A team of European astronomers says it has made the first confirmed observation of stars forming in the powerful outflow of material hurled out by a galaxy's central supermassive black hole. This outflow represents an extreme environment for star formation, and the presence of stars within it could help answer a number of puzzling questions, including how our galaxy, and others like it, got their distinctive central bulge.
Under ideal circumstances, stars such as our Sun form in vast interstellar clouds of dust and gas located within the disk of a galaxy. Over time, turbulence within these clouds creates density inconsistencies, and the gravitational attraction of these subtle clumps causes the clouds to slowly collapse and form stellar cores known as protostars. Over the course of roughly 50 million years, a protostar could grow to become a Sun-like body, or to follow a different branch of stellar evolution.
That is a classic example of how a star could come to form. A new study, which focused on a pair of colliding galaxies collectively designated IRAS F21128-5919, is proving that stars can be born in far more extreme circumstances, and with far-reaching implications.
The team behind the study used MUSE and X-shooter, a pair of powerful spectrograph instruments mounted aboard the European Southern Observatory's Very Large Telescope (VLT), to observe the colliding galaxies.
The scientists focused their attention on the supermassive black hole embedded in the center of the southernmost of the two galaxies, which is believed to be the driving force behind a powerful outflow of dust and gas. These outflows are created as a byproduct of black-hole feeding. The material around a black hole becomes super heated as it falls toward the event horizon. This in turn heats the surrounding gas, and expels it in powerful winds known as outflows.
The VLT observations of the outflow contained clear signatures of youthful stars – of no more than 10 million years of age – embedded within the stream of material travelling at high speeds away from the galactic center. The stars detected within the feature are thought to be much hotter and brighter than those known to form in more stable environments such as, for example, on the galactic plane.
It is estimated that stars equaling roughly 30 times the mass of our Sun are birthed in the outflow every year, and that the feature accounts for over a quarter of the galaxy's stellar production levels.
The fates of the stars embedded in the outflow could depend largely on their position within the stream of material. Stars closer to the black hole may have a greater chance of slowing down, and being drawn back into the galaxy, however those forming near the leading edge of the stream could be ejected from their host entirely, and left abandoned to wander intergalactic space.
This galactic expulsion could help to explain why the space between galaxies, known as the intergalactic medium, is so enriched in heavy elements. Stars that are thrown out of their host galaxies could wander through this void until they end their lives in dramatic supernovae, distributing their enriched materials in the process.
The enrichment of the intergalactic medium could in turn explain why some galaxies abruptly stop producing stars. The powerful black hole outflows are effectively bleeding off star-forming gas, which would otherwise be used in the creation of stellar bodies and recycled into new stellar nurseries once these bodies explode. The expulsion of the gas in large enough quantities could effectively stall this process.
The discovery of stars within the outflow could also explain certain structural features of spiral galaxies such as our own Milky Way. Stars partially expelled by the black hole winds, and subsequently drawn back in by the gravitational influence of a galaxy, could explain the bulge located in the center of many spiral galaxies. Furthermore, the "halo" of stars known to surround these galaxies could be formed from the stellar bodies that managed to escape from the main body of a galaxy, but are still under the influence of its gravity.
Scroll down to view an artist's impression of a black hole outflow.
The study has been published online in the journal Nature.
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