Characteristics of radio halos, cosmic ray electron propagation, and the warm ionized medium as determined through observations of radio synchrotron emission from the Milky Way and edge-on spiral galaxies
Observations of radio synchrotron emission from the Milky Way and other galaxies can be used as a powerful probe of the intrinsic properties and distribution of both the emitting material and absorbing material along the line of sight. In the case of our own Galaxy, low radio frequency measurements (∼100 kHz 100 MHz) exhibit the telltale signs of free-free absorption. By assuming a reasonable form of the cosmic ray electron (CRE) spectrum and its distribution with height above the galactic plane, propagation simulations imply that the warm ionized medium of the Galaxy is clumpy on scales of the order of a parsec with electron density of 0.225 cm-3, a filling factor of ∼0.1, and temperature of 7000 K. In order to explain the shape of the radio spectrum at the lowest end of the frequency range, the simulations further predict a “local cloud” with the same properties of the clumps in which the solar system is embedded. The radio synchrotron emission from edge-on galaxies proves to be useful in examining the vertical distribution of CREs above the galactic plane. Several methods have been proposed to extract as much information as possible from the outer halo of these galaxies, such as the multiscale clean algorithm employed in this project. For the seven sample galaxies chosen for this project, medium resolution observations reveal radio emission distributed exponentially above the galactic plane with a typical scale height of about 1 kpc. These halos are seen to extend up to ∼10 kpc from the plane, although these estimates appear to be affected by a distance related bias. Spectral index profiles derived from observations at 20 cm and 6 cm are compared with those predicted by cosmic ray propagation models to constrain the values of parameters governing the propagation of CREs throughout the galaxies, particularly: D, the diffusion coefficient (D ≈ 6.8 ± 5.1 cm 2/s), γ, the CRE injection spectral index (γ ≈ 2.0 ± 0.2), and B, the magnetic field strength ( B ≈ 5.5 ± 1.8 μG). These models are also consistent with a Gaussian source distribution in the galactic plane with a FWHM of 5.4 ± 1.4 kpc.
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- Physics: Astronomy and Astrophysics