The Heterogeneous Nature of the Warm Ionized Medium in the Galaxy

Details The Heterogeneous Nature of the Warm Ionized Medium in the Galaxy The Heterogeneous Nature of the Warm Ionized Medium in the Galaxy

Dec 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004aas...205.4105m&link_type=abstract

American Astronomical Society Meeting 205, #41.05; Bulletin of the American Astronomical Society, Vol. 36, p.1409

Astronomy and Astrophysics

Astronomy

Scientific paper

We report on Wisconsin H-Alpha Mapper (WHAM) observations of optical emission lines from H I, He I, [NII], [SII], and [OIII] toward several classical O-star HII regions in the Galaxy, as well as a variety of newly revealed faint, large-scale Hα -emitting structures that span a wide range in surface brightness, scale, environment, and morphology. We compare the relative intensities of these emission lines to explore the temperature and ionization conditions within the emitting gas and their variations between the different emission regions. We find that the fainter, more diffuse gas identified with the warm ionized medium (WIM) is warmer, in a lower ionization state, and ionized by a softer spectrum than the gas in classical HII regions surrounding O stars, the presumed ionization source for the WIM. Furthermore, we find that compared to the more diffuse WIM, the large filamentary Hα structures are regions of higher density, not geometrical projections of folds in large sheet-like or shell-like structures.
We also discuss the detection of optical emission from ionized gas near the Galactic plane toward a large, low extinction window into the inner Galaxy, out to the tangent point at a heliocentric distance of ˜ 6 kpc. We find that the scale height and r.m.s. midplane density of the gas increases toward the inner Galaxy. Within 4 kpc of the Sun, we find that the extinction per unit neutral gas column density, (E(B-V)/NH), is near the local value and increases toward the inner Galaxy. We also find preliminary evidence for an increase in temperature, and the ionization state of oxygen, with increasing height above the plane in the inner Galaxy.
This work has been supported by the Wisconsin Space Grant Consortium, and the National Science Foundation through grants AST 96-19424 and 02-04973.

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