Magnetic collimation of meridional-self-similar general relativistic MHD flows

We present a model for the spine of relativistic MHD outflows in the Kerr geometry. Meridional self-similarity is invoked to derive semi-analytical solutions close to the polar axis. The study of the energy conservation along a particular field line gives a simple criterion for the collimation of jets. Such parameter have already been derived in the classical case by Sauty et al. 1999 and also extended to the Schwarzschild metric by Meliani et al. 2006. We generalize the same study to the Kerr metric. We show that the rotation of the black hole increases the magnetic self-confinement.Comment: 16 pages, 6 figures, accepted for publication in Physical Review

Stellar Population Constraints on the Dark Matter Content and Origin of Ultra-Compact Dwarf Galaxies

We analyse intermediate-resolution VLT FLAMES/Giraffe spectra of six ultra-compact dwarf (UCD) galaxies in the Fornax cluster. We obtained velocity dispersions and stellar population properties by full spectral fitting against PEGASE.HR models. Objects span a large range of metallicities (-0.95 to -0.23 dex), 4 of them are older than 8 Gyr. Comparison of the stellar and dynamical masses suggests that UCDs have little dark matter at best. For one object, UCD3, the Salpeter initial mass function (IMF) results in the stellar mass significantly exceeding the dynamical one, whereas for the Kroupa IMF the values coincide. Although, this object may have peculiar dynamics or/and stellar populations, the Kroupa IMF seems more realistic. We find that UCDs lie well above the metallicity-luminosity relation of early-type galaxies. The same behaviour is demonstrated by some of the massive Milky Way globular clusters, known to contain composite stellar populations. Our results support two following UCD formation scenarii: (1) tidal stripping of nucleated dwarf elliptical galaxies; (2) formation of tidal superclusters in galaxy mergers. We also discuss some of the alternative channels of the UCD formation binding them to globular clusters.Comment: accepted to MNRAS, 7 pages, 4 figures, 3 table

Conformal representation of Kerr space–time poloidal sub-manifolds

International audienceWe use two conformal transformations to represent eld lines in the poloidal sub-manifold of Kerr space-time. The rst one is based on an embedding in R 3 of a manifold which is conform to the poloidal submanifold. The second one is a planar representation using quasi-isotropic coordinates. We compare plots of the poloidal magnetic eld lines in the usual Boyer-Lindquist Cartesian coordinates and in the conformal representation based on quasi-isotropic coordinates. In a conformal representation these lines appear entering in the horizon perpendicularly to it as deduced through a mathematical perspective conversely to the usual physical approach. We also compare the value of the conformal factor in these two representations

The effects of ram pressure stripping on cluster galaxies

International audienceWe investigate the effects of ram pressure stripping of spiral galaxies using a numerical model that is directly confronted with interferometric observations. The current status of our research is presented

Magnetic collimation of relativistic jets: the role of the black hole spin

International audienceAn ideal engine for producing ultrarelativistic jets is a rapidly rotating black hole threaded by a magnetic field. Following the 3 1 decomposion of spacetime of Thorne et al. (1986), we use a local inertial frame of reference attached to an observer comoving with the frame-dragging of the Kerr black hole (ZAMO) to write the GRMHD equations. Assuming theta-self similarity, analytical solutions for jets can be found for which the streamline shape is calculated exactly. Calculating the total energy variation between a non polar streamline and the polar axis, we have extended to the Kerr metric the simple criterion for the magnetic collimation of jets developed by Sauty et al. (1999). We show that the black hole rotation induces a more efficient magnetic collimation of the jet

On Magnetic Self-Collimation of Relativistic Jets

International audienceWe present a semi-analytical model using the equations of general relativistic magnetohydrodynamics (GRMHD) for jets emitted by a rotating black hole. We assume steady axisymmetric outflows of a relativistic ideal fluid in Kerr metrics. We express the conservation equations in the frame of the FIDucial Observer (FIDO or ZAMO) using a 3+1 space-time splitting. Calculating the total energy variation between a non-polar field line and the polar axis, we extend to the Kerr metric the simple criterion for the magnetic collimation of jets obtained for a nonrotating black hole by Meliani et al.10 We show that the black role rotation induced a more efficient magnetic collimation of the jet

