January 29, 2014
Early, Short-Lived Galaxies Formed Through Collisions
Lee Rannals for redOrbit.com - Your Universe Online
Astronomers writing in the Astrophysical Journal say that massive galaxies in the early universe were formed through collisions.
Scientists studied galaxies that existed when the universe was just between 1 and 2 billion years old. The smallest of these galaxies contain a few million stars, while the largest can contain several hundred billion stars.
The first stars in the universe showed up about 200 million years after the Big Bang from the gases hydrogen and helium. Giant clouds of gas and dust contract and eventually become so compact that the pressure heats the matter to make stars.
Astronomers believe that the structure of the universe was built by baby galaxies gradually growing larger and more massive by constantly forming new stars and by colliding with neighboring galaxies to form a larger galaxy. The largest galaxies in the universe today were thought to have been under construction throughout the history of the universe. However, there were massive galaxies when the universe was just 3 billion years old, some of which were as large as spiral galaxies and the largest elliptical galaxies today.
“Even more surprisingly, the stars in these early galaxies were squeezed into a very small area, so the size of the galaxies were three times smaller than similar mass galaxies today. This means that the density of stars was 10 times greater. Furthermore, the galaxies were already dead, so they were no longer forming new stars. It was a great mystery," said Sune Toft of the Dark Cosmology Centre at the Niels Bohr Institute at the University of Copenhagen.
Toft developed a theory that the massive galaxies were formed by the fusion of smaller galaxies. However, that alone does not explain how they became so massive so quickly.
“My theory that it must have been some galaxies with very specific properties that were part of the formation process made me focus on the special SMG galaxies, which are dominated by intense star formation hidden under a thick blanket of dust," explains Toft.
When these gas-rich galaxies merge, all of the gas is driven into the center of the system where it ignites an explosion star formation reaction. Stars in the center begin to form, and the galaxy becomes very compact, very quickly. However, with the gas being all used up by new star formation it leads to a dead galaxy rather quickly.
The astronomers compared samples of two galaxy populations and discovered an evolutionary link between the compact elliptical galaxies and the submillimeter galaxies.
"I discovered that there was a direct evolutionary link between two of the most extreme galaxy types we have in the universe – the most distant and most intense star forming galaxies which are formed shortly after the Big Bang – and the extremely compact dead galaxies we see 1-2 billion years later," says Toft.
The observations show that the violent starbursts in the dusty galaxies had the same characteristics that would have been predicted for progenitors of the compact elliptical galaxies. The team found that the intense starburst activity inside the submilimeter galaxies only lasted for about 40 million years before the interstellar gas supply ran out.