TNG 35.0+28.0 , the SIMBAD biblio

We present a high-resolution 5-GHz radio continuum map of the starburst galaxy NGC 2146 made with MERLIN and the VLA (A-array), in a search of radio supernovae and supernova remnants expected to be already produced by the most massive stars in the starburst. At 5GHz, about 20 point sources were detected earlier by Glendenning & Kronberg (1986BAAS...18.1006G) in the central 800pc of NGC 2146. Our observations with higher sensitivity and resolution made with MERLIN and the VLA confirm the detection of 18 sources, and resolve 7 of them. Additional 1.6-GHz MERLIN observations disclose 9 sources coincident in position with those detected at 5GHz, which allows us to derive their spectral indices α_1.6⁵, (S_ν~ν^α). Only 3 sources have indices (α<0) consistent with synchrotron emission from supernova remnants or radio supernovae, while the others have very steep inverted spectra (α>0). We suggest that the sources with positive spectral index are optically thick ultra-compact and/or ultra-dense HII regions with high electron densities and high emission measures (EM>10⁷cm^–6.pc). Minimum energy requirements indicate that these regions may contain up to 1000 equivalent stars of type O6, comparable to the number of stars found in super starclusters. 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. We may, however, not exclude an alternative scenario in which strong free-free absorption at 1.6GHz in foreground ionized gas with very high emission measures (EM>10⁸cm^–6.pc) hides a certain number of supernova remnants, thus rendering for some sources the observed inverted spectra.