Deep in the heart of every large galaxy is a supermassive black hole. The lightest of those monsters weigh in at hundreds of thousands of solar masses (M); the heaviest tip the scales at more than 10 billion M. For most of their existence, they simply sit there, each quietly exerting a gravitational force on the stars and gas that safely orbit far beyond its Schwarzschild radius. That quiescent lifestyle belies the fact that in a cosmic heartbeat, the vicinity of one of those monsters can brighten into the most luminous object in the universe. For a while—a million to a hundred million years—those galactic cores can shine at a trillion solar luminositiesbefore blinking out. While they shine, they take on a new name—quasars—and when they go quiet, all they leave behind is a remnant of light. Hanny’s Voorwerp, a green cloud of ionized gas found 45 000 to 70 000 light-years from the galaxy IC 2497, is the first detected light echo from a now-dormant quasar.

Hanny van Arkel, a young schoolteacher in the Netherlands, discovered the voorwerp (Dutch for“object”) in 2007 while she classified galaxies as part of the Galaxy Zoo citizen science project. (You can see the image that caught her eye at http://www.galaxyzooforum.org/index.php?topic=3802.0.) Galaxy Zoo was originally designed to allow the public to help scientists classify the shapes of nearly 1 million galaxies in the Sloan Digital Sky Survey. Like many good science tools, it was able to do far more than it was designed to do, and the voorwerp was just one of its many serendipitous results. Today, Galaxy Zoo asks people to help astronomers study galaxies observed with the Hubble Space Telescope. One of the new science topics it will enable is the study of active galaxies—those whose central black holes are consuming matter—and the voorwerpjes (“small objects”)they occasionally produce.

Once upon a time, 400 000 years after the Big Bang, the universe was boring. Consisting of neutral hydrogen and helium mixed with a tiny bit of lithium and beryllium, space was filled with a distribution of gas so consistently consistent that the largest variations were just 1 part in 105. Although that doesn’t seem like a lot of variation, it was enough to allow gravity to concentrate the slightly denser regions into much denser regions. In those gravitationally contracting landscapes, stars lit up and supernovae went off. (See the article by Tom Abel in PHYSICS TODAY, April 2011, page 51.) Galaxies and protogalaxies began to grow, and in many but not all cases, they incorporated black holes at their centers.

Collisions frequently occurred as all that structure formed in the still-small cosmos. When the young, gas-rich systems merged, they formed larger galaxies. During the merger some scattered gas and dust collapsed into star-forming regions while other material was driven into the gravitational core of the merging galaxies. Conservation of angular momentum prevented the infalling material from descending directly onto the giant (and becoming supermassive) central black hole. Instead, it piled up in a dense disk, called an accretiondisk, that spiraled around the black hole. The density in the accretion disk built up until temperatures and pressures there began to resemble those in stellar atmospheres. At the same time, the black hole tore at the material on the inner edge of the disk, slowly devouring it. The meal consumed 10–1000 M per year and, in combination with the density buildup, typically released 1040 W of power—on the order of 1013 solar luminosities. Those systems were quasars, the brightest of all active-galaxy cores.

The golden age of quasarswas in the distant past, 2 billion to 3 billion years after the Big Bang. Though some quasars did form late and linger into more recent times, with each passing eon they have become increasingly rare. Now, 13.6 billion years after the Big Bang, we see no quasars nearby.

Those giant monsters brought on their own death. As they fed on gas and dust, they grew so bright that the pressure of their light overcame the black hole’s gravitational pull on the infalling material. With the light clearing out the central region around each quasar, the accretion disks stopped growing and eventually got completely gobbled up.

Occasionally, as massive galaxies interact, the central supermassive black holes are given a new chance to shine. As we have seen, galaxy mergers can drive gas and dust into the galactic cores. But, in all likelihood, so can galaxy harassment, a less catastrophic process by which the close passage of two galaxies disrupts the galaxies’ gas and dust while leaving the overall structures mostly undistorted. While often at lower luminosities than quasars—and then called active galactic nuclei—the reignited systems obey the same physics as quasars. They shine brightly because a black hole is feeding.

