Observations of the extended distribution of ionized hydrogen in the plane of M31

We have used the Wisconsin H-Alpha Mapper to observe the spatially extended distribution of ionized hydrogen in M31 beyond the stellar disk. We obtained five sets of observations, centered near the photometric major axis of M31, that extend from the center of the galaxy to just off the edge of the southwestern H I disk. Beyond the bright stellar disk, but within the H I disk, weak H alpha is detected with an intensity I-H alpha = 0.05(-0.02)(+0.01) R.(1) Since M31 is inclined 77 degrees with respect to the line of sight, this implies that the ambient intergalactic ionizing flux on each side of M31 is Phi (o) less than or equal to 1.6 x 10(4) photons cm(-2) s(-1). Just beyond the outer boundary of the H I disk, we find no significant detection of H alpha and place an upper limit I-H alpha less than or equal to 0.019 R.