Astronomers around the world are watching as a mysterious object heads for a close pass around the Milky Way's black hole.
The gaseous snack inbound for our galaxy’s supermassive black hole might be starting to swing around the beast, like a waitress on roller skates leading with her hors d’oeuvres tray. Called G2, the mystery object was discovered in 2011 to be dive-bombing toward the black hole. It appears to have about 3 Earth masses in gas, but whether it’s merely a cloud or a young enshrouded star is fiercely debated.
Astronomers had forecast that G2’s closest approach to the black hole, a point in its orbit called peribothron (from the Greek bothros, a type of sacrificial pit), would be around September 2013. This spring, Kim Phifer and Andrea Ghez (both University of California, Los Angeles) and their colleagues pushed that estimate back to March 2014 after analyzing Keck observations spanning June 2006 through August 2012.
Now Stefan Gillessen (Max Planck Institute for Extraterrestrial Physics, Germany) and his colleagues think they’ve spotted a turnaround. Observations done this April with the Very Large Telescope in Chile show a shift in part of G2’s emission: instead of being entirely redshifted — which means the emitting object is moving en masse away from us — a fraction of the diffuse gas is now blueshifted. That implies that the front-most bit of gas has swung around the black hole and begun moving back toward us along our line of sight.
That doesn’t mean the estimate for peribothron is off, Ghez says. It’s hard to talk about an exact date because while G2’s compact “head” is roughly 100 Earth-Sun distances wide, the diffuse part is stretched across 10 times that distance. Gillessen’s team estimates that only 7% of the cloud had swung around the black hole when they observed in April. The main chunk of gas is therefore still en route.
Astronomers will have to wait until G2’s head swings by the black hole before they know for sure what it is. If it’s just a cloud, the object should be torn apart; if a star hides inside, it should survive. Until then, researchers are at loggerheads.
Whatever G2 is, observers hope that it might nudge some material into the black hole’s maw or even drop some crumbs itself. If so, it could encourage the beast to take a break from its diet and swallow a big meal, possibly triggering a fanfare of flares and activity that could last several years. Teams across the world already have more than three dozen observing proposals approved to follow G2 in as many wavelengths as possible, making G2 one of the most intensive observing campaigns in recent memory.
S. Gillessen et al. "Pericenter passage of the gas cloud G2 in the Galactic Center."
K. Phifer et al. "Keck Observations of the Galactic Center Source G2: Gas Cloud or Star?"