New research led by Dr. James De Buizer at the SETI Institute and Dr. Wanggi Lim at IPAC at Caltech revealed surprising results about the rate at which high-mass stars form in the Galactic Center of the Milky Way. The researchers based their study primarily on observations from NASA’s now-retired SOFIA airborne observatory, focusing on three star-forming regions — Sgr B1, Sgr B2, and Sgr C — located at the heart of the Galaxy. Although the central part of our Galaxy has a much higher density of star-forming material than the rest of the Milky Way, in the Galactic Center, the current rate of formation of massive stars (those larger than 8 times the mass of our Sun) appears to be lower compared to the rest of the Galaxy.
The team compared these three Galactic Center star-forming regions to similar-sized regions further out in the Galaxy, including those closer to our Sun, and confirmed that the rate of star formation is below average near the Galactic Center. Their study finds that despite the Galactic Center’s dense clouds of gas and dust, conditions that typically produce stars with high masses, these star-forming regions struggle to form high-mass stars. Furthermore, the studied areas appear to lack sufficient material for continued star formation, suggesting such regions effectively produce just one generation of stars, unlike typical star-forming regions.
“Recent studies have concluded that star formation is likely depressed near the Galactic Center, and even that there may be no present star formation occurring there,” said De Buizer, lead author of the study. “Since presently-forming massive stars are brightest at long infrared wavelengths, we obtained the highest resolution infrared images of our Galaxy’s central-most star-forming regions. The data show that, contrarily, massive stars are presently forming there, but confirm at a relatively low rate.”
The study suggests that the reason for the slowdown in star formation is due to the extreme conditions in the Galactic Center. These regions orbit swiftly around the black hole at the center of the Galaxy, interacting with older stars and possibly with other material falling toward the black hole. These conditions could inhibit gas clouds from holding together long enough to form stars in the first place and prevent those that do form stars from staying together long enough for continued future star formation.
However, Sgr B2 appears to be the exception. Although its rate of present massive star formation is unusually low, like the other Galactic Center regions studied, it seems to have maintained its reservoir of dense gas and dust, allowing for a future emergent star cluster to be born.
Traditionally, astronomers have viewed giant H II regions — large clouds of gas, mainly hydrogen, in space like Sgr B1 and Sgr C — as hosts of massive star clusters still embedded in their birth clouds. This study challenges that assumption. The team argues these two regions may not fit the classical definition at all, or they may represent a new, previously unrecognized category of stellar nursery.
Enshrouded in gas and dust that obscure these star-forming regions from view in all but the longest infrared wavelengths, SOFIA’s high-resolution infrared eyes allowed the team to identify more than six dozen presently-forming massive stars within the Galactic Center regions. However, these regions formed fewer stars — and topped out at a lower stellar mass — than the Galactic average.
“These Galactic Center star-forming regions are in many ways very similar to the massive star-forming regions in the relatively calm backwaters of our galaxy,” said Lim. “However, the most massive stars we are finding in these Galactic Center regions, though still remarkably large, fall short in both size and quantity compared to those found in similar regions elsewhere in our Galaxy. Furthermore, such star-forming regions typically hang on to large reservoirs of star-forming material and continue to produce multiple epochs of stars, but that appears to not be the case for these Galactic Center regions.”
Lim will present the results of this study at the 246th meeting of the American Astronomical Society in Anchorage, AK.
