Weekly Science Update

Friday, November 21, 2008
Colliding Galaxies in the Early Universe
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 warm dust. Astronomers are quite sure that the energy to heat the dust comes from giant bursts of star formation that are hidden from optical view by the dust itself, but they do not know what triggers these bursts. These bright objects have been detected in the very early universe, 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. At least, that is what astronomers conclude from studying nearby interacting galaxies. More distant ones in the early universe, however, are so far away that such interactions are much harder to verify. Perhaps a massive black hole at the nucleus is responsible for the luminosity, in whole or in part?

A team of SAO astronomers, Steve Willner, Mark Gurwell, and Matt Ashby, along with six of their colleagues, have used the Submillimeter Array and other observing facilities to see 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% of the age of the universe. The object had been known because of its powerful radio wavelength emission, thought to be the result of a massive black hole.

The team discovered that the source is actually 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 material between them, produced by their collision. Their data reveal one galaxy and the bridge; the other galaxy is hidden even at infrared and submillimeter wavelengths but is seen with the radio data. The results contradict the commonly held view that a black hole nucleus is responsible for both the radio and the luminous infrared emission in distant galaxies -- at least in this case the powerful infrared emission comes from a burst of new stars.

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