Aims. We present a new numerical code, ECHO, based on a Eulerian conservative high-order scheme for time dependent threedimensional general relativistic magnetohydrodynamics (GRMHD) and magnetodynamics (GRMD). ECHO is aimed at providing a shock-capturing conservative method able to work at an arbitrary level of formal accuracy (for smooth flows), where the other existing GRMHD and GRMD schemes yield an overall second order at most. Moreover, our goal is to present a general framework based on the 3 + 1 Eulerian formalism, allowing for different sets of equations and different algorithms and working in a generic space-time metric, so that ECHO may be easily coupled to any solver for Einstein's equations. Methods. Our finite-difference conservative scheme previously developed for special relativistic hydrodynamics and MHD is extended here to the general relativistic case. Various high-order reconstruction methods are implemented and a two-wave approximate Riemann solver is used. The induction equation is treated by adopting the upwind constrained transport (UCT) procedures, appropriate to preserving the divergence-free condition of the magnetic field in shock-capturing methods. The limiting case of magnetodynamics (also known as force-free degenerate electrodynamics) is implemented by simply replacing the fluid velocity with the electromagnetic drift velocity and by neglecting the contribution of matter to the stress tensor. Results. ECHO is particularly accurate, efficient, versatile, and robust. It has been tested against several astrophysical applications, like magnetized accretion onto black holes and constant angular momentum thick disks threaded by toroidal fields. A novel test of the propagation of large-amplitude, circularly polarized Alfvén waves is proposed, and this allows us to prove the spatial and temporal high-order properties of ECHO very accurately. In particular, we show that reconstruction based on a monotonicity-preserving (MP) filter applied to a fixed 5-point stencil gives highly accurate results for smooth solutions, both in flat and curved metric (up to the nominal fifth order), while at the same time providing sharp profiles in tests involving discontinuities.

Abstract. Multidimensional shock-capturing numerical schemes for special relativistic hydrodynamics (RHD) are computationally more expensive than their correspondent Euler versions, due to the nonlinear relations between conservative and primitive variables and to the consequent complexity of the Jacobian matrices (needed for the spectral decomposition in most of the approximate Riemann solvers of common use). Here an efficient and easy-to-implement three-dimensional (3-D) shockcapturing scheme for ideal RHD is presented. Based on the algorithms developed by P. Londrillo & L. Del Zanna (2000, ApJ, 530, 508) for the non-relativistic magnetohydrodynamic (MHD) case, and having in mind its relativistic MHD extension (to appear in a forthcoming paper), the scheme uses high order (third) Convex Essentially Non-Oscillatory (CENO) finite difference interpolation routines and central-type averaged Riemann solvers, which do not make use of time-consuming characteristic decomposition. The scheme is very efficient and robust, and it gives results comparable to those obtained with more sophisticated algorithms, even in ultrarelativistic multidimensional test problems.

We present a general framework to design Godunov-type schemes for multidimensional ideal magnetohydrodynamic (MHD) systems, having the divergence-free relation and the related properties of the magnetic field B as built-in conditions. Our approach mostly relies on the Constrained Transport (CT) discretization technique for the magnetic field components, originally developed for the linear induction equation, which assures [∇ · B]num = 0 and its preservation in time to within machine accuracy in a finite-volume setting. We show that the CT formalism, when fully exploited, can be used as a general guideline to design the reconstruction procedures of the B vector field, to adapt standard upwind procedures for the momentum and energy equations, avoiding the onset of numerical monopoles of O(1) size, and to formulate approximate Riemann solvers for the induction equation. This general framework will be named here Upwind Constrained Transport (UCT). To demonstrate the versatility of our method, we apply it to a variety of schemes, which are finally validated numerically and compared: a novel implementation for the MHD case of the second order Roe-type positive scheme by Liu and Lax (J. Comp. Fluid Dynam. 5, 133, 1996), and both the second and third order versions of a central-type MHD scheme presented by Londrillo and Del Zanna (Astrophys. J. 530, 508, 2000), where the basic UCT strategies have been first outlined.

A general method for constructing high-order upwind schemes for multidimensional magnetohydrodynamics (MHD), having as a main built-in condition the divergence-free constraint $ AE B \ 0 for the magnetic Ðeld vector B, is proposed. The suggested procedure is based on consistency arguments, by taking into account the speciÐc operator structure of MHD equations with respect to the reference Euler equations of gasdynamics. This approach leads in a natural way to a staggered representation of the B Ðeld numerical data in which the divergence-free condition in the cell-averaged form, corresponding to second-order accurate numerical derivatives, is exactly fulÐlled. To extend this property to higher order schemes, we then give general prescriptions to satisfy a (r ] 1)th order accurate $ AE B \ 0 relation for any numerical B Ðeld having a rth order interpolation accuracy. Consistency arguments lead also to a proper formulation of the upwind procedures needed to integrate the induction equations, assuring the exact conservation in time of the divergence-free condition and the related continuity properties for the B vector components. As an application, a third-order code to simulate multidimensional MHD Ñows of astrophysical interest is developed using essentially nonoscillatoryÈbased reconstruction algorithms. Several test problems to illustrate and validate the proposed approach are Ðnally presented.

