Exploring the Galactic Warm Ionized Medium with WHAM

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Scientific paper

The interstellar medium of the Milky Way contains a pervasive, nearly fully ionized (n_H^+/n_H^0 ˜ 10), low density (ne ˜ 0.1 cm-3), warm (T ˜ 10^4 K) plasma. This Warm Ionized Medium (WIM) contains roughly 90% of the total ionized gas mass and contributes 30%--50% of the total Balmer-α (Hα) luminosity in late-type spiral galaxies. The powering of the WIM is not completely understood; however, of the sources examined to date, only radiation from massive stars contains enough ionizing energy to easily maintain the layer. The WIM has now been fully surveyed from the Northern Hemisphere by the Wisconsin H-Alpha Mapper (WHAM). WHAM utilizes a 0.6-m objective lens coupled to a high-throughput, 15-cm diameter double-etalon Fabry-Perot spectrometer and sensitive CCD detector. The Northern Sky Survey (WHAM-NSS) has one-degree angular resolution and provides the first kinematically resolved map of the WIM. With 12 km s-1 (0.26 Å near Hα) spectral resolution, we remove atmospheric emission, including the bright, exospheric geocoronal Hα line, and Hα zodiacal absorption features from each of the 37,565 spectra. With sensitivity to emission below 0.1 R (EM ˜ 0.2 cm-6 pc), WHAM detects Galactic emission in nearly every spectrum. The WHAM-NSS reveals complex filamentary structure in the local WIM and nearest spiral arms, some clearly associated with active star formation in the Galactic plane. An initial analysis of the relationship between high latitude ionized and neutral emission suggests that although the spatial extent and velocity profiles are quite similar, the intensities are uncorrelated. Today, WHAM continues to explore the physical parameters, heating, and ionization of the WIM by observing other spectral diagnostic lines. Data and further information about WHAM are available for download at http://www.astro.wisc.edu/wham/. WHAM is supported by NSF.

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