Colloquium: Interstellar Ionized Gas (Greg Madsen, University of Sydney)

The interstellar medium (ISM) of the Milky Way is pervaded by an extensive layer of faint, diffuse ionised gas. This low density plasma is a principal component of the ISM and significantly impacts our understanding of the structure and physics of all phases of the ISM. Despite its importance, many questions about the origin, energetics, and physical conditions of the ionised gas remain unanswered. Significant progress has been made through observations from the Wisconsin H-Alpha Mapper (WHAM), a large diameter, high resolution Fabry-Perot spectrograph designed specifically to study extremely faint, diffuse optical emission. WHAM has produced the first spectroscopic sky survey of diffuse ionised gas in the northern Galaxy comparable to all-sky surveys of neutral gas. We have extended the H-Alpha survey work to other diagnostic emission lines. This allows us to characterise the physical conditions of the gas and to compare the results with photoionisation models. The unique capabilities of WHAM have been used to study a range of other phenomena including the geocorona of the Earth, the motion of zodiacal dust, large planetary nebulae, high-velocity clouds, and dwarf spheroidal galaxies. WHAM has now been relocated to Cerro Tololo and will provide important new views of the southern sky. In this talk, I will review highlights of WHAM science to date and will discuss future projects and opportunities with WHAM-South.