Low-frequency absorption and radio recombination line features of the Galactic Center Lobe

TL;DR

This study investigates the Galactic Center Lobe using radio and infrared observations, revealing its properties, distance constraints, and electron temperature, suggesting it is closer than previously thought, likely around 2 kpc.

Contribution

The paper provides new radio continuum and recombination line data that constrain the GCL's distance and physical properties, challenging previous interpretations of its origin.

Findings

GCL shows thermal absorption incompatible with synchrotron self-absorption.

Estimated GCL distance is about 2 kpc based on cosmic ray emissivity.

Electron temperature and line widths suggest a closer location for the GCL.

Abstract

The Galactic center lobe (GCL) is a $\sim 1^\circ$ object located north of the Galactic center. In the mid-infrared (MIR), the GCL appears as two 8.0-micron filaments that roughly define an ellipse. There is strong 24-micron and radio continuum emission in the interior of the ellipse. Due to its morphology and location in the sky, previous authors have argued that the GCL is created by outflows from star formation in the central molecular zone or by activity of the central black hole Sgr~A$^*$. We present images of the GCL from the GaLactic and Extragalactic All-sky Murchison Widefield Array survey in radio continuum that show thermal absorption against the Galactic center, incompatible with an interpretation of synchrotron self-absorption. Estimates of the cosmic ray emissivity in this direction allow us to place a distance constraint on the GCL. To be consistent with standard emissivity assumptions, the GCL would be located 2kpc away. At a distance of 8kpc, the synchrotron background emissivity is enhanced by $\sim75$% in the direction of the GCL. We also present radio recombination line data from the Green Bank Telescope that constrains the electron temperature and line widths in this region, which are also more explicable if the GCL lies relatively close.