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 the interstellar medium.
In spite of those important roles of dust in radiation and planet formation, nobody knows what is predominantly producing dust grains; in other words, what produces dust efficiently is still one of the largest mysteries in the Universe. There are some processes of dust production: dust is produced in supernovae or in stellar winds, and is dispersed into the interstellar medium; dust subsequently grows in the interstellar medium.
We calculated the evolution of dust content in galaxies by taking into account these dust sources (dust supply from stars and dust growth in the interstellar medium). We also took into account dust destruction by supernova shocks. Because dust grains are composed of metals, the relation between metallicity (fraction of metals in gas) and dust-to-gas ratio (fraction of dust to gas) is useful to examine the evolution of dust content in a galaxy. Moreover, as a galaxy evolve, stars formed in the system enrich metals at their death. Therefore, as a galaxy evolve, the metallicity also evolve; in other words, the metallicity is an indicator of the evolutionary stage of a galaxy.
In the figure below, we show our calculation results. In the low-metallicity regime, the dust is predominantly supplied by stars, so dust-to-gas ratio increases in proportion to metallicity. Above a certain metallicity, the dust-to-gas ratio rapidly increases because the dust growth in the interstellar medium becomes dominant (note that the dust growth occurs efficiently in metal-rich environments). The rapid increase is indeed consistent with the observational data of nearby galaxies.
We also show that the dust growth is sensitive to the grain size distribution. In the figure we show three cases for the grain size distribution: a large r indicates a larger number of small grains. If the abundance of small grains is large, the surface-to-volume ratio of the grains is large; therefore, the grain growth becomes dominant earlier (i.e., at a lower metallicity). (Note that a large surface per unit volume means a large sticking efficiency of gas-phase metals on the grain surface.) Our results indicate that the grain size distribution should have an imprint on the evolution history of the dust abundance in galaxies.
This above results are published in our paper, Hirashita & Kuo (2011).
The relation between dust-to-gas ratio and metallicity. The metallicity is an indicator of galaxy evolution; a galaxy evolves from low to high metallicity. The solid, dotted, and dashed lines present the results for r = 2.5, 3.5, and 4.5, where r is the slope of the grain size distribution (the abundance of small grains is larger for larger r). The points show observational data of nearby galaxies (filled squares: nearby blue compact dwarf galaxies whose dust mass is derived from far-infrared luminosities; asterisks: nearby spiral galaxies whose dust mass is derived from far-infrared luminosities; open diamonds: nearby spiral galaxies whose dust mass is derived from dust extinction). We observe that the dust-to-gas ratio rapidly increases at a certain metallicity level where the dust growth in the interstellar medium becomes the most dominant mechanism of grain production. The metallicity level at which the rapid increase of dust-to-gas ratio occurs is sensitive to the grain size distribution (r).