Close encounter with a supermassive black hole
By Brian Dodson
September 23, 2013
As you read this, the eyes of the astrophysical world are focused on about one-trillionth of the sky, watching as the calm existence of G2, a three-Earth mass gas cloud near the galactic center, is viciously disrupted by a close encounter with Sagittarius A*, the galaxy's supermassive black hole. Careful observation of this rare event is expected to provide an enormous amount of information on the environment of the central light month (about 6,000 AU) immediately surrounding the black hole.
The penetration of black holes into popular culture has changed from a trickle into an avalanche. We have heard about micro through ultramassive black holes, Universe-illuminating gamma-ray bursts caused by formation of black holes, wormholes, evaporating black holes, firewalls and more, but the image that is perhaps most prevalent is that of the voracious void in space eating any matter that ventures closely enough to the event horizon.
Astronomers now have a rare chance to actually see what happens when a cloud of gas passes close by a supermassive black hole. In 2011, a small cloud of interstellar gas, named G2 by the discoverers, was found using the Very Large Telescope observatory of the European Science Organization. The rapid motion of G2 is clearly apparent in the figure above, where the blue image marks G2's position in 2006, the green in 2010, and the red in 2013. Located less than a few light-days from Sagittarius A*, the Milky Way's 4 million-solar mass supermassive black hole, G2 has a mass about three times that of the Earth, and is on a near-collision course with Sagittarius A*. The date of closest passage of the cloud is expected in early 2014.
G2 is not falling directly into the black hole. Rather, its highly eccentric orbit will miss the black hole's 24 million km diameter event horizon by about by about triple the diameter of the Solar System. The original size of the cloud is similar to the nominal closest approach, so that some of G2's material is expected to pass close to the black hole, and may be captured therein. X-ray emissions from capture or interaction with the accretion disk of the black hole will be studied by the Chandra, SWIFT and NuSTAR orbital x-ray telescopes.
The character of G2 has been under discussion, with some believing it to be a simple gas cloud, and others that it has a central star providing a gravitational core to the cloud. The recent observations indicate that the black hole's tidal forces have spaghettified G2 from its original size of about 50 billion km to a length some 50 times greater. This observation is not consistent with simulations of G2 as a cloud surrounding a star, so the tentative conclusion is that G2 is simply gas and dust, having no central body to hold it together.
Given that G2 is not held together through gravity, the question of its origin arises. It seems clear, both from simulations and the current partial observations, that G2 could not have survived a previous pass by the black hole on its current orbit. Reinforcing this is the presence of stars orbiting Sagittarius A* which approach as close as 15 billion km. Any significant period of gravitational interaction with these nearby and rapidly moving stars would very quickly destroy a simple gas cloud. The current opinion is that something happened quite recently (perhaps as recently as 1995) to form this cloud, but exactly what remains unclear.
The region immediately surrounding Sagittarius A* is expected to be teeming with small black holes, with possibly as many as 20,000 orbiting within one parsec of the central black hole. Estimates suggest that G2 may encounter several (perhaps more than 10) of these on its trip around Sagittarius A*, and that these encounters may trigger enough accretion of matter onto these small black holes to be detected at Earth by x-ray space telescopes. As yet there appears to be no clear signature of such an encounter, but the day is young, and a great deal of data has not yet been analyzed (or yet collected, for that matter).
The leading edge of G2 has already swung around Sagittarius A*, and is moving back toward us at a speed of about 1 percent of the speed of light. At present, only about 10 percent of G2's material has swung around the black hole. Despite the prediction of a number of phenomena associated with G2 running into other accumulations of gas and dust near Sagittarius A*, preliminary data does not show unambiguous signs of such activity. It may be that the collected data is not yet sufficient to make such a call, or possibly there is considerably less gas and dust than expected surrounding the black hole. This might be related to the remarkable "quietness" of our galactic nucleus. Only a fraction of the expected high-energy emissions are actually generated.
Yet another issue that might be addressed by careful analysis of the G2 encounter is the spin of this supermassive black hole. A black hole with angular momentum (spin) pulls spacetime into an inwardly spiraling pattern, rather like a draining sink. The estimated magnitude of the spin of Sagittarius A* ranges from near zero to nearly the maximum possible for a black hole (roughly the point at which spacetime is spinning at the speed of light at the event horizon of the black hole). The manner in which G2's gas flows near the black hole will be influenced by the gravitational effects of such spin, and we might be able to obtain an independent estimate of the spin magnitude from these observations. The complexity of the flow of the dust and gas of G2 is suggested by the attached video, which shows a Lawrence Livermore 3D simulation of the upcoming encounter.
We are remarkably lucky to get the chance at observing G2's descent into the supermassive black hole at the center of our galaxy. Over the next few years, the accumulated observations of this event should answer a lot of questions about black holes and how they interact with their surroundings.
Source: European Southern Observatory
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