A black hole is like a scary monster from children’s literature. It’s vividly imagined but never actually seen in real life.
This is true even for the largest black holes we know — the ones that reside at the centers of galaxies. The nearest of these lies some 30,000 light-years away, in the core of the Milky Way. If you placed it in our solar system it would probably span the orbit of Mercury. Yet, because of its great distance, it’s a mere speck against the sky, about 36 million times smaller than the full Moon. How could anyone see any detail when looking at something with an apparent size that small?
Amazingly, there is a way. And now it’s promising not only to reveal the giant black hole in our own galaxy, but also a much larger and more active one in the galaxy known as M87 in Virgo.
The nifty trick that puts this ambitious goal within reach is called very long baseline interferometry. VLBI involves two or more radio dishes that are spaced as far apart as possible — for example, in Arizona and Hawaii. The dishes observe the same radio sources in the sky, and when their signals are combined they form an image that’s as sharp as what you would get from a single receiver as big as the separation between the dishes. The idea is to show the way radio emission is spatial distributed across a small region of sky. It’s just what you need to “see” a black hole.
But what does that mean? Aren’t black holes supposed to be, well, black?
Yes and no. If completely isolated in space, a black hole would indeed be well camouflaged. But in the densely populated center of the Milky Way, there is plenty of hot gas swirling around the giant black hole there.
The energized ions in the gas give off radio waves. Seen up close, there should also be a dark sphere at the center of that swirl of gas, where matter funnels in and never comes out. The dark sphere is the infamous event horizon. It’s the point of no return, from which not even light can escape. In this case, “seeing” the black hole means seeing the event horizon silhouetted against the glowing gas.
Shep Doeleman of the Massachusetts Instistute of Technology has been leading the charge to image the Milky Way’s central black hole, and he and his collaborators have made astounding progress. They’ve looked deep into the heart of the radio source known as Sagittarius A*, where the black hole is believed to be lurking. What they’ve got is not exactly an image but a sense that there is some structure on the scale of a supermassive black hole deep in the heart of the radio source.
The complication in mapping out an image of this object is that Sgr A* is changing on a regular basis, presumably as clumps of gaseous matter go a-whirling around the black hole.
This is not going to be a problem when it comes to looking at even bigger black holes, like the giant monster at the center of the galaxy M87 in Virgo. It devours vast quantities of gas and spits out a spectacular jet that extends far into extragalactic space.
Recently, Karl Gebhardt of the University of Texas at Austin and Jens Thomas of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, set about measuring the size of M87 by running existing data through a new model that mimics the galaxy star by star. Unlike earlier efforts, their model also takes into account the unseen halo of dark matter that surround the visible portion of the galaxy. This turned out to have an unexpected affect on the way the model calculates the mass in the stars that illuminate the galaxy’s core. In the final analysis, it allocates far more mass — a whopping 6.4 billion suns — to the galaxy’s monstrous central black hole.
The surprising corollary to this is that M87’s black hole, if viewed from Earth, should be the same apparent size as the black hole in Sgr A* — just as the Sun and the Moon appear roughly the same size, though the Sun is larger and much farther way. That puts M87’s black hole within reach of Doeleman’s radio telescopes. It may even be easier to image than Sgr A* because it’s larger size means it doesn’t change nearly as rapidly.
What’s particularly exciting to theorists like Avery Broderick, of the Canadian Institute for Theoretical Astrophysics in Toronto, is that M87’s black hole is also violently active, with a vast disk of gas around it and a big jet of shooting out in one direction. A radio image of this black hole might not only reveal the event horizon but show us the region where the jet is launched.
“It’s kind of exciting as an alternate object because it is so different from the supermassive black hole in our backyard,” says Avery. “Between them, they span the range of what we expect from these objects.”
Stay tuned. Modern astronomy is increasingly become the science in which the unseen becomes seeable. Before long we’ll be adding black holes to that list.
Ivan Semeniuk is host of the podcast The Universe in Mind and a science journalist in residence at the Dunlap Institute for Astronomy and Astrophysics, University of Toronto.