The galaxy NGC 5972 is wreathed in glowing green bands of ionized oxygen

Green, glowing whorls of ionized oxygen around the galaxy NGC 5972 may be due to a supermassive black hole's ancient feeding frenzy. Credit: NASA, ESA, and W. Keel (University of Alabama, Tuscaloosa)

Green as a color can mean animal, vegetable or mineral. It is the stuff of crocodiles, chlorophyll and copper patina, the essence of serpentine or of snakes in the grass, the hue of a glacial lake, a stagnant pond and the Chicago River on St. Patrick’s Day. Green seems to be everywhere you look—everywhere, that is, but up. As ubiquitous as it is on Earth, green is almost entirely absent from the heavens.

There are exceptions. At high latitudes, green occasionally graces Earth’s sky in undulating bands of aurorae, produced by oxygen atoms tickled by charged particles from the sun. Of the planets, only Uranus could be called green, thanks to the red-absorbing methane in its atmosphere, but its true color is closer to turquoise. Comets can look green when their carbon bakes in ultraviolet sunlight, and the setting sun can spark a vivid green flash when its light refracts through the thickest part of Earth’s atmosphere. But green is not a color for the stars, which, due to quantum mechanics and quirks of human vision, we only perceive from coolest to hottest as red, orange, yellow, white, or light blue. No star will ever shine so green that its viridescence can be seen, and so astronomers had long assumed that the galaxies where stars live wouldn’t be green, either.

Which is why Hanny van Arkel’s discoveries in 2007 were so deeply weird. Van Arkel, a young Dutch schoolteacher, was a volunteer sifting through images from the Sloan Digital Sky Survey, classifying galaxies based on their color and shape for the citizen-science project, Galaxy Zoo. Spiral galaxies, cosmic sites of star formation, shined blue from all the bright, hot, young suns they contained. Elliptical galaxies, made of colder, older stars, shined yellow and red. But here and there, van Arkel found little green dots that defied categorization. She called them “Green Peas,” and notified other Galaxy Zoo users about them in an online forum.

Within months, Galaxy Zoo volunteers had discovered more than a hundred green blobs in the Sloan data. Soon, some of the world’s most powerful telescopes were scouring the skies looking for more. Spectroscopic observations showed that the green color van Arkel and other Galaxy Zoo volunteers were seeing came from ionized oxygen, the same source of the verdant aurorae that wash over Earth’s skies. But ionizing an entire galaxy’s worth of oxygen is not an easy thing to do. Astronomers determined that some of the green blobs were a new class of compact, highly luminous galaxies – “Green Pea galaxies” – where intense bouts of star formation created pan-galactic oxygen-ionizing baths of ultraviolet radiation.

In honor of van Arkel, astronomers began calling the first object she found “Hanny’s Voorwerp,” Dutch for “Hanny’s Object.” However, Hanny’s Voorwerp proved to not be a Green Pea galaxy—it was something much stranger. Rather than being compact and filled with stars, Hanny’s Voorwerp is a spread-out, largely star-free cloud of gas, ionized from outside rather than from within. Something more exotic than star formation was causing its green glow.

Previous observations have shown that Hanny’s Voorwerp and many other similar “Voorwerpjes” (“small objects”) are in the outskirts of merging pairs of galaxies that harbor active, monstrously energetic supermassive black holes at their cores. Now, new data from the Hubble Space Telescope as well as from ground-based observatories suggest that those supermassive black holes, each with a mass of hundreds of millions of suns, are the likely power plants for the Voorwerpjes. The findings will be published in The Astronomical Journal. (View all the new Hubble images here.)

The new Hubble observations reveal new details for eight carefully selected Voorwerpjes. They show that the green, ionized oxygen is looped around the outer edges of the luminous galaxies, more than 30,000 light-years from their centers, in wispy, tangled braids stretched more than 10,000 light-years long by tidal forces. These tidal streams of oxygen and other light elements are signs of long-ago galactic cannibalism, when one galaxy tore apart and engulfed another. The lead author of the paper, astronomer Bill Keel of the University of Alabama in Tuscaloosa, speculates that the green glows of the Voorwerpjes are literally echoes of immense burps after each extended meal.

In each case, the echo is in light, not in sound: When two galaxies containing supermassive black holes merge, the two black holes can end up orbiting each other, and each can be fed by huge avalanches of material poured onto them by the merger. Infalling material is heated by friction as it piles up in a spinning disk around one or both black holes, becoming so hot that it creates intense beams of hard radiation that shine like searchlights out into the wider galaxy. Traveling at the speed of light, the ultraviolet rays in those beams will take tens of thousands of years to reach the edge of the merged galaxies, where they will ionize the streamers of oxygen, illuminating them like galaxy-spanning television screens. By the time that light reaches the oxygen in the tidal streams, the outburst that produced it will be long over, perhaps quelled by interactions between superheated disks surrounding the spiraling supermassive black holes. But the ionized oxygen will record the event with an afterglow that lasts thousands of years.

As long as these timescales seem, they are very short for interacting galaxies and feasting supermassive black holes. Hanny’s Voorwerp and the other Voorwerpjes thus may be among the best probes available for transient phenomena taking place within the active, energetic cores of merging galaxies. And, if they are in fact fueled by binary pairs of supermassive black holes, we may eventually have a front-row seat for the fireworks. The Milky Way harbors a supermassive black hole, just like our nearest neighboring island universe, the great spiral galaxy of Andromeda, and the two galaxies are set to collide in about four billion years. After the merger, the newly combined galaxies—Milkomeda—could be wreathed in their own ghostly, echoing glow. For a brief cosmic moment, a stargazer in that far future might see a memory of that epochal merger, a night sky swathed in green.