Halpha and [S II] emission from warm ionized gas in the Scutum-Centaurus Arm

Abstract

We present Wisconsin H-Alpha Mapper [S II] {\lambda}6716 and H{\alpha} spectroscopic maps of the warm ionized medium (WIM) in the Scutum-Centaurus Arm at Galactic longitudes 310{\deg} < l < 345{\deg}. Using extinction-corrected H{\alpha} intensities (IH{\alpha}c), we measure an exponential scale height of electron density-squared in the arm of H_ne^2 = 0.30 kpc (assuming a distance of 3.5 kpc), intermediate between that observed in the inner Galaxy and in the Perseus Arm. The [S II]/H{\alpha} line ratio is enhanced at large |z| and in sightlines with faint IH{\alpha}c. We find that the [S II]/H{\alpha} line ratio has a power law relationship with IH{\alpha}c from a value of ~=1.0 at IH{\alpha}c < 0.2 R (Rayleighs) to a value of ~=0.08 at IH{\alpha}c >= 100 R. The line ratio is better correlated with H{\alpha} intensity than with height above the plane, indicating that the physical conditions within the WIM vary systematically with electron density. We argue that the variation of the line ratio with height is a consequence of the decrease of electron density with height. Our results reinforce the well-established picture in which the diffuse H{\alpha} emission is due primarily to emission from in situ photoionized gas, with scattered light only a minor contributor.