Hyperstrong radio-wave scattering and free electrons in the galactic center

Abstract

The scattering [index interstellar scattering! angular broadening] diameters of Sgr A* [index Source! Sgr A*] and several nearby OH masers (≅ 1 arcsec at 1 GHz) indicate that a region of enhanced scattering [index interstellar scattering] is along the line of sight to the Galactic center. We have combined radio-wave scattering data and free-free emission [index free-free emission] measurements in a likelihood analysis that constrains the GC-scattering region separation to be ΔGC = 133-80+200 pc and the angular extent of the region to be 0.5 ° <= ψell <-- 1°. The close correspondence between ΔGC and ψell DGC suggests that the scattering region encloses the GC. A uniform scattering region would cover G359.87 + 0.18 [index Source G359.87 + 0.18], a Fanaroff-Riley II radio galaxy 15 arcmin from Sgr A* [index Source Sgr A*], producing a scattering diameter of at least 600 arcsec at 0.33 GHz. Its observed diameter is 20 arcsec. We therefore conclude that the GC scattering region is patchy, with a typical separation between scattering clumps of approximately 5 arcmin (≅ 10 pc at a distance of 8.5 kpc). This separation is comparable to the size scale of GC molecular clouds. The similarity between these two length scales is an indication that the scattering region is linked to the GC molecular clouds. We favor a model in which the scattering arises in the interfaces between molecular clouds and the 107 K ambient gas; in this model the scattering medium is hot (Te ~106 K) and dense (ne ~10 cm-3). Pulsars [index pulsars] seen through this region will have a dispersion measure of approximately 2000 pc cm -3, of which approximately 1500 pc cm-3 arises from the GC, and will have a pulse broadening [index interstellar scattering ! pulse broadening] time of 350νGHz-4 seconds. We stress the uniqueness of the GC scattering region, probably resulting from the high-pressure environment in the GC.