When galaxies collide, their powerful gravitational forces and vast quantities of matter may combine in a way that turns the supermassive but invisible black holes at their centers into the most brilliantly radiant objects in the universe, quasars.
That is the view emerging from newly published analyses of three pairs of colliding galaxies, each independently discovered by one of three teams of astronomers. If the interpretations are correct, the scientists have caught galaxies at three sequential stages in the process of plowing through one another, with startling results.
Close encounters of galaxies, each containing hundreds of billions of stars along with dark but vast quantities of drifting gas and dust, are stately events, happening over tens of millions to hundreds of millions of years. The three observations are essentially snapshots of a phenomenon that could never be studied in real time.
The astronomers say they may represent events that were far more common billions of years ago when the expanding universe was young and galaxies were, therefore, closer together and more likely to bump.
Black holes and quasars, though still mysterious inhabitants of the cosmological zoo, have become considerably better understood in recent years.
A black hole is the result of matter becoming so densely compressed that its gravitational force is strong enough to prevent anything, even light, from escaping.
Typical black holes are thought to form when stars exhaust the nuclear fuel that keeps them inflated and experience a cataclysmic death as a supernova. Much of the star's mass is blasted into space, but if the remnant is heavy enough, its weight will cause it to collapse into a state of matter so dense that even the space within atoms is squeezed out. Many astronomers believe that black holes containing the mass of several million suns act as gravitational hubs at the centers of galaxies.
Quasars, starlike objects that give off electromagnetic radiation detectable in radio telescopes, are puzzling because they emit far more energy for their size than is possible in conventional stars. Astronomers have also been intrigued that the number of quasars increases the farther they look into space. Because distance in space is also time, this means, for example, that a quasar 10 billion light years away, a typical distance, existed 10 billion years ago. Because there are relatively few quasars close by, astronomers assume they are less likely to form nowadays.
The new discoveries may link all these ideas.
The three pair of colliding galaxies are all relatively near Earth, between 1 billion and 4 billion light years away. Hence, as astronomers reckon, they are young and are being seen at early stages in their life history.
An early stage of a collision can be seen in the first pair of galaxies. According to Jay A. Frogel of the Kitt Peak National Observatory, the galaxies are just beginning to merge. The centers of the two galaxies are still separated by about one-third of the larger galaxy's diameter. Nonetheless, there is a quasar burning brightly at the host galaxy's center.
Frogel, who published his findings in the Astronomical Journal along with collaborators from Yale University and the State University of New York at Stony Brook, speculates that enormous quantities of dust and gas from the second galaxy are streaming toward the black hole at the center of the host galaxy.
Astronomers have calculated that if thick enough clouds of dust and gas were to be pulled toward a black hole, much of it would go into orbit, forming a huge cloud around the black hole.
The friction of the rapidly moving dust streams would generate enough heat to make the dust's atoms begin to glow, giving off powerful levels of electromagnetic radiation. Astronomers detect this as a quasar. Although the dust is continually sucked into the black hole, the continuing collision yields a steady supply of dust and gas.
The second collision is farther along. According to astronomers at Kitt Peak and the University of Montreal, who also published in the Astronomical Journal, the host galaxy appears sheared off on one side, as if the gravity of the visitor had pulled the stars out of their normal position.
The host in the second pair is also rich in newly formed blue stars. It is known that major disruptions in a gravitational field can press dust and gas particles closer together, forming clumps whose gravity pulls in still more matter. When the mass of the clump gets big enough, its internal pressure and heat ignite the thermonuclear reaction that makes stars shine.
The third pair appear to be galaxies that have nearly merged. According to astronomers at Palomar Observatory and the University of Alabama, who published in the Astrophysical Journal, a quasar is burning at the center and there are signs of a burst of not-so-recent star formation.
Donald Hamilton of Palomar said the merged galaxies both appear severely distorted by the gravitational disruptions. He suspects that one galaxy orbited the other for millions of years, gradually spiraling closer until they collided.