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

We present a high-resolution 5-GHz radio continuum map of the starburst gal axy NGC 2146 made with MERLIN and the VLA (A-array), in a search of radio s upernovae and supernova remnants expected to be already produced by the mos t massive stars in the starburst. At 5 GHz, about 20 point sources were det ected earlier by Glendenning & Kronberg (1986) in the central 800 pc of NGC 2146. Our observations with higher sensitivity and resolution made with ME RLIN 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 p osition with those detected at 5 GHz, which allows us to derive their spect ral indices alpha(1.6)(5) (S-nu similar to V-alpha). Only 3 sources have in dices (alpha < 0) consistent with synchrotron emission from supernova remna nts or radio supernovae, while the others have very steep inverted spectra (alpha > 0). We suggest that the sources with positive spectral index are optically thic k ultra-compact and/or ultra-dense H II regions with high electron densitie s and high emission measures (EM > 10(7) cm(-6) pc). Minimum energy require ments indicate that these regions may contain up to 1000 equivalent stars o f type O6, comparable to the number of stars found in super starclusters. W hen 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 st ar formation stronger than that in M 82, but being in a younger phase. In t his phase, only few stars have already exploded, whereas the others cause s trong thermal emission from compact, optically thick ionized gas regions, a round the young super starclusters. We may, however, not exclude an alternative scenario in which strong free-f ree 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.