Galactic collision may have triggered Solar System formation

Sgr has also been an important actor in the build-up of the stellar mass of the Milky Way disk.


The formation of the Solar System has puzzled scientists for centuries. 

Now, it seems that scientists have finally known what causes the solar system’s emergence.

According to a new study, an event of galactic collision triggered a chain of events that are believed to have led to the birth of the Earth.

It is very well known that a so-called dwarf galaxy called Sagittarius repeatedly crushes through the Milky Way‘s disc as its orbits around the galaxy’s core tighten because of gravitational forces.

Now, the data obtained from the ESA’s galaxy mapping powerhouse, Gaia, uncovered that the influence of Sagittarius on the Milky Way might be even more substantial.

The ripples brought about by the collisions appear to have activated significant star formation episodes, one of which generally matched with the time of the formation of the Sun some 4.7 billion years ago.

Tomás Ruiz-Lara, a researcher in Astrophysics at the Instituto de Astrofísica de Canarias (IAC) in Tenerife, Spain, said“It is known from existing models that Sagittarius fell into the Milky Way three times – first about five or six billion years ago, then about two billion years ago, and finally one billion years ago.”

“When we looked into the Gaia data about the Milky Way, we found three periods of increased star formation that peaked 5.7 billion years ago, 1.9 billion years ago and 1 billion years ago, corresponding with the time when Sagittarius is believed to have passed through the disc of the Milky Way.”

The analysts observed luminosities, distances, and colors of stars within a sphere of around 6500 light a long time around the Sun and contrasted the data with existing stellar evolution models. As indicated by Tomás, the notion that the dwarf galaxy may have had such an effect makes a lot of sense.

Tomás said, “In the beginning, you have a galaxy, the Milky Way, which is relatively quiet. After an initial violent epoch of star formation, partly triggered by an earlier merger as we described in a previous study, the Milky Way had reached a balanced state in which stars were forming steadily. Suddenly, you have Sagittarius fall in and disrupt the equilibrium, causing all the previously still gas and dust inside the larger galaxy to slosh around like ripples on the water.”

These ripples caused higher concentrations of dust and gas in some regions of the milky way. The high density of material in those areas would then trigger the formation of new stars.

Carme Gallart, a co-author of the paper, also of the IAC said“It seems that not only did Sagittarius shape the structure and influenced the dynamics of how stars are moving in the Milky Way, it has also led to a build-up of the Milky Way. It seems that an important part of the Milky Way’s stellar mass was formed due to the interactions with Sagittarius and wouldn’t exist otherwise.”

Carme said, “The Sun formed at the time when stars were forming in the Milky Way because of the first passage of Sagittarius. We don’t know if the particular cloud of gas and dust that turned into the Sun collapsed because of the effects of Sagittarius or not. But it is a possible scenario because the age of the Sun is consistent with a star formed as a result of the Sagittarius effect.”

Each collision stripped Sagittarius of some of its gas and residue, leaving the galaxy smaller after every passage. Existing data recommend that Sagittarius may have gone through the Milky Way’s disc again quite recently, in the last few million years, and is currently very close to it. Indeed, the new examination found a continuous burst of star formation, suggesting a potentially new and constant wave of stellar birth.

According to ESA Gaia project scientist Timo Prusti, such detailed insights into the Milky Way’s star formation history wouldn’t be possible before Gaia, the star-mapping telescope launched in late 2013, whose two data releases in 2016 and 2018 revolutionized the study of the Milky Way.

Timo said, “Some determinations of star formation history in the Milky Way existed before based on data from ESA’s early 1990s Hipparcos mission. But these observations were focused on the immediate neighborhood of the Sun. It wasn’t representative, and so it couldn’t uncover those bursts in star formation that we see now.”

“This is the first time that we see a detailed star formation history of the Milky Way. It’s a testament to the scientific power of Gaia that we have seen manifest again and again in countless ground-breaking studies in a period of only a couple of years.”

Journal Reference:
  1. Ruiz-Lara, T., Gallart, C., Bernard, E.J. et al. The recurrent impact of the Sagittarius dwarf on the star formation history of the Milky Way. Nature Astronomy 4, 965–973 (2020). DOI: 10.1038/s41550-020-1097-0
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