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Session 59 - Milky Way, Center & Halo.
Oral session, Tuesday, January 16
Salon del Rey Central, Hilton
We use angular broadening measurements to trace the strength of microtubulence in, and the associated energy deposition (e.g. stellar winds, supernovae) into, the warm ionized medium (WIM) in the Galaxy. \begindescription \item[Galactic center] The radial\/ distance to the region responsible for the enhanced, anisotropic angular broadening of Sgr A and nearby OH/IR stars is unconstrained. If this region is an unrelated superposition \gtrsim 1 kpc from Sgr A^*, extragalactic sources seen through the region should be \sim 1\arcsec, roughly double the OH/IR source sizes. If the region is a consequence of the extreme properties of the Galactic center and \sim 100 pc from Sgr A^*, extragalactic sources should be much\/ larger, \sim 2 arcmin. Using VLA observations of 10 fields within 5\deg\ of Sgr A^*, we have identified over 100 sources. Within \sim 1\deg of Sgr A^*, we find a paucity of sources; this deficit indicates a scattering region local to the Galactic center, < 500 pc, resulting in sources being resolved out\/ by our observations. We discuss likely agents for this enhanced scattering.
\item[Anticenter] The radial scale length of the WIM, which appears to extend well beyond the solar circle, remains largely unconstrained, because the vast majority of scattering observations has been toward the inner Galaxy. Ionized gas at distances \sim 50 kpc is suggested by the appearance of the H\,\sc i disks of nearby galaxies and models of low-redshift quasar absorption systems and Galactic ``fountains.'' We have used the VLBA to observe twelve extragalactic sources between 155\deg < \ell < 210\deg. Seven sources are at b < 1\deg; these lines of sight probe considerable path lengths, if the WIM extends to large distances. Five sources are at b < 10\deg; these lines of sight are sensitive to flaring or warping of the WIM in the outer Galaxy. We find source sizes consistent with a scale length \approx 20 kpc. \enddescription
