The grand machinery of galaxies is set in motion by some of the most basic forces in the universe. The gravitational energy between atoms and molecules in interstellar gas is continually converted into light, heat, and high-speed particles in the cores of stars. The by-products of this ongoing process become the essential ingredients of planets and life itself. In addition, the formation of stars and the release of matter and energy during their lifetimes regulates the inevitable crush of gravity, setting up a complex feedback loop that allows galaxies to continue this cycle for billions of years.

The Wisconsin H-Alpha Mapper (WHAM) group is studying one important component of the interstellar medium (ISM) in our own Milky Way to help answer important questions about how galaxies work. Where does the energy produced in the star-forming regions of our Galaxy go? How does that energy propagate away from these birth sites? How does this energy change as it travels, and how is it deposited back into the Galaxy?

The entire disk of the Milky Way is encompassed in a thick (many thousands of light-years) layer of ionized gas, dubbed the Warm Ionized Medium (WIM). This layer appears to be powered by active star formation. Ultraviolet and X-ray light leaking from the dense star-forming regions in the disk appears to be the primary source of energy. But some evidence suggests that a portion of the power may come from converting energy released in supernovae, which occur on average about once a century in the Milky Way. Due to the WIM's diffuse nature—only about 100,000 atoms per cubic meter—it is difficult to detect and characterize with traditional astronomical instruments.

WHAM is a custom-built observatory located in Arizona on Kitt Peak. Designed for studying the WIM in detail, it has produced the first map that traces not only the distribution but also the motion of the gas. To achieve this goal, WHAM obtains spectra, rather than images, of very faint Hα (H-alpha) emission from the gas. Our survey, the WHAM Northern Sky Survey, is now the basis for a number of projects as we continue to explore the properties of the WIM and the origin of the energy needed to sustain it. Most of these ongoing projects combine new WHAM emission-line observations of elements other than hydrogen (primarily sulfur, nitrogen, oxygen, and helium) to begin an exploration of the physics involved in powering the WIM.

Want to know more? Follow one of the links below:

• History
• Fabry-Perot Spectroscopy
• Technical Description