Properties of free–free, dust and CO emissions in the starbursts of blue compact dwarf galaxies

Abstract

The central star-forming regions in three blue compact dwarf galaxies (He 2-10, NGC 5253, and II Zw 40) were observed in the 340 GHz (880 micron) band at 5 arcsec resolution with the Submillimetre Array (SMA). Continuum emission associated with the central star-forming complex was detected in all these galaxies. The SMA 880 micron flux is decomposed into free-free emission and dust emission by using centimetre-wavelength data in the literature. We find that free-free emission contributes half or more of the SMA 880 micron flux in the central starbursts in those three galaxies. In spite of the dominance of free-free emission at 880 micron, the radio-to-far infrared (FIR) ratios in the central star-forming regions are not significantly higher than those of the entire systems, showing the robustness of radio-FIR relation. Based on the robustness of the radio-FIR relation, we argue that the free--free fraction in the 880 micron emission is regulated by the dust temperature. We also analyze the CO (J = 3--2) emission data. We find that CO is a good tracer of the total gas mass in solar-metallicity object He 2-10. Low-metallicity objects, NGC 5253 and II Zw 40, have apparently high star formation efficiencies; however, this may be an artifact of significant dissociation of CO in the low-metallicity environments. We also point out a potential underestimate of dust mass, since the dust traced by emission is biased to the most luminous high-temperature regions, particularly when a system hosts a compact star-forming region where the dust temperature is high.