The Galactic Center Lobe Filled with Thermal Plasma

Abstract

An observational result of a radio continuum and H92$α$ radio recombination line of the Galactic Center Lobe (GCL), using the Yamaguchi 32 m radio telescope, is reported. The obtained spatial intensity distribution of the radio recombination line shows two distinctive ridge-like structures extending from the galactic plane vertically to the north at the eastern and western sides of the galactic center, which are connected to each other at a latitude of $1.2^{\circ}$ to form a loop-like structure as a whole. This suggests that most of the radio continuum emission of the GCL is free-free emission, and that the GCL is filled with thermal plasma. The east ridge of the GCL observed with the radio recombination line separates 30 pc from the radio arc, which has been considered as a part of the GCL, but coincides with a ridge of the radio continuum at a galactic longitude of $0^{\circ}$. The radial velocity of the radio recombination line is found to be between $-4$ and $+10$ km s$^{-1}$ across the GCL. This velocity is much smaller than the one expected from the galactic rotation, and hence indicates that the GCL exists apart from the galactic center. These characteristics of the GCL suggest that the long-standing hypothesis that the GCL was created by an explosive activity in the galactic center is unlikely, but favor that the GCL is a giant HII region.