Scientific Highlights with the SMA

Observations of Blue Compact Dwarf Galaxies

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. We 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. 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.

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[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.