Astrophile: A-List black hole gets a face

Like the real thing (Image: Dexter, McKinney and Agol)

Astrophile is our weekly column on curious cosmic objects, from the solar system to the far reaches of the multiverse

Object type: Black hole
Location: M87 galaxy, 50 million light years from Earth

Being photographed is pretty much a way of life for human celebrities. Not so astronomical ones. The most massive black hole ever measured, which lies at the heart of the galaxy M87, 50 million light years from Earth, is the closest thing we have to a celestial bigwig. Yet no picture of it, or any other black hole, has ever been snapped.

That's not just a problem for fans lusting after a pin-up. Seeing the shadow of a black hole would provide the first direct evidence that these bizarre objects really do exist. What's more, looking at the way light bends around the edges of the shadow could also turn up deviations from Einstein's theory of general relativity, the reigning theory of gravity that some physicists want to replace.

The trouble is not that black holes are invisible: as great absorbers of light, they should appear as a black spot when viewed against a bright background. Rather, all known black holes are too far away for ordinary telescopes to make out the shadow. However, plans are afoot to take a picture of a black hole using radio telescopes set far apart on Earth but which together act like a single huge telescope thousands of kilometres across.

In the meantime, Jason Dexter of the University of California in Berkeley and colleagues have created the next best thing – the most realistic preview yet of what the black hole at the heart of M87 looks like. As well as putting a face on this A-list object, their simulations reveal details that suggest glimpsing M87's black hole for real may well be feasible.

Smeared jets

The team simulated how matter and light behave near M87's black hole. Unlike previous efforts, their calculations fully incorporated general relativity and the effects of powerful magnetic fields near the black hole.

They simulated the disc of gas and dust that is swirling around it as well as the powerful jet of electrically charged particles shooting into space from the black hole's vicinity.

Both the disc and jet emit radio waves. But rather than travelling in straight lines, the radio waves are bent by the black hole's powerful gravity, which acts a little bit like a lens.

This would radically distort the appearance of the disc and jet in radio images, the researchers found, smearing them to make a bright crescent surrounding the dark shadow of the black hole.

In a real image, the existence of this shadow would provide direct evidence for an event horizon, the defining feature of a black hole. Once light, or anything else, passes inside this boundary, it can never escape.

Target: M87

The simulations suggest the black hole's shadow should be observable with telescopes that researchers are planning to link up for this purpose. The researchers concluded this from calculations of the shadow's size and the size of the smallest details discernable by such linked telescopes.

With a mass that was recently revised upwards to 6.4 billion times that of the sun, M87's black hole seems like an ideal photographic target, because its shadow should look relatively large in the sky.

Though the black hole at the heart of our own galaxy would appear slightly larger because it is so much closer, M87's position in the sky means it is easier to observe using some of the best radio telescopes. Its black hole could well be the first to be seen directly.

These images will be the next best thing to actually travelling across the event horizon into a black hole – an experience that has been simulated but will remain forever in the realms of science fiction.

Journal reference: Monthly Notices of the Royal Astronomical Society, DOI: 10.1111/j.1365-2966.2012.20409.x

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