An Investigation of Diffuse Interstellar Gas toward a Large, Low‐Extinction Window into the Inner Galaxy

Abstract

H? and H? spectroscopy with the Wisconsin H? Mapper (WHAM) reveals a strong concentration of high-velocity emission in a ?5? ? 5? 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 H? to H? emission as a function of velocity suggests that dust along these lines of sight produces a total visual extinction of AV ? 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 RG ? 4 kpc, for example, we find that the H+ has an rms midplane density of ?1 cm-3 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)/NH to be near the local mean of 1.7 ? 10-22 cm2 mag, with evidence for an increase in this ratio at RG ? 4 kpc. Finally, our observations of [N II]??6583, [S II]??6716, and [O III]??5007 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.