A team of astronomers looking to find the origin of the so-called “nuclear star cluster” (NSC) near our galaxy’s center has discovered an interesting anomaly. The astronomers say this group of millions upon millions of stars likely owes part of its extraordinary mass and brightness to “immigrant” stars. That is, stars that were formed elsewhere, but were eventually trapped by the already established main cluster of stars.
Science News reported on the findings, which the astronomers outlined in two papers; both of which were recently published in The Astrophysical Journal Letters. To make the discovery, one part of the team snapped infrared images of the NSC and analyzed them. The other modeled the physical events that could’ve led to the NSC as we see it now.
Where did the millions of stars at the Galactic center come from? In a new paper, we find evidence that some of the stars probably came from a globular cluster or dwarf galaxy that fell in and got trapped.— Tuan Do (@quantumpenguin) September 10, 2020
“We noticed a very curious thing about our data, which is that the stars with less metals than our sun seem to be moving differently than the stars with more metals,” Tuan Do, an astronomer at UCLA, told Science News. Do, who led the team that captured the infrared images, and his colleagues, glimpsed 700 red giant stars. Based off this grouping, the astronomers determined that seven percent of the NSC’s stars were unlike the rest. Compared to their peers, these stars revolve around the galactic center faster, and do so around a different axis.
The metallicity of these stars—which describes the abundance of elements heavier than hydrogen and helium—is also relatively low. All of these characteristics combined, the astronomers say, point to a percentage of the NSC’s stars originating from elsewhere.
In a companion paper, my theory colleagues used computer simulations throwing star clusters and small galaxies into the center of our galaxy to explore this process. The simulations show that this infall might have happened over 3 billion years ago. https://t.co/zA05uMjVRS— Tuan Do (@quantumpenguin) September 10, 2020
“Among other things, our goal was to find out how long ago a star cluster of this type could have entered the region around the Galactic Center …” Manuel Arca Sedda said in a press release. Sedda, at Heidelberg University in Germany, led the team that modeled different simulations of how a star cluster falling into the Milky Way’s NSC would look; as well as how they’d compare to what Do et al. imaged.
Based on their models, Sedda and his colleagues say the immigrant stars likely arrived between three and five billion years ago. They say that the stars likely come from a dwarf galaxy or from the collision of globular clusters of stars. (Globular clusters are large, compact, spherical star clusters.)
“Although an extragalactic origin of the stars cannot be completely excluded, it is rather unlikely,” Arca Sedda says. He adds this evidence supports the idea that part of the NSC is the result of stars migrating from elsewhere in the Milky Way.
Feature image: asa/Jpl-Caltech/S. Solovy