We examine the outer Galactic H i disk for deviations from the b ¼ 0 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 ¼ 1 and 2, all of which grow with galactocentric radius. Adding the m ¼ 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 ¼ 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.

We produce a detailed map of the perturbed surface density of neutral hydrogen in the outer Milky Way disk demonstrating that the Galaxy is a nonaxisymmetric multi-armed spiral. Spiral structure in the southern half of the Galaxy can be traced out to at least 25 kpc, implying a minimum radius for the gas disk. Overdensities in the surface density are coincident with regions of reduced gas thickness. The ratio of the surface density to the local median surface density is relatively constant along an arm. Logarithmic spirals can be fit to the arms with pitch angles of 20• -25• .Mapping the Milky Way's spiral structure is traditionally difficult because the Sun is imbedded in the Galactic disk; absorption by dust renders optical methods ineffective at distances larger than a few kpc. Radio lines like the 21 cm hyperfine transition of atomic hydrogen (HI) are not affected by this absorption, and are therefore well-suited to looking through the disk. The density of HI is roughly proportional to the intensity of the emission, barring optical depth effects. Maps are constructed by using the Doppler shift of the emission line in combination with the rotation structure to determine where in the Galaxy the emission originates. The first maps of the HI density in the midplane of the Milky Way (1) made using the 21 cm transition offered the promise that HI mapping of the Galactic plane would reveal its spiral structure by highlighting regions with HI overdensities. The task is easiest outside of the Sun's orbit around the Galactic center because velocities in this region map into distances uniquely; in the inner Galaxy, however, each velocity corresponds to two distinct distances from the Sun, so an unambiguous velocity-to-position mapping is impossible. When observations of the southern plane of the Galaxy made in Australia were combined with northern sky data from the Netherlands (2), a picture of spiral structure of the inner Milky Way was seen, but it did not clearly 0 Science, in press. Embargoed for discussion in the popular press until publication by ScienceXpress.

Using the Fisher matrix formalism, we quantitatively investigate the constraints on a 10 dimensional space of cosmological parameters which may be obtained with future cluster surveys. We explore the dependence of the Ω m constraint on both angular coverage and depth of field. We show that in each case there is a natural cutoff beyond which the constraints on Ω m do not significantly improve. We also investigate the sensitivity of the constraints to changes in our knowledge of the Mass-Temperature (M-T) relation by including its normalization and scatter as two of the parameters in the Fisher matrix.To make our analysis more realistic, we have added, as priors, the Fisher matrices from hypothetical supernova and CMB experiments. We find that X-ray cluster surveys actually help to constrain the M-T relation, and explore the implications of this result.

We measure the Galactic rotation curve and its first two vertical derivatives in the first and fourth quadrants of the Milky Way using the 21 cm VGPS and SGPS. We find tangent velocities of the atomic gas as a function of Galactic longitude and latitude by fitting an analytic line profile to the edges of the velocity profiles. The shape of the analytic profile depends only on the tangent velocity and the velocity dispersion of the gas. We use two complementary methods to analyze the tangent velocities: a global model to fit typical parameter values and a local fitting routine to examine spatial variations. We confirm the validity of our fitting routines by testing simple models. Both the global and local fits are consistent with a vertical falloff in the rotation curve of À22 AE 6 km s À1 kpc À1 within 100 pc of the Galactic midplane. The magnitude of the falloff is several times larger than what would be expected from the change in the potential alone, indicating some other physical process is important. The falloff we measure is consistent in magnitude with that measured in the halo gas of other galaxies.

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