The Galactic Center Lobe Filled with Thermal Plasma
  An observational result of a radio continuum and H92$\alpha$ 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.
