2005A&A...436..837H

We present 1.4, 4.8 and 8.4GHz Very Large Array observations of the lowest metallicity blue compact dwarf (BCD) galaxy known, IZw18, with a heavy element abundance of ∼2% that of the Sun. The 1.4 and 4.8GHz 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 1.4GHz to 4.8GHz and -0.13 from 4.8 to 8.4GHz. The radio luminosity of IZw18 has a thermal to total emission fraction of 0.30 at 1.4GHz. This fraction increases to 0.41 at 4.8GHz and to 0.47 at 8.4GHz. The thermal radio luminosity gives a total of 1200 O7 V stars and a star formation rate of 0.1M_☉/yr. Unlike the BCD SBS0335-052 which has a similar metallicity and forms stars with a high rate in regions which are dense and compact, IZw18 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.