Why were there old, enormously massive galaxies no longer forming new stars in the very early universe?

The first stars already emerged in the very early universe about 200 million years after the Big Bang. Gas is the raw material used to form stars and giant clouds of hydrogen and helium and dust (and whatever "dark matter" will eventually be) contracted and eventually the gas became so compact that the pressure heated the matter so that glowing gas balls were formed and new stars were born. 

Stars are collected in galaxies and as long as there is gas in the galaxy, new stars are being formed.  But by 3 billion years after the Big Bang there were already old galaxies no longer forming stars, and that is a puzzle.  By comparison, our 12 billion-year-old Milky Way galaxy is still pumping them out.

Mysteries in the childhood of the universe

A new hypothesis is 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 new, larger galaxies.

The largest galaxies in today's universe were therefore believed to have been under construction throughout the history of the universe. 

The size of the extremely compact dead galaxies in the early universe with the size of our own galaxy, the Milky Way. The two galaxy types have approximately equal amounts of stars, which means that the density of stars in the compact galaxies is more that 10 times higher than in the Milky way. Graphic courtesy of NASA, European Space Agency, and S. Toft og A. Feild

"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 center. 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?  

Solving the mystery

To find out what happened, Toft and colleagues 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 hypothesis 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 Toft. 

He explains that when such gas-rich galaxies merge, all of the gas is driven into the center of the system where it ignites an explosion of new star formation. A lot of stars are formed in the center 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. 

The evolutionary sequence in the growth of massive elliptical galaxies over 13 billion years, as gleaned from space-based and ground-based telescopic observations. The growth of this class of galaxies is driven by rapid star formation in the so called SMG galaxies and mergers with other galaxies. Graphic courtesy of NASA, European Space Agency, and S. Toft og A. Feild

"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. 

They say the new research is a breakthrough in discovering the formation process of the enormously massive and dead galaxies in the early universe.

Preprint: S. Toft, V. Smolcic, B. Magnelli, A. Karim, A. Zirm, M. Michalowski, P. Capak, K. Sheth, K. Schawinski, J.-K. Krogager, S. Wuyts, D. Sanders, A. W. S. Man, D. Lutz, J. Staguhn, S. Berta, H. Mccracken, J. Krpan, D. Riechers, 'Sub-millimeter galaxies as progenitors of compact quiescent galaxies', arXiv:1401.1510. Published in the Astrophysical Journal