Global enhancement and structure formation of the magnetic field in spiral galaxies

Khoperskov, Sergey; Khrapov, S. S.

doi:10.1051/0004-6361/201629988

Cited by 14 publications

(11 citation statements)

References 103 publications

(129 reference statements)

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“…Instead density waves have been historically treated as a process that could shape large-scale fields already generated. Only recent MHD simulations of galactic discs have suggested some link between the dynamo and the spiral arms (Dobbs et al 2016;Khoperskov & Khrapov 2018). Our GI-simulations show that such waves can, in principle, act as a dynamo and produce spiral magnetic patterns, similar to those observed in galaxies.…”

Section: Applications To Galaxiessupporting

confidence: 52%

Gravitoturbulent dynamos in astrophysical discs

Riols

Latter

2018

Monthly Notices of the Royal Astronomical Society

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The origin of large-scale and coherent magnetic fields in astrophysical discs is an important and long standing problem. Researchers commonly appeal to a turbulent dynamo, sustained by the magneto-rotational instability (MRI), to supply the large-scale field. But research over the last decade in particular has demonstrated that various non-ideal MHD effects can impede or extinguish the MRI, especially in protoplanetary disks. In this paper we propose a new scenario, by which the magnetic field is generated and sustained via the gravitational instability (GI). We use 3D stratified shearing box simulations to characterise the dynamo and find that it works at low magnetic Reynolds number (from unity to ∼ 100) for a wide range of cooling times and boundary conditions. The process is kinematic, with a relatively fast growth rate ( 0.1Ω), and it shares some properties of mean field dynamos. The magnetic field is generated via the combination of differential rotation and spiral density waves, the latter providing compressible horizontal motions and large-scale vertical rolls. At greater magnetic Reynolds numbers the build up of large-scale field is diminished and instead small-scale structures emerge from the breakdown of twisted flux ropes. We propose that GI may be key to the dynamo engine not only in young protoplanetary discs but also in some AGN and galaxies.

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Section: Applications To Galaxiessupporting

confidence: 52%

Gravitoturbulent dynamos in astrophysical discs

Riols

Latter

2018

Monthly Notices of the Royal Astronomical Society

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“…The equilibrium state of the gaseous disk is found according to the radial balance between the gas rotation versus radial gradient of gas pressure, gravitational forces (external potential and self-gravity) and magnetic field pressure. To mimic the turbulent structure of the magnetic field in the disk plane, its components are established as a superposition of two modes with (pseudo-) random location in the disk and various amplitudes [7].…”

Section: Methods and Modelmentioning

confidence: 99%

High Performance Computing of Magnetized Galactic Disks

Khoperskov

Venichenko

Khrapov

et al. 2018

JSFI

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A parallel implementation of the magneto-hydrodynamical code for global modeling of the galactic evolution is reported. The code is parallelized by using MPI interface, and it shows ideal scaling up to 200-300 cores on Lomonosov supercomputer with fast interconnect. In the benchmarking of this code, we study the dynamics of a magnetized gaseous disk of a galaxy with a bar. We run a high-resolution 3D magnetohydrodynamic simulation taking into account the Milky Way-like gravitational potential, gas self-gravity and a network of cooling and heating processes in the interstellar medium. By using this simulation the evolution of morphology and enhancement of the magnetic field are explored. In agreement to hydrodynamical models, when the bar is strong enough, the gas develops sharp shocks at the leading side of the bar. In such a picture we found that when typically the magnetic field strength traces the location of the largescale shocks along the bar major axis, the magnetic field pressure weakens the shocks and reduces the inflow of gas towards the galactic center.

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“…Since any spiral arms are supposed to be weakly unstable in our linear analysis even in infinitesimal magnetic fields, perturbations can grow slowly (see Section 2.1) although the arms in this run do not fragment in early stages of the simulation. Moreover, magnetic fields in spiral arms can be amplified because of accretion of gas coupled with magnetic fields onto the arms, transport of magnetic fields (Khoperskov & Khrapov 2018) and/or dynamo mechanisms by differential rotation of a galactic disc (Shukurov et al 2006) and small-scale turbulence (Schober et al 2013). The arms finally fragment at t = 800-900 Myr even in the run with the weak magnetic fields (βini = 100).…”

Section: The Single-component Runsmentioning

confidence: 99%

Spiral-arm instability – II. Magnetic destabilization

Inoue

Yoshida

2019

Monthly Notices of the Royal Astronomical Society

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Fragmentation of spiral arms can drive the formation of giant clumps and induce intense star formation in disc galaxies. Based on the spiral-arm instability analysis of our Paper I, we present linear perturbation theory of dynamical instability of selfgravitating spiral arms of magnetised gas, focusing on the effect of toroidal magnetic fields. Spiral arms can be destabilised by the toroidal fields which cancel Coriolis force, i.e. magneto-Jeans instability. Our analysis can be applied to multi-component systems that consist of gas and stars. To test our analysis, we perform ideal magnetohydrodynamics simulations of isolated disc galaxies and examine the simulation results. We find that our analysis can characterise dynamical instability leading arms to fragment and form clumps if magnetic fields are nearly toroidal. We propose that dimensionless growth rate of the most unstable perturbation, which is computed from our analysis, can be used to predict fragmentation of spiral arms within an orbital time-scale. Our analysis is applicable as long as magnetic fields are nearly toroidal. Using our analytic model, we estimate a typical mass of clumps forming from spiralarm fragmentation to be consistent with observed giant clumps ∼ 10 7−8 M . Furthermore, we find that, although the magnetic destabilisation can cause low-density spiral arms to fragment, the estimated mass of resultant clumps is almost independent from strength of magnetic fields since marginal instability occurs at long wavelengths which compensate the low densities of magnetically destabilised arms.

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Global enhancement and structure formation of the magnetic field in spiral galaxies

Cited by 14 publications

References 103 publications

Gravitoturbulent dynamos in astrophysical discs

Gravitoturbulent dynamos in astrophysical discs

High Performance Computing of Magnetized Galactic Disks

Spiral-arm instability – II. Magnetic destabilization

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