Milky Way could be catapulting stars into its outer halo, astronomers say

Though mighty, the Milky Way and galaxies of similar mass are not without scars chronicling turbulent histories. University of California, Irvine astronomers and others have shown that clusters of supernovas can cause the birth of scattered, eccentrically orbiting suns in outer stellar halos, upending commonly held notions of how star systems have formed and evolved over billions of years.

Hyper-realistic, cosmologically self-consistent computer simulations from the Feedback in Realistic Environments 2 project enabled the scientists to model the disruptions in otherwise orderly galactic rotations. The team’s work is the subject of a study published today in the Monthly Notices of the Royal Astronomical Society.

“These highly accurate numerical simulations have shown us that it’s likely the Milky Way has been launching stars in circumgalactic space in outflows triggered by supernova explosions,” said senior author James Bullock, dean of UCI’s School of Physical Sciences and a professor of physics & astronomy. “It’s fascinating, because when multiple big stars die, the resulting energy can expel gas from the galaxy, which in turn cools, causing new stars to be born.”

Bullock said the diffuse distribution of stars in the stellar halo that extends far outside the classical disk of a galaxy is where the “archeological record” of the system exists. Astronomers have long assumed that galaxies are assembled over lengthy periods of time as smaller star groupings come in and are dismembered by the larger body, a process that ejects some stars into distant orbits. But the UCI team is proposing “supernova feedback” as a different source for as many as 40 percent of these outer-halo stars.

Lead author Sijie Yu, a UCI Ph.D. candidate in physics, said the findings were made possible partly by the availability of a powerful new set of tools.

Bullock said he did not expect to see such an arrangement because stars are such tight, incredibly dense balls that are generally not subject to being moved relative to the background of space. “Instead, what we’re witnessing is gas being pushed around,” he said, “and that gas subsequently cools and makes stars on its way out.”

The researchers said that while their conclusions have been drawn from simulations of galaxies forming, growing and evolving to the present day, there is actually a fair amount of observational evidence that stars are forming in outflows from galactic centers to their halos.

Bullock added that mature, heavier, metal-rich stars like our sun rotate around the center of the galaxy at a predictable speed and trajectory. But the low-metallicity stars, which have been subjected to fewer generations of fusion than our sun, can be seen rotating in the opposite direction.

Source: “Milky Way could be catapulting stars into its outer halo, astronomers say” PhysOrg, 7 April 2020.
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