Supernova Spectacular

Peering deep into the maelstrom of two colliding galaxies, astronomers have discovered a cluster of massive stars exploding like firecrackers. From what they’ve seen, researchers estimate that in this pair of merging galaxies, dubbed Arp 299, a star dies in a supernova explosion every 2 years. In quiescent galaxies such as the Milky Way, an entire century can go by between such spectacles.

Studies of Arp 299 offer astronomers a rare opportunity to examine a collection of stars exploding in an environment dense with gas and dust, where the rates of both stellar birth and death are high. But it’s not just the fireworks that have drawn astronomers to Arp 299 and similar cosmic venues, says Susan G. Neff of NASA’s Goddard Space Flight Center in Greenbelt, Md. Although these objects, known as starburst galaxies, are relatively near to our Milky Way, they may be revealing what star birth was like billions of years ago. Scientists are also looking to starburst galaxies to learn how supermassive black holes form and merge and how elliptical galaxies, one of the most common galaxy types, take on their distinctive shape.

Residing 140 million light-years from Earth, Arp 299 isn’t the closest starburst galaxy, but it’s the nearest example of an unusually bright one. It packs more than a million newborn stars into a region just 10 light-years across and harbors some 25 other pockets of prolific star formation.

Neff’s team, which includes James S. Ulvestad of the National Radio Astronomy Observatory in Socorro, N.M., and Stacy Teng of the University of Maryland in College Park, presented its observations of Arp 299 in late May at a meeting of the American Astronomical Society in Nashville.

The type of collision that created Arp 299 and its firestorm of activity is rare today because most galaxies are whizzing past each other too quickly to interact, notes theorist Chris Mihos of Case Western Reserve University in Cleveland. However, billions of years ago, when the universe was smaller and galaxies moved much more slowly, they more often succumbed to the gravitational pull of their neighbors.

Although their collision probably began several hundred million years ago, the two galaxies that formed Arp 299 can still be distinguished, and their hubs are 15,000 light-years apart. Both are spiral galaxies, which are rich in gas, the raw material for making stars. Some 8 million years before the current observations, the galaxies’ interaction was so violent that it triggered the birth of millions of stars, Neff estimates. Now, the shortest-lived of these stars–the heaviest ones, some 10 to 20 times the mass of the sun–are dying spectacular deaths, Neff’s team has found.

At the same time, the collisions are scrambling the orderly paths of the stars already present in each of the spiral galaxies. This is how elliptical galaxies–football-shape or spherical galaxies featuring a swarm of stars going every which way–are generated, according to a leading model of the process.

“We believe that in Arp 299 we’re witnessing how elliptical galaxies were made in the distant past,” says Mihos.

Supernova factory

Shrouded by dust and dense gas, the star clusters in Arp 299 can’t be seen in visible light. To observe them, Neff’s team used a network of radio telescopes. Although both radio waves and X rays can penetrate dust and travel out of the galaxy, only state-of-the-art radio telescopes can discern features small enough to resolve individual supernova explosions, Neff notes.

Two decades ago, astronomers recognized Arp 299 as a prolific star maker and coined the term starburst for this galaxy. Observations last year with the Very Large Array, a network of radio telescopes in Socorro, N.M., indicated that a region near the center of one of the colliding galaxies was producing stars at a furious rate. Neff and her colleagues homed in on this region, dubbed “Source A,” with the Very Large Baseline Array (VLBA), a continent-wide network of 10 radio telescopes. The team combined the sensitivity of the Greenbank (W. Va.) Telescope, whose radio-receiving dish covers 2.5 acres, with the resolution of VLBA. They discerned four individual supernovas within Source A, none of which was more than 10 to 20 years old.

When Neff’s team took a second look with the VLBA last February, it found a fifth supernova. It lies within 7 light-years of one of the other four and therefore is likely to belong to the same star cluster.

The researchers say that the newly observed supernova is likely to be part of a compact group of so-called super star clusters that collectively produce one supernova every 2 years. These clusters are a “supernova factory,” says Ulvestad.

In Arp 299, such clusters appear to be forming in much the same way that globular clusters, crowded groupings of stars that rank among the oldest in the cosmos, formed early in the universe.

The clusters in Arp 299 “provide us with a unique opportunity to learn how stars formed billions of years ago,” Neff says. “There have been all sorts of deductions and assumptions about how starbursts work, but this is . . . the first direct confirmation that [such galaxies] have millions of massive stars all close together, forming together and dying together.”

Mimicking the dense environments in which stars presumably formed in the early universe, these clusters produce a higher proportion of massive stars than more typical, lower-density regions of star formation do. Because only stars more massive than about eight suns can die as supernovas, the number of these explosions in Arp 299 indicates how many heavyweights are made in a starburst galaxy, notes Mihos.

In a galaxy such as the Milky Way, the fierce winds blown by supernovas drive the heavy elements forged in these explosions to the outskirts of the galaxy. But in compact star-forming regions such as those in Arp 299, the supernova winds may be no match for the high density of gas and dust. In these locales, the heavy elements stick closer to home, which is often near the center of the galaxy. This could explain why the cores of elliptical galaxies, proposed to arise from collisions, just as starbursts do, are rich in heavy elements, Mihos says. Astronomers include carbon, nitrogen, and oxygen in that category.

Black hole dynamics

Geometry provides one reason Arp 299 produces so many supernovas. One of the colliding galaxies rotates in a direction opposite to that of its orbit. When such a galaxy collides with another galaxy, its retrograde rotation tends to push gas toward its center rather than pull it out as a tail or streamer. As a result, says Neff, “there’s lots more gas available for making new stars.”

Gas moving to the center of a galaxy that already contains a supermassive black hole would set the stage for some of the most powerful pyrotechnics the cosmos can muster. The energy emitted by this torrent of material falling into the black hole could be enough to power a quasar, notes Mihos.

A collision of two galaxies might also hurl two black holes toward each other, creating a single, much heavier black hole. In this scenario, hundreds or thousands of so-called middleweight black holes could coalesce into a supermassive black hole–or a supermassive black hole could grow even bigger.

In a starburst galaxy designated NGC 3256, Neff, Ulvestad, and Scott Campion of the University of Maryland have recently seen hints of such activity. Neff reported their findings at a conference in Marstrand, Sweden, in late June.

This starburst, whose colliding galaxies have merged into a single body, lies about 120 million light-years from Earth. The new observations reveal that the two brightest and most compact sources of radio waves coincide with the two most powerful X-ray sources, a strong indication that each is a black hole. Additional measurements with the Hubble Space Telescope indicate that one of the candidate black holes is supermassive, weighing more than 100,000 times as much as the sun. Separated by less than 3,000 light-years, the two candidate black holes seem destined to merge.

In another starburst, Arp 220, two colliding galaxies have coalesced, too. Carol J. Lonsdale of the California Institute of Technology in Pasadena and her collaborators estimate that Arp 220 produces two new supernovas each year, which is four times the rate in Arp 299. However, because Arp 220 lies farther from Earth than Arp 299 does, fewer of its supernovas can be detected.

Observing Arp 299 and other nearby starbursts, “we can watch [galaxies] change over human timescales,” says Neff. With cosmic life and death playing out over relatively short times, starburst galaxies are becoming the next best thing to viewing the history of the universe on fast forward.

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