Warm Ionized Medium throughout the Sagittarius–Carina Arm

Abstract

Wisconsin H-Alpha Mapper observations of H α  and [S ii ]  λ 6716 emission are used to trace the vertical distribution and physical conditions of the warm ionized medium along the Sagittarius–Carina arm. CO emission, tracing cold molecular gas in the plane of the Galaxy, is used as a guide to isolate H α  and [S ii ]  λ 6716 emission along individual spiral arms. Exponential scale heights of electron density squared (or emission measure) are determined using H α  emission above (below) the midplane to be along the near Sagittarius arm, along the near Carina arm, and along the far Carina arm. The emission measure scale height tends to increase as a function of Galactocentric radius along the Sagittarius–Carina arm for . Physical conditions of the ionized gas are analyzed using the [S ii ]/H α  line ratio, which more closely traces I H α  than height above the plane, z , suggesting a stronger relationship with the in situ electron density. We interpret this result as further evidence for the majority of the observed diffuse emission originating from in situ ionized gas as opposed to scattered light from classical H ii regions in the plane.