Star formation surges in our Milky Way

Star formation surges in our Milky Way
The Sagittarius dwarf galaxy has passed through the Milky Way several times. (Image: ESA)

When galaxies collide or get too close to each other, it can have a formative impact – and so can our Milky Way. Now astronomers have discovered that one of our small neighbors, the Sagittarius dwarf galaxy, may have had a decisive influence on star formation in our galaxy. This dwarf galaxy has migrated through the Milky Way three times in the last six billion years. This caused a strong boost in star formation each time, the researchers report. Our sun may also have formed in the first of these stellar surges about 5.7 billion years ago.

The Milky Way does not move through space in isolation, but has experienced close approximations or even violent collisions with neighboring galaxies over the course of its history. In some of these carambolages, both partners got away with it and exchanged only gases and stars with each other. About ten billion years ago, however, the Milky Way experienced such a violent collision with a dwarf galaxy that it was completely torn apart. From the remnants and the stars hurled around our galaxy probably only the dense “star bump” in the Milky Way center and also much of the surrounding halo originated. Observations of collisions of other galaxies also show that such events often lead to periods of increased star formation in the collision partners. The strong turbulence promotes the collapse of gas clouds and thus the formation of new stars.

Star formation surges after galaxy passage

Astronomers led by Tomas Ruiz-Lara of the Astrophysical Institute of the Canary Islands in Tenerife have now identified an important factor influencing the star formation of our Milky Way. “Existing models show that the Sagittarius dwarf galaxy has crashed into the Milky Way three times – almost six billion years ago, two billion years ago, and about a billion years ago,” explains Ruiz-Lara. Currently, this elliptical dwarf galaxy is about 50,000 light-years from the Milky Way center and contains about one billion mostly old stars. According to the theory, it moved through the star disk of our galaxy from above or below when it collided. To find out what the consequences of these passages were, astronomers looked at the star catalog created using data from ESA’s Gaia satellite and analyzed the luminosity, distances, and temperature colors of stars within a radius of about 6,500 light-years around the sun.

From this data, astronomers were able to conclude the age and thus the time of origin of these stars. “We then compared the times when we noticed increased star formation with those at which the Sagittarius dwarf galaxy fell into the Milky Way,” the astronomers explained. In fact, they found a link: “We found three periods of strong star formation that peaked 5.7 billion years ago, 1.9 billion years ago, and one billion years ago,” Ruiz-Lara reports. “This corresponds to the time when the Sagittarius galaxy is said to have migrated through the disc of the Milky Way.” These collisions may have set gas and dust in motion in our galaxy in such a way that gas clouds collapsed in many places and thus gave birth to new stars. “This is the first time we have broken down the star-forming history of the Milky Way in such detail,” said Timo Prusti, a Gaia project scientist who was not involved in the study.

Stimulation for the formation of our sun?

The first of these collisions could be particularly exciting, around 5.7 billion years ago. This is because it occurred just before the star-forming push, in which our sun was formed about 4.7 billion years ago. “The sun formed at the time when the stars formed in the aftermath of the first passage of Sagittarius,” says Ruiz-Lara’s colleague Carme Gallart. “We do not know whether or not the gas and dust cloud in which the sun originated collapsed in the course of the Sagittarius Passage. But it’s a very conceivable scenario because the age of the sun fits well with a star that was created as a result of this collision.” Thus, in the end, we could only owe our existence to this galactic carambola with our smaller neighbor. “Sagittarius has not only influenced the structure and stellar dynamics of our Milky Way, but it has also helped it grow,” Gallart says.

For the Sagittarius dwarf galaxy, however, these repeated collisions were far less positive. The observations suggest that she lost a large part of her gas on her first passage. As a result, the material supply for new stars was missing and the formation of stars in the dwarf galaxy received a strong dampener, as the astronomers explained. With each passage, Sagittarius also lost further gas and became smaller.

Source: Tomas Ruiz-Lara (Instituto de Astrofésica de Canarias, Tenerife) et al., Nature Astronomy, doi: 10.1038/s41550-020-1097-0