It Came From the Center of the Galaxy!

University of Chicago

source:University of Chicago

Deep in the heart of the Milky Way lurks an ever-hungry force, consuming planets, stars, and nebulae with voracious appetite. The monster in question is Sagittarius A*, thought to be the location of a massive black hole. Its 25,000 light year distance from Earth and relatively tiny size make SgrA* difficult to observe. A recent study led by MIT astronomer Sheperd Doeleman, published in the Sept. 4 issue of Nature, offers the closest look yet.

Doeleman’s project made use of Very Long Baseline Interferometry, or VLBI. This technique uses multiple telescopes distributed across the planet, which is like using a telescope equal in size to the distance between them. The researchers sought to maximize resolving power by linking telescopes from California, Arizona, and Hawaii, and measuring wavelengths of 1.3mm, the smallest ever attempted. This allowed the scientists to map the size of SgrA* to 37 microarcseconds. To give some idea of the scale, Doeleman said, “it’s like seeing a baseball on the moon with the naked eye.”

But how were the telescopes able to see something from which even light cannot escape? Scientists measured radio, infrared, and x-ray emissions from SgrA*, but these do not originate from the black hole itself. As Alan Marscher, professor of astronomy at Boston University, describes, matter drawn towards the event horizon, the outer limit of the black hole, doesn’t just fall straight in- it spins around like water circling a drain. As it rotates, it loses energy in the form of radiation, which can be picked up by the telescope array.

Mapping SgrA* has bolstered the case for identifying the object at the galactic center as a black hole, given its extremely small size and its extremely large mass of four million solar masses. No other phenomenon is known to have such a high density. There is, Doeleman said, a slight chance that SgrA* could be a more exotic construct known as a Boson star, which is composed of unique particles that exist much closer to each other than in normal stars. The Boson star, however, is only a theoretical possibility, and has yet to be observed.

Furthermore, scientific principles rule out the possibility that SgrA* is an aggregate of matter. According to the laws of physics, “any object that was originally at the center of the galaxy, whether it was a planet, some gravel, or a collection of Hello Kitty dolls, would have collapsed into a black hole after at least 500 years”, said Doeleman.

Future studies will attempt to push the resolving power even further, with telescopes placed even further apart and measuring even smaller wavelengths. Doeleman hopes to eventually make use of the newly constructed ALMA array, a collection of 50 dishes located in Chile. The addition of more telescopes will ultimately allow scientists to construct a real image of a black hole, rather than a mathematical representation.

Still, Marscher mused, there is a point beyond which even the most powerful telescope cannot see. Little is known about the character of what lies beyond the event horizon, he said, and at this point we lack the capability to describe it. “The universe,” he said, “conceals a lot from us.”

Original Article: Doeleman et al.  “Event-horizon-scale structure in the supermassive black hole candidate at the Galactic Centre”.  Nature 455:78-80 (4 September 2008).


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