The giant galaxy in the center of the image is ten billion light years from Earth
Photo: ESA / Hubble & NASA, A. Newman, M. Akhshik, K. Whitaker
In around half of all large galaxies in the young cosmos, the rapid formation of stars suddenly comes to a standstill around three billion years after the Big Bang.
Astronomers encountered this strange phenomenon almost a decade ago and were faced with a riddle: What was going on in space?
Stars normally form according to a typical pattern: matter and molecules condense in gigantic gas nebulae, hundreds of light years in size. Due to their own gravitation, they contract and rotate until a protostar, a young sun, is finally born. At some point the fire of nuclear fusion starts up due to pressure and heat, which turns stars into powerful energy suppliers. But apparently something works differently with the observed phenomenon.
Observations by experts with the Hubble space telescope and the large Alma radio telescope facility in Chile are now providing clues as to the cause of the sudden cessation of star formation: the galaxies are apparently running out of gas that they need to produce new stars.
What leads to this deficiency remains a mystery, however, since there is plenty of gas in the vicinity of the star systems, the scientists write in the journal "Nature".
Explosive star formation
"The largest galaxies in the cosmos produced their stars in a remarkably short time after the Big Bang," explains Katherine Whitaker from the University of Massachusetts in Amherst.
"About a hundred billion stars in a billion years." That is no wonder, according to the astronomer, because at that time the raw material for the formation of stars - hydrogen gas - was abundant.
Due to the almost explosive formation of stars, the galaxies shine brightly and can be easily observed with terrestrial telescopes even over billions of light years.
Astronomers were all the more surprised when they came across large galaxies in the young cosmos that only shine weakly - and in which hardly any new stars are formed.
The experts originally thought that an unknown effect had, so to speak, blown out star formation in these systems.
“But that's not correct,” said Whitaker.
"Our observations show that the galaxies are running out of supplies." The extinct galaxies contain a hundred times less hydrogen gas than comparable systems with rapid star formation.
Like a natural telescope
In order to track down the extinct - and thus faint - galaxies, Whitaker and her team not only resorted to large telescopes, but also made use of a natural phenomenon: the magnification of the galaxies through a gravitational lens.
A galaxy cluster lying between the distant galaxies and the earth deflects their light.
"So the galaxy cluster acts like a natural telescope," explains co-author Justin Spilker from the University of Texas.
"This makes the extinct galaxies appear larger and brighter - and this enables us to see what is going on in them."
The team with Hubble and Alma observed a total of six extinct galaxies - in all cases with the same result: the galaxies contain almost no hydrogen gas.
And without this gas, no new stars can arise.
Experts had previously calculated in a study that at least 5,000 hydrogen molecules per cubic centimeter had to cavort in a cosmic gas cloud in order for a new sun to form in it.
Why no gas flows in from the vicinity of the galaxies, as is the case with other large galaxies, remains an open question.
Perhaps, so the experts speculate, supermassive black holes in the centers of the galaxies prevent the influx of new gas with their radiation.
With the help of further observations through gravitational lenses, they hope to expose the physical processes that lead to the extinction of the galaxies.
The solution to one riddle has given rise to a new one.
joe / dpa