WHAM Observations of Hα, [Sii], and [Nii] toward the Orion and Perseus Arms: Probing the Physical Conditions of the Warm Ionized Medium

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Abstract
A large portion of the Galaxy (l = 123°-164°, b = -6° to -35°), which samples regions of the Local (Orion) spiral arm and the more distant Perseus arm, has been mapped with the Wisconsin Hα Mapper (WHAM) in the [S II] λ6716 and [N II] λ6583 lines. By comparing these data with the maps from the WHAM Hα Sky Survey, we begin an investigation of the global physical properties of the warm ionized medium (WIM) in the Galaxy. Several trends noticed in emission-line investigations of diffuse gas in other galaxies are confirmed in the Milky Way and extended to much fainter emission. We find that the [S II]/Hα and [N II]/Hα ratios increase as absolute Hα intensities decrease. For the more distant Perseus arm emission, the increase in these ratios is a strong function of Galactic latitude, b, and thus of height, z, above the Galactic plane, while the [S II]/[N II] ratio is relatively independent of Hα intensity. Scatter in this ratio appears to be physically significant, and maps of [S II]/[N II] suggest that regions with similar ratios are spatially correlated. The Perseus arm [S II]/[N II] ratio is systematically lower than local emission by 10%-20%. With [S II]/[N II] fairly constant over a large range of Hα intensities, the increase of [S II]/Hα and [N II]/Hα with |z| seems to reflect an increase in temperature. Such an interpretation allows us to estimate the temperature and ionization conditions in our large sample of observations. We find that WIM temperatures range from 6000 to 10,000 K, with temperature increasing from bright to faint Hα emission (low to high [S II]/Hα and [N II]/Hα), respectively. Changes in [S II]/[N II] appear to reflect changes in the local ionization conditions (i.e., the S+/S++ ratio). We also measure the electron scale height in the Perseus arm to be 1.0 ± 0.1 kpc, confirming earlier, less accurate determinations.