arXiv subjects

Anvar Shukurov

Publications and source records attributed to Anvar Shukurov.

76 records · Page 5Linked to original sources

arXiv2001

Galactic dynamos with captured magnetic flux and an accretion flow

We examine the behaviour of an axisymmetric galactic dynamo model with a radial accretion flow in the disc. We also introduce a vertical magnetic flux through the galactic midplane, to simulate the presence of a large scale magnetic field trapped by the galaxy when forming. The trapped vertical flux is conserved and advected towards the disc centre by the radial flow. We confirm that accretion flows of magnitude several km/s through a significant part of the galactic disc can markedly inhibit dynamo action. Moreover, advection of the vertical flux in general results in mixed parity galactic fields. However, the effect is nonlinear and non-additive -- global magnetic field energies are usually significantly smaller that the sum of purely dynamo generated and purely advected field energies. For large inflow speeds, a form of `semi-dynamo' action may occur. We apply our results to the accumulation and redistribution, by a radial inflow, of a vertical magnetic flux captured by the Galactic disc. Taking representative values, it appears difficult to obtain mean vertical fields near the centre of the Milky Way that are much in excess of 10 microgauss, largely because the galactic dynamo and turbulent magnetic diffusion modify the external magnetic field before it can reach the disc centre.

astro-ph↗

arXiv2001

Hydrostatic equilibrium in a magnetized, warped Galactic disc

Hydrostatic equilibrium of the multiphase interstellar medium in the solar vicinity is reconsidered, with the regular and turbulent magnetic fields treated separately. The regular magnetic field strength required to support the gas is consistent with independent estimates provided energy equipartition is maintained between turbulence and random magnetic fields. Our results indicate that a midplane value of $B_{0}=4\mkG$ for the regular magnetic field near the Sun leads to more attractive models than $B_{0}=2\mkG$. The vertical profiles of both the regular and random magnetic fields contain disc and halo components whose parameters we have determined. The layer at $1\la|z|\la4\kpc$ can be overpressured and an outflow at a speed of about $50\kms$ may occur there, presumably associated with a Galactic fountain flow, if $B_{0}\simeq 2\mkG$. We show that hydrostatic equilibrium in a warped disc must produce asymmetric density distributions in $z$, in rough agreement with \HI observations in the outer Galaxy. This asymmetry may be a useful diagnostic of the details of the warping mechanism in the Milky Way and other galaxies. We find indications that gas and magnetic field pressures are different above and below the warped midplane in the outer Galaxy and quantify the difference in terms of turbulent velocity and/or magnetic field strength.

astro-ph↗

arXiv2000

Global Magnetic Structures in Spiral Galaxies: Evidence for Dynamo Action

Observational evidence for dynamo action in spiral galaxies is reviewed, and the capabilities of various theories in explaining the basic features of galactic magnetic fields are discussed. Mean-field dynamo models appear to be unique in providing a coherent explanation of a wide variety of magnetic features in spiral galaxies. Reliable modelling of global magnetic structures, such as the magnetic ring in M31, requires detailed knowledge of the rotation curve, the magnitude and radial profile of turbulent and noncircular systematic velocities, the scale height of the warm ionized layer, the total gas density, the turbulent scale and their variations with galactocentric radius. More detailed models involving the effects of the spiral arms on magnetic field require the knowledge of the arm-interarm contrasts in the above quantities.

astro-ph↗

arXiv2000

A Dynamic Interstellar Medium: Recent Numerical Simulations

Recent numerical simulations of the interstellar medium driven by energy input from supernovae and stellar winds indicate that HI clouds can be formed by compression in shock waves and colliding turbulent streams without any help from thermal and gravitational instabilities. The filling factor of the hot phase in these models does not exceed 20-40% at the midplane. The hot gas is involved in a systematic vertical outflow at |z|<1-3 kpc, similar to that expected for galactic fountains, whereas the warm component may remain in hydrostatic equilibrium. The turbulent velocity is larger in the warmer phases, being 3, 10 and 40 km/s in the cool, warm and hot phases, respectively, according to one of the simulations. The models exhibit global variability in the total kinetic and thermal energy and star formation rate at periods of (0.4-4)x10^8 yr. Current models are still unable to reproduce dynamo action in the interstellar gas; we briefly discuss implications of the dynamo theory for turbulent interstellar magnetic fields.

astro-ph↗