We present a high-resolution 5-GHz radio continuum map of the starburst
galaxy NGC 2146 made with MERLIN and the VLA, in a search of radio supernovae
and supernova 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 confirms the detection of 18 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 HII 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. 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. 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.Comment: 10 pages, including 2 figures. Accepted for publication in Astronomy
and Astrophysic
