WHAT’S ORBITING OUR GALAXY’S BLACK HOLE?
Six strange objects have been found in the vicinity of Sagittarius A* – and there may be more
At the heart of the Milky Way is a supermassive black hole called Sagittarius A*. We know it is 26,000 light years from Earth and that it has 4 million times the mass of the Sun. We also found out last year that it is surrounded by a cool gas halo and is perhaps quieter than its counterparts in other galaxies because of magnetic fields.
What we’re less certain about, however, is the make-up of six strange objects that have been discovered relatively close by. Found to orbit Sagittarius A*, or Sgr A* as it is also known, they are a few hundred astronomical units away from the galactic centre and take anywhere between 170 and 1,600 years to complete their circuit.
“These objects look like gas and behave like stars,” says Andrea Ghez, a professor of physics and astronomy at UCLA, who co-authored a recent study into the objects. Yet the exact identification of what is said to be a new class of object is still being worked on, and teams coming up with potential theories appear to be split.
The objects began to truly puzzle astronomers after one of them, named G2, was seen to make a close approach to the supermassive black hole in 2014. It had originally been discovered in 2011 by a team based in Germany, and the expectation was that it would be ripped to shreds and cause a flash of radiation.
Instead it was observed becoming elongated, and while much of its gassy outer shell was torn apart, it survived and continued on its way, becoming more compact once again. “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,” Ghez explains.
At this stage, G2 could have been treated as an outlier – a strange object behaving oddly alone. But some years earlier in 2005, astronomers had also discovered another strange object close to Sgr A*. In light of G2, this became known as G1, and it set Ghez wondering whether G1 and G2 could be part of a larger class of objects.
As such, together with her colleagues, Ghez subsequently went back over 13 years of data taken from observations from the W.M. Keck Observatory in Hawaii, and found evidence linking four more. These became known as G3, G4, G5 and G6, collectively known as the G objects. Certainly, they were looked upon by astronomers with great excitement. “Such objects had not been seen before in any other environment,” says Mark Morris, a UCLA professor of physics and astronomy who worked with Ghez on the study.
“By stellar standards their dust photospheres are very cold, and because they are in a region with a strong background of ultraviolet radiation they are partially ionised and show up in emissions from very hot hydrogen atoms, which again hasn’t been seen elsewhere. They also appear to be present only in the entourage of the black hole.”
G3 through to G6 follow a different orbit to G1 and G2 around the supermassive black hole, which points to them having developed independently of each other, even though their behaviour in relation to Sgr A* was determined to be the same. “In every passage the tidal forces of the black hole can remove some of the material from the outer layers of some of the G objects,” Morris says. “We actually observed that happening for both G1 and G2.”
“The tidal forces of the black hole can remove some of the material from the outer layers of some of the g objects”
Mark Morris
He says there can be two main possible explanations for this. “I think they’re either gas clouds or stars, which reflects the ongoing debate between the German group at the Max Planck Institute for Extraterrestrial Physics and ours at UCLA,” he tells All About Space.
Morris says that his group are strong proponents of the hypothesis that the G objects are stars surrounded by a cocoon of dust and gas. “Can they be something else entirely?” he asks. “Yes, of course, but I think the realm of possibilities has been well scoured, so alternative hypotheses would need to be quite surprising and unexpected.”
He certainly rejects the rival theory put forward by the German team that they are compact clouds of gas and dust because he says this would cause the objects to be torn apart. “They’d be tidally disrupted as they pass through the point of closest approach to the black hole, and yet two of the G objects have gone through their close approach and survived intact,” he says.
In fact, he notes that the outer shell of G2 stretches dramatically, whereas the dust inside the gas doesn’t stretch much when it gets close to Sgr
A*. It points to there being stellar objects with a strong gravitational pull inside the G objects, and this would explain why they are compact and able to survive the immense gravitational pull of the black hole. But how did they get to be that way? Well, according to UCLA astronomers the six objects were most likely binary stars that merged due to the supermassive black hole’s strong gravitational force. In other words, a system of two stars orbiting each other would have been pulled together into one entity over the course of more than a million years, Ghez says.
“The merged-binary hypothesis fits the theoretical expectations for what should happen to binary stars orbiting fairly closely around a supermassive black hole,” explains Morris. “Binary stars are commonplace in the galaxy, so one should expect that to be true in the galactic centre as well, and their abundance is something that we are closely looking into.”
He says another hypothesis to consider is one that suggests stars in the galactic centre are so close to each other because of the high stellar density that they occasionally encounter each other at high speeds in glancing collisions. “This could levitate much of the stellar atmosphere, causing it to become distended, cold and dusty, which is what the G objects are,” Morris says. “It would take a long time for such bloated stars to settle down.”
Others have suggested that the objects are a protostar surrounded by an accretion disc. “But that would necessarily be very young, and there are reasons to question whether star formation has taken place sufficiently recently for that hypothesis to be viable,” Morris counters.
As for the German team, Morris says: “They are not willing to admit that we can rule out the gas cloud hypothesis. They have argued that such a cloud can stay together during its close passage because of the high external pressure of the gas there. I don’t think that prevents the cloud from being tidally disrupted.”
The evidence for them being cocooned stars is backed up by the angular momentum of the objects in their orbits around the black hole. “Because they have angular momentum in their orbits around the black hole, they will keep going in the same way that the Earth keeps going in its orbit around the Sun without getting pulled in, especially if they are stars, because that angular momentum cannot be removed,” he says.
If the G objects are indeed merged binary stars, then it has wide ramifications for astronomy and scientific understanding. In a statement announcing the discovery of G3 through to G6 and the study into such objects, it was pointed out that the black hole may be driving binary stars to merge.
Ghez said in the statement: “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.”
Morris claims G objects are likely relevant to the growth of supermassive black holes in galactic nuclei. “There is a long-standing problem of how such black holes get fed and how they can grow to have the masses that they have,” he tells us.
“Both stars and interstellar gas contribute to their growth, but in spite of their strong gravity black holes are so small that it is very hard to get matter to reach them and to go all the way down to their event horizon to become part of the black hole.
“Part of the problem is angular momentum. Orbiting gas and orbiting stars can’t usually get too close because of their inherent angular momentum, which presents what is called a centrifugal barrier and makes it hard to feed the black hole. But gas can apparently be pulled off the G objects as they orbit close to the black hole, and that gas can ultimately be drawn into the black hole as it joins the accretion flow of matter onto the black hole.”
If correct, it would mean that G objects are contributing to the growth of black holes. They’re probably not the dominant contributor, but it might help to explain why, in September last year, the UCLA team was also able to observe the brightest light in 24 years of supermassive black hole observation, pointing to the galaxy having a scrumptiously large feast of interstellar gas and dust. The team is now looking for more G objects, and it says it has identified some candidates. “We will continue to identify more G objects and to determine their orbits more precisely with more measurements in coming years,” Morris says.
“The next generation of big telescopes will be very useful for seeing in much improved detail what happens as G objects swing closely around the black hole in their orbits, and they will help to tell
“The stars we’ve been watching and not understanding may be the end product of mergers that are calm now” Andrea Ghez
us how much mass they lose in each passage.” Key quests will involve trying to find binary systems which haven’t yet merged but which may later do so and become G objects. They will also look at whether the black hole becomes more active after a G object passes nearby.
“The answer to that is something we are pursuing now,” Morris says. “We recently reported an infrared observation last year that the black hole underwent an enormous flare. Can we associate that with a G object passage a few years earlier? Or is that just some random lump of matter falling into the black hole?”
One thing’s for sure, the issue of G objects is not about to be forgotten, and they represent another mysterious puzzle to solve.