How does the Galaxy work? (eBook)

The Milky Way: Four Centuries of Discovery of the Galaxy.- Hi Clouds Beyond the Galactic Disk.- The Wham Northern Sky Survey and the Nature of the Warm Ionized Medium in the Galaxy.- The Structure of Galactic Hydrogen on the Largest Scales.- The Bu-Fcrao Galactic Ring Survey.- The View of the Interstellar Medium with the Canadian Galactic Plane Survey.- Dark Hydrogen in the Galactic Plane.- Diffuse Infrared Excess Features in the Canadian Galactic Plane Survey.- The Oxygen Abundance in Our Galaxy.- Ionization of the Galactic Halo.- Diffuse Ionized Gas and the Disk-Halo Interaction in Spiral Galaxies.- Are All the DIGs the Same?.- Radio Halos in Star Forming Galaxies.- Brackett-? Line Survey of the Ionized Hydrogen in the Galactic Plane.- Leaky H II Regions and Heliumionization in the Reynolds Layer.- Reynolds Layers in Nearby Galaxies: The Face-On View.- Volume Filling Factors of the Dig.- Latest Results of the Sethi Survey at Arecibo.- Job Accomplished? The Escape of Ionizing Radiation from the Disks of Spiral Galaxies.- Modeling the Diffuse Ionized Gas with Cloudy.- Diffuse Ionized Medium Panel Discussion.- Absorption Line Studies in the Halo.- Dynamical Evolution of High Velocity Clouds in the Galactic Halo.- Pulsar Bow Shocks as Probes of Warm Neutral Gas.- Highly Ionized HVCs Towards Pks 2155–304: Galactic or Extragalactic?.- Towards an Understanding of the Dynamics of the Milky Way.- Fuse Observations of Ovi Emission from the Galactic Fountain.- Pattern Speeds of Three Spirals Using Co and the Tremaine-Weinberg Method.- Gas Infall Onto the Superthin Ic 2233.- High-Velocity Gas Observed Towards the Shajn 147 Supernova Remnant.- Hot Gas in the Galaxy.- Bubbles and Superbubbles.- The Chandra View of M101: What Can We Learn About the Milky Way?.- Fuse Observations of the Vela Supernova Remnant.- Circumstellar Interaction in Young Radio Supernovae.- Chandra Archive of Nearby Galaxies. I. Normal Galaxies.- Hi Shells in the 2nd Galactic Quadrant.- Key Science Programs for the Spear Mission.- Chandra Archive of Nearby Galaxies. II. Galaxies with Nuclear Activity.- Heating and Cooling in the ISM.- The Galactic Centre and Its Black Hole.- Stellar Metallicities and the Formation of the Galactic Bulge.- The Galactic Bar.- Stellar Distribution in the Galactic Disk from NIR Color-Magnitude Diagrams.- Stellar Clusters in the Nucleus of Galaxie.- Massive Young Clusters.- Deep Multicolor NIR Surver of the Galactic Plane.- Tracing the History of the Galaxy with Globular Clusters.- Tidal Debris in the Galactic Halo.- Star Formation in High-Redshift ISM.- Gas Inflow in Spiral Galaxies from Kiloparsec to Parsec Scale.- Open Clusters as Tracers of the Cygnus ARM.- Triaxial Bulge and In-Plane Bar of the Milky Way.- Tidal Shocks on Globular Clusters.- Evolution of Globular Cluster Populations in Compact Galaxy Groups.- Near IR Color-Magnitude Diagrams of Bulge Globular Clusters.- Magnetic Fields in the Milky Way and Other Spiral Galaxies.- The Origin of Galactic Magnetic Fields.- Magnetic Turbulence in the WIM.- Magnetic Fields and Spiral Structure.- Calculations of the Dynamo Coefficients in Galactic Magnetic Fields with Shear.- Galactic Magnetic Fields, from Radio Polarimetry of the WIM.- Radio Polarimetry in the Sothern Galactic Plane Survey.- The Truncation of the Stellar Disk of the Milky Way: A Magnetic Effect?.- The Parker Instability.- 3D MHD Modeling of the Galaxy.- The Frequency Evolution of Interstellar Pulse Broadening from Radio Pulsars.- 3D HD and MHD Adaptive Mesh Refinement Simulations of the Global and Local ISM.- The Turbulent Interstellar Medium: Insights and Questions from Numerical Models.- Hi: Temperatures, Magnetism, Turbulence, Morphology.- Cosmic Rays and ? Rays in the Galaxy.- Probing the Disk-Halo Connection Through the Local Interstellar Chimney.- The Spear (Spectroscopy of Emission from Astrophysical Radiation) Mission.- Dense Gas Contours Surrounding the Local Cavity: Comparison with the Soft X-Ray Background Emission Map.- Fuse Observations of the Hot Post-AGB Star ZNG 1 in the Globular Cluster M5.- 3D Simulations of the Local Bubble: How Much Ovi Can We Expect?.- XMM-Newton Observations of the Local Bubble and the Galactic Halo.- The Temperature of Massive Stars.- Fuse Survey Of The Local Interstellar Medium.- Diffuse Ovi Emission Towards the Loop I Superbubble.- The Local Bubble: Where is the ?300,000 K Gas?.- Is the Local Bubble Dead?.- Closing Remarks.

