Observations of Milky Way’s chaotic center show hints of stars forming just two light-years away from our galaxy’s supermassive black hole.
The supermassive black hole lurking at the center of the Milky Way’s brilliant bulge wreaks gravitational havoc on its immediate surroundings. And that doesn’t make life easy for would-be stars: the 4-million-solar-mass behemoth stretches gas clouds into taffy-like streamers before they have the chance to collapse into denser objects. Yet bright and massive stars less than 10 million years old are surprisingly common in the black hole’s neighborhood.
Over the past decade, astronomers have debated how these cosmic youngsters came to be. Did they migrate inward after they formed? Or did a set of unusual circumstances allow the stars to form where they are? Recent observations hinting at early stages of star formation advance the theory that stars regularly form within reach of the supermassive black hole.
In a recent paper, Farhad Yusef-Zadeh (Northwestern University) and his colleagues took advantage of public data to search for signatures of star formation near the black hole. As part of its “science verification” process, the still-under-construction Atacama Large Millimeter/submillimeter Array (ALMA) Observatory had pointed 12 of its radio dishes, each the size of a five-story building, toward the galactic center.
Searching the millimeter-wavelength images, Yusef-Zadeh’s team found 11 clumps of the molecular gas silicon oxide within two light-years of the supermassive black hole. They confirmed the clumps exist using additional images taken by the Combined Array for Research in Millimeter-wave Astronomy (CARMA) in California. The molecular gas lies inside a cavity surrounding the black hole that astronomers previously thought was filled with hot, ionized gas.
Silicon oxide typically appears in warm, dense environments such as those around forming stars. Before they begin to shine by fusion, protostars accrete gas from their surroundings and in the process fling some of that gas outward along their poles. Yusef-Zadeh and his colleagues draw on examples of silicon oxide clumps in nearby star formation regions to suggest that those in the galactic center also come from such protostellar outflows.
If so, Yusef-Zadeh’s team has found evidence for one of the earliest stages of star formation, hinting that stars form right under the beast’s nose. Star formation in the presence of the black hole’s immense gravitational field might not require exceptional circumstances after all.
But Reinhard Genzel (Max Planck Institute for Extraterrestrial Physics, Germany), who was not involved in the study, urges caution in interpreting the results. Though the protostellar interpretation makes sense in the quieter outer reaches of the Milky Way, Genzel suggests another interpretation might be warranted in the chaotic galactic center. Collisions between fast-moving gas clouds could be enough to make silicon oxide clumps glow — no protostars required.
“I am not saying that Farhad’s interpretation is wrong,” Genzel explains. “I am rather suggesting that one ought to perhaps be cautious about transferring interpretations won from much more benign environments into this hostile region.” Genzel suggests follow-up observations, including high-resolution spectroscopy of the dense gas clumps, would prove whether the protostars are really there.