Modélisation des jets relativistes et de l'accélération du rayonnement cosmique

Cette thèse explore divers enjeux liés aux jets relativistes issus des trous noirs. Leur formation ainsi que le processus d'accélération et de collimation sont étudiés par une description fluide dans une approche globale de l'écoulement. Les équations magnétohydrodynamiques en relativité générale peuvent être intégrées en utilisant le formalise "3+1" afin de passer de l'écriture covariante des équations à une écriture vectorielle, où les grandeurs physiques sont mesurées par un observateur eulérien localement non tournant. Ce formalisme permet d'étudier la phyqiue de la magnétosphère autour du trou noir de Kerr, c'est-à-dire la physique d'un plasma fortement magnétisé dans un espace-remps courbe et en rotation différentielle. L'écriture des équations dans ce formalisme et la construction d'un modèle auto-similaire méridien en métrique de Kerr ont permis d'obtenir des solutions de jets relativistes et de décrire la dynamique et la géométrie de l'écoulement proche de l'axe de rotation. En particulier, j'ai étudié le rôle de la rotation du trou noir dans le processus de collimation. J'ai également calculé des solutions dans la limite newtonienne applicables aux jets issus d'étoiles T Tauri afin d'étudier le freinage magnétique et de les comparer à leur généralisation en métrique Kerr. Les jets relativistes seraient également le siège d'émission de particules ou de photons très énergétiques. En particulier, les sursauts gamma seraient considérés comme une source possible du rayonnement cosmique de ultra haute énergie. L'étude de ce problème nécessite une description locale de l'écoulement. Une autre partie de ce travail s'attache à la modélisation des chocs relativistes en fonction des différentes configurations magnétiques ainsi que du rayonnement émis. L'analyse des processus d'accélération et des différents processus de perte d'énergie en compétition permet de voir si l'on peut effectivement accélérer des particuules aux énergies observées.This thesis explores various issues related to relativistic jets associated with black holes. Their formation as well as the acceleration and collimation processes, are studied using fluid approximation within a global description of the flow. The general relativistic magnetohydrodynamic equations can be integrated using the "3+1" formalism, and the covariant equations can be transposed to a vectorial form, where the physical vectorial quantities are mesured by an Eulerien observer comoving with the rotation, the sco-called zero angular momentum observer. This formalism allows us to study the physics of the magnetosphere surrounding a Kerr black hole, i.e., the physics of a strongly magnetized plasma in differential rotation in curved space-time. This formalism enabled me to develop a self-similar meridional model in the Kerr metric that allows us to obtain relativistic jet solutions, and to describe the dynamics, and the geometry of the flow close to the rotational axis. In particular, I have investigated how the rotation of the black hole affects the collimation process. I have found solutions for T Tauri stars in the newtonian approximation in order to study magnetic braking, and to compare them with their generalization in the Kerr metric. Relativistic jets could also be the seat of high-energy corpuscular and photon emission. In particular, gammay-ray bursts could be possible sources of ultra-high-energy cosmic rays. This problem requires the use of a local descritopn of the flow. Finally, we have investigated the propagation of ultra high energy cosmic rays in these shocks, taking into account the competition between relativistic Fermi acceleration and energy loss due to interaction with the gama ray background, in order to ascestain whether the observed energies can be obtained in this way.PARIS-Observatoire (751142302) / SudocSudocFranceF

An Energetic Criterion for Astrophysical Winds and Jets

International audienc

Quasi-isometric embedding of Kerr poloidal submanifolds

International audienceWe propose two approaches to obtain an isometric embedding of the poloidal Kerr submanifold. The first one relies on the convex integration process using the corrugation from a primitive embedding. This allows us to obtain one parameter family of embeddings reaching the limits of an isometric embedding. The second one consists in consecutive numerical resolutions of the Gauss-Codazzi-Mainardi and frame equations. This method requires geometric assumptions near the equatorial axis of the poloidal submanifold to get initial and boundary conditions. The second approach allows to understand some physical properties in the vicinity of a Kerr black hole, in particular the fast increasing ergoregion extent with angular momentum