MAGNETIC FIELDS IN THE MILKY WAY AND OTHER SPIRAL GALAXIES (p. 277-278)
R. Beck MPI für Radioastronomie, Germany

Abstract

The average strength of the total magnetic field in the Milky Way, derived from radio synchrotron data under the energy equipartition assumption, is 6µG locally and 10µG at 3 kpc Galactic radius. Optical and synchrotron polarization data yield a strength of the local regular field of 4µG (an upper limit if anisotropic fields are present), while pulsar rotation measures give 1.5µG (a lower limit if small-scale fluctuations in regular field strength and in thermal electron density are anticorrelated). In spiral arms of external galaxies, the total [regular] field strength is up to 35µG [ 15µG]. In nuclear starburst regions the total field reaches 50µG. Little is known about the global field structure in the Milky Way. The local regular field may be part of a "magnetic arm" between the optical arms, a feature that is known from other spiral galaxies. Unlike external galaxies, rotation measure data indicate several global field reversals in the MilkyWay, but some of these could be due to field distortions. The Galaxy is surrounded by a thick radio disk of similar extent as around many edge-on spiral galaxies. While the regular field of the local disk is of even symmetry with respect to the plane (quadrupole), the regular field in the inner Galaxy or in the halo may be of dipole type. The Galactic center region hosts highly regular fields of up to milligauss strength which are oriented perpendicular to the plane.


1. Motivation

Magnetic fields are a major agent in the interstellar medium. They contribute significantly to the total pressure which balances the ISM against gravitation. They affect the gas flows in spiral arms and around bars. Magnetic fields are essential for the onset of star formation as they enable the removal of angular momentum from the protostellar cloud during its collapse. MHD turbulence distributes energy from supernova explosions within the ISM. Magnetic reconnection is a possible heating source for the ISM and halo gas. Magnetic fields also control the density and distribution of cosmic rays in the ISM.

2. Observing magnetic fields

Polarized emission at optical, infrared, submillimeter and radio wavelengths is the clue to interstellar magnetic fields. Radio continuum emission at centimeter wavelengths, emitted by cosmic-ray electrons in the interstellar medium, by pulsars and by background quasars, has higher degrees of polarization than in the other spectral ranges and provides the most extensive and reliable information on large-scale interstellar magnetic fields in our Galaxy (Heiles 1996, Han et al. 1999a) and about 70 external galaxies (see list in Beck 2000).

The observable degree of polarization is reduced by the contribution of unpolarized thermal emission which may dominate in star-forming regions, by Faraday depolarization (Sokoloff et al. 1998) and by geometrical depolarization within the beam. The orientation of polarization vectors is changed in a magneto-ionic medium by Faraday rotation which is generally small below about ?6 cm so that the B–vectors (i.e. the observed E–vectors rotated by 90.) directly trace the orientation of the regular (or anisotropic) fields in the sky plane. Polarization angles are ambiguous by ±180. and hence insensitive to field reversals. Compression or stretching of turbulent .elds generates incoherent anisotropic .elds which reverse direction frequently within the telescope beam, so that Faraday rotation is small while the degree of polarization can be high.