Galactic Structure From Infrared Surveys

By combining the 2MASS and DENIS infrared surveys with the USNO-B proper-motion catalog, it will be possible to map the structure of the Galaxy in unprecedented detail. The key parameter that these surveys measure together, but neither measures separately, is t_*=r_*/v_\perp, the time it takes a star moving at transverse speed v_\perp to cross its own radius r_*. This parameter cleanly separates white dwarfs, main sequence stars, and giants, and simultaneously separates disk from spheroid stars. The infrared photometry then yields photometric parallaxes for the individual stars, while the proper motions give kinematic information. Hence, it becomes possible to measure the physical and velocity distributions of the known Galactic components and to identify new ones. The analysis of such a huge data set poses a major technical challenge. I offer some initial ideas on how to meet this challenge.