Scientists around the world are tracking a doomed cloud of gas as it makes a daring approach toward the monster black hole at the center of our Milky Way galaxy, a cosmic encounter that might reveal new secrets on how such supermassive black holes evolve.
The G2 space cloud, and its ultimate death by black hole, have been under close scrutiny since the cloud's fate was first identified in 2011. Now, the cloud is destined to be shredded by a supermassive black hole.
For scientists Stefan Gillessen and Daryl Haggard, the excitement is mounting over the impending death of G2.
"We get to watch it unfolding in a human lifetime, which is very unusual and very exciting," said Haggard, a researcher at Northwestern University in Illinois, during a presentation this month at the American Physical Society in Savannah, Georgia. Gillessen is a researcher with the Max Planck Institute for Extraterrestrial Physics in Germany.
Milky Way's black hole heart
The supermassive black hole at the center of the Milky Way is known as Sagittarius A* (pronounced "Sagittarius A-star" and known as Sgr A*). It is 4 million times as massive as the sun and visible only by its effects on the surrounding stars.
In 2011, Gillessen and a team of astronomers found that a small gas cloud with a mass roughly three times that of Earth is on a collision course with the black hole in the Milky Way's core. Scientists quickly determined that the cloud would begin to interact with Sgr A* around the end of March 2014 — one month ago — and have been monitoring it continuously in various wavelengths.
Gas Cloud G2 Position-Velocity Diagrams 2004-2013
Pin It Series of position-velocity diagrams from 2004 to 2013, which were scaled to identical peak luminosities. Over time, the gas cloud becomes increasingly stretched, as can clearly be seen, due to the gravitational shear of the black hole. Image released July 16, 2013.
Credit: MPEView full size image
A space cloud's doom
At its closest, G2 will pass the Sgr A* black hole at a range about 150 times the distance from Earth to the sun. (The Earth-sun distance is about 93 million miles, or 150 million kilometers). If Sgr A* were located where the sun is, the G2 cloud would be located within the boundaries of the solar system.
"There's a lot of action in a small space," Haggard said.
Haggard serves as the principal investigator on a project that monitors the interaction using NASA's Chandra X-ray Observatory and the NRAO's Very Large Array, combining the X-ray and radio wavelengths to learn more about the days ahead. Gillessen keeps the European Southern Observatory's Very Large Telescope tuned to the heart of the Milky Way.
"The object is being accelerated — it's getting faster and faster," Gillessen said.
Already, parts of the cloud have begun to shift. Turbulence and the tidal forces of the black hole combine to mix up the gas cloud as it approaches the black hole.
"It looks like a drop of milk in your morning coffee," Gillessen added.
The front of the cloud has begun to move faster than the back as gravity affects the region closer to the black hole. Gillessen compared it to a train whose back was moving slower than the front—"not very healthy," he said.
But while some wavelengths have begun to show the effects, G2 has remained silent in the X-ray regions observed by Chandra.
"So far in the X-ray, there is no sign of the G2 interaction," Haggard said. "We're hoping that will change in time."
Sgr A* itself hasn't been quiet; in 2013, NASA's Swift Gamma-Ray Burst mission detected the brightest flare ever observed from the black hole. According to Haggard, the flare is probably not connected to G2, but it's possible that as the gas cloud gets shredded, it could potentially give rise to similar flares, though from a greater distance.
A proud lion or a growing child?
As the gas cloud interacts with Sgr A*, Haggard expressed hope that it would help scientists to understand how black holes grow so large. While supermassive black holes can reach masses millions of times that of the average star, other black holes can pack a single stellar mass into a small space.
Scientists know that supermassive black holes grow primarily by accreting stars, gas and dust, but they are uncertain as to how often the behemoths require feeding.
