Continued Exploration of the Warm Ionized Medium using the Wisconsin H-Alpha Mapper

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

AST 0204973 Reynolds An accurate knowledge of interstellar matter and processes is crucial to understanding the cycle of stellar birth and death and the evolution of galaxies. The Wisconsin H-Alpha Mapper (WHAM), a remotely controlled Fabry-Perot facility dedicated to the detection and study of faint diffuse interstellar emission lines, has opened a new window to the interstellar medium by making it possible for the first time to explore the warm ionized phase of the gas with as much or more accuracy and detail as the other principal phases of the medium have been explored. These optical emission line studies provide unique new information about the nature of the interstellar medium and the principal sources of ionization and heating within the disk and halo of the Galaxy, information that cannot be obtained through other techniques at other wavelengths. Using the WHAM facility, Dr. Ronald Reynolds and his colleagues, at the University of Wisconsin, will carry out a comprehensive study of the variations in ionization and excitation conditions within the ionized gas and explore the strength and spectrum of the radiation field. Their recently completed WHAM Northern Sky Survey has given the first comprehensive picture of the distribution and kinematics of this gas over the sky and thus provides the basic foundation for this investigation. The complete set of survey data has been released to the community and is available through the website: http://www.astro.wisc.edu/wham/. In this second phase of the program the investigators will observe fainter diagnostic emission lines to explore the nature of the new emission features discovered by the survey and to study the surprising differences in temperature and ionization state between this widespread, low-density phase of the interstellar medium and the bright, classical H II regions near hot stars. From observations of faint emission lines of oxygen, nitrogen, sulfur, and helium, they will investigate the physical conditions of the diffusely emitting gas, the faint WHAM "point sources", the enormous filaments and loops of ionized gas that reach 1000 pc or more from the Galactic midplane, and the relationship of the ionized gas to the warm neutral hydrogen phase of the medium. By comparing these observations with those toward classical O star H II regions and extended regions of ionized gas associated with high latitude hot white dwarf stars, B stars, and supernova remnants, they will study the relationship between these known sources of ionization within the Galaxy and the warm ionized phase of the medium. Observations of the intermediate and high velocity clouds will be used to trace the changing conditions in the ionized gas and radiation field from the Galactic midplane to the Galactic halo, while observations of H Beta/H Alpha will provide information about the influence of interstellar dust along the lines of sight of these optical line studies. The ability of WHAM to carry out the observations of these and other faint emission lines was verified during the first phase of this program. The diverse observational program in this second phase takes full advantage of this powerful, unique facility and will produce a substantial advancement in our understanding of interstellar matter and processes within the disk and halo of the Milky Way. ***