Optical Emission Lines

Since most of the Hα emission we detect arises from hydrogen recombination, atomic physics dictates a set ratio of Hα to Hβ emission from ionized interstellar gases. Although it is a slight function of temperature, near 10,000 K, the ratio is about 3:1 in favor of Hα. However, interstellar dust absorbs more blue light than red so that ratios greater than this are typical in observations. Observed ratios of Hα/Hβ provide an interesting probe of dust in front of and within the ionized gas. Madsen & Reynolds 2005 present our first application of this technique toward a hole in the local dust toward the northern inner Galaxy, which generally has a substantial amount of obscuration.

[S II] 6717Å & [N II] 6583Å

These lines are nearly as bright as Hα in the WIM and are good tracers to discriminate between the diffuse background emission and H II regions. Combined with Hα, they begin to trace the physics of this ionized phase in addition to its distribution. In both the Milky Way and other spiral galaxies these lines tend to increase in intensity relative to Hα as the Hα emission decreases. In several of our papers, we propose that these rises are due to increasing temperatures. Both the collisional excitation of theses forbidden lines and the recombination that produces Hα are functions of the gas density squared (the emission measure). However near 10,000 K, the emissivity of the forbidden lines increases much more rapidly with temperature than that of Hα decreases. Thus, the smooth increase of [N II]/Hα and [S II]/Hα ratios with decreasing Hα intensity seems to indicate a gradual rise in the gas temperature. This argument can be taken one step further since, in many cases, the decrease in Hα intensity is due to a decrease in electron density. For example, as we look toward regions above the Galactic plane, the Hα intensity is decreases smoothly with distance from the plane due to the exponential scale height of the ionized layer. In this particular case, we then infer that the temperature of the WIM rises into the halo of our Galaxy.

[O I] 6300Å

Due to similar first ionization potentials, the fraction of neutral and singly-ionized oxygen and hydrogen are locked together from charge-exchange reactions in many astrophysical plasmas. WHAM detected this line from the WIM for the first time near the Galactic plane. Measurements of this line relative to Hα provide a good estimate of the average fraction of neutral oxygen (and thus hydrogen) along the line of sight. See Reynolds et al. 1998 and Hausen et al. 2001 in our publications list for details.

[N II] 5755Å

Like the 4363Å "auroral" line of [O III], this upper level transition from the isoelectrically similar [N II] spectrum provides a direct measurement of the temperature of an ionized region. Since [O III] is quite faint in the WIM, the 5755Å line is more likely to be detected from the diffuse background. Although the emission is still very faint, we have several detections that confirm that the WIM has high temperatures for photoionized gas, particularly when compared to diffuse H II regions ionized by single stars (see Reynolds et al. 2001 and Madsen et al. 2006 in our list of papers).

He I 5876Å

Using WHAM, we have detected this line for the first time from the WIM (see Steve Tufte's PhD thesis and Madsen et al. 2006). This recombination line probes the degree of helium ionization in the WIM. Comparing the helium ionization fraction to the hydrogen ionization fraction yields valuable information on the spectrum of the WIM's unknown source of ionization. 

[O III] 5007Å

Emission from the WIM of this classic H II region line had only been detected in the Galactic plane (b = 0) prior to WHAM. Observations of this gas at even higher latitudes provides upper limit measurements of the contribution of 5007Å emission from hot, Galactic coronal gas. Madsen et al. 2006 provides a recent summary of some detections and upper limits of [O III] from the WIM.


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