We present a high-resolution 5-GHz radio continuum map of the starburst galaxy NGC 2146 made with MERLIN and the VLA (A-array). Our observations detect 18 compact sources, and resolves 7 of them. Additional 1.6-GHz MERLIN observations disclose 9 sources coincident in position with those detected at 5 GHz, which allows us to derive their spectral indices. Only 3 sources have indices consistent with synchrotron emission from supernova remnants or radio supernovae, while the others have very steep inverted spectra. We suggest that the sources with positive spectral index are optically thick ultra-compact and/or ultra-dense H II regions with high electron densities and high emission measures (EM > 107 cm-6 pc). When compared with M 82, the galaxy NGC 2146 lacks however a large number of supernova remnants. We suggest that NGC 2146 is experiencing a burst of star formation stronger than that in M 82, but being in a younger phase. In this phase, only few stars have already exploded, whereas the others cause strong thermal emission from compact, optically thick ionized gas regions, around the young super starclusters. An alternative scenario with strong free-free absorption at 1.6 GHz from foreground ionized gas with very high emission measures (EM > 108 cm-6 pc) is discussed.
Galaxies and their Constituents at the Highest Angular Resolutions
- Pub Date:
- January 2001