A simulation of gas flow into a model submillimeter galaxy, fueling extreme star formation in its center.

A simulation of gas flow into a model submillimeter galaxy, fueling extreme star formation in its center.

Desika Narayanan

Why are some galaxies a thousand times brighter than the Milky Way?

Our universe was once home to galaxies so bright they make the Milky Way look like a mere candle next to a searchlight. But what can possibly have fueled such brilliance? A new simulation suggests that these radiant beasts are not the result of galaxies smashing into each other—as some astronomers suspect—but rather the consequence of a galaxy recycling fuel to igniting hundreds of suns per year.

When the universe was one-fifth of its current age—about 3 billion years old—galaxies were pumping out stars like mad, the equivalent of 100 suns per year—100 times the rate in our Milky Way today. Over the past 2 decades, thanks to telescopes that pick up so-called submillimeter radiowaves to spy on galaxies shrouded in dust, astronomers have spotted some hyperactive examples in this already overactive period: superbright galaxies forming stars at 1000 times the Milky Way’s output.

Astronomers figured this must be a short-lived phase, because a normal galaxy forming stars that fast would soon run out of fuel (gas and dust). The brightest galaxies in today’s universe are those undergoing a merger with another galaxy, which triggers a short burst of star formation. These superbright submillimeter galaxies, they decided, must also be the result of mergers.  

It would be easy to tell whether that’s true if astronomers could get a good look at them. But most submillimeter observatories so far have been single dishes or small arrays without the resolution to pick out star-forming regions in such distant objects. Simulations of galaxy formation suggest that such bright galaxy mergers could form, but not in the numbers seen during that active epoch.

In the past few years, some astronomers have entertained the possibility that these superbright galaxies aren’t due to mergers but are true powerhouses, somehow continuing to suck in dust and gas from the surrounding universe to form stars over a longer period—a billion years or more. A number of groups have been trying to simulate how such a monster could form and keep burning for so long. Most efforts have failed to model anything as bright as what observers are seeing.

Now, researchers have used a galaxy simulation called FIRE (Feedback In Realistic Environments), developed at Northwestern University, Evanston, in Illinois, to model a submillimeter galaxy with as fine a resolution as they could achieve. As they report online today in Nature, a process known as cosmic recycling plays an important role. In this process, massive stars formed early in the life of a galaxy shine so brightly that the pressure of their radiation pushes lots of gas and dust out of the galaxy altogether. Later the galaxy’s gravity pulls the material back in, where it lengthens the period of intense star formation. Incorporating this mechanism, the team modeled a galaxy that was creating the equivalent of between 500 and 1000 suns per year for a billion years.

Researchers welcome the new result, but they say the issue of submillimeter galaxies remains unsettled. Ian Smail of Durham University in the United Kingdom says that, even by the standards of submillimeter galaxies, the simulated galaxy is very large, productive, and long-lived. And because the simulation doesn’t run on to the present day, he says, we don’t know whether the simulated galaxy would end up as something that looks familiar in today’s universe. “This is an interesting attempt to address an important problem, but I am not convinced that this is the final statement in this regard,” Smail says.

“This model is very intriguing with respect to one galaxy I’ve worked on a lot, which is particularly bright,” says Jacqueline Hodge of the Leiden Observatory in the Netherlands. “We would need more observations to see if it will work with fainter galaxies.”

Astronomers shouldn’t have to wait too long for that. The Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, a collection of 66 movable dishes spread across 16 kilometers, is nearing completion. Researchers have already trained it on submillimeter galaxies with up to 50 dishes but its fullest resolution—with dishes spread farthest apart—will soon become available. “We’re at a rare time as ALMA’s capabilities are increasing. We will definitely make progress in the next few years,” Hodge says. Study author Desika Narayanan of Haverford College in Pennsylvania agrees: “ALMA is going to be our best bet. It ought to be able to do it.”

*Correction, 24 September, 11:10 a.m.: This article has been corrected to say that when the universe was one-fifth its current age, it was 3 billion years old, not  3 billion years ago.