The Radio Continuum of the Metal‐deficient Blue Compact Dwarf Galaxy SBS 0335−052

We present new Very Large Array observations at five frequencies, from 1.4 to22GHz, of the extremely low-metallicity blue compact dwarf SBS0335-052. Theradio spectrum shows considerable absorption at 1.49GHz, and a compositethermal+non-thermal slope. After fitting the data with a variety of models, wefind the best-fitting geometry to be one with free-free absorptionhomogeneously intermixed with the emission of both thermal and non-thermalcomponents. The best-fitting model gives an an emission measure EM ~ 8x10^7pccm^{-6} and a diameter of the radio-emitting region D ~17pc. The inferreddensity is n_e ~ 2000 cm^{-3}. The thermal emission comes from an ensemble of\~9000 O7 stars, with a massive star-formation rate (>=5Msun) of 0.13-0.15yr^{-1}, and a supernova rate of 0.006 yr^{-1}. We find evidence for ionizedgas emission from stellar winds, since the observed Bralpha line fluxsignificantly exceeds that inferred from the thermal radio emission. Thenon-thermal fraction at 5GHz is ~0.7, corresponding to a non-thermal luminosityof ~2x10^{20} W Hz^{-1}. We attribute the non-thermal radio emission to anensemble of compact SN remnants expanding in a dense interstellar medium, andderive an equipartition magnetic field of ~0.6-1 mG, and a pressure of\~3x10^{-8}-1x10^{-7} dyne cm^{-2}. If the radio properties of SBS0335-052 arerepresentative of star formation in extremely low-metallicity environments,derivations of the star formation rate from the radio continuum in highredshift primordial galaxies need to be reconsidered. Moreover, photometricredshifts inferred from ``standard'' spectral energy distributions could beincorrect.Comment: 25 pages, including 3 figures, accepted for publication in Ap