Central free-free dominated 880-μm emission in II Zw 40

The central star-forming region in a blue compact dwarf galaxy, II Zw 40, was observed in the 340-GHz (880-?) band at ∼5 arcsec (250 pc) resolution with the Submillimetre Array (SMA). A source associated with the central star-forming complex was detected with a flux of 13.6 ± 2.0 mJy. The structure is more extended than the beam in the east-west direction. The SMA 880-? flux is analysed by using theoretical models of radio spectral energy distribution along with centimetre interferometric measurements in the literature. We find that (i) the SMA 880-? flux is dominated (∼75 per cent) by free-free emission from the central compact star-forming region and (ii) the contribution from dust emission to the SMA 880-? flux is at most 4 ± 2.5 mJy. We also utilize our models to derive the radio-far-infrared (FIR) relation of the II Zw 40 centre, suggesting that free-free absorption at low frequencies (? several GHz; ? several cm) and the spatial extent of dust affect the radio-FIR relation.