Stars and bars

Bars are a common feature of disc galaxies and, as such, must be taken into account in understanding the evolution of structure within galaxies, argues Johan H Knapen.     

Dynamos with a flat α-effect distribution

Abstract

In order to obtain a better insight into the excitation conditions of magnetic fields in flat objects, such as galaxies, we have calculated critical dynamo numbers of different magnetic field modes for spherical dynamos with a flat α-effect distribution. A simple but realistic approximation formula for the rotation curve is employed. In most cases investigated a stationary quadrupole-type solution is preferred. This is a consequence of the flat distribution of the α-effect. Non-axisymmetric fields are in all cases harder to excite than axisymmetric ones. This seems to be the case particularly for flat objects in combination with a realistic rotation curve for galaxies. The question of whether non-axisymmetric (bisymmetric) fields, which are observed in some galaxies, can be explained as dynamos generated by an axisymmetric αω-effect is therefore still open.     

Magma-clustering and galaxy-clustering: a comparison

The analogies between clustering processes in magmas and clustering ones in galaxies are pursued. By using reasonable assumptions about the properties of the fluids in magmas and in the Earth's mantle, it is shown that: (1) in both processes instabilities are formed by density fluctuations; (2) in both phenomena the process of clusters is of the hierarchical kind; (3) magmas and galaxies both exhibit a simultaneous superposition of mass, momentum and energy transports; (4) coalescence occurring in magmas can be considered the corresponding phenomenon to the close encounters in the galactic groups; and (5) vorticity of the Taylor Proudman type in magmatic systems can be regarded as the corresponding phenomenon to cannibalism in the galaxies. It is suggested that there is a single transport mechanism that may be universally applicable.     

Extended horizontal branch stars

Astronomers met at Keele University, Staffordshire, in June to discuss how observations of extended horizontal branch stars have changed our view of hot star atmospheres, stellar winds, single and binary star evolution, stellar pulsations and the age estimates for elliptical galaxies. Pierre Maxted reports.     

Analysis of large scale disturbances in spiral galaxies

Summary A scale analysis is made of large scale eddying motions superimposed upon the state of mean rotation of spiral galaxies. By this means a balance relationship is derived between the fields of potential on the one hand and the horizontal eddy velocity on the other. This should provide a useful kinematical relationship for observational astronomy. It is obvious to the meteorological reader that the large scale irregular motions of galaxies are analogous to those which occur in our own atmosphere.     

Torus dynamo in the outer rings of galaxies

ABSTRACT

The magnetic fields in the inner parts of some spiral galaxies are understood quite well. Their generation is connected with the dynamo mechanism that is based on the joint action of turbulent diffusion and the α-effect. Usually the galactic dynamo is described with the so-called no-z approximation which takes into account that the galaxy disc is quite thin, with the implication that some spatial derivatives may be replaced by algebraic expressions. Some galaxies have outer rings that are situated at some distance from the galactic centre. The magnetic field can be described there also using the no-z model. As the thickness of such objects is comparable with their width, it is necessary to take into account the z-dependence of the field. We have studied the magnetic field evolution using the no-z approximation and torus dynamo model for the torus with rectangular cross-section in the axisymmetric case.     

Black holes at work

Andrew C Fabian reviews the role that black holes play in the evolution of the galaxies that surround them, in his 2009 Presidential Address to the Royal Astronomical Society.     

MERLIN: today and tomorrow

The MERLIN team together with Melvin Hoare and Karen Wills , review MERLIN's revealing role in studying star formation throughout the universe – from nearby stars to the most distant galaxies.     

Donate a telescope in IYA

A combination of NASA/ESA's Hubble Space Telescope and the European Southern Observatory's Very Large Telescope has revealed the 3-D movements of gas in distant galaxies, and sprung some surprises for observers.     

Core-collapse supernovae and their massive progenitors

In this summary of the 10 November 2006 RAS Specialist Discussion Meeting, organizers Paul Crowther and Steve Smartt consider aspects of massive stellar evolution, their dependence upon stellar winds, progenitor models and explosion mechanisms. In addition, observations of core-collapse supernovae were discussed alongside the question of whether supernovae form significant quantities of dust, of relevance to the detection of dust in star-forming galaxies in the early universe.     

Small galaxies are growing smaller

New telescope technology and major sky surveys are finding more and more dwarf galaxies. Steve Phillipps discusses how this growing population may hold the clues to understanding a range of galaxy collisions and interactions.     

