At the center of the Milky Way, there is a massive persistent radio source known as Sagittarius A*. Since the 1970s, astronomers have known that this source is a supermassive black hole (SMBH) roughly 4 million times the mass of our Sun. Thanks to advancements in optics, spectrometers, and interferometry, astronomers have been able to peer into Galactic Center. In addition, thanks to the international consortium known as the Event Horizon Telescope (EHT), the world got to see the first image of Sagittarius A* (Sgr A*) in May 2022.
These efforts have allowed astronomers and astrophysicists to characterize the environment at the center of our galaxy and see how the laws of physics work under the most extreme conditions. For instance, scientists have been observing a mysterious elongated object around the Sgr A* (named X7) and wondered what it was. In a new study based on two decades’ worth of data, an international team of astronomers with the UCLA Galactic Center Group (GCG) and the Keck Observatory have proposed that it could be a debris cloud created by a stellar collision.
The research effort was led by the Galactic Center Initiative, an international project made up of scientists from the Mani L. Bhaumik Institute for Theoretical Physics, the University of California Los Angeles (UCLA), the W. M. Keck Observatory, the Observatoire de Paris (Sorbonne Universite), the University of California Berkeley, and the Instituto de Astrofísica de Andalucía (CSIS). The paper that describes their findings recently appeared in The Astrophysical Journal.
Using the Keck Observatory’s 10-meter (32.8 ft) Telescopes on Mauna Kea, the GCG team has been measuring the star closest to Sgr A* (S0-2) for more than twenty years (since 1995). They are one of only two groups in the world to have observed S0-2 make a full orbit of Sgr A* – a process that takes 16 years – for the sake of testing Einstein’s Theory of General Relativity. The team has spent that same time monitoring the object known as X7, a dust and gas cloud of about 50 Earth masses that takes 170 years to orbit the SMBH.
As they report in their study, X7 has become elongated and stretched by tidal forces as it has been pulled closer to Sag A*. Within the next few decades, they anticipate that X7 will disintegrate as the dust and gas that make it up are accreted onto the face of the SMBH. As Anna Ciurlo, a UCLA assistant researcher and the paper’s lead author, said in a UCLA press release:
“No other object in this region has shown such an extreme evolution. It started off comet-shaped and people thought maybe it got that shape from stellar winds or jets of particles from the black hole. But as we followed it for 20 years, we saw it becoming more elongated. Something must have put this cloud on its particular path with its particular orientation.”
The team also notes that X7 has similar properties to other strange dusty objects orbiting Sag A* (aka. G objects). These objects look like dust clouds but behave like stars and were identified using 12 years of spectroscopic measurements made using Keck’s OH-Suppressing Infrared Imaging Spectrograph (OSIRIS). The results of this study (also led by Ciurlo) were presented in 2018 at the 232nd American Astronomical Society Meeting. However, X7’s shape and velocity have changed more dramatically than Gobjects, reaching speeds of up to 1,126.5 km/s (700 mps).
These results are the most robust analysis to date of X7’s changes in appearance, shape, and behavior and the first estimate of X7’s slightly elliptical orbit. While the origins of X7 are still the subject of debate, the team’s finding suggests that it resulted from a collision between two stars orbiting Sgr A*. Such mergers are very common, especially in the vicinity of black holes. This merger is likely to have ejected gas and dust, which could have formed a shell that is concealing the merged star while the rest became the X7 object.
“The stars circle each other, get closer, merge, and the new star is hidden within a cloud of dust and gas,” said Ciurlo. “X7 could be the dust and gas ejected from a merged star that’s still out there somewhere.” Said Randy Campbell, the science operations lead at the Keck Observatory and a co-author of the paper:
“It’s exciting to see significant changes of X7’s shape and dynamics in such great detail over a relatively short time scale as the gravitational forces of the supermassive black hole at the center of the Milky Way influences this object. It’s a privilege to be able to study the extreme environment at the center of our galaxy. This study can only be done using Keck’s superb capabilities, and performed at the very special and revered Maunakea, with honor and respect to the mauna.”
Based on its trajectory, the team estimates that X7 will make its closest approach to Sgr A* sometime in 2036 and then spiral inward to be devoured. In the meantime, the research team will continue to monitor X7 using the Keck Observatory and watch as the powerful gravity of Sgr A* pulls it apart.