This project comprises an observational campaign to determine the mass distributions in the disks of large spiral galaxies, and thereby to constrain the importance of dark matter in the inner regions of these systems. The Principal Investigator has, with previous NSF support, led the development of special-purpose integral-field spectrographs (or integral-field units, IFUs) for mapping the motions of stars and gas in galaxies. With this instrumentation, he has recently completed the Diskmass Survey, acquiring multi-color broad-band imaging and integral-field rotation and velocity dispersion maps of galaxy disks oriented face-on to the line of sight. These data suggest that galaxy disks have lower mass-to-light ratios than expected, which implies that there is no large population of faint, very low-mass stars, and that dark-matter halos are dynamically important even in the galaxies' inner regions. However, the conclusions are weakened by uncertainties in the actual thicknesses of the disks along the line of sight. To remedy the situation, the Principal Investigator, his collaborators, and graduate students will conduct further observations, focusing on similar galaxies that are oriented edge-on to the line of sight. Specifically, they will (1) make photometric measurements of 20 galaxies, in strategically chosen wide and narrow infrared spectral bands that will permit the separation of stellar populations of different age; (2) make IFU measurements of the rotation speed and velocity dispersions in edge on disks, as functions of both distance from the galactic center and distance from the disk midplane, and then compare the rates of change of these quantities against dynamical models to constrain the gravitating mass distributions in the disk and the dark halo; and (3) re-examine the Diskmass Survey data to determine if earlier suggestions in one galaxy for systematic variation in stellar motions with age of the population are borne out in the entire sample. The broader impact of this project includes the launching of science programs on NSF-supported instrumentation, graduate student training and mentoring and inclusion of research results in undergraduate course development, and public presentations that disseminate results to a wide audience.
