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.

A long ionized filament

A 50°-long ionized filament rising out of the Galactic plane, as observed in Hα by WHAM.

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 disk of the Milky Way contains a thick (many thousands of light-years) layer of ionized gas, dubbed the Warm Ionized Medium (WIM). This layer appears to be powered by ongoing, 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 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 instead of images of very faint Balmer-alpha (Hα) emission from ionized hydrogen. Its primary mission is to produce the first spectral, all-sky survey of this emission from the Milky Way. While on Kitt Peak in Arizona, WHAM obtained the data for the northern portion of this survey and followed up with a variety of other projects to explore the properties of the WIM and the origin of the energy needed to sustain it. In 2009, we moved WHAM to Cerro Tololo in Chile so that it can observe from the southern hemisphere and complete the all-sky survey. We will then return to exploring the detailed physics of the WIM as well as gas associated with two of the Milky Way's satellite galaxies, the Large and Small Magellanic Clouds. Most of these ongoing projects combine new WHAM emission-line observations of elements other than hydrogen (primarily sulfur, nitrogen, oxygen, and helium) to measure the physical conditions of the gas and explore the processes involved in powering the WIM.

WHAM (west view)

WHAM at Kitt Peak

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