The Gaseous ISM : Observations with the Wisconsin H α Mapper ( WHAM )

The Wisconsin Hα Mapper (WHAM) is a new facility dedicated to the study of faint optical emission lines from diffuse interstellar gas. During its first 18 months of operation, WHAM carried out a survey of the interstellar Hα emission associated with the warm, ionized component of the interstellar medium. The observations consisted of 37,000 spectra obtained with a one degree diameter beam on a 0 o .98 × 0 o .85 grid (l × b), covering the sky above declination –30 o. This survey provides for the first time a detailed picture of the distribution and kinematics of the diffuse ionized hydrogen through the Hα line comparable to surveys of the neutral hydrogen obtained through the 21 cm line. Preliminary reduction of the data from selected portions of the sky reveal that the interstellar H II has a complex distribution, with long filaments and loop-like structures extending to high Galactic latitudes and superposed on a more diffuse background. Apart from the Hα sky survey, WHAM also has detected for the first time faint diagnostic emission lines in selected directions, [O I] λ6300, [O III] λ5007, and He I λ5876, which provide information about the physical state of the gas and clues about the source of the ion-ization. Maps of [S II] λ6716 and [N II] λ6584 over limited regions of the sky are providing information about variations in the temperature and ionization conditions within the Galactic disk, and the detection of faint optical emission lines from high velocity clouds is probing conditions in the halo. Finally, WHAM has the capability to explore the smaller scale structure of the medium through very narrow band (12 km s −1), 1 ′ angular resolution images within selected 1 o fields. This facility is currently located at Kitt Peak National Observatory near Tucson, Arizona and operated remotely from Madison, Wisconsin. Diffuse ionized gas is a major, yet little understood component of the interstellar medium, which consists of regions of warm (10 4 K), low-density (10 −1 cm −3), nearly fully ionized hydrogen that occupy approximately 20% of the volume within a 2 kpc thick layer about the Galactic midplane (e.g., Reynolds 1991, 1993a). Hoyle & Ellis (1963) were the first to suggest the existence of an extensive layer of warm H II surrounding the Galactic disk, based on the spatial and spectral characteristics of the Galactic synchrotron background at very low 1