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The study reports that X7 is being pulled apart as the supermassive black hole, called Sagittarius A*, pulls it closer. Credit: Anna Ciurlo/UCLA
Now, after examining the evolution of X7 using 20 years of data collected by the Galactic Center Orbit Initiative, astronomers from the UCLA Galactic Center Group and Keck Observatory suggest that it may be a cloud of dust and gas ejected during the collision of two stars.
Over time, they report, X7 has stretched, being pulled apart as the black hole pulls it closer, exerting its tidal force on the cloud. They expect that over the next few decades X7 will disintegrate and the gas and dust it contains will eventually be pulled towards the black hole, which is called Sagittarius A*, or Sgr A*.
The study was published on 21 February i The
“No other object in this region has shown such extreme evolution,” said Anna Ciurlo, assistant scientist at the University of California, Los Angeles (UCLA) and the paper’s lead author. “It started out 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 become more elongated. Something must have set this cloud on its particular path with its special orientation.”
X7 has a mass of about 50 Earths and is on an orbit around Sgr A* that will take 170 years to complete.
But that may never happen. Based on its trajectory, the team estimates that X7 will come closest to Sgr A* around the year 2036, then likely spiral towards Sgr A* and disappear.
“We expect that the strong tidal forces exerted by the galactic black hole will eventually tear X7 apart before it completes another orbit,” said co-author Mark Morris, UCLA professor of physics and astronomy.
Tidal forces are the gravitational force that causes an object approaching a black hole to stretch; the side of the object closest to the black hole is pulled much more strongly than the opposite end.
X7 exhibits some of the same characteristics as the other strange dusty objects orbiting Sgr A*. The so-called G-objects look like gas, but behave like stars. But the X7’s shape and speed have changed more dramatically than G-objects have. As it accelerates toward the black hole, X7 moves quickly, clocking in at speeds up to about 700 miles per second.
“It is exciting to see significant changes to 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 affect this object,” Randy Campbell, a co-author. of the paper and science operations manager at Keck Observatory, said in a statement.
Although X7’s origin is still a matter of debate, the finding suggests that it formed after two stars collided.
“One possibility is that X7’s gas and dust were ejected at the moment two stars merged,” Ciurlo said. “In this process, the merged star is hidden inside a shell of dust and gas, which may fit the description of the G objects. And the ejected gas may have produced X7-like objects.”
The merger of two stars is very common, especially when they are near black holes, Ciurlo said.
“This is a very messy process: The stars circle each other, get closer, merge, and the new star is hidden in a cloud of dust and gas,” she said. “X7 could be the dust and gas ejected from a merged star that’s still out there somewhere.”
The findings are the first estimate of X7’s mildly elliptical orbit and the most robust analysis to date of the remarkable changes in its appearance, shape and behavior. The research team will continue to use the Keck Observatory to monitor X7’s dramatic changes as the force of the black hole’s gravity pulls it apart.
“It is a privilege to be able to study the extreme environment at the center of our galaxy,” Campbell said in the statement. “This study can only be done using Keck’s superb abilities, and is performed on the very special and revered Maunakea, with honor and respect for the mauna.”
Reference: “The Swansong of the Galactic Center Source X7: An Extreme Example of Tidal Evolution near the Supermassive Black Hole” by Anna Ciurlo, Randall D. Campbell, Mark R. Morris, Tuan Do, Andrea M. Ghez, Eric E. Becklin , Rory O. Bentley, Devin S. Chu, Abhimat K. Gautam, Yash A. Gursahani, Aurélien Hees, Kelly Kosmo O’Neil, Jessica R. Lu, Gregory D. Martinez, Smadar Naoz, Shoko Sakai, and Rainer Schödel, 21 February 2023, The Astrophysical Journal.
DOI: 10.3847/1538-4357/acb344
