Across the universe there are trillions of black holes—objects so compact that their gravitational pull prevents anything from escaping, even light. Some of them formerly blazed brilliantly as stars before collapsing catastrophically. Others, far more massive and ancient, sit at the center of galaxies, including our own. When they draw in material from their surroundings, such monster black holes can shoot out jets of energetic particles that stretch for hundreds of thousands of light-years, as in the galaxy Hercules A.
Theoretical predictions about the existence of black holes date back nearly a century, to Albert Einstein’s general theory of relativity. In 1915 one of Einstein’s colleagues, Karl Schwartzchild, interpreted the relativity equations and showed that a sufficiently massive object could curve space and time in on itself, cutting itself off from the rest of the universe. Even Einstein had a hard time imagining such a possibility. It took another half-century before scientists started accepting that black holes exist, and only recently have they grasped the full cosmological importance of these extreme objects.
Because black holes, by definition, cannot emit any light, researchers such as myself are forced to study them indirectly. Much of our knowledge comes from observing gas as it is pulled into or ejected from the black hole. But we really want to know what is happening right at the event horizon, the outer boundary that defines a black hole’s point of no return. That is the goal of the Event Horizon Telescope, which links together radio antennas around the world to create, in effect, a new telescope the size of the Earth. Even in its preliminary form it can resolve details as small as 60 microarcseconds, equivalent to seeing a baseball on the surface of the Moon.
The next year will be particularly exciting for studying black holes: A cloud of gas is nearing our galaxy’s black hole, called Sagittarius A*, and may interact strongly with it. Using the Event Horizon Telescope and other instruments, astronomers will be able to see directly how a black hole gets fed. Stay tuned for the results, which will certainly be groundbreaking—or maybe universe-breaking.
Sagittarius A*: The Beast in Our Back Yard
Sagittarius A*, the massive black hole at the heart of the Milky Way, is an oddly quiescent beast. Stars orbit around it, but not much matter reaches the hole itself. It is so subdued that it is almost always invisible in optical images or x-ray maps of the center of our galaxy (below). Astronomers only became aware of Sagittarius A* in 1974 from its radio emissions.
It is in many ways easier to study smaller black holes, weighing just a few times as much as the Sun. In contrast to monsters like Sagittarius A*, whose formation was probably intertwined with the birth of our galaxy, the more modest black holes began their lives as bright stars but then exploded and collapsed. When matter gets close to one of these stellar-mass black holes, it settles into a flattened formation called an accretion disk. The extreme gravity near the black hole then causes the gas to heat up by tens of millions of degrees Celsius and emit x-rays. If the black hole has a nearby companion star, the star can get shredded and slowly consumed, creating a brilliant x-ray beacon. Around fifty such systems have been found so far. A schematic illustration (below) shows the key features of one of these x-ray binaries, called XTE J1550-564. It turns out to share basic traits with other, far more massive active black holes.
Many active black holes also shoot off supersonic jets of particles above and below the accretion disk. The jets produce copious amounts of radio emissions because they contain high-speed particles and strong magnetic fields. When you combine the two, you get synchrotron radiation (produced when charged particles are accelerated in the presence of a magnetic field) whose intensity peaks at radio frequencies. Exactly how the jets are produced, or even what they are made of, remains a mystery, so they are the focus of intensive study.
Sagittarius A* does not make itself seen in these ways. Much of what astronomers know about it comes from tracking the movement of nearby stars (above). The dots in the image show the shifting position of stars at the galactic center over the past 20 years; the lines show their reconstructed trajectories. The map also shows G2, the gas cloud currently plunging toward the black hole. The motions of the stars reveal they are orbiting around a compact object that weighs 4.3 million solar masses. Only a black hole could invisibly contain so much matter in such a small space. Einstein’s equations of relativity indicate that Sagittarius A* has an event horizon 25 million kilometers across.
Centaurus A and M87: Black Holes Blasting Out Huge Jets
The nearest active supermassive black hole resides in Centaurus A, a galaxy less than 13 million light-years away—just five times farther than our neighboring Andromeda galaxy. The dynamics of the gas and stars near the central regions reveal that this black hole weighs 50 million solar masses, more than 10 times as massive as Sagittarius A*.
