An article published in the journal “Nature” describes a research on star formation in the Milky Way. According to Masafumi Noguchi of Tohoku University there were two star formation periods separated by 2 billion years. In essence, our galaxy had a first period in which it was vital then it died and after about 2 billion years star formation restarted in a sort of second life. According to this theory, during that period of death the gas present in the Milky Way got enriched with iron, the reason why stars like the Sun have a greater amount of it than others.
Avishai Dekel of Hebrew University recently proposed the theory called cold flow accretion in which a galaxy grows collecting gas around it during its formation, but this can occur in two phases separated by a long period of stasis. The idea was suggested by other researchers such as Yuval Birnboim for massive galaxies but according to Masafumi Noguchi it’s possible to apply it to the Milky Way as well.
In the reconstruction proposed by Masafumi Noguchi, 13 billion years ago the Milky Way was born and the cold gas that formed it started giving life to the first stars. This first generation is rich in what are called alpha elements such as oxygen, magnesium and silicon generated in type-II supernovae with their short life cycle and dispersed in space. That star formation stopped about 7 billion years ago due to cosmic shockwaves that heated up the gas in such a way as to not allow it any more to coalesce.
For 2 billion years no new stars formed but the existing ones continued their cycle and some with a long cycle exploded into type-I supernovae, scattering elements like iron into space. The gas, which in the meantime had cooled down, restarted forming new stars rich in that element in the Milky Way’s second life.
The bottom image (Courtesy M. Noguchi, Nature, M. Haywood et al., A & A, 589, 66 (2016), ESO) shows the two stages of star formation in the Milky Way according to this theory. In the illustration above, blue (cold) and red (hot) indicate the gas. The color map in the bottom panel shows the distribution of the stars’ elemental composition calculated from Masafumi Noguchi’s model with the purple line that indicates how the elemental composition of the gas changes over time. The superimposed contours show the distribution of the stars in the solar neighborhood observed by the APOGEE spectroscope at the Apache Point Observatory.
This model could also be valid for the Andromeda galaxy, while in dwarf galaxies or in galaxies that are much smaller than the Milky Way stars formed continuously. More studies will be needed to try to understand if the theory proposed by Masafumi Noguchi is correct and what’s the lowest limit in a galaxy’s size for that model to be applicable. If it turns out to be correct, it will be an important step forward in the study of galaxy evolution.