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A simulation of the G2 dust cloud approaching the black hole at the center of the Milky Way. Stellar orbits around the black hole are traced in blue. Credit: M. Schartmann and L. Calcada/ European Southern Observatory and Max-Planck-Institut fur Extraterrestrische Physik.
For the past year, astronomers around the world have been watching the center of the Milky Way in anticipation of a once-per-eon event. Right around now (or, technically, 24,000 years ago–that’s roughly how far away the galactic center is in light years), a cloud of gas and dust plummeting toward our galaxy’s supermassive black hole, Sagittarius A*, is expected to make its closest approach. The black hole’s tidal forces should then rip the cloud to shreds. Eventually, Sagittarius A* might even swallow some of those shreds. And if that happens? Fireworks. X-rays and other radiation should rain down on all those watchful telescopes back on Earth. For the first time, we will have witnessed a black hole eating in real time.
But we’re so far away from the galactic center that it’s difficult to know exactly what’s happening there, and the nature of this cloud, which astronomers call G2, has been the subject of debate for quite a while. When Stefan Gillessen, an astronomer with the Max Planck Institute for Extraterrestrial Physics, and his colleagues announced the discovery of G2 in Nature in 2011, they described it as a cold, dense cloud of gas and dust. Andrea Ghez at UCLA, meanwhile, has argued that G2 probably has some sort of star in its center. That sounds like a pretty academic distinction until you consider that if G2 is in fact a dusty, windy star, it will interact with the black hole in a much different way than a dust cloud. Namely, the fireworks could be cancelled. The star’s gravity will bind the cloud together tightly enough that the black hole can’t rip it to pieces. Instead, it will pull away the envelope of gas and dust around the central star and reveal what is inside.
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On March 19th and 20th, Ghez and her colleagues watched the galactic center from the W. M. Keck Observatory on Mauna Kea, Hawaii. Before the observation, Ghez and her colleagues took bets on the outcome. “If it’s a dust cloud, it won’t be there,” Ghez said, because it will have been torn to pieces. “If it’s a star, it will.”
Both nights, there it was. “We saw G2, clear as day,” Ghez said.
Ghez and her colleagues describe the observation in a dispatch posted this afternoon to the online forum Astronomer’s Telegram. At the moment, all they can say is that G2 is still there, intact and approaching the black hole. “That doesn’t mean there’s no gas associated with it,” Ghez said. Astronomers know more in a month or so, after they’ve observed G2 in different wavelengths. “But the simple gas cloud hypothesis can be ruled out,” she said.
Ghez has been tracking stars in the galactic center for nearly two decades. In fact, her discovery that massive stars orbit a blank spot at the heart of the Milky Way much like planets orbit their sun is the strongest evidence to date that the galactic center contains a 4-million-solar mass black hole. She hypothesizes that G2 might be ultimately become another one of these stars.
The argument goes like this: The stars that Ghez has recorded orbiting around Sagittarius A* are so young that we don’t understand how they got to the galactic center; they should have had trouble forming there. One theory is that each of these stars (called S stars) was once a member of a binary system–a pair of stars orbiting around a single center of mass. When the pair fell into the galactic center, the black hole tore them asunder. “The black hole grabs one and flings the other out,” Ghez said. “You end up with one object that’s bound to the black hole in a very eccentric orbit. G2 has the absurdly high eccentricity that is predicted.” Perhaps, then, G2 is a binary on its way to becoming an S star.
Ghez’s argument continues like so: To explain what we’re seeing at the galactic center, you need an object that has gas at large distances from the central binding object–otherwise, the tidal forces from the black hole wouldn’t be able to shear away the gas. And G2 is definitely being tidally sheared, Ghez said. “Typical stars in that region are sufficiently compact that there should be no tidal interaction. It’s a competition between the black hole and the central object. If the gravity of the black hole wins, that material gets stripped off.” Scientists have proposed several theories that could explain this weird arrangement, Ghez said, but all of them predict that G2 would get brighter as it got closer to the black hole. “That’s not happening,” she said. “Not only do you see it and it’s compact, but it hasn’t changed brightness at all.”
All of this is speculation, Ghez admitted. “We’re trying to come up with a theory that passes Occam’s Razor.”
So far, evidence from other spring observations–at least what’s been reported so far–is ambiguous. Daryl Haggard at Northwestern University is leading this year’s observations of Sagittarius A* with the Chandra x-ray satellite telescope. “We are just not seeing G2 in the x-ray yet,” Haggard said in late March. “If we really don’t detect anything from G2 in the next couple months, that will probably support a stellar interpretation,” Haggard said.
A stellar interpretation–that is, G2 as a star surrounded by gas, rather than a pure dust cloud–still won’t fully solve the mystery of what the object is, and how it will interact with Sagittarius A* in the months and years ahead. Haggard pointed out that even if G2 is a star, it could still unleash x-rays as it plows through the got gases surrounding the black hole. G2 is “a cold little bullet slamming through hot diffuse medium” surrounding Sagittarius A*. That impact could create a shock wave that would accelerate particles until they emit x-rays. “It’s not entirely clear why a star wouldn’t drive one of those shocks,” she said.
Plenty of people will no doubt be disappointed if it turns out we don’t get to see Sagittarius A* shred dust. Ghez won’t be one of them.
“The reason I’m excited about this is that most stars in our galaxy are binaries,” she said. “So far we’ve completely ignored the impact of binaries.” That is a huge blind spot, she said. “It would be like modeling society as if every family unit were composed of one adult. You can’t understand the family unless you understand that there are two people involved.”
Ultimately it won’t matter much what G2 turns out to be–we still get to see something we’ve never seen before. “Our galaxy is the only place where we can understand how black holes interact with their environment,” Ghez said. “This is first time to track something like this going through closest approach.”
A side effect of the novelty is a little bit of mayhem in the astronomy community. “It’s because we’ve never been able to see this before,” she said, “that there’s confusion.”