Since the disk of the Milky Way comprises the bulk of the luminous mass of the Galaxy, its formation and evolutionary history provide critical clues to galaxy formation in general and to the interpretation of spatially unresolved observations of distant galaxies. Despite its significance, our knowledge of the structure of the stellar disk on size scales of tens of kpc is relatively poor due to our vantage point within the disk. As a result, disk stars can be obscured by large columns of intervening dust, and the disk subtends a dauntingly large solid angle on the sky (fig. 7). There are, nevertheless, fundamental and compelling questions about the nature, formation, and evolution of the disk that can be addressed by densely sampling the stellar kinematics and metallicities in the Galactic plane. For example, what does the stellar disk of the Milky Way look like from above? -- the kinematic (e.g., spiral) structure of the disk is better traced by stars than gas since distances can be obtained for individual stars and the result is model independent. The stellar metallicity distribution in the disk contains fossil clues to both the star formation and merger history of the disk.
Simulated mergers of satellite galaxies with the Milky Way show that such encounters can produce vertical heating and thickening of the disk, as well as tilts and warps that persist for many dynamical times (> 10 Gyr; fig. 10). As the satellite is torn apart, debris is left at all disk radii as the satellite first sinks into the plane, then spirals in (e.g., Quinn et al. 1993; Walker et al. 1996; Huang & Carlberg 1997). Since the disk can be damaged extensively in fairly major mergers (satellite mass >10% of the disk mass), with the extent of the damage a strong function of the mass and density of the satellite, measurements of the structure of the Milky Way disk can constrain the role of major mergers in its evolutionary history. Measurements of the ``thinness" of the thin disk, when combined with the ages of the oldest objects in the thin disk, can be used to determine the time since the last major merger.
A Representative Project: Galactic Plane Survey of K-giants
Clues to the Milky Way's merging history can be obtained from searches for (1) merger remnants that have spiraled in to the disk, and counter-rotating streams from retrograde mergers (isolated through kinematics and chemical abundances), (2) tilts and warps, and (3) the radial variation of the vertical velocity dispersion. K giants are excellent tracers of disk structure since they can be seen across the entire Galaxy, are not restricted to young stellar populations, and are abundant enough that studies of large samples are possible. Along a given line-of-sight that probes the entire Galaxy (>~20kpc in depth), we require a sample of >1000 stars in order to achieve a spatial resolution of <~1 kpc with >~100 stars in each radial bin in order to characterize the mean and spread in metallicity, the stellar kinematics, and any relation between the two to a few percent. K giants have the requisite density: at , b=0, the K giant density is 10,000 per sq.deg., decreasing to 1000 per sq.deg. at b~6o. A survey of the inner Galaxy (l=35-75o, b=+/-5o) would yield a sample of >~400,000 stars.
K giants (to I~22, assuming a maximum AV~12.5) can be identified (using, e.g., the CO index) and proper motions measured from near-IR wide-field imaging surveys. Radial velocities, metallicities (from Ca triplet), and line-of-sight gas column densities would be measured with SWIFT at a spectral resolution of ~5000. The entire spectroscopic program can be completed in clear nights. The comparison with the observing time required with other observing systems is shown in table 1.
Beyond providing valuable constraints on the merger history of the disk, the potential results and legacy of such a study would also include: (1) Chemical evolution of the Galactic plane, as extracted from the metallicity distribution for different kinematic populations as a function of position in the Galaxy. (2) Mass distribution of the Galaxy, as constrained by the phase space distribution of 400,000 stellar test particles. (3) A map of extinction and gas/dust ratio along 400,000 lines of sight through the Galaxy.