Collaborative Research: Distances to High Velocity Clouds

Grants and Contracts Details


A proposal is made to continue our 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. In the past decade. observations of some 20 extra-galactic targets with the spectrographs on the Hubble Space Telescope (HST) and the Far-Ultraviolet Spectroscopic Explorer (FUSE) have yielded metaIIicities for 6 of the 20 main HVC complexes, with values ranging from OJ to 1 times solar. We will continue to work on this aspect of the study of HVCs, especially with the COS instrument on HST. HVC distances can be measured by finding detections and significant non-detections of interstellar absorption lines in the spectra of stars projected on the clouds. Suitable stars are blue (to minimize confusion between stellar and interstellar lines), they are numerous, and they can be identified at large distances. This leads to Blue Horizontal Br20000) Ca II H & K :;;pechvscopy ofconfirmed BHBIRRLvrae stars, used to search for the HVC in absorption. We succeeded in obtaining 37+32 hours of VLT time, as well as three nights (for a net 16 hours) of NOAO's slice of Keck. This resulted in spectra for 87 stars. giving distance brackets to douds in 6 HVC complexes (Wakker et a1. 2007, 2008. 2009) (with data for 3 HVCs unanalyzed). Broader 1mpac/ The results of our project will be important for estimates of several fundamental parameters of Galactic evolution. Specifically: ra) The rate ofin(all onow-me/alliciN gas. We can now identifiy low-metallicity HVCs that repesent 0.2-0.3 ~yr-1 of infalling material. There are several other clouds with as-yet unknown metallicity and distallce that are likely to fit in this category. (b) The potential of lite dark mailer halo. Accurate models for the HVCs forming the Magellanie Stream depend on the details of the potential of the Galaxy at distances of 10 to 100 kpc. A direct determination of the Stream's distance would be a major further constraint on future models. (d The rate ofcirClilatiol1 orgas between Disk alld Halo. Distances to HVCs with near-solar metallicity allow an estimate of the z-height to which hot gas rises, which is a measure of the strength of the underlying supernova activity. Assume a Galaxy covering factor on the order of 50%. the implied Disk-Halo circulation rate is a few M(') yr-1• (d) The kactiol1 ofiol1izillg photons escaping the Disk. HVCs are an ideal probe of the ionizing radiation field near the Milky Way, as they are neutral and lie in different directions at a range of distances. Therefore, they can be used to directly calibrate the number of ionizing photons escaping the disk, and (for very distant douds) the number of extragalactic ionizing photons, allowing us to detemline whether star-fonning galaxies or AGN dominate the production of the ionizing background at low redshift. . Finally, this project lends itself well to participation by undergraduate and graduate student. as there are many self-contained pieces. So far. nine students have already participated.
Effective start/end date8/28/097/31/12


  • National Science Foundation: $11,720.00


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