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Maunakea Astronomers Spot ʻBizarreʻ Objects Near Black Hole

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Photo courtesy: W.M. Keck Observatory.

Astronomers from UCLA and W. M. Keck Observatory have discovered four more bizarre objects at the center of our galaxy, not far from the supermassive black hole called Sagittarius A*, that are now forming a class of their own.

The study, which is part of UCLA’s Galactic Center Orbits Initiative, consists of 13 years of data taken from Keck Observatory on Maunakea in Hawaii; the results published online today in the journal Nature.

“These objects look like gas but behave like stars,” said co-author Andrea Ghez, UCLA’s Lauren B. Leichtman and Arthur E. Levine Professor of Astrophysics and director of the UCLA Galactic Center Group.

This new class of objects, called G objects, look compact most of the time and stretch out when their orbits bring them closest to the black hole. Their orbits range from about 100 to 1,000 years, said lead author Anna Ciurlo, a UCLA postdoctoral researcher.


Ciurlo led the study while participating in Keck Observatory’s Visiting Scholars Program and labeled the four new objects G3, G4, G5 and G6. This set is in addition to the first pair of G objects found near the Galactic Center; G1 was discovered by Ghez’s research group in 2005, followed by G2, which was discovered by astronomers in Germany in 2012.

“The fact that there now several of these objects observed near the black hole means that they are, most likely, part of a common population,” said co-author Randy Campbell, science operations lead at Keck Observatory.

The researchers have determined orbits for each of the newly discovered G objects. While G1 and G2 have similar orbits, G3, G4, G5, and G6 all have very different orbits. Ghez and her research team believe that G2 is most likely two stars that had been orbiting the black hole in tandem and merged into an extremely large star, cloaked in unusually thick gas and dust.

“At the time of closest approach, G2 had a really strange signature,” Ghez said. “We had seen it before, but it didn’t look too peculiar until it got close to the black hole and became elongated, and much of its gas was torn apart. It went from being a pretty innocuous object when it was far from the black hole to one that was really stretched out and distorted at its closest approach and lost its outer shell, and now it’s getting more compact again.”


“One of the things that has gotten everyone excited about the G objects is that the stuff that gets pulled off of them by tidal forces as they sweep by the central black hole must inevitably fall into the black hole,” said co-author Mark Morris, UCLA professor of physics and astronomy. “When that happens, it might be able to produce an impressive fireworks show since the material eaten by the black hole will heat up and emit copious radiation before it disappears across the event horizon.”

Ghez believes all six objects were binary stars — a system of two stars orbiting each other — that merged because of the strong gravitational force of the supermassive black hole. The merging of two stars takes more than 1 million years to complete, Ghez said.

“Mergers of stars may be happening in the universe more often than we thought, and likely are quite common,” Ghez said. “Black holes may be driving binary stars to merge. It’s possible that many of the stars we’ve been watching and not understanding may be the end product of mergers that are calm now. We are learning how galaxies and black holes evolve. The way binary stars interact with each other and with the black hole is very different from how single stars interact with other single stars and with the black hole.”

Ciurlo noted that while the gas from G2’s outer shell got stretched dramatically, its dust inside the gas did not get stretched much.


“Something must have kept it compact and enabled it to survive its encounter with the black hole,” Ciurlo said. “This is evidence for a stellar object inside G2.”

“The unique dataset that Professor Ghez’s group has gathered during more than 20 years is what allowed us to make this discovery,” Ciurlo said. “We now have a population of ‘G’ objects, so it is not a matter of explaining a ‘one-time event’ like G2.”

The researchers made the observations using powerful technology that Ghez helped pioneer at Keck Observatory called adaptive optics (AO), which corrects the distorting effects of the Earth’s atmosphere in real time. AO, combined with Keck Observatory’s OH-Suppressing Infrared Imaging Spectrograph (OSIRIS), allowed the team to obtain spectroscopic measurements of the Galactic Center’s gas dynamics.

“The challenge was trying distinguish G objects from a crowded cluster of stars,” Campbell said. “Because their spectra are different from standard stars, we were able to separate them using a tool called the OSIRIS-Volume Display, or OsrsVol.”

The OsrsVol software Campbell developed produces a 3-D spectral data cube that consists of two spatial dimensions plus a wavelength dimension that contains velocity information. This allowed the team to clearly isolate the G-objects and track their movement to see how they behaved around the Milky Way’s supermassive black hole.

In September 2019, Ghez’s team reported that the black hole is getting hungrier and it is unclear why. The stretching of G2 in 2014 appeared to pull off gas that may recently have been swallowed by the black hole, said co-author Tuan Do, a UCLA research scientist and deputy director of the Galactic Center Group.

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