2005ApJ...630..925M

Hα and Hβ spectroscopy with the Wisconsin Hα Mapper (WHAM) reveals a strong concentration of high-velocity emission in a ~5°x5° area centered near (l, b)=(27°, -3°), known as the Scutum cloud. The high velocities imply that we are detecting optical emission from near the plane of the Galaxy out to the tangent point at heliocentric distances of D_☉≳6 kpc, assuming that the gas participates in circular Galactic rotation. The ratio of the Hα to Hβ emission as a function of velocity suggests that dust along these lines of sight produces a total visual extinction of A_V~3 at D_☉∼6 kpc. This makes it possible to use optical emission lines to explore the physical conditions of ionized gas in the inner Galaxy. At a Galactocentric distance R_G~4 kpc, for example, we find that the H⁺ has an rms midplane density of ~1/cm3 with a vertical scale height of ~300 pc. We also find evidence for an increase in the flux of Lyman continuum photons and an increase in the ratio of ionized to neutral hydrogen toward the inner Galaxy. We have extended the measurements of E(B-V) in this direction to distances far beyond what has been accessible through stellar photometry and find E(B-V)/N_H to be near the local mean of 1.7x10^–22 cm² mag, with evidence for an increase in this ratio at R_G~4 kpc. Finally, our observations of [N II] λ6583, [S II] λ6716, and [O III] λ5007 toward the window reveal that in the inner Galaxy the temperature of the gas and the ionization state of oxygen increase with increasing height from the midplane.