For decades, many astronomers believed in a cookie-cutter universe. Orderly, well-behaved, predictable. The mold for galaxies, the large systems where stars and planets reside, came in two shapes: spirals and ellipticals. They were "island universes" that evolved in "splendid isolation" just a few million years after the Big Bang. To these astronomers, colliding galaxies were merely an oddity, an anomaly.
But there was a group of astronomers who had a less kind view of the universe. They believed that the universe was a violent place, full of collisions, cannibalism, and mergers. Galaxies, they proposed, may not have been created in cookie-cutter fashion early in the universe. Maybe collisions between spirals spawned ellipticals.
The debate over the role colliding galaxies play in galaxy evolution has continued for decades. In the 1940's, just a few years after American astronomer Edwin Hubble defined galaxy shapes, Swedish astronomer Erik Holmberg wondered what would happen if a couple of galaxies encountered one another. So he constructed an analog computer using about 200 light bulbs to simulate galaxy encounters. Based on this seemingly primitive computer simulation, Holmberg concluded that some galaxies may indeed collide, inducing tides or distortions that rob them of energy, thus causing them to slow down and eventually merge into a single galaxy. The Swedish astronomer's computer simulations also foreshadowed the important role that computers would play in studying galaxy interactions.
The astronomical community largely ignored Holmberg's work. The snubbing, however, didn't stop some astronomers from pursuing these enigmatic galaxies. Swiss astrophysicist Fritz Zwicky at the California Institute of Technology was the first to systematically photograph interacting galaxies in the 1950's. He noticed wispy tails in these galaxies that were similar to those that Holmberg had discovered in his simulations, and concluded that they must stem from gravitational interaction. Zwicky also guessed that these tails must consist of stars.
Still, most astronomers paid little attention to the subject of colliding galaxies, mainly because they believed that the chance of galaxy encounters was relatively small. They didn't understand that galaxies, like stars, often orbit in double and multiple systems, creating a dense environment where collisions are more likely. Some astronomers proposed that the wispy tails were the remnants of gigantic explosions.
Many astronomers believed, as Hubble did, that most galaxies were orderly and symmetrical. Astronomer Allan Sandage emphasized those galaxies in his 1961 book "The Hubble Atlas of Galaxies." He also was among a group of astronomers who proposed that the blobby ellipticals were formed before the disk-shaped spirals.
But astronomer Halton Arp believed in a different kind of universe, one filled with violence. In 1966 he published a catalogue of 338 interesting systems called the "Atlas of Peculiar Galaxies." Arp was convinced that colliding galaxies were more than just oddball systems: He was the first to suggest that these galaxies could form stars in bursts.
Colliding galaxy research received a boost in the late 1960's when scientists made significant improvements in computer technology. Faster, more powerful computers meant more sophisticated simulations of galaxy interactions, which could furnish astronomers with details about these collisions.
Soon after, several astronomers using computer simulations to study colliding galaxies published scientific papers on their work. The paper with the most-developed theory was written in 1972 by the Toomre brothers, Alar and Juri. Instead of plugging in a couple of generic interacting galaxies into their computer to see the results, they also chose four well-known colliding spiral galaxies, including M51 and the Antennae. They wanted to know whether their computer results would match observational evidence. The brothers discovered that they did. Their models showed that galaxy collisions cause strong gravitational interactions, which produce features similar to the bridges and tails of dust and stars found in many of the galaxies in Arp's atlas of galaxies.
After colliding, these galaxies slow down and are drawn closer together until they eventually merge. The offspring of these mergers are star piles resembling elliptical galaxies. There must have been many more mergers in the past when the universe was younger and denser. Alar Toomre, in his classic 1977 paper, estimated that about 10 percent of all galaxies should be merger remnants, a percentage that roughly matches the number of ellipticals observed in the universe. Their conclusion was a salvo shot at a popular theory that ellipticals came before spirals.
The Toomres also were among the first astronomers to suggest that debris stirred up from galaxy interactions could provide fuel for black holes, which power quasars. They penned the phrases "stoking the furnace" and "feeding the monster," descriptions that are now indelibly linked with black holes and quasars.
