1997/11/15 by T. Joseph W. Lazio, James M. Cordes
Physics and Astronomy · #Astronomy and Astrophysical Research #Astrophysics and Star Formation Studies #Electron scattering #Flux (metallurgy) #Galaxies: Formation, Evolution, Phenomena #Galaxy #Intergalactic travel #Milky Way #Scale height #Scattering #Source counts #Spiral galaxy #astro-ph
paper · pdf · doi:10.1086/305448
34 pages, LaTeX2e with AASTeX aaspp4 macro, 9 figures in 9 PostScript files, accepted for publication in ApJ
arxiv created 1997/11/15 · openalex publication_date 1998/04/10 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/06
We use radio-wave scattering data for extragalactic sources and pulsars to constrain the distribution of ionized gas in the outer Galaxy. Like previous models, our model for the H II disk includes parameters for the radial scale length and scale height of the ionized gas. In addition, we have used the known H I distribution in the outer Galaxy in constructing our model, and we allow the H II disk to warp and flare. We also include the Perseus arm in our model. We use a likelihood analysis of 18 anticenter sources with measured scattering observables: 11 extragalactic sources and 7 pulsars. We find that the strength of scattering in the Perseus arm is no more than 60% of the level contributed by spiral arms in the inner Galaxy and is equivalent to a scattering diameter of 1.5 mas at 1 GHz. Our analysis favors an unwarped, nonflaring disk with a scale height of 1 kpc, though this may reflect the nonuniform and coarse coverage of the anticenter provided by the available data. One extragalactic source has a size a factor of 2 smaller than predicted by our model, possibly indicating the existence of holes in the scattering material. The lack of a warp in the scattering material indicates that VLBI observations near 1 GHz with an orbiting station having baseline lengths of a few Earth diameters will not be affected by interstellar scattering at moderate Galactic latitudes, | b | ≈ 15°. The radial scale length is 15-20 kpc, but the data cannot distinguish between a gradual decrease in the electron density and a truncated distribution. We favor a truncated one because we associate the scattering with massive star formation, which is also truncated near 20 kpc. A radial extent of 20 kpc is also comparable to the radial extent of Hα emission observed for nearby spiral galaxies. We find that the distribution of electron density turbulence must decrease more rapidly with Galactocentric distance than does the distribution of hydrogen. Alternate ionizing and turbulent agents—the intergalactic ionizing flux and the passage of satellite galaxies through the disk—are unlikely to contribute significant amounts to scattering in the anticenter. We cannot exclude the possibility that a largely ionized but quiescent disk, similar to that inferred for some Lyα absorbers, extends to ≳100 kpc.