Astronomy and Astrophysics – Astrophysics
Scientific paper
2006-09-08
Astron.J.132:2383-2397,2006
Astronomy and Astrophysics
Astrophysics
48 pages, 6 figures. Accepted for publication in AJ
Scientific paper
10.1086/508373
We report subarcsecond-resolution VLA imaging of four nearby spiral galaxies: IC 342, Maffei II, NGC 2903, and NGC 6946. In each galaxy, compact radio continuum sources are identified in the central ~ 15" x 15" region. These compact sources are responsible for 20 - 30 % of the total emission from the central kpc of the host galaxies at 2 cm, but only ~ 5 - 10 % at 6 cm. More than half of the compact sources appear to be HII regions. The HII regions with rising spectra must be fairly dense (n_i ~ 10^4 cm^(-3)) and are presumably very young. The largest of these HII regions require the excitation of 500 - 800 O stars, within regions of only few parsecs extent. These clusters approach the sizes expected for globular clusters. Thermal free-free emission from compact sources contributes more significantly at 2 cm, while diffuse synchrotron emission dominates at 6 cm. The radio HII regions are found near the centers of giant molecular clouds in projection, and do not have obvious visual counterparts.
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Beck Sara C.
Crosthwaite Lucian P.
Ho Paul T. P.
Meier David Stuart
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