Radio Recombination Lines toward the Galactic Center Lobe

C. J. Law(1,2,3), D. Backer(3), F. Yusef-Zadeh(1), R. Maddalena(4)

(1) Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208, USA;
(2) Astronomical Institute ``Anton Pannekoek'', University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
(3) Radio Astronomy Lab, University of California, Berkeley, CA 94720, USA;
(4) National Radio Astronomy Observatory, Green Bank, WV, 24944, USA;

Paper: ApJ accepted


EPrint Server: 09011480


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 H106 alpha to H191\epsilon 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 p! ressure broadening in the higher electronic states, implying a gas density of ne=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 105 \msol, similar to ionized outflows seen in dwarf starbursts.

Preprints available from the authors at , or the raw TeX (no figures) if you click here.

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