The radio continuum of the extremely metal-poor blue compact dwarf galaxy I Zw 18

We present 1.4, 4.8 and 8.4 GHz Very Large Array observations of the lowest metallicity blue compact dwarf (BCD) galaxy known, I Zw 18, with a heavy element abundance of ~2% that of the Sun. The 1.4 and 4.8 GHz images reveal a halo of mostly non-thermal extended emission, with asymmetric lobes extending laterally in the east-west direction. We interpret the radio halo as produced by a superbubble carved out in the interstellar medium (ISM) by supernovae, with a bipolar outflow oriented along the galaxy's rotation axis. The overall spectral index is -0.39 from 1.4 Ghz to 4.8 Ghz and -0.13 from 4.8 to 8.4 GHz. The radio luminosity of I Zw 18 has a thermal to total emission fraction of 0.30 at 1.4 GHz. This fraction increases to 0.41 at 4.8 GHz and to 0.47 at 8.4 GHz. The thermal radio luminosity gives a total of 1200 O7 V stars and a star formation rate of 0.1 yr-1. Unlike the BCD SBS 0335-052 which has a similar metallicity and forms stars with a high rate in regions which are dense and compact, I Zw 18 makes stars at a smaller rate in complexes which are diffuse and extended. Star formation in BCDs thus appears to occur in a bimodal fashion, independently of the metallicity of the interstellar medium.