Radio Recombination Lines toward the Galactic Center Lobe
  The Galactic Center lobe is a degree-tall shell seen in radio continuum
images of the Galactic center (GC) region. If it is actually located in the GC
region, formation models would require massive energy input (e.g., starburst or
jet) to create it. At present, observations have not strongly constrained the
location or physical conditions of the GC lobe. This paper describes the
analysis of new and archival single-dish observations of radio recombination
lines toward this enigmatic object. The observations find that the ionized gas
has a morphology similar to the radio continuum emission, suggesting that they
are associated. We study averages of several transitions from H106alpha to
H191epsilon and find that the line ratios are most consistent with gas in local
thermodynamic equilibrium. The radio recombination line widths are remarkably
narrow, constraining the typical electron temperature to be less than about
4000 K. These observations also find evidence of pressure broadening in the
higher electronic states, implying a gas density of n_e=910^{+310}_{-450}
cm^{-3}. The electron temperature, gas pressure, and morphology are all
consistent with the idea that the GC lobe is located in the GC region. If so,
the ionized gas appears to form a shell surrounding the central 100 parsecs of
the galaxy with a mass of roughly 10^5 Msun, similar to ionized outflows seen
in dwarf starbursts.
