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Computer models reveal how the Universe's biggest and brightest galaxies formed

It all has to do with a big buildup of gas

Dr. Robert Thompson (NCSA)

In the far-off depths of our Universe, there are giant, luminous galaxies that are, essentially, star farms. Our home, the Milky Way, produces one to two new stars every year, but these galaxies create more than 1,000 new stars annually. How they do this has been a mystery — until now.

These ancient galaxies, called submillimeter galaxies (SMGs) because of the infrared wavelengths at which they can be seen, accumulate a steady buildup of gas over millions of years, according to a paper published in the journal Nature. This gas gets pushed to the edges of the galaxies, where it becomes the main ingredient for the formation of many new stellar objects, computer simulations show.

These giant, luminous galaxies are, essentially, star farms

The findings help explain how SMGs pop out new stars so quickly. SMGs are thought to have formed not very long after the Big Bang. That's stumped scientists, because it usually takes a really long time for galaxies to grow to such a massive size. No one was sure how these galaxies accumulated so many stars during a time when the Universe was so young. "We hope that we have a model now that can explain what has been a fairly enigmatic astrophysical source for 20 years now," said study author Desika Narayanan, an assistant professor of astronomy for Haverford College.

Before this study, the leading theory was that an SMG is the result of a merger between two galaxies. It explained why the galaxies are so bright: a doubled star count. "That's more or less been the pervasive view, but it has a few nagging problems," said Narayanan. For one, galaxy mergers that have been observed by astronomers tend to only be bright for a relatively short amount of time — only tens of millions of years. SMGs are brighter for 10 times longer than that. Also, galaxy mergers tend to result in relatively compact galaxies, while SMGs are 10 times bigger than a typical merger.

To figure out where SMGs come from, Narayanan and his team modeled how galaxies might form around 2 billion years after the Big Bang. Using computers, they reproduced a small chunk of the early Universe, showing how it could grow and evolve over millions of years. The simulation took into account a variety of physical equations — such as those related to gravity, the flow of fluids, how stars evolve, and more. "You put this in the computer simulation and press go and watch what happens," said Narayanan. "From doing this we track the galaxies that form, and at the end of the day we had some model galaxies."

A simulation of how gas flows in a "submillimeter galaxy." (Desika Narayanan and Matthew Turk)

At first, it was hard to know what these galaxies looked like; all the astronomers had were the objects' physical characteristics, like their underlying temperatures or velocities. They didn’t know how many stars the galaxies had or how bright they were. To fix this, Narayanan applied what is known as a radiative transfer code, which models how light flows through the galaxies. This illuminated the galaxy models, showing that some were bright enough to be SMGs.

The movement of light also indicated how the SMGs were able to create so many new stars. Because an SMG is so massive, it has a huge gravitational pull that attracts a lot of gas from the surrounding intergalactic space. The gas is pulled into the galaxy — and then pushed back outward by the light emanating from all the stars. It’s an interaction known as feedback, where light exerts a pushing pressure on fluids. This process causes the gas to slowly accumulate at the galactic edge, acting like an ingredient bank that the SMG can use later on to form new stars.

Galaxies seem to turn into SMGs when they become massive enough to pull in gas

Galaxies seem to turn into SMGs when they become massive enough to pull in this gas, said Narayanan. They then enter this feedback loop that allows them to constantly fill up their gas reservoir. "It’s possible that all galaxies of this mass would become submillimeter galaxies, and if that’s true, the numbers work out right to match the abundances of how many people have actually observed," said Narayanan.

The models’ proposed explanation is satisfying for astronomers. "We can form one of the Universe’s most extreme galaxies in a cosmological simulation, and it’s actually a pretty accurate description of what observers like me see when we’re at the telescope," said Caitlin Casey, an astronomer at the University of Texas, who was not involved in the study. Of course, these are still just computer simulations. Joaquin Vieira, an astronomer at the University of Illinois, hopes the models can be further verified by data gathered by the Atacama Large Millimeter Array (ALMA) observatory in northern Chile.

With more data, we can gain a better understanding about these rare objects in the universe that fart out so many stars.