Clusters of supernovas can cause the birth of scattered, eccentrically orbiting stars in outer halos of Milky Way-mass galaxies, according to new research published in the Monthly Notices of the Royal Astronomical Society.
A simulated galaxy image from the FIRE-2 project, representing a structure spanning more than 200,000 light-years, shows the prominent plumes of young blue stars born in gas that was originally rotating and then blown radially outward by supernova explosions. Image credit: Sijie Yu / University of California Irvine.
“Highly accurate numerical simulations from the Feedback in Realistic Environments 2 (FIRE-2) project have shown us that it’s likely the Milky Way has been launching stars in circumgalactic space in outflows triggered by supernova explosions,” said University of California Irvine’s Professor James Bullock, co-author of the study.
“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.”
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 Professor Bullock and colleagues are proposing ‘supernova feedback’ as a different source for as many as 40% of these outer-halo stars.
“The FIRE-2 simulations allow us to generate movies that make it seem as though you’re observing a real galaxy,” said University of California Irvine’s Dr. Sijie Yu, lead author of the study.
“They show us that as the galaxy center is rotating, a bubble driven by supernova feedback is developing with stars forming at its edge. It looks as though the stars are being kicked out from the center.”
The team didn’t 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, and that gas subsequently cools and makes stars on its way out,” Dr. Yu said.
While the 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.
“In plots that compare data from ESA’s Gaia mission — which provides a 3D velocity chart of stars in the Milky Way — with other maps that show stellar density and metallicity, we can see structures similar to those produced by outflow stars in our simulations,” Dr. Yu said.
“Mature, heavier, metal-rich stars like our Sun rotate around the center of the galaxy at a predictable speed and trajectory,” Professor Bullock said.
“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.”
“Over the lifespan of a galaxy, the number of stars produced in supernova bubble outflows is small, around 2%. But during the parts of galaxies’ histories when starburst events are booming, as many as 20% of stars are being formed this way.”
“There are some current projects looking at galaxies that are considered to be very ‘starbursting’ right now,” Dr. Yu said.
“Some of the stars in these observations also look suspiciously like they’re getting ejected from the center.”
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Sijie Yu et al. 2020. Stars made in outflows may populate the stellar halo of the Milky Way. MNRAS 494 (2): 1539-1559 doi: 10.1093/mnras/staa522
