Billions of years ago a small galaxy plunged through the Milky Way and out the other side. The ripples it created caused an enormous outbreak of star formation. While we have no way yet to be sure if our own Sun was the result of this galactic intervention, the timing looks right.
The Sagittarius dwarf galaxy (Sgr) is among the closest galaxies to our own Milky Way. However, it was only discovered in the 1990s, being both small and, from our location, hidden behind the dense stars of the center of the Milky Way.
Once thought too small to make much difference, there is increasing recognition Sgr's movements created ripples in the Milky Way's gas clouds that induced fragmentation and the formation of pockets dense enough to become stars.
“It is known from existing models that Sagittarius fell into the Milky Way three times – first about 5 or 6 billion years ago, then about 2 billion years ago, and finally 1 billion years ago,” Dr Tomás Ruiz-Lara of the Instituto de Astrofísica de Canarias, Spain said in a statement.
Dr Ruiz-Lara has taken advantage of the wealth of information the European Space Agency's Gaia satellite has provided on stars' movements and composition to report the timing of major bursts of star formation in Nature Astronomy.
“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 disk of the Milky Way,” Ruiz-Lara said.
As another paper in the same edition reported, the passage of a galaxy through a larger one can destroy the entire structure, leaving a "ring of fire" behind. Sgr's size and angle of impact made for something much less dramatic.
Ruiz-Lara's team previously showed the merger between the Milky Way and another galaxy set off a dramatic period of star formation. However, this had passed by the time of the first Sgr encounter, with the galaxy a quiet place in between
“The Sun formed at the time when stars were forming in the Milky Way because of the first passage of Sagittarius,” said co-author Dr Carrme Gallart. “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."
Sagittarius lost some of its mass in every encounter with its large neighbor, making each passage less influential than the last. The first starburst lasted 2 billion years to include the Sun's birth, but the others were much shorter. The authors think a small burst of star formation in the last 70 million years may be the result of Sgr making a fourth passage through the Milky Way, but this time it carried so little material the effects are hard to detect.