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 ne ~ 2000 cm-3. The thermal emission comes from an ensemble of ~9000 O7 stars, with a massive star formation rate (≥5 M☉) 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.