Warm Ionized Medium Throughout the Sagittarius-Carina Arm

TL;DR

This study maps the vertical distribution and physical conditions of the warm ionized medium along the Sagittarius-Carina arm using H-Alpha and [S II] emission, revealing scale heights and ionization characteristics.

Contribution

It provides detailed measurements of the WIM's scale heights and physical conditions along specific spiral arms, highlighting variations with Galactocentric radius and in-situ ionization evidence.

Findings

Electron density squared scale heights range from 300 to over 1000 pc.

Emission measure scale height increases with Galactocentric radius.

[S II]/H-Alpha ratio correlates with in-situ electron density.

Abstract

Wisconsin H-Alpha Mapper (WHAM) observations of H-Alpha and [S II]$\lambda6716$ emission are used to trace the vertical distribution and physical conditions of the warm ionized medium (WIM) 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-Alpha and [S II] emission along individual spiral arms. Exponential scale heights of electron density squared (or emission measure) are determined using H-Alpha emission above (below) the midplane to be $330 \pm 80$ pc ( $550 \pm 230$ pc) along the near Sagittarius arm, $300 \pm 100$ pc ($250 \pm 30$ pc) along the near Carina arm, and $>1000$ pc 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 $R_G > 8$ kpc. Physical conditions of the ionized gas are analyzed using the [S II]/H-Alpha line ratio, which more closely traces H-Alpha Intensity 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.