Since the 1930s, physicist and radio engineer Karl Jansky has reported the discovery of a persistent radio source from the center of our galaxy. This source became known as Sagittarius A* (Sgr A*), and in the 1970s astronomers determined that it was a supermassive black hole (SMBH) about four million times the mass of the Sun. Since then, astronomers have used increasingly advanced radio telescopes to study Sgr A* and its surroundings. This has led to many exotic discoveries, such as the many “stellar stars” and gaseous “G objects” that orbit it.
Studying these objects and how Sgr A*’s strong gravity has allowed scientists to test the laws of physics in the most extreme conditions. In a recent study, an international team of researchers led by the University of Cologne made an amazing discovery. Based on data collected from multiple observatories, they observed what appeared to be a newly formed star (X3a) near Sgr A*. This discovery raises important 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 postdoc at the Institute for Astrophysics at the University of Cologne. He was joined by colleagues from Masaryk University, the Institute of Astro- and Particle Physics, the JAXA Institute for Space and Astronautical Sciences (ISAS), the Astrophysics and Planetology Research Institute (IRAP), the Max Planck Institute for Radio Astronomy (MPIA), the Czech Astronomical Institute of the Academy of Sciences and the Observatoire de Paris. The paper describing their findings, “X3: a high-mass Young Stellar Object near the supermassive black hole Sgr A*,” recently appeared in the Astrophysical Journal.
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This environment of Sgr A* is characterized by highly dynamic processes and hard radiation, the very conditions that counteract star formation. Therefore, astronomers have long assumed that only older stars, formed billions of years ago and put into orbit by sliding friction, are found near SMBHs. However, astronomers have been observing very young stars near Sgr A* for the past twenty years. This raised the obvious question of where and how they formed and found their way into their current orbits.
In observing X3a, the team found that it was not only very young (tens of thousands of years old), but also ten times the size and fifteen times the mass of the Sun. For their study, the team relied on data from multiple telescopes to make observations at multiple wavelengths. This consisted of near- and mid-infrared (NIR/MIR) measurements with the SINFONI, NACO, ISAAC and VISIR instruments on ESO’s Very Large Telescope (VLT), the SHARP instrument on the New Technology Telescope (NTT) and the near-infrared Camera-2 (NIRC-2) on the WM 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 believes X3a formed in a dense cloud of dust and gas further orbiting from Sgr A* and then descended to its current orbit. As first author, Dr. Florian Peißker in a press release from the University of Cologne:
“It turns out that there is a region a few light-years away from the black hole that meets the conditions for star formation. This region, a ring of gas and dust, is sufficiently cold and shielded from damaging radiation. This so-called fall time roughly corresponds to the age of X3a.”
The galactic center at about 30000 light-years away (left) and the baby star X3a (shown in blue) in its envelope (right). Photo 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 grow up to a hundred solar masses, causing them to collapse under their own gravity and form one or more protostars. Observations have also shown that there are many such clouds in the outer ring, which are likely to interact with each other. This could (theoretically) cause some of them to lose angular momentum and fall inward (in the direction of the black hole) over time.
This scenario would explain X3a’s stellar evolution phase, as current observations (showing what it looked like 300,000 years ago) indicate that it is evolving into a mature star. Therefore, it is very plausible that the ring of gas and dust acts as the birthplace of the young stars at the center of our galaxy. As such, the existence of X3a could close the gap between star formation and YSOs in the immediate vicinity of Sgr A*. like dr Michal Zajacek from Masaryk University (a co-author of the study) added:
“With its high mass of about 10 times the mass of the Sun, X3a is a giant among the stars, and these giants are evolving to maturity very quickly. We were lucky enough to spot the massive star amid the comet-shaped circumstellar envelope. We then identified key features associated with young age, such as the compact circumstellar envelope that revolves around it.”
Since similar rings of dust and gas are found in other galaxies, this mechanism could also apply to other SMBHs – meaning many massive galaxies could have very young stars near their centers. Follow-up studies are currently planned using next-generation telescopes such as NASA’s James Webb Space Telescope (JWST) and ESO’s Extremely Large Telescope (ELT) in Chile. These observations will test this model of star formation in our galaxy and possibly others.
Further reading: University of Cologne, The Astrophysical Journal