now. Observations in the 1990's with OSSE/CGRO showed that the emission is strongly concentrated towards the Galactic bulge. In the 2000's, the SPI instrument aboard ESA's INTEGRAL gamma-ray observatory allowed scientists to measure that emission across the entire Galaxy, revealing that the bulge/disk luminosity ratio is larger than observed in any other wavelength. This mapping prompted a number of novel explanations, including rather "exotic ones (e.g. dark matter annihilation). However, conventional astrophysical sources, like type Ia supernovae, microquasars or X-ray binaries, are still plausible candidates for a large fraction of the observed total 511 keV emission of the bulge. A closer study of the subject reveals new layers of complexity, since positrons may propagate far away from their production sites, making it difficult to infer the underlying source distribution from the observed map of 511 keV emission. However, contrary to the rather well understood propagation of high energy (>GeV) particles of Galactic cosmic rays, understanding the propagation of low energy (~MeV) positrons in the turbulent, magnetized interstellar medium, still remains a formidable challenge. We review the spectral and imaging properties of the observed 511 keV emission and we critically discuss candidate positron sources and models of positron propagation in the Galaxy.Comment: 62 pages, 35 figures. Review paper to appear in Reviews of Modern Physic

Rôle hydrogéologique des linéaments structuraux en milieu cristallin et cristallophyllien : cas du bassin versant du Sassandra, Sud-Ouest de la Côte d’Ivoire

La zone d’étude est située en milieu tropical humide au sud-ouest de la Côte d’Ivoire dans les formations plutoniques et métamorphiques précambriennes. L’objectif est d’étudier l’effet des linéaments structuraux dans l’hydrodynamisme souterrain dans un tel environnement en se basant sur des données de télédétection, de forages, des teneurs SO42- et Cl- des eaux souterraines. Le traitement des images satellitaires a donné la carte des linéaments qui a servi au choix des sites d’échantillonnage d’eau souterraine et à l’individualisation des « blocs ». Les rapports de SO42- et Cl- a permis d’identifier les linéaments participant à la circulation de l’eau souterraine à l’échelle régionale. L’analyse des données géométriques des blocs et des débits des forages a permis de mettre en évidence le rôle hydrogéologique prépondérant des petits linéaments par rapport aux linéaments majeurs. Confirmant ainsi l’importance de l’horizon fissuré et la topographie dans l’écoulement souterrain en milieu de socle cristallin et cristallophyllien.Mots-clés: aquifères de socle, bloc, débit, horizon fissuré, hydrodynamisme.Hydrogeological role of structural lineaments in plutonic and metamorphic environment: Case of Sassandra Watershed (South-Western Côte d'Ivoire)The survey area is located south-west of Ivory Coast in Precambrian metamorphic and plutonic zone. The aim is to study the contribution of structural lineaments in the underground hydrodynamics in such environment. To highlight the network of kilometer-scale fractures in hard-rock, remote sensing techniques, drilling and concentrations SOSO42- and Cl- in groundwater were used. The map of lineaments obtained after processing of satellite images has been discretized into blocks aquifers which are delimited by opened or clogged fractures that can have a conducting or watertight function. Analysis of the geometric block parameters and the drilling instantaneous discharge helped highlight the leading role hydrogeological small fractures (fissured layer) compared to major lineaments in groundwater flow of in hard-rock. Thus confirming the importance of fissured topography and groundwater flow in crystallophyllian crystalline basement.Keywords: hard-rock aquifer, block, hydrodynamism, fissured zone, and yield

The Evolution of Adiabatic Supernova Remnants in a Turbulent, Magnetized Medium

(Abridged) We present the results of three dimensional calculations for the MHD evolution of an adiabatic supernova remnant in both a uniform and turbulent interstellar medium using the RIEMANN framework of Balsara. In the uniform case, which contains an initially uniform magnetic field, the density structure of the shell remains largely spherical, while the magnetic pressure and synchrotron emissivity are enhanced along the plane perpendicular to the field direction. This produces a bilateral or barrel-type morphology in synchrotron emission for certain viewing angles. We then consider a case with a turbulent external medium as in Balsara & Pouquet, characterized by $v_{A}(rms)/c_{s}=2$. Several important changes are found. First, despite the presence of a uniform field, the overall synchrotron emissivity becomes approximately spherically symmetric, on the whole, but is extremely patchy and time-variable, with flickering on the order of a few computational time steps. We suggest that the time and spatial variability of emission in early phase SNR evolution provides information on the turbulent medium surrounding the remnant. The shock-turbulence interaction is also shown to be a strong source of helicity-generation and, therefore, has important consequences for magnetic field generation. We compare our calculations to the Sedov-phase evolution, and discuss how the emission characteristics of SNR may provide a diagnostic on the nature of turbulence in the pre-supernova environment.Comment: ApJ, in press, 5 color figure

Strong magnetic fields and cosmic rays in very young galaxies

We present a scenario for efficient magnetization of very young galaxies about 0.5 Gigayears after the Big-Bang by a cosmic ray-driven dynamo. These objects experience a phase of strong star formation during this first $10^9$ years. We transfer the knowledge of the connection between star formation and the production rate of cosmic rays by supernova remnants to such high redshift objects. Since the supernova rate is a direct measure for the production rate of cosmic rays we conclude that very young galaxies must be strong sources of cosmic rays. The key argument of our model is the finding that magnetic fields and cosmic rays are dynamically coupled, i.e. a strong cosmic ray source contains strong magnetic fields since the relativistic particles drive an efficient dynamo in a galaxy via their buoyancy. We construct a phenomenological model of a dynamo driven by buoyancy of cosmic rays and show that if azimuthal shearing is strong enough the dynamo amplification timescale is close to the buoyancy timescale of the order of several $10^7 \div 10^8$ yr. We predict that young galaxies are strongly magnetized and may contribute significantly to the gamma-ray-background.Comment: 9 pages, 1 figure, accepted for Astronomy and Astrophysic

Evidence for dark matter in the inner Milky Way

The ubiquitous presence of dark matter in the universe is today a central tenet in modern cosmology and astrophysics. Ranging from the smallest galaxies to the observable universe, the evidence for dark matter is compelling in dwarfs, spiral galaxies, galaxy clusters as well as at cosmological scales. However, it has been historically difficult to pin down the dark matter contribution to the total mass density in the Milky Way, particularly in the innermost regions of the Galaxy and in the solar neighbourhood. Here we present an up-to-date compilation of Milky Way rotation curve measurements, and compare it with state-of-the-art baryonic mass distribution models. We show that current data strongly disfavour baryons as the sole contribution to the galactic mass budget, even inside the solar circle. Our findings demonstrate the existence of dark matter in the inner Galaxy while making no assumptions on its distribution. We anticipate that this result will compel new model-independent constraints on the dark matter local density and profile, thus reducing uncertainties on direct and indirect dark matter searches, and will shed new light on the structure and evolution of the Galaxy.Comment: First submitted version of letter published in Nature Physics on Febuary 9, 2015: http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys3237.htm