Galaxy collisions produce a warm, fuzzy glow
28 Nov 2008
The universe contains many fabulously luminous galaxies, some of them more than a thousand times brighter than our own Milky Way. Most of them are practically invisible at optical wavelengths, however, because their light is predominantly at infrared wavelengths, and comes not from stars but from copious amounts of warm dust the galaxies contain.
Astronomers are quite sure that the energy to heat the dust comes from giant bursts of star formation that are hidden from view by the dust itself, but they don't know what triggers these starbirth bursts.
These bright galaxies are seen as they were when the universe was very young, prompting speculation that perhaps our own Milky Way is in some ways descended from galaxies like them.
Galaxies frequently collide with one another, and evidence for their stupendous interactions is found from their distorted shapes and their bright infrared emission. These collisions are thought to trigger the production of massive stars that heat the galactic dust. That, at least, is what astronomers conclude from studying nearby interacting galaxies.
More distant galaxies in the early universe, however, are so far away that such interactions are much harder to verify. Could a massive black hole at a galaxy's nucleus be responsible for the energy production, in whole or in part?
A team of Smithsonian Astrophysical Observatory astronomers, Steve Willner, Mark Gurwell, and Matt Ashby, along with six of their colleagues, have used a telescope system called the Submillimetre Array and other facilities to study the problem. And they have seen for the first time the spatial geometry of a bright galaxy so old that its light has been en route to us for over eleven billion years, nearly 80 percent of the age of the universe. Astronomers had already known of the object because of its powerful radio wavelength emission, thought to be coming from the vicinity of a massive black hole.
The team found that the radio emission is actually coming from two bright objects, consistent with the idea of two galaxies in collision. Surprisingly, they found that the two sources are slightly offset from the bright radio source. They concluded that the system consists of two colliding galaxies and a bridge of warm gas between them, produced by the collision.
The data they have collected reveals one galaxy and the bridge—the other galaxy remains hidden at infrared and submillimetre wavelengths even though it is seen with the radio data.
These results have contradicted the commonly held view that a black hole is responsible for both the radio and the infrared emission in distant galaxies. In this case at least, the powerful infrared emission comes from a burst of new stars caused by a the collision of two galaxies.
Adapted from information issued by the SAO.
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