Discovery of Cloud of Antimatter Alters Ideas on Galactic Formation
The surprising discovery of a large cloud of antimatter hovering above the spiral disk of the Milky Way is forcing astronomers to refashion their theories of how our galaxy evolved, researchers from the Naval Research Laboratory and Northwestern University announced Monday in Virginia.
In the not-so-distant past, these astronomers believe, our galactic environment was a far more violent neighborhood than previously thought, with enormous stars bursting into being, flaming out quickly, then blowing up in supernova explosions--scattering antimatter ashes into interstellar space.
Astronomers at a symposium in Williamsburg were hailing the finding as “the biggest astronomy story of the year,” according to Sonoma State astronomer and press spokeswoman Lynn Cominsky. “We never expected to see anything like this.”
One of the cloud’s discoverers, astronomer Charles Dermer of the Naval Research Laboratory, said the discovery made him feel like a 16th century explorer. “There’s this huge stream out there, like the mighty Mississippi, right in the center of the galaxy, and no one knew it was there.”
“The picture of what’s going on inside our galaxy needs to be revised,” said Northwestern astrophysicist William Purcell, another member of the group. “We can’t explain why the [antimatter particles] are there.”
Scientists aren’t sure what’s creating the antimatter cloud. The center of the galaxy is a very hard place to see, obscured--from Earth’s point of view--by thick clouds of gas and dust. “We know something’s out there,” said James Kurfess, an astronomer with the Naval Research Lab. “But we’re not going to find out [exactly what it is] any time soon.’
The form of antimatter discovered is not the sort that could form antimatter planets or people. Rather, the particles are the antimatter counterparts of familiar electrons, and materialize out of energy at the edges of black holes or during explosions of dying stars, according to Einstein’s famous formula for turning energy into mass, and vice versa (E=MC2). These antimatter electrons, called positrons, and electrons are always created in pairs. When the two kinds of particles meet, they immediately annihilate, effervescing into energy.
Interstellar space is so rarefied, however, that positrons spewed out by exploding stars could hang around for millions of years before colliding with an electron, said Kurfess, one of the researchers who mapped the cloud. And clouds of positrons are thought to be common in the plane of the galaxy, which is densely populated by stars, exploding stars and probably black holes.
But no one expected to find such a cloud 3,000 light years above the galaxy’s center, where nothing much is thought to go on.
“It’s quite a surprise,” Kurfess said. “What does it mean? What created it?”
Dermer believes that the persisting cloud must be continually replenished by a fountain of matter shooting up from the center of the galaxy--about 25,000 light years from Earth. If the antimatter were simply floating alone in space, it would not annihilate and send out the distinctive gamma ray signal. Antimatter has to hit normal matter to be visible.
“It [the antimatter] has to have a target,” Dermer said. “I think it’s a continuous fountain of hot gas.” Over the past million years, he said, matter ejected by exploding stars has built up enormous pressure near the center of our galaxy, like steam rising in a closed pot. “When you remove the lid, it shoots out,” Dermer said, creating what he calls an “annihilation fountain.”
The antimatter cloud was detected by the Compton Gamma Ray Observatory, launched in 1991. One of the observatory’s primary missions was to map the origin of the gamma ray signals, which were first detected 20 years ago. As its name suggests, the orbiting observatory detects gamma rays--the highest energy form of light, typically released by the most violent events in the cosmos.
The gamma ray beacons produced by matter and antimatter annihilation have a very specific frequency--like a bell ringing with a pure tone. Until now, astronomers believed the signals were coming from a huge black hole near the center of the Milky Way known as the Great Annihilator because it appears to be gobbling up stars and gas around it at a prodigious rate.
But the new map, created by an instrument aboard the observatory called Oriented Scintillation Spectrometer Experiment, shows that the signals are coming from a diffuse cloud rather than a point source. What’s more, the cloud is about eight degrees removed from the center of the galaxy--suggesting that the radiation is not coming from a black hole after all.
The successful mapping of the cloud, Purcell said, heralds the dawn of a new way of seeing inside the galactic center. “We’ve developed a new technique for using this instrument,” Purcell said. “It’s not something that was originally planned.”
Gamma rays are much too strong to be focused by lenses or reflected by mirrors. So the detector makes a map by moving from one point in the sky to another, taking time exposures, then moving onto another spot.
Astronomers still believe that a massive black hole sits at the center of our galaxy, based on the way stars are pulled around the center by the huge gravitational force of the invisible object, thought to be as massive as a million suns. But that central black hole “is not sucking up a lot of matter,” Kurfess said. If it were, he said, “We’d have millions of times more gamma rays.”
The Great Annihilator, which is thought to be another, much less massive black hole, is sucking in matter, but is apparently not the source of the gamma ray signal.
If the antimatter cloud was, in fact, created in the violent pressure-cooker environment envisioned by Dermer, it would mean that our own galaxy is a lot like other, so-called “starburst” galaxies. These previously discovered galaxies harbor regions of intense star birth, quite unlike normal, more sedate, stellar nurseries.
A typical starburst galaxy, said Dermer, is “a caldron of activity. Everything’s whipping around. Star birth is disrupted. The only [stars] that survive are very massive; they have very short lifetimes, and burn fuel very rapidly and drive off intense winds. Then they explode.”
But it’s possible that the cloud has entirely different origins, researchers say. For example, the cloud may be the leftover million-year-old glow of a cosmic gamma ray burst, one of the most mysterious phenomena in the sky.
“We know that when gamma ray bursts go off, it is a profound event,” Kurfess said. The gamma ray “fireball” creates copious amounts of positrons and electrons, which would expand at tremendous rates into interstellar space. Some, Kurfess said, can hang around for tens of thousands or millions of years before they meet an electron, and annihilate.
It’s possible, he said, that the astronomers are mapping the site where a gamma ray burst occurred a million years ago, “and all we’re seeing is the ashes that are still kind of glowing a million years later.”
Discovery of Cloud of Antimatter Alters Ideas on Galactic Formation
Computer model shows antimatter atop the Milky Way, a finding that alters the way scientists see the galaxy.
Sources: UC Riverside, Northwestern University
