(Guest post by Hirokuni Oda)
The winner of the 2014 EPS award was Dr. Ryosuke S. Asano, the first author of the article “Dust formation history of galaxies: A critical role of metallicity dust mass growth by accreting materials in the interstellar medium” (Asano et al., 2013) published in Earth, Planets and Space. A revolutionary theory of the evolution of dust amount in galaxies based on the metallicity, dust-to-gas ratio, has been presented. Since the initial publication, the article has received 32 citations to date.
The importance of research for dust mass growth in galaxies
From observations of the Cosmic Microwave Background (CMB), we can understand that the early Universe was alsmost isotropic and homogeneous. On the other hand, we can see a variety of some objects, such as galaxies, stars, planets, and so on. This means that the formation and evolution of various structures occur throughout the evolution of the Universe. Then, the history of the evolution of the galaxies is one of the most important issue in astrophysics, since galaxies are the fundamental object of cosmic structure.
In galaxies, stars form by collapsing interstellar gas, and then, heavy elements and dust grains are formed and released by stellar mass loss during stellar evolution. Especially, the star formation history and the spectral energy distribution (SED) of galaxies are strongly affected by the amount of dust grains. Thus, the understanding of dust evolution is very important to understand the formation and evolution of galaixes.
Main driver of dust mass growth in galaxies
The dust grains are formed by stars, such as Asymptotic giant branch (AGB) stars and type II Supernovea. Additionally, the dust grains grow by metal accretion on the surface in the interstellar medium (ISM), this process is called “grain growth.” On the other hand, dust grains are destroyed by supernovae blast waves. The physical timescales of their processes are different from each other more than two orders of magnitude. Because of the complexity of the processes, although the importance of the research of the dust evolution in galaxies, some of the previous works investigate the dust evolution in galaxies by simplified models. Thus, they contruct the coherent model by considering all of them described above.
The important point in their model is that the relationship of these dust formation/destruction processes and metallicity is considered. Metallicity is the ratio of the amount of dust and heavy elements, and is thought to be the key factor to understand the evolution of galaixes since dust grains consist of heavy elements. And, it is believed that galaxies evolve from the state with low metallicity and a small amount of dust grain to high metals and dust grains. Thus, it is crucial to investigate the evolution of dust amount in galaxies along with the evolution of metallicity.
From their model, they found that the dominant sources of dust grains in young galaxies are stars, however, the main contribution to the total dust amount change to the grain growth when a certain metallicity threshould which is called critical metallicity in this article is reached (shown by Fig. 1). This result naturally explains the total dust amount in galaxies at various stages, such as dusty and massive Quasi-Stellar Objects (QSOs) at high redshift and evolved galaxies like the Milky Way in a coherent manner. Thus, it is proved that the metallicity is the important clue leading into understanding what is the dominant contributor of the dust amount in galaxies. It is a revolutionary conclusion which leads to a large shift from previous theory of the evolution of dust amount in galaxies.
(Modified from the original positing by Ryosuke Asano)