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Why the Brightest Galaxies Create 1,000 Times More Stars Than Ours

Submillimeter galaxies are insane star-making dynamos. Why?

For scientists who study the deep cosmic past, perplexity is written into the job description. Nowhere is that more true than in the ongoing debate over the origins of submillimeter galaxies (SMGs)—the brightest galaxies ever observed in the universe.

SMGs first appeared on the cosmic scene about three billion years after the Big Bang, and the secret behind their hyperactive rate of star formation—they churn out 1,000 times as many stars as the Milky Way, for comparison—has puzzled scientists for years. Many experts think that SMGs were formed in giant galactic mergers, but according to astrophysicist Desika Narayanan, lead author of a new Nature study on SMGs published today, there are a lot of glitches in this hypothesis.

Animation of SMG. Video: NPG Press/YouTube

"The canonical picture is that major mergers between galaxies cause these really bright and short-lived events," Narayanan told me over the phone. ("Short-lived" in this case means about 50 to 100 million years.)

"The problem has been that for this particular class of the brightest starbursts in the universe—the submillimeter galaxies—there are too many of these galaxies given the number of mergers that we think exist in the early universe," he continued. "There are also other issues with the merger model, like the sizes of these galaxies. They are too big, too bright, and too far out. Mergers tend to make very compact bright sources."

That said, in the absence of other explanations, the merger model has become the default. "The issue has been that no model thus far that doesn't invoke mergers has been able to form a submillimeter galaxy," Narayanan said.

Until today, that is. In their new paper, Narayanan and his colleagues demonstrate that the explosive starbursts observed in SMGs could be generated by something called "stellar feedback," a catchall term for the energy that stars dump into their environment through solar winds and supernovae.

The team showed that this feedback interacts with massive amounts of interstellar dust observed in SMGs to sustain long periods of wildly active star formation.

The gas density distribution of one instance in time of the model starburst galaxy, spanning approximately 650,000 light years across. Image: Desika Narayanan

"The light from the massive stars starts to push back on some of the accreting gas, and it banks that gas for later usage," Narayanan told me. "The gas sort of sits out in the halo of this galaxy and waits. The time it takes to cool is about a billion years or so, and then that gas rains back down on the galaxy and is available for star formation."

"Then, you have enough gas at the right time to form one of these galaxies," he said.

Essentially, SMGs may be powered by enormous gas reservoirs that steadily pipe in raw material for the formation of new stars. Not only does this explanation account for the unusual size, shape, and radiance of these galaxies, it also suggests that we are actually not observing short-lived periods of starburst in SMGs, but rather long-lasting, sustained activity that may last one billion years or more.

This changes the picture of how massive galaxies form and evolved in the early universe, and could help shed light on some of the other bizarre contemporaries of SMGs.

"At the same redshift—or time in cosmic history—there are a lot of different types of enigmatic galaxy populations," Narayanan said. "There are galaxy populations that host the biggest black holes we know about in the universe. There are galaxy populations that are super compact and seem like they are dead from star formation, which is an odd thing so early in the universe."

"It's tempting to want to try to construct a model where many of these galaxy populations overlap in some way, especially since their abundances seem to be kind of similar," he continued. "In order to be able to do this, you need a model for any of these galaxy populations. That is one direction where we are going to try to push forward."

Distribution of galaxies across the infrared luminous region, at a given instance in time. The colours denote the gas density. Image: Robert Thompson (NCSA)

For the time being, however, proponents of the merger hypothesis will have to get used to the new model in town. Narayanan can certainly empathize with that, because interestingly enough, he was one of the merger model's biggest defenders only a few years ago.

"From 2008 and 2010, I wrote maybe five papers or something similar, with some of the same co-authors [as today's Nature paper] saying they are all mergers—they have to be mergers!" he told me.

"Then over time, I started thinking there's this hole and that hole, and in my opinion, it's sort of science at its best. You can take the attitude of 'oh no, I wasted 5 years,' but it's part of the game. Science evolves and you revise the picture and try to come up with a new converged story as opposed to sticking to your guns."