Astronomy and Astrophysics – Astronomy
Scientific paper
Apr 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998aps..apr..i804r&link_type=abstract
American Physical Society, APS/AAPT Joint April Meeting, April 18-21, 1998 Columbus, Ohio, abstract #I8.04
Astronomy and Astrophysics
Astronomy
Scientific paper
The Wisconsin H-Alpha Mapper (WHAM) is a new facility for the detection and study of faint optical emission lines from ionized gas in the Galaxy. The instrument consists of a large aperture Fabry-Perot spectrometer coupled to a dedicated 0.6 m ``telescope''. This combination provides a one degree diameter beam on the sky and produces a 12 km s-1 radial velocity resolution spectrum within a 200 km s-1 spectral window that can be centered on any wavelength between 4800 Å and 7200 ÅWHAM is located at Kitt Peak National Observatory in Arizona and operated remotely from Madison, Wisconsin. The first year of operation has been devoted almost exclusively to a sky survey of the low density (0.1 cm-3) ionized component of the interstellar medium. This survey is providing for the first time a detailed picture of the distribution and kinematics of the ionized hydrogen through the optical Hα line that is comparable to earlier surveys of the neutral hydrogen made through the 21 cm line. Early results from the WHAM survey reveal long (60^o -- 80^o) loop-like filaments extending up to 50^o from the Galactic plane superposed on a more diffuse Hα background. In addition to hydrogen recombination emission, WHAM has detected a He I recombination line and collisionally excited lines of [S II], [N II], [O III], and [O I], which provide information about the temperature and ionization state of the gas. The results indicate that the interstellar medium contains wide spread regions of nearly fully ionized hydrogen (i.e., n(H^+)/n(H^o) ~ 10) at a temperature near 10^4 K that extend far above the Galactic midplane. The nature of the filamentary structures and the more diffuse interstellar Hα emission covering the sky is not yet known, and their existence appears to challenge traditional ideas about the principal mechanisms of ionization and heating within the Galaxy. Speculation about the source of the ionization has been wide ranging, including in addition to traditional sources such as hot stars, more exotic possibilities like an intense flux of cosmic-ray electrons, galactic magnetic flares, and even the decay of dark matter particles. This research has been supported primarily by grants from the National Science Foundation with supplemental funding from the University of Wisconsin Graduate School, the Department of Astronomy, and the Department of Physics.
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