In visible light, Centaurus A looks like a roundish galaxy with an unusual dust lane going through its center (above). If we examine the same galaxy in radio waves instead of light, however, we see something completely different: two powerful jets shooting out from the center (below).
Looking at x-rays from Centaurus A provides yet another perspective. The jets light up, but more important, at the center there is a bright pointlike x-ray source, which coincides with the location of the supermassive black hole (below).
A combined view (below) beautifully shows all of these components together. What is truly shocking is that the black hole is almost a billion times smaller than the galaxy, yet produces jets that extend past the visible edge of Centaurus A.
More clues about supermassive black holes come from M87, a giant elliptical galaxy 50 million light-years away, at the center of the nearby Virgo cluster of galaxies. Like Centaurus A, M87 looks fairly normal (below), but it contains one of the biggest black holes known, with a mass several billion times that of the Sun.
This black hole in M87 is highly active, as can be seen in a dramatic composite image (below) of radio emissions shown in red, and x-rays, shown in blue. Notice the mushroom shapes at the top of the image: These are similar to the mushroom clouds seen in nuclear explosions.
Bigger galaxies systematically have more massive central black holes (below). And the more massive the black hole, the larger its event horizon. Interestingly, the “size” of a black hole scales directly with its mass: Double its mass, and its radius doubles as well. The event horizon for M87’s black hole is as large as our entire solar system. Because M87 harbors one of the most massive black holes known and is relatively nearby, the apparent size of its black hole is just large enough that the Event Horizon Telescope has been able to image the base of its jet. (The only other black hole that appears larger in the sky is Sagittarius A*.) The latest studies indicate that the jet in M87, which extends far beyond the edge of the host galaxy, originates from a region no larger than six times the size of the black hole!
The Biggest Black Holes in the Universe
Some 20 years ago, astronomers made a fascinating discovery: The more massive the galaxy, the more massive its central black hole. The biggest galaxies lie at the centers of clusters of galaxies, like M87. In this setting, supermassive black holes such as the one in M87 can affect not just their own galaxies but the cluster as a whole.
Here we see an x-ray view (above) of a large grouping of galaxies called the Perseus Cluster. Such clusters contain large amounts of gas between their galaxies—and because the cluster is so massive, this gas gets compressed and heated enough to glow in x-rays, similar to what happens in accretion disks. This image shows that the x-ray-emitting gas in the Perseus Cluster has a lot of structure: There is a central point source—which coincides with the supermassive black hole of the cluster’s dominant central galaxy, NGC 1275—but there are also peculiar “bubbles.”
To make sense of these structures, let’s look at the Perseus Cluster a different way (above). This composite is a combination of a visible-light image highlighting the galaxies (white), an x-ray image (violet), and a radio image (pink). Note that the dark bubbles in the x-ray image are largely filled with bright emission in the radio image. It seems that the supermassive black hole at the center of NGC 1275 has the strength to push away the x-ray-emitting gas that fills the cluster. We are witnessing the immense power black holes can have on their surrounding medium, influencing the evolution of galaxies and entire clusters of galaxies.
Another galaxy cluster, known as MS 0735.6+7421 and located 2.8 billion light-years away, shows features on an even larger scale (above). The bubbles in MS 0735.6+7421 are more than 10 times the size of the bubbles in the Perseus Cluster. Their size points to the presence of an ultramassive black hole in the central galaxy. But how massive? About 10 years ago astronomers discovered a new way to find out: No matter the mass of a black hole, if it is accreting at low rates, its ratio of x-ray to radio emission depends solely on its mass (see graph below)
This discovery means that supermassive black holes are simply scaled-up version of stellar-mass ones. By measuring this radiation ratio, astronomers estimate that the black holes in MS 0735.6+7421 (and in many other similar clusters of galaxies) must weigh up to several tens of billions of solar masses. These are the most massive black holes in the universe.
Discoveries such as these are what make me so passionate about astronomy. Modern technology has pushed our understanding of the universe to a level that we never thought possible a couple of decades ago. Black holes exist, and the more we study them, the more they surprise us.