The vertical structure of the outer Milky Way HI disk

We examine the outer Galactic H I disk for deviations from the b = 0 degrees plane by constructing maps of disk surface density, mean height, and thickness. We find that the Galactic warp is well described by a vertical offset plus two Fourier modes of frequency m = 1 and 2, all of which grow with galactocentric radius. Adding the m = 2 mode accounts for the large asymmetry between the northern and southern warps. We use a Morlet wavelet transform to investigate the spatial and frequency localization of higher frequency modes; these modes are often referred to as scalloping.'' We find that the m = 10 and 15 scalloping modes are well above the noise, but localized; this suggests that the scalloping does not pervade the whole disk, but only local regions.