Hyperstrong radio-wave scattering in the galactic center. II. A likelihoodanalysis of free electrons in the galactic center

The scattering diameters of Sgr A* and several nearby OH masers (approximat e to 1 " at 1 GHz) indicate that a region of enhanced scattering is along t he line of sight to the Galactic center. We combine radio-wave scattering d ata and free-free emission and absorption measurements in a likelihood anal ysis that constrains the following parameters of the GC scattering region: The GC-scattering region separation, A,,; the angular extent of the region, psi(l), and psi(b); the outer scale on which density fluctuations occur, l (o); and the gas temperature, T-e. The maximum likelihood estimates of thes e parameters are Delta(GC) = 133+(+200)(-80) pc, 0.degrees 5 less than or e qual to psi(l) less than or similar to 1 degrees, and (l(o)/1 pc)T-2/3(e)-1 /2 = 10(-7+/-0.8). The parameter psi(b) was not well constrained, and we ad opt psi(b) = 0.degrees 5. The close correspondence between Delta(GC) and ps i(l)D(GC) suggests that the scattering region encloses the GC. As host medi a for the scattering, we consider the photoionized surface layers of molecu lar clouds and the interfaces between molecular clouds and the 10(7) K ambi ent gas. We are unable to make an unambiguous determination, but we favor t he interface model in which the scattering medium is hot (T-e similar to 10 (6) K) and dense (n(e) similar to 10 cm(-3)). The GC scattering region prod uces a 1 GHz scattering diameter for an extragalactic source of 90 ", if th e region is a single screen, or 180 ", if the region wraps around the GC, a s appears probable. We modify the Taylor-Cordes model for the Galactic dist ribution of free electrons in order to include an explicit GC component. We predict that pulsars seen through this region will have a dispersion measu re of approximately 2000 pc cm(-3), of which approximately 1500 pc cm(-3) a rises from the GC component itself. We stress the uniqueness of the GC scat tering region, probably resulting from the high-pressure environment in the GC.