Observations of metal-poor galaxies by AKARI

How has the interstellar matter evolved? In the Milky Way, dust grains are one of the important components of the interstellar medium. Dust grains are composed of heavy elements such as C, O, Si, Mg, Fe, and so on (we call these elements metals), and they not only absorb and scatter the interstellar light, but also become ingredients of planets if they are incorporated into protoplanetary disks. Therefore, it is important to study when and how dust grains form and accumulate in interstellar medium.

Unfortunately, the interstellar medium in the Milky Way has already been enriched by metals and dust. The ongoing dust enrichment can be better traced if we observe less evolved galaxies where significant ongoing metal enrichment is occurring. We focused on blue compact dwarf galaxies (BCDs), whose metal abundance is still around 1/10 times the Milky Way metal abundance. Moreover, they are actively forming stars, so that the stars are expected to be producing metals and ejecting them at their death. In the metal ejection, dust is also produced.

We observed a sample of BCDs by the AKARI satellite (Japanese infrared satellite launched in 2006). We measured the fluxes from the BCDs at 65, 90, 140, and 160 micron. As shown in the figure, we found that typical dust temperature of the BCD sample is systematically higher than that of the Milky Way and even higher than that of Magellanic Clouds. This confirms the concentrated intense star-formation activities which strongly heat the dust in BCDs.

Finally, we also quantified the dust mass from the observational data, and found that around 10 percent of the metals are condensed into dust. This fraction is indeed consistent with some theoretical works on dust production in supernovae. In other words, our observational results are consistent with the picture that dust grains form in supernovae.

Left: The relation between two far-infrared flux ratios. The far-infrared flux ratios are indicators of dust temperature, and the points at the top-left tend to have high dust temperatures. The points with error bars show our observational data of blue compact dwarf galaxies, while the contour, crosses, and squares show the data of Milky Way, the Large Magellanic Cloud, and the Small Magellanic Clouds, respectively, taken by DIRBE on the COBE satellite. We observe that the blue compact dwarf galaxies have higher dust temperatures than the other samples. This is interpreted as strong star formation activities in concentrated regions in the blue compact dwarf galaxies. Right: Images of blue compact dwarf galaxies observed by AKARI at 90 micron.

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