Where do the disks of spiral galaxies end?

In spiral galaxies, the HI surface density declines with increasing radius to a point where it is seen to truncate dramatically in the best observed cases. It was anticipated that if the ambient radiation field is sufficiently strong, there exists a maximum radius beyond which the cold gas is unable to support itself against ionization. We have now succeeded in detecting ionized gas beyond the observed HI disk in spirals. Here, we report on our findings for the Sculptor galaxy NGC 253. The HI disks in Sculptor galaxies extend to only about 1.2 R_25 although we have detected ionized gas to the limits of our survey out to 1.4 R_25. This has important ramifications for spiral galaxies in that it now becomes possible to trace the gravitational potential beyond where the HI disk ends. The detections confirm that the rotation curve continues to rise in NGC 253, as it appears to do for other Sculptor galaxies from the HI measurements, but there is a hint that the rotation curve may fall abruptly not far beyond the edge of the HI disk. If this is correct, then it suggests that the dark halo of NGC 253 may be truncated near the HI edge, and provides further support for the link between dark matter and HI. The line ratios are anomalous with [NII]6548/H-alpha ratios close to unity. While metallicities at these large radii are uncertain, such enhanced ratios compared to solar-abundance HII regions ([NII]/H-alpha = 0.05-0.2) are likely to require selective heating of the electron population without further ionization of N^++. We discuss the most likely sources of ionization and heating, and the possible role of refractory element depletion (e.g. Ca, Si, Fe) onto dust grains.