This collaborative award will fund continuation of a project to measure distances to the Galactic high velocity clouds (HVCs), massive clouds of neutral hydrogen gas moving at velocities incompatible with a simple model of differential galactic rotation. Since they stand out from the gas in the Galactic Disk, they can be used as test particles for energetic phenomena in the Milky Way. The program will study HVC metallicities and distances using interstellar absorption lines in the spectra of stars projected against the clouds. The results of this project will be important for estimates of several fundamental parameters of Galactic evolution, including the rate of infall of low metallicity gas, the potential of the dark matter halo, the rate of circulation of gas between Disk and Halo, and the fraction of ionizing photons escaping the Disk.
The Broader Impacts of this program include training of graduate and undergraduate students.
This project sought to obtain measures of distances to, or brackets on the distance to the co-called High Velocity Clouds of HI in the halo of the Milky Way galaxy. Such measurements are critical for determinations of the masses of these clouds -- the masses in turn can be used in order to place the high velocity clouds in the appropriate context for the formation and evolution of the halo of the Milky Way, and for large spiral galaxies in general. The primary activities in this work involved (1) the identification of likely A-type main-sequence and horizontal-branch stars in the halo of the Galaxy, based on photometric measurements and follow-up medium-resolution spectroscopy. The photometric measurements were also required in order to obtain estimates of the distance to each star, based on a comparison of its observed apparent magnitude with the expected absolute magnitude for the assigned evolutionary state. (2) High-resolution spectroscopy of suitable tracer stars, which was needed in order to detect the presence (or absence) of the expected insterstellar metallicity lines (such as singly ionized calcium) that should appear in the spectrum if the chosen tracer is behind the cloud, or be absent if the cloud is in front of the tracer. By obtaining at least two (ideally more) such tracers for each cloud studied, we attempted to obtain a bracketed distance between which the high velocity cloud must lie. We determined (or are in the process of refining) distance estimates for some 10 such high velocity clouds. These results will appear in a series of publications once the final determinations are made. Students and post doctoral researchers who were involved with this work gained valuable observing experience on a number of telescopes, and were directly involved with the reduction and analysis of the data obtained. The problem of the nature of high velocity clouds has existed for almost half a century, but now appears to have been resolved, at least in part, by the results obtained from this study.