The galaxy IC 2497 appears to be harassed. When it is viewed in radio frequencies, long streams of stripped-off gas stretch away from the visible disk of IC 2497 and reach toward a nearby companion galaxy. The observation suggests that in the not too distant past, the two galaxies experienced a close flyby. As shown in the figure, the companion tore out a stream of material—some of which was destined to shine as the voorwerp.

Genesis of a voorwerp. (a) The process culminating in Hanny’s Voorwerp begins when the spiral galaxy IC 2497 (bigger and brighter in the figure) nearly collides with another galaxy. (b) The gravitational interaction between the two galaxies pulls a large “tidal” tail of gas out from IC 2497. Meanwhile, the black hole at the center of the galaxy feeds on nearby galactic material. (c) Eventually, the vicinity of the black hole shines as a quasar and emits a powerful beam of light that ionizes a portion of the tidal tail. The glowing, ionized tail gas is Hanny’s Voorwerp. (Images courtesy of NASA.)

Genesis of a voorwerp. (a) The process culminating in Hanny’s Voorwerp begins when the spiral galaxy IC 2497 (bigger and brighter in the figure) nearly collides with another galaxy. (b) The gravitational interaction between the two galaxies pulls a large “tidal” tail of gas out from IC 2497. Meanwhile, the black hole at the center of the galaxy feeds on nearby galactic material. (c) Eventually, the vicinity of the black hole shines as a quasar and emits a powerful beam of light that ionizes a portion of the tidal tail. The glowing, ionized tail gas is Hanny’s Voorwerp. (Images courtesy of NASA.)

Close modal

High luminosity isn’t the only characteristic of a quasar;jets also emanate from the feeding monster. As the accretion disk material spirals in toward its death, a magnetic field is generated that points away from the disk. And just as the field of a common magnet can move small objects, the magnetic field associated with a quasar has the ability to drive material through space. The black hole and accretion disk thus spew out a small fraction of the infalling material as collimated jets of plasma held together by magnetic fields. Put concisely, black holes are messy eaters.

Not all quasars have jets, but all galactic jets are associated with a feeding black hole. A radio view of IC 2497 shows not only stripped gas but also clear evidence of a jet reaching from the galacticcenter toward the green, ionized voorwerp. Such a jet would normally indicate that IC 2497 has a quasar. The green light results from a forbidden transition of O2+ at a wavelength of 5007 Å (this forbidden spectral line is often written [OIII]), combined with light from forbidden transitions of He+ ([HeII]) and Ne4+ ([NeV]). The particular elements observed and the narrowness of the emission lines all add up to indicate that the voorwerp’s gas is being photoionized by the type of high-energy continuum radiation that would be associated with a quasar in IC 2497.

Radio observations paint IC 2497 as an active galaxy, but x-ray observations tell an entirely different story. In that higher-energy radiation, the jets disappear. In fact, the x-ray radiation from IC 2497 is no more than would be expected from a generic extragalactic source. That result initially baffled team scientists, led by William Keel, Chris Lintott, and Kevin Schawinski. There is no easy way to explain the presence of radio emission and emission lines from high-energy continuum light in the absence of x-ray emission. Or at least none invoking commonly observed phenomena. The team’s explanation, which suggested a never-before-seen phenomenon, was so amazing that they had to go to great lengths to publish their results. It appears that Hanny discovered the remnant light of a now quiet quasar. In that scenario, the radio jet is a detached beam, still traveling away from its now dramatically faded source, and the emission lines are a light echo of leftover high-energy continuum photons that are still traveling through space.

The picture isn’t easy to imagine. The separation between IC 2497 and the voorwerp is 45 000 to 70 000 light-years, and the beam that emanated from IC 2497 is large enough that it lights up the entirety of the object. So, if the last x-ray photon to leave IC 2497 is illuminating the voorwerp right now, then the quasarturned off no more than 70 000 or so years ago. Over cosmic time, we’ll see the voorwerp fade away,with the side closest to IC 2497 disappearing first as the beam finishes passing through the cloud of gas.

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Pamela Gay is an assistant research professor at Southern Illinois University Edwardsville. As one of her projects in using new media to engage the public in science and technology, she and Fraser Cain host the Astronomy Cast podcast.