Since the 1930s, physicists and radio engineer Karl Jansky reported discovering a persistent radio source coming from the center of our galaxy. This source came to be known as Sagittarius A* (Sgr A*), and by the 1970s, astronomers determined that it was a supermassive black hole (SMBH) roughly four million times the mass of our Sun. Since then, astronomers have used increasingly-advanced radio telescopes to study Sgr A* and its surrounding environment. This has led to many exotic discoveries, such as the many “Stars stars” and gaseous “G objects” that orbit it.
The study of these objects and how the powerful gravity of Sgr A* has allowed scientists to test the laws of physics under the most extreme conditions. In a recent study, an international team of researchers led by the University of Cologne made a startling discovery. Based on data collected by multiple observatories, they observed what appears to be a newly-formed star (X3a) in the vicinity of Sgr A*. This discovery raises significant questions about how young stellar objects (YSOs) can form and survive so close to an SMBH, where they should be torn apart by violent gravitational forces.
The research was led by Florian Peißker, a postdoctoral researcher at the University of Cologne’s Institute of Astrophysics. He was joined by colleagues from Masaryk University, the Institute for Astro and Particle Physics, JAXA’s Institute of Space and Astronautical Science (ISAS), the Astrophysics and Planetology Research Institute (IRAP), the Max Planck Institute for Radioastronomy (MPIA), the Czech Academy of Sciences Astronomical Institute, and the Observatoire de Paris. The paper that describes their findings, “X3: a high-mass Young Stellar Object close to the supermassive black hole Sgr A*,” recently appeared in The Astrophysical Journal.
This vicinity of Sgr A* is characterized by highly dynamic processes and hard radiation, the very conditions that act against star formation. As a result, astronomers have assumed for a long time that only older stars – which formed billions of years ago and settled into orbit via dynamical friction – would be found in the vicinity of SMBHs. However, astronomers have observed very young stars in the vicinity of Sgr A* for the past twenty years. This raised the obvious question of where and how they formed and found their way to their current orbits.
When observing X3a, the team noted that it was not only very young (several tens of thousands of years old) but also ten times the size and fifteen times as massive as the Sun. For their study, the team relied on data from multiple telescopes to conduct observations in multiple wavelengths. This consisted of near- and mid-infrared (NIR/MIR) measurements using the SINFONI, NACO, ISAAC, and VISIR instruments on the ESOs Very Large Telescope (VLT), the SHARP instrument on the New Technology Telescope (NTT), and the Near-Infrared Camera-2 (NIRC-2) on the W.M. Keck Telescopes.
These were combined with radio domain observations using the Atacama Large Millimeter-submillimeter Array (ALMA) to identify components at different temperatures and locations. Based on their observations, the team thinks that X3a formed in a dense could of dust and gas orbiting farther from Sgr A* and sank to its current orbit afterward. As first author Dr. Florian Peißker explained in a University of Cologne press release:
“It turns out that there is a region at a distance of a few light years from the black hole which fulfills the conditions for star formation. This region, a ring of gas and dust, is sufficiently cold and shielded against destructive radiation. This so-called fall time approximately corresponds to the age of X3a.”
The Galactic center at a distance of about 30000 light-years (left), and the baby star X3a (shown in blue) in its envelope (right). Credit: Florian Peißker
According to the team, the formation process begins in X3, a gaseous envelope in the outer ring surrounding the center of Sgr A*. These clouds could become up to one hundred solar masses, which would cause them to collapse under their own gravity to form one or more protostars. Observations have also shown that there are many such clouds in the outer ring that likely interact with each other. This could (in theory) cause some of them to lose angular momentum and fall inwards over time (the direction of the black hole).
This scenario would explain X3a’s stellar development phase since current observations (which show how it looked 300,000 years ago) indicate it appears to be evolving into a mature star. Therefore, it is highly plausible that the gas and dust ring acts as the birthplace of the young stars in the center of our Galaxy. In this respect, the existence of X3a could close the gap between star formation and YSOs in the immediate vicinity of Sgr A*. As Dr. Michal Zajacek at Masaryk University (a co-author of the study) added:
“With its high mass of about ten times the Solar mass, X3a is a giant among stars, and these giants evolve very quickly towards maturity. We have been lucky to spot the massive star in the midst of the comet-shaped circumstellar envelope. Subsequently, we identified key features associated with a young age, such as the compact circumstellar envelope rotating around it.”
Since similar dust and gas rings can be found in other galaxies, this mechanism could apply to other SMBHs – meaning that many massive galaxies could have very young stars near their centers. Follow-up studies are currently planned using next-generation telescopes like NASA’s James Webb Space Telescope (JWST) and the ESO’s Extremely Large Telescope (ELT) in Chile. These observations will test this model of star formation in our galaxy and potentially others.