A Tale of Three Gas Clouds Orbiting the Supermassive Black Hole at the Center of the Milky-Way Galaxy
The black hole at the center of our Milky-Way galaxy, Sagittarius A*, is 4 million times more massive than the Sun. It is continuously being fed by gas from stellar winds (as suggested in my 2004 paper here), including a handful of gas clumps with masses on the order of a few times the Earth mass in its immediate vicinity. One of these compact gas clouds, labeled G2, has an extreme orbit which passes within 100 times the Earth-Sun separation from the black hole. It dissipated some of its orbital energy via drag accretion flow around Sagittarius A*. Thirteen years prior to G2, a similar gas cloud, called G1, passed Sagittarius A* on a similar orbit.
The origin of G2 is unknown. In 2012, I published a paper in collaboration with Ruth Murray-Clay (accessible here), where we suggested that G2 can naturally be produced by a proto-planetary disc surrounding a star, which was scattered from the observed ring of young stars orbiting Sagittarius A*. As the young star approaches the black hole, its disc experiences both photoevaporation and tidal disruption, producing a cloud. Our model implied that planets form in the Milky-Way center, and that tidal debris from proto-planetary discs can flag stars, which are otherwise too faint to be detected.
A new paper (accessible here) reports about the orbit of a third gas clump moving along nearly the same orbits as G1 and G2. Since the probability of finding three stars on such similar orbits is very low, the authors argue against stellar-based source models. Instead, they suggest that the trio of clouds: G1, G2 and G3, originated from the stellar wind of the massive binary star IRS 16SW.
The main argument of the new paper is that the likelihood of having three stars on similar orbits as the origin of the G1, G2 and G3 system is low. However, they miss the point that stars often form in groups and most massive stars are born in triples (as reported in another recent paper here). The main challenge of the starless scenario proposed by the authors is in making compact dense clouds out of the interaction of a diffuse wind with a diffuse ambient medium. Given the high velocities of stars and gas in the Galactic center, it is rather challenging to account for the formation of dense and compact gas clumps with the specific properties of G1, G2 and G3. A simpler explanation might be that a triple star system was tidally disrupted by the black hole and we are witnessing the clouds of gas around each of them.
Just as in human relationships, three-body systems are often chaotic and unstable. However, it is possible that the gravitational intervention of the supermassive black hole Sagittarius A* separated the triple system of stars shortly after formation. In that case, the three gas clouds G1, G2 and G3 flag the separated system of stars in this tumultuous environment.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.
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