AAS 196th Meeting, June 2000
Session 28. Nearby Galaxies
Display, Tuesday, June 6, 2000, 10:00am-6:30pm, Empire Hall South

## [28.02] The Extended Distribution of Ionized Hydrogen in M31

G.J. Madsen, R.J. Reynolds, S.L. Tufte, L.M. Haffner (U. Wisconsin - Madison), P.R. Maloney (CASA, U. Colorado)

We have used the Wisconsin H-Alpha Mapper (WHAM) 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 M31 to just off the edge of the observed southeastern HI disk. We find an H\alpha intensity of IH\alpha \approx 0.12R beyond the bright stellar disk but within the HI disk (1R = 106 / 4\pi~photons~cm-2~s-1~ster-1~= 2.41 \times 10-7~erg~cm-2~s-1~ster-1). Just within and off of the outer HI disk, we find no significant detection of H\alpha, and place an upper limit on the intensity equal to our sensitivity limit of IH\alpha \lesssim 0.04R. Our sensitivity limit is dominated by the systematic uncertainty in the shape of the spectral baseline generated by weak spatially and temporally varying atmospheric emission lines. Although a 2\sigma uncertainty due to Poisson noise is at the \approx 0.008R level, an emission line with an intensity up to \approx 0.04R may be present due to systematic errors associated with the subtraction of the atmospheric lines. Our results show a gradient in IH\alpha within the HI disk out to radii well beyond the stellar disk. This indicates that any uniform source of ionization, such as the intergalactic Lyman continuum flux \Phi0, produces less than \approx 0.04R of H\alpha radiation. If we assume that the outer HI disk aborbs all incident Lyman continuum photons, that this disk is optically thin to H\alpha photons, and use the geometry of the inclined disk, an IH\alpha \lesssim 0.04R implies that \Phi0 \lesssim 1.0 \times 104~~photons~cm-2~s-1. This upper limit is well below the required photoionization flux incident upon high-velocity clouds in the Milky Way's halo (Tufte et al. 1998) and in the Magellanic Stream (Weiner & Williams 1996). Therefore an additional source of ionization, such as the stellar disk of the Milky Way, is present in the Galactic halo (Bland-Hawthorn & Maloney 1999). This work was supported by the NSF through grant AST96-19424.