We explore the effects of dissipationless merging on the fundamental plane of elliptical galaxies using an N-body code based on a new, high-performance numerical scheme. We investigate the two extreme cases of galaxy growth by equal-mass merging and accretion of small stellar systems; in a subset of simulations we also consider the presence of dark matter haloes around the merging galaxies. Curiously, we found that the fundamental plane is preserved by major merging, while in the accretion scenario its edge-on thickness is only marginally reproduced, with substantial thickening in the case of merging with low angular momentum. We also found that both the Faber-Jackson and Kormendy relations are not reproduced by the simulations, in accordance with the results of a preliminary analysis based on a simple application of the virial theorem. Finally, we discuss the implications of our results for the origin of the M BH -σ 0 and Magorrian relations. We found that dissipationless merging is unable to reproduce the M BH -σ 0 relation, if the black hole masses add linearly (while the Magorrian relation is nicely reproduced); in contrast, a black hole merging with substantial emission of gravitational waves reproduces the M BH -σ 0 relation but fails to reproduce the Magorrian relation. We argue that our results strongly point towards a major role of dissipation in the formation of early-type galaxies and in the growth of their central supermassive black holes, thus supporting the idea of a link between galaxy formation and quasi-stellar object activity.

In galaxies like the Milky Way, cold (~ 10^4 K) gas ejected from the disc by stellar activity (the so-called galactic-fountain gas) is expected to interact with the virial-temperature (~ 10^6 K) gas of the corona. The associated transfer of momentum between cold and hot gas has important consequences for the dynamics of both gas phases. We quantify the effects of such an interaction using hydrodynamical simulations of cold clouds travelling through a hot medium at different relative velocities. Our main finding is that there is a velocity threshold between clouds and corona, of about 75 km/s, below which the hot gas ceases to absorb momentum from the cold clouds. It follows that in a disc galaxy like the Milky Way a static corona would be rapidly accelerated: the corona is expected to rotate and to lag, in the inner regions, by ~ 80-120 km/s with respect to the cold disc. We also show how the existence of this velocity threshold can explain the observed kinematics of the cold extra-planar gas.Comment: 10 pages, 6 figures, 1 table. Accepted for publication in MNRAS. Several typos correcte

Abstract. The nonlinear evolution of monochromatic large-amplitude circularly polarized Alfvén waves subject to the decay instability is studied via numerical simulations in one, two, and three spatial dimensions. The asymptotic value of the cross helicity depends strongly on the plasma beta: in the low beta case multiple decays are observed, with about half of the energy being transferred to waves propagating in the opposite direction at lower wave numbers, for each saturation step. Correspondingly, the other half of the total transverse energy (kinetic and magnetic) goes into energy carried by the daughter compressive waves and to the associated shock heating. In higher beta conditions we find instead that the cross helicity decreases monotonically with time towards zero, implying an asymptotic balance between inward and outward Alfvénic modes, a feature similar to the observed decrease with distance in the solar wind. Although the instability mainly takes place along the propagation direction, in the two and three-dimensional case a turbulent cascade occurs also transverse to the field. The asymptotic state of density fluctuations appears to be rather isotropic, whereas a slight preferential cascade in the transverse direction is seen in magnetic field spectra. Finally, parametric decay is shown to occur also in a non-periodic domain with open boundaries, when the mother wave is continuously injected from one side. In two and three dimensions a strong transverse filamentation is found at long times, reminiscent of density ray-like features observed in the extended solar corona and pressure-balanced structures found in solar wind data.

Abstract. Star-forming disc galaxies such as the Milky Way need to accrete > ∼ 1 M ⊙ of gas each year to sustain their star formation. This gas accretion is likely to come from the cooling of the hot corona, however it is still not clear how this process can take place. We present simulations supporting the idea that this cooling and the subsequent accretion are caused by the passage of cold galactic-fountain clouds through the hot corona. The Kelvin-Helmholtz instability strips gas from these clouds and the stripped gas causes coronal gas to condense in the cloud's wake. For likely parameters of the Galactic corona and of typical fountain clouds we obtain a global accretion rate of the order of that required to feed the star formation.Keywords: turbulence -ISM: kinematics and dynamics -Galaxy: kinematics and dynamics -Galaxy: structure -galaxies: formation PACS: 98.35.Ac, 98.38.Am, 98.58.Ay, 98.62.Ai THE PROPOSED SCENARIOStar-forming disc galaxies like the Milky Way must accrete > ∼ 1 M ⊙ of fresh gas each year [see 1, and references therein] and have built their discs gradually over the last 10 Gyr [e.g. 2]. A central question is the origin of the accreted gas and how this gas reaches the thin disc whitin which the process of star formation takes place. The virial-temperature corona, in which disc galaxies are embedded, is the only reservoir of baryons capable of sustaining an accretion rate of ∼ 1 M ⊙ yr −1 for a Hubble time. We present the results of a set of grid-based hydrodynamical simulations supporting the idea that the gas needed by the disc to form stars is drawn from this corona.Coronae of disc galaxies are similar in many respects to the hot atmospheres of giant elliptical galaxies and galaxy clusters, but with lower gas temperature and density [e.g. 3, 4]. As the hot gas of these more massive systems, the coronal gas is unlikely to fragment into clouds via thermal instability [5], but it is expected to cool monolithically and feed the central black hole rather than produce an extended cold disc in which stars can form. However, if the gas needed to feed star formation has to be drawn from the corona, a mechanism that makes the hot gas accrete onto the disc must be at work.There is abundant evidence that star formation in galaxies like the Milky Way powers a galactic fountain: ejection of gas from the mid-plane by supernova explosions [6]. Through the fountain a significant fraction (from 10 to 25 %) of the whole HI content of the galaxy is carried into its halo [see 7, and references therein]. In this work, supported by several lines of argument, we hypothesise that the transfer of gas from the corona to the star-forming disc is effected by the HI clouds ejected by the galactic fountain [8].Our hydrodynamical simulations suggest that the gas accretion proceeds through the following steps: (i) stripping of gas from fountain clouds by the corona as a result of the Kelvin-Helmholtz instability, (ii) mixing of the (high metallicity) stripped gas with a comparable amount of coronal gas in the turbulent ...