Structure of magnetic fields generated by galactic dynamos

Abstract

We investigated global axisymmetric (m = 0) and non-axisymmetric (m = 1) modes of magnetic fields generated by the galactic dynamo including the α²-dynamo. The α²-dynamo is responsible for the field generation in the central region of galaxies where the shear of galactic rotation is weak (e.g. M51). The highest growth rate of m = 1 modes is always smaller than that of m = 0 modes; thus m = 1 modes of the standard galactic dynamo cannot explain the dominance of the bisymmetric fields in spiral galaxies. Radial extent of each m = 1 mode is too narrow to reproduce the observed bisymmetric structure extending over a disk.     

Starbirth in galaxy collisions

When galaxies collide, stars are born. But many such close encounters also result in the formation of clusters with many of the characteristics of ancient globular clusters. Richard de Grijs argues the case that this process represents continuing formation of globular clusters in a hierarchical scheme of galaxy formation.     

Hydrodynamic conservation integrals for galaxies

Summary This paper supplements and extends previous discussions of galaxies in the light of meteorological theory by including explicitly the effect of random motions in a star gas. The conservation integrals for momentum and kinetic energy are formulated and the kinetic energy is further partitioned into kinetic energy of the large scale flows and kinetic energy of the random star motions. The kinetic energy of the random star motions is analogous to the internal energy in an ordinary gas and is transformed into kinetic energy of the fluid flow according to a law which is a generalization of the first law of thermodynamics as applied to fluids. It is possible to make more detailed division of the kinds of energy and their laws of transformation.     

Rings of radio continuum,Co, Hα and magnetic fields in galaxies

Abstract

Observations are reviewed that indicate the existence of rotating rings in a number of galaxies that possess poloidal magnetic fields in their nuclear regions, including our own Galaxy. Jets from these, possibly aligned with the poloidal field, may also be present. The role of these rings in dynamo processes is briefly discussed.     

The DRESDYN project: liquid metal experiments on dynamo action and magnetorotational instability

ABSTRACT

Magnetic fields of planets, stars and galaxies are generated by self-excitation in moving electrically conducting fluids. Once produced, magnetic fields can play an active role in cosmic structure formation by destabilising rotational flows that would be otherwise hydrodynamically stable. For a long time, both hydromagnetic dynamo action as well as magnetically triggered flow instabilities had been the subject of purely theoretical research. Meanwhile, however, the dynamo effect has been observed in large-scale liquid sodium experiments in Riga, Karlsruhe and Cadarache. In this paper, we summarise the results of liquid metal experiments devoted to the dynamo effect and various magnetic instabilities such as the helical and the azimuthal magnetorotational instability and the Tayler instability. We discuss in detail our plans for a precession-driven dynamo experiment and a large-scale Tayler–Couette experiment using liquid sodium, and on the prospects to observe magnetically triggered instabilities of flows with positive shear.     

Magnetic fields in galaxy halos and their importance

Abstract

The observational facts about magnetic fields in galactic halos are reviewed. The existence, origin and significance of poloidal field components are described. Observational evidence that magnetic fields channel winds from active galactic centres is discussed. Field strengths adduced from the radio polarizations of edge-on galaxies are given. Priorities for future research are suggested.     

Scaling Law for Galactic Magnetic Fields

A nonlinear mean field dynamo in turbulent disks and spherical shells is discussed. We use a nonlinearity in the dynamo which includes the effect of delayed back-reaction of the mean magnetic field on the magnetic part of the — effect. This effect is determined by an evolutionary equation. The axisymmetric case is considered. An analytical expression (in a single-mode approximation) is derived which gives the magnitude of the mean magnetic field as a function of rotation and the parameters for turbulent disks. The value obtained for the mean magnetic field is in agreement with observations for galaxies.     

On a general fluid dynamical theory of discrete unstable spiral modes in disk-shaped galaxies

Abstract

A general fluid dynamical theory of discrete unstable spiral modes in disk-shaped galaxies is described. This formulation of modes includes a radiation boundary condition and an exact numerical treatment of the Poisson equation. Thus, the modes are maintained by an outward transport of angular momentum, but they may be composed of both leading and trailing waves. A numerical scheme based on this formulation is described, and examples of modes obtained with this scheme are presented. These examples compare favorably with calculations based on the original asymptotic theory of Bertin, Lau, Lin and Mark. The implications of the present formulation of modes in galactic models support the hypothesis of a quasi-stationary spiral structure.     

The galactic dynamo: Axisymmetric and non-axisymmetric modes

Abstract

We discuss recent developments in the theory of large-scale magnetic structures in spiral galaxies. In addition to a review of galactic dynamo models developed for axisymmetric disks of variable thickness, we consider the possibility of dominance of non-axisymmetric magnetic modes in disks with weak deviations from axial symmetry. Difficulties of straightforward numerical simulation of galactic dynamos are discussed and asymptotic solutions of the dynamo equations relevant for galactic conditions are considered. Theoretical results are compared with observational data.