Nearby blue compact dwarf galaxies (BCDs) are a unique category of galaxies that are still in the early stage of galaxy evolution in the following two aspects: (i) low metallicity, and (ii) rich gas content. Some BCDs are experiencing the most active class of star formation associated with the formation of super star clusters. Such intense star formation is occurring in dense and compact regions and thus can only be traced with optically thin star formation indicators. Far-infrared (FIR) and radio continuum luminosities are suitable for such indicators.

In observing intense starbursts associated with the formation of super star clusters, a high spatial resolution is crucial to spot the young star-forming component. While high-resolution data of the central star-forming regions in BCDs are available through various radio interferometric observations with arcsecond resolutions, information of FIR--submillimeter (submm) dust emission on such a small scale was lacking for BCDs. We thus performed SMA observations of three BCDs (He 2-10, NGC 5253, and II Zw 40) to resolve their submm emission at 340 GHz (880 μm) with a subcompact configuration. Since He 2-10 has a higher metallicity than the other two galaxies, we may be able to obtain a hint for metallicity effects.

Figure 1 shows the obtained continuum brightness distribution overlaid with the Hubble Space Telescope Advanced Camera for Surveys data in the optical. The central active star-forming regions are detected by our SMA observation in all the galaxies. The center of submm emission coincides with that of optical for II Zw 40, while there is a significant shift in He 2-10. The optical image of NGC 5253 has a complicated morphology, and the peak of the SMA image does not match the brightest region in the optical. The poor positional coincidence can be attributed to the extinction in the optical image.

We find that the contribution from free-free emission is comparable to or larger than that from dust emission in the SMA 880 μm flux of the central starbursts in those three galaxies. In contrast, the total luminosity over the entire BCD is usually dominated by dust emission at 880 μm. Thus, it appears that the contribution from free-free emission is enhanced if we pick up the central starburst. However, as shown in the figure, if we compare the radio luminosity (free-free emission is dominated) and the FIR luminosity (total dust luminosity integrated for all wavelengths), the central starbursts in our sample just lie on the relation determined by the entire emission from BCDs. Therefore, although free-free is dominated at a particular submm wavelength in the central star-forming regions, the radio--FIR luminosity relation robustly holds. Noting the large difference in metallicity between He 2-10 (12 + log (O/H) = 8.93) and NGC 5253 (12 + log (O/H) = 8.14), we conclude that metallicity cannot be the dominant factor that causes a variation in the radio--FIR relation. In other words, even a low metallicity environment can reprocess the stellar light into FIR with a similar efficiency to a solar metallicity environment.

See Hirashita 2013 for details.

[The first three panels] Contour: SMA 880 μm continuum brightness of (a) He 2-10, (b) NGC 5253, and (c) II Zw 40. Solid contours are 2, 3 σ, ..., while dotted contours are -2 and -3 σ. The beam is shown in the lower right corner. Grey scale: HST ACS optical images. [The last panel] Radio-FIR relation for the central star-forming regions (the diamonds connected by the solid lines for He 2-10 and NGC 5253 and the diamond with an arrow for II Zw 40), in terms of the global (entire-galaxy) relations for BCDs (squares and crosses, fitted with the soid line). The observational data for the global emission from BCDs are taken from (Hunt et al. 2005) (squares) and (Klein et al. 1991) (crosses). The dotted lines show 1 σ for the fitting. We observe that radio–FIR relation robustly holds also for our SMA sample.