Radio supernovae, supernova remnants and H II regions in NGC 2146 observed with MERLIN and the VLA

We present a high-resolution 5-GHz radio contin- uum map of the starburst galaxy NGC 2146 made with MERLIN and the VLA (A-array), in a search of radio supernovae and su- pernova remnants expected to be already produced by the most massive stars in the starburst. At 5 GHz, about 20 point sources were detected earlier by Glendenning & Kronberg (1986) in the central 800 pc 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 5 GHz, which al- lows us to derive their spectral indices 5:6 ,( S ). Only 3 sources have indices ( 0). 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 > 10 7 cm 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 how- ever 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 1.6 GHz in foreground ionized gas with very high emission measures (EM > 10 8 cm 6 pc) hides a certain number of supernova remnants, thus rendering for some sources the observed inverted spectra.