Early galaxies: More grazing cows than cannibals

New findings contradict common theory of only short-term phases of explosive star formation

This split view shows on the left the image of a normal galaxy in the universe today. On the right, a treatment that shows how this galaxy might have looked in the early Universe: Instead of predominantly old, reddish shimmering stars, it would have contained a dense carpet of young, massive, blue-shining stars. These evidently did not arise at short intervals of skyrocketing star formation, but rather evenly over several hundred million years. © NASA / JPL-Caltech / STScI
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Astronomers have discovered that galaxies in the early Universe resembled grazing cows rather than hungry tigers: most star accumulations gradually grew by gas uptake over long periods of time. This contradicts the current theory of short-lived phases of explosive star formation - triggered by the collision and merging of galaxies.

"We find that this type of galactic cannibalism was very rare. Instead, we see evidence of galaxy growth in which a typical galaxy fed on a steady stream of gas, forming stars at a much faster rate than previously thought, "the researchers say. Using the NASA Spitzer Space Telescope, they studied 70 galaxies more than ten billion light-years away in the infrared range for the first time. The dust clouds penetrating infrared light revealed a sixfold higher star formation rate in these galaxies, as determined in previous studies. However, a high amount of gas, as typical for mergers, was missing.

Mystery about galaxy growth in the early cosmos

Galaxies are huge collections of stars, gas and dust. The hydrogen gas is the raw material for today rather slowly occurring star formation: Only about ten new stars per year, for example, arise in our Milky Way. By contrast, in the early universe, many more stars must have formed in a very short time to form the first galaxies.

But where did they get their raw material from? According to the most popular theory, galaxies grew in the first billion years after the Big Bang, mainly due to collisions. In the process, the hydrogen gas contained in them was compressed. This led to an explosive increase in star formation.

Slow growth instead of sudden jumps

The new data from the Spitzer telescope now draw a completely different picture: the galaxies, which date back to Zeitnur one to two billion years after the Big Bang, did not appear to grow in abrupt jumps. Instead, they showed a relatively constant rate of star formation over hundreds of millions of years. This was about a hundred times higher than in the Milky Way and other nearby galaxies. display

"This is the first time we have identified galaxies that grow by" grazing ", says astronomer Hyunjin Shim of NASA's Spitzer Science Center. "Our study shows that the fusion of massive galaxies was not the dominant method of galaxy growth in the distant universe, " adds his colleague Ranga-Ram Chary, lead author of the study. (Astrophysical Journal)

(NASA, 05.07.2011 - NPO)