SDSS-V LVM: Verifying what, and where, the 'Galactic Center' Lobe is

TL;DR

This study uses new optical spectroscopic data to confirm the Galactic Center Lobe is a foreground HII region at about 2 kpc, not directly associated with the Galactic center, and provides detailed analysis of its morphology and kinematics.

Contribution

The paper provides the first detailed optical spectroscopic analysis confirming the GCL as a foreground HII region and constrains its distance, morphology, and ionization sources.

Findings

GCL is a foreground photoionized HII region at ~2 kpc.

Ionized gas forms a closed outer loop, not filling the GCL interior.

Kinematic data shows a uniform velocity structure across the GCL.

Abstract

The so-called 'Galactic Center' Lobe (GCL) is an extended (~1 deg) radio continuum feature situated above the Galactic Plane, for which the literature contains varying claims about both its nature and location. Using new optical integral field spectroscopic observations from the SDSS-V Local Volume Mapper, we confirm the characterization of the GCL as a foreground photoionized HII region, not associated with the Galactic center. We present a new analysis of the ionized gas morphology, line ratio diagnostics, and kinematics. From our [SIII]9532 emission line map, which suffers the least extinction, we identify ionized gas emission throughout a closed outer loop, which does not fill the GCL interior. All optical line ratio diagnostics are consistent with photoionization. By comparing the ionized gas reddening from the Balmer decrement with 3D dust maps, we directly constrain the distance to the GCL to ~2 kpc. [NII]6583 line kinematics show a uniform velocity structure across the GCL, further confirming that the entire bubble is one structure. The size and emission line morphology is strongly reminiscent of that seen in the nearby Barnard's Loop, providing a possible analog to explain how this outer shell may be photoionized by a more distant and off-center embedded young cluster. We suggest the acronym GCL be repurposed to instead abbreviate the name 'Greatly Confused Loop'.