Presented to the AAS at the June 2006 meeting in Calgary, Alberta. [AAS link]
This poster is a sequel to this one.
Our internal perspective makes it difficult to determine the spiral structure of our own galaxy. Although H I emission renders the disk transparent, its very ubiquity and low optical depth complicate its use as a spiral tracer, because emission features are often difficult to separate, and velocity crowding along the line of sight can mimic density enhancements. H I absorption, on the other hand, traces colder, denser gas that should be more confined in discrete clouds and perhaps discrete spiral arms. In addition, H I self-absorption (HISA) requires background H I emission at the same velocity as the foreground absorbing gas, thus introducing a much-needed constraint on the large-scale sightline geometry and kinematics. Recent high-resolution maps of the HISA longitude-velocity distribution in the Canadian and VLA Galactic Plane Surveys contain significant HISA in both the inner and outer Galaxy. These provide an excellent test bed for exploring simple models of Galactic spiral structure, including density-wave velocity perturbations and possible gas-phase evolution downstream of spiral arm shocks. I will present initial results of such model experiments, focusing on the plausibility of HISA tracing actual spiral shocks and the visibility of this tracer amid the lumpy and turbulent structure of the Galactic ISM.
This work is supported by the National Astronomy and Ionosphere Center, operated by Cornell University under Cooperative Agreement with the National Science Foundation.