Where Do the Disks of Spiral Galaxies End?

Abstract

In spiral galaxies, the H I surface density declines with increasing radius to a point at which it is seen to truncate dramatically in the best observed cases. 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 H I disk in spirals. Here we report on our findings for the Sculptor galaxy NGC 253. The H I disks in Sculptor galaxies extend to only about 1.2R₂₅, although we have detected ionized gas to the limits of our survey, out to 1.4R₂₅. This has important ramifications for spiral galaxies in that it now becomes possible to trace the gravitational potential beyond where the H I 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 H I measurements, but there is a hint that the rotation curve may fall abruptly not far beyond the edge of the H I disk. If this is correct, it suggests that the dark halo of NGC 253 may be truncated near the H I edge and provides further support for the link between dark matter and H I. The line ratios are anomalous with [N II] λ6548 to Hα ratios close to unity. While metallicities at these large radii are uncertain, such enhanced ratios, compared to solar-abundance H II regions ([N II] λ6548/Hα = 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, and Fe) onto dust grains.