Although astronomers debated the Toomres' work, they began to take the study of colliding galaxies more seriously. But they still had objections. Among them was this puzzle: spirals are full of gas, but contain relatively few globular clusters (dense spherical clusters of about 100,000 stars). Ellipticals, on the other hand, contain very little gas but possess many globular clusters. How, then, can two merging spiral galaxies produce an elliptical? It's almost like saying 2 plus 2 equals 8. Our Milky Way galaxy, a spiral, has about 150 globular clusters while an elliptical with the same brightness would contain about 600 globulars.
These dissenting astronomers did not consider the important role that gas plays in mergers. Most mergers involve gas being compressed, which triggers intense star formation. Perhaps this burst of star formation could produce new globular clusters?
Astronomers studying colliding galaxies hoped that a new infrared satellite would provide some clues. They weren't disappointed. The satellite, called the Infrared Astronomical Satellite (IRAS), was launched in 1983 to take an infrared survey of the sky. This survey revealed that the most luminous galaxies in the infrared part of the spectrum were always colliding galaxies, illuminated by dust surrounding a burst of infant stars. The images provided evidence that interacting galaxies showed signs of unusually vigorous star formation, a theory that originally had been proposed by Zwicky and Arp.
When two galaxies collide, their interstellar gas is compressed into thick clouds. These clouds of gas collapse even more under gravity's intense force to form new stars. The resulting star burst uses up nearly all of the interstellar gas and expels most of the remaining gas through supernovae explosions, leaving a gas-poor system similar to that of an elliptical galaxy.
Some astronomers believed this intense star formation might produce globular clusters. These clusters would shine with the blue light of hot stars. Other astronomers, however, argued that there was no such evidence for young globular clusters. They contended that globular clusters, such as the ones in our Milky Way, are old. But astronomer Francois Schweizer of the Carnegie Institution of Washington disagreed. Schweizer had teamed up with Alar Toomre to probe several interacting galaxies. In 1982 Schweizer studied the interacting galaxy NGC 7252 (the Atoms for Peace galaxy) using ground-based telescopes and observed six bluish knots of light near the galactic nucleus. He interpreted these knots as young star clusters formed during the merger. He and other astronomers (e.g. Keith Ashman at the Space Telescope Science Institute and Steve Zepf at Johns Hopkins University) suggested that the formation of young globular clusters by colliding spirals might explain why ellipticals have so many globular clusters.
But Schweizer and other astronomers couldn't provide solid evidence for the existence of new star clusters in interacting galaxies. Ground-based telescopes didn't have the resolution to completely define these clusters. Enter the Hubble Space Telescope, with its high-resolution capabilities and its great location above the Earth's atmosphere. In the Antennae, for example, one giant star- forming knot from a ground-based telescope often turns into 10 to 12 star clusters through the eyes of the Hubble telescope, each with the size of a normal globular cluster.
Even a Hubble telescope without corrective vision found plenty of young star clusters. Peering into the core of the interacting galaxy NGC 1275, the Hubble telescope's Wide Field and Planetary Camera found in 1992 what astronomer Jon Holtzman of Lowell Observatory described as 50 young clusters less than several hundred million years old. He concluded that the clusters were spawned by a merger.
In 1993 a team of astronomers, including Schweizer of Carnegie and led by Brad Whitmore of the Space Telescope Science Institute, provided conclusive evidence that mergers produce new star clusters. Using the Hubble telescope, the team identified 40 young clusters, mostly between 50 and 500 million years old, near the center of NGC 7252.
Since then, Whitmore and Schweizer and their collaborators, Miller of the Carnegie Institution of Washington, and Fall and Leitherer of the Space Telescope Science Institute, have continued to probe colliding galaxies. The Wide Field and Planetary Camera 2 with its corrective vision has penetrated more than 10 times deeper into the heart of colliding galaxies than earlier observations. Recent observations of NGC 7252, for example, have revealed more than 500 star clusters, compared with only 40 in 1993.
Whitmore now believes he can tell how long ago these collisions occurred by measuring the colors and brightness of young globular clusters. These clusters, many astronomers agree, may play an essential role in understanding how galaxies evolve.
From oddball galaxies to galaxy building blocks: the part colliding galaxies play in galaxy evolution has changed dramatically over the decades.
"Tales of ... A history of colliding galaxies: From oddballs to galaxy building blocks" explains how scientists determined the role of colliding galaxies in the story of galaxy evolution. The story begins with an early attempt to model galaxy interactions and concludes with using the Hubble Space Telescope to find young clusters of stars.
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HubbleSite press release images of interacting galaxies: "Cosmic Collisions Galore!"