Atomic hydrogen in the outer Milky Way

The H I in the outer Galaxy is reanalyzed using the CO rotation curve and the full latitude extent of the gas from the Weaver and Williams survey. Gas with a surface density not less than 0.1 solar mass/sq pc is found to a distance of 30 kpc from the center. Three distinct, well-defined spiral features of roughly constant surface density are seen, two of which extend at least 20-25 kpc along their length and can be traced to 20 kpc from the center. If they are logarithmic spirals, the major arms have a pitch angle of approximately 22-25 deg. A nearly circular radial corrugation is seen as a deviation from the large-scale warping at R = 11 kpc. The outermost parts of the Galaxy show a remarkable scalloping with a large azimuthal wavenumber (m = 10). The scale height of the gas shows an almost linear increase from the solar vicinity to R = 30 kpc. This increase implies that the large mass in the outer Galaxy implied by the rotation curve does not reside in the disk.