  The VLBA was used at 1.6 GHz to observe a target field 50 arcminutes in
diameter including the core of M31. Novel VLBI correlation techniques were used
to observe 200 sources simultaneously, of which 16 were detected. We classify
all 16 as background AGN based on their X-ray properties and arcsecond- and
mas-scale morphology. The detected sources were then analyzed for evidence of
scatter-broadening due to the ionized ISM of M31. The detection of a compact
background source only 0.25 kpc projected distance from M31 places a constraint
on the extent of any extreme scattering region associated with center of M31.
However, the two sources closest to the core show evidence of scatter
broadening consistent with that which would be observed if a compact source
were observed through the inner disk of our Galaxy at the inclination of M31.
We interpret this as a detection of the ionized ISM of M31 along two lines of
sight. With the increases in bandwidth and sensitivity envisaged for future
long-baseline interferometers, this should prove to be a remarkably powerful
technique for understanding the ionized ISM in external galaxies.
