Observations of the Extended Distribution of Ionized Hydrogen in the Plane of M31

We have used the Wisconsin H-Alpha Mapper (WHAM) to observe the spatiallyextended distribution of ionized hydrogen in M31 beyond the stellar disk. Weobtained five sets of observations, centered near the photometric major axis ofM31, that extend from the center of the galaxy to just off the edge of thesouthwestern HI disk. Beyond the bright stellar disk, but within the HI disk,weak H-alpha is detected with an intensity I(H-alpha) = 0.05 (+0.01 / -0.02)Rayleighs. Since M31 is inclined 77 degrees with respect to the line of sight,this implies that the ambient intergalactic ionizing flux onto each side of M31is Phi_0 <= 1.6 x 10^4 photons cm^-2 s^-1. Just beyond the outer boundary ofthe HI disk we find no significant detection of H-alpha and place an upperlimit I(H-alpha) <= 0.019 Rayleighs.Comment: To appear in ApJ Letters; 12 pages, 4 figure