It has long puzzled scientists that there were enormously massive galaxies that were already old and no longer forming new stars in the very early universe, approx. 3 billion years after the Big Bang. Now new research from the Niels Bohr Institute, among others, shows that these massive galaxies were formed by explosive star formation that was set in motion by the collision of galaxies a few billion years after the Big Bang.
The astronomers' theory is therefore 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 neighbouring galaxies to form new, larger galaxies. The largest galaxies in today's universe were therefore believed to have been under construction throughout the history of the universe.
"That is why it surprised us that we already when the universe was only 3 billion years old, found galaxies that were just as massive as today's large spiral galaxies and the largest elliptical galaxies, which are the giants in the local universe. 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," explains Sune Toft, Dark Cosmology Centre at the Niels Bohr Institute at the University of Copenhagen.
The extremely massive and compact galaxies were not flattened spiral galaxies where stars and gas rotate around the centre. Rather, they resembled elliptical galaxies where stars move more hither and thither and where the gas for new star formation has been used up. But how could the galaxies become so massive and so burnt out so early? How were they formed?
To find out what happened, Sune Toft had to look even further back in time. Based on the ages of the galaxies, he knew that they had to have formed very early in the history of the universe, but at that point there was simply not enough time for the galaxies to have grown so massive through normal star formation. He had a theory that the massive galaxies were formed by the fusion of smaller galaxies, but that alone could not explain how they had become so massive so quickly and were already dead. The theory was therefore, that there must have been some especially extreme galaxies 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," explains Sune Toft.
He explains that when such gas-rich galaxies merge, all of the gas is driven into the centre of the system where it ignites an explosion of new star formation. A lot of stars are formed in the centre and the galaxy quickly becomes very compact. But with the explosive star formation, the gas to form new stars is also used up extremely quickly and then you get a dead galaxy.
"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 Sune Toft.
The new research is a breakthrough in discovering the formation process of the enormously massive and dead galaxies in the early universe. The results are published in the scientific journal, Astrophysical Journal.
In a recent study using Herschel data, astronomers have captured the onset of this process between two massive spiral galaxies colliding and merging to produce a vast elliptical galaxy, with the collision triggering such a massive burst of star formationseen when the Universe was just 3 billion years old shown in the image below.
The galaxy pair was initially identified in the Herschel data as a single bright source, named HXMM01. Follow-up observations showed that it is in fact two galaxies, each boasting a stellar mass equal to about 100 billion Suns and an equivalent amount of gas. The galaxies are linked by bridge of gas, indicating that they are merging.
“This monster system of interacting galaxies is the most efficient star-forming factory ever found in the Universe at a time when it was only 3 billion years old,” says Hai Fu from University of California, Irvine, USA, who led the study published in Nature.
“The HXMM01 system is unusual not only because of its high mass and intense star-forming activity, but also because it exposes a crucial, intermediate step of the merging process, providing valuable insight that will help us constrain models for the formation and evolution of galaxies,” adds co-author Asantha Cooray, also from University of California, Irvine.
The Daily Galaxy via University of Copenhagen - Niels Bohr Institute and ESA