Supermassive black holes are voracious. Clumps of dust and gas are often disrupted by turbulence and radiation when they get pulled too close. So why are some of them orbiting the edge of the Milky Way’s own supermassive monster, Sgr A*? Maybe these mysterious blobs are hiding something.
After analyzing observations of the dusty objects, an international team of researchers led by astrophysicist Florian Peißker from the University of Cologne identified these clumps as potentially harboring young stellar objects (YSOs) shrouded in a haze of gas and dust. Even stranger, these young stars are younger than an unusually young and bright cluster of stars already known to orbit Sgr A*, known as S stars.
It is unusual for the two groups to orbit each other so closely, as stars orbiting supermassive black holes are expected to be faint and much older. Peißker and his colleagues “reject the popular idea that [these] objects like coreless clouds in the high-energy radiation field of the supermassive black hole Sgr A*,” they said in a study recently published in Astronomy & Astrophysics.
More than just space dust
To figure out what the objects near Sgr A* might be, researchers had to rule out things they weren’t. Embedded in envelopes of gas and dust, they maintain particularly high temperatures, don’t evaporate easily, and each orbits only the supermassive black hole.
The researchers determined their chemical properties from the photons they emitted, and their mid- and near-infrared emissions matched those of stars. They used one of them, object G2/DSO, as a case study to test their ideas about what the objects might be. This object's high brightness and especially strong emissions make it the easiest to study. Its mass is also comparable to the masses of known low-mass stars.
YSOs are low-mass stars that have outgrown the protostar phase but have not yet evolved into main-sequence stars, with cores that convert hydrogen to helium. These objects are favored as YSO candidates because they cannot possibly be clumps of gas and cosmic dust. Gas clouds without objects inside them to hold them together gravitationally would not be able to survive very long so close to a supermassive black hole. The intense heat causes the gas and dust to evaporate quite quickly, with the excited particles crashing into each other and flying off into space.
The team found that a cloud similar in size to G2/DSO would evaporate in about seven years. A star orbiting at the same distance from the supermassive black hole would not be destroyed nearly as quickly due to its much higher density and mass.
Another class of object that the dust clouds could hypothetically be, but aren't, is a compact planetary nebula, or CPN. These nebulae are the expanding outer gaseous envelopes of small to medium-sized stars in their final death throes. While CPNs share some characteristics with stars, the gravity of a supermassive black hole would easily rip their gaseous envelopes apart, tearing them apart.
It is also unlikely that the YSOs are binaries, even though most stars form in binary systems. The scorching temperatures and turbulence of SGR A* would likely cause stars that were once in binary systems to migrate.
Seeing stars
Further observations revealed that some of the dust-covered objects are emerging stars. Others are thought to be stars, but have not yet been definitively identified.
The properties that made G2/DSO an exceptional case study are also the reason it was identified as a YSO. D2 is another high-luminosity object, about the mass of a low-mass star, that is easy to observe in the near- and mid-infrared. D3 and D23 also have similar properties. These are the blobs near the black hole that researchers believe are most likely YSOs.
There are other candidates that require further analysis. These include additional objects that may or may not be YSOs but still exhibit stellar features: D3.1 and D5, which are difficult to observe. D9's mid-infrared emissions are particularly low compared to the other candidates, but it is still thought to be some kind of star, though possibly not a YSO. Objects X7 and X8 both exhibit a bow shock, the shock wave created by a star's stellar wind pushing against other stellar winds. Whether any of these objects are actually YSOs is still unknown.
Where these dusty objects came from and how they formed are still unknown. The researchers suggest that the objects formed together in molecular clouds that fell toward the center of the galaxy. They also think that regardless of where they were born, they migrated to Sgr A*, and that any objects in binary systems were separated by the immense gravity of the black hole.
Although it is unlikely that the YSOs and potential YSOs originate from the same cluster as the slightly older S stars, they may be related in some way. They may have undergone similar formation and migration journeys, and the younger stars may eventually reach the same stage.
“Speculatively, the dust sources will evolve into low-mass S stars,” Peißker's team said in the same study.
Even black holes look better with a necklace of sparkling diamonds.
Astronomy and Astrophysics, 2024. DOI: 10.1051/0004-6361/202449729
