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

Abstract

We present new Very Large Array observations at five frequencies, from 1.4 to 22 GHz, of the extremely low metallicity blue compact dwarf SBS 0335-052. The radio spectrum shows considerable absorption at 1.49 GHz and a composite thermal plus nonthermal slope. After fitting the data with a variety of models, we find the best-fitting geometry to be one with free-free absorption homogeneously intermixed with the emission of both thermal and nonthermal components. The best-fitting model gives an emission measure EM~8×10⁷ pc cm^-6 and a diameter of the radio-emitting region D~17 pc. The inferred density is n_e~2000 cm^-3. The thermal emission comes from an ensemble of ~9000 O7 stars, with a massive star formation rate (>=5 M_solar) of 0.13-0.15 yr^-1 and a supernova (SN) rate of 0.006 yr^-1. We find evidence for ionized gas emission from stellar winds, since the observed Brα line flux significantly exceeds that inferred from the thermal radio emission. The nonthermal fraction at 5 GHz is ~0.7, corresponding to a nonthermal luminosity of ~2×10²⁰ W Hz^-1. We derive an equipartition magnetic field of ~0.6-1 mG and a pressure of ~3×10^-8 to 1×10^-7 dynes cm^-2. Because of the young age and compact size of the starburst, it is difficult to interpret the nonthermal radio emission as resulting from diffusion of SN-accelerated electrons over 10⁷-10⁸ yr timescales. Rather, we attribute the nonthermal radio emission to an ensemble of compact SN remnants expanding in a dense interstellar medium. If the radio properties of SBS 0335-052 are representative of star formation in extremely low metallicity environments, derivations of the star formation rate from the radio continuum in high-redshift primordial galaxies need to be reconsidered. Moreover, photometric redshifts inferred from ``standard'' spectral energy distributions could be incorrect.