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G.J. Madsen, R.J. Reynolds, L.M. Haffner (Univ. of Wisconsin - Madison)
The Wisconsin H-Alpha Mapper (WHAM) has been used to investigate the range of conditions present in the warm ionized medium (WIM) of the Milky Way. The WHAM Northern Sky Survey has revealed the presence of remarkable large-scale H\alpha-emitting structures, including loops, filaments, and bubbles throughout the WIM superposed on a more diffuse H\alpha~background. In an effort to understand the origin and conditions associated with these structures within the WIM, observations of the relatively strong optical emission lines of H\alpha, H\beta, [\ion{S}{2}]~\lambda6716, and [\ion{N}{2}]~\lambda6583, as well as the much weaker lines of [\ion{O}{1}]~\lambda6300, [\ion{O}{3}]~\lambda5007, [\ion{N}{2}]~\lambda5755, and \ion{He}{1}~\lambda5876~have been made toward directions sampling both the diffuse and filamentary WIM. Analysis of the strength of these lines and their ratios reveal important clues about the temperature and ionization state of the gas, and indirectly reveal information about the ionizing spectrum, extinction, density, and heating of the WIM.
This paper presents the results of WHAM spectra obtained toward two directions: l=130\circ, b=-7.5\circ~and l=133\circ, b=+18\circ. The spectra reveal multiple emission line components in each direction, interpreted as ionized regions spread along the line of sight out to 3 kpc or more. We find significant variations in the ratio of these emission lines among the different radial velocity components along a single line of sight, as well as between the two lines of sight. Preliminary analysis suggests interesting correlations and anti-correlations among the line ratios and raises new questions about the heterogeneous nature of the WIM. For example, toward the sightline with four radial velocity components (l=130\circ, b=-7.5\circ), we find an anti-correlation between [\ion{O}{3}]~\lambda5007/[\ion{N}{2}]~\lambda6583~and [\ion{N}{2}]~\lambda5755/[\ion{N}{2}]~\lambda6583, indicating that the regions with a lower O++/O+ ionization fraction have a significantly higher electron temperature. Potential implications of these correlations in the spectra will be discussed.
This work was supported by the NSF through grant AST96-19424.
The author(s) of this abstract have provided an email address for comments about the abstract: madsen@astro.wisc.edu