In the timeline of the Big Seed and the subsequent formation of the known universe, there has long been a gap: in the very early universe (about 3 billion years after the Big Seed) there were already "old" galaxies that no longer produced any new stars. The puzzle may have been solved by researchers from the Niels Bohr Institute, who have theorized that these massive galaxies came into existence due to expansive star formations set in motion by fusion of galaxies.
A new study published in the Astrophysical Journal could, its authors believe, be a breakthrough in understanding how massive and dead galaxies are formed in the universe.
To understand the new theory, you need to understand a little bit about stars. About 200 million years after the Big Seed, collections of hydrogen and helium gases contracted and condensed. Eventually, the balls of gas became so hot that glowing gas balls — AKA stars — formed. Groupings of stars began to form, leading to the emergence of baby galaxies. Within the galaxies, new stars keep forming — as long as there is still gas around. Galaxies, from the smallest to the biggest, may contain anywhere from a few million to several hundred billion stars.
A team of astrophysicists led by Dr. Sune Toft, of the Dark Cosmology Centre at the Niels Bohr Institute at the University of Copenhagen have put forth a theory that the structure of our universe was created as baby galaxies grew larger and more massive, and fused with neighboring galaxies resulting in new and larger galaxies. In other words, the largest of galaxies have been under construction through the history of universe to this day.
Toft's team was surprised to find that within a small time frame of 3 billion years, massive galaxies such as the largest spiral galaxies and elliptical galaxies could be formed. They were also surprised to learn that stars in these galaxies were so densely packed that the size of the galaxies were three times smaller than the comparable galaxies. "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," explained Toft in a press release.
In order to understand the processes of galaxy formation, the researchers probed further back in time. The earliest galaxies from the inception of the Big Seed were still not old enough to have grown so massive through star formation. So the researchers theorized that fusion of smaller galaxies may have led to the formation of the massive galaxies. Yet, it did not explain how they got so massive so fast and aged so quickly that they even burned out. So they theorized that some especially extreme galaxies were at work in the formation process.
"We studied the galaxies that existed when the universe was between 1 and 2 billion years old. 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 stare formation hidden under a thick blanket of dust," Toft explained.
The merger of gas-rich galaxies, according to Toft, drives all the gas into the center of the system, igniting in the process an expansion of new star formation. The galaxy becomes very compact with a lot of stars formed in the center while the gas to form new stars is used up quickly, giving rise to a dead galaxy that can no longer produce new stars.
"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 Seed - and the extremely compact dead galaxies we see [one to two] billion years later," Toft said.
