WASHINGTON, D.C.—In 1987, astronomers spotted a spectacular supernova in the Large Magellanic Cloud, a dwarf galaxy 168,000 light-years away. Called SN 1987A, it was the closest supernova seen since the 17th century and shone brightly enough to be seen with the naked eye. Now, observing it through the world’s most sophisticated radio telescope, astronomers have discovered a massive cloud of dust in the heart of the supernova remnant. The finding, presented here Monday at the 223rd annual meeting of the American Astronomical Society (AAS), suggests that such supernova explosions may have supplied the huge volumes of dust required for the development of galaxies in the early universe.
Dust may sound dull, but for astronomers it's pretty important. Not only is dust scattered throughout the diffuse interstellar medium, but it also swarms within molecular gas clouds. And dust in gas clouds helps cool them down enough to collapse into stars. So if the universe didn’t have so much dust, it would have a harder time making stars and galaxies. Stars themselves spew out some dust, but early in cosmic history, when the universe was just 500 million to a billion years old, there were too few stars around to have supplied very much of it. Yet there had to have been enough dust to facilitate the birthing and proliferation of galaxies that began not long after. Where did it all come from?
Possibly supernovas, say Rémy Indebetouw, an astronomer with the National Radio Astronomy Observatory and the University of Virginia in Charlottesville, and colleagues. When they trained the newly commissioned Atacama Large Millimeter/submillimeter Array (ALMA) in Chile on 1987A’s remnant, ALMA enabled them to glimpse cold regions of the remnant that telescopes tuned to shorter visible and infrared wavelengths could not see.
Optical and x-ray images of 1987A show the shock wave from the event, which has been expanding outward from the center ever since the explosion occurred. The dust that ALMA imaged in the remnant lies at the center of this expanding ring. The telescope’s high resolution enabled the researchers to pin down the location of the cold dust with such confidence that they were convinced that the dust had formed in the aftermath of the explosion. “Since it’s so well within the ring, it had to have been made there,” Indebetouw says. “This is the first time that we have imaged copious amounts of dust unambiguously formed inside a supernova.” The researchers estimate that the total load of newly formed dust in the remnant is about 25% as massive as our sun.
The finding excites Indebetouw and others because it supports a theory that supernovas can help explain how the universe got so dusty. Some of the dust in the heart of 1987A will probably blow out of the remnant and into the surrounding space, Indebetouw says. “It’s hard to predict exactly how much,” he says. A few percent would be too little to clear up the early universe’s dust mystery, but “if at least a third of it makes it out, then we’re in good shape.”
Robert Kirshner, an astrophysicist at Harvard University who was one of the first to study 1987A in detail, is impressed with the finding. Although it's not surprising to see that heavy elements get made in the explosion, he says, "it's amazing that they get cold enough to form dust." He adds: "The big picture is that these solids become the stuff for planets, and—after a lot we don't understand yet—astronomers."
*Correction, 8 January, 10:23 a.m.: This item originally stated that SN 1987A was the closest supernova seen since the 16th century. It was, in fact, the 17th century. Thanks to one of our readers for pointing this out.