The brightest galaxies in the Universe are known as submillimeter galaxies (SMGs). That’s because they were discovered using light with wavelengths longer than a microwave, below infra-red on the electromagnetic spectrum. While they're very bright at visible wavelengths, most of their output there is blocked out by dust.

SMGs existed in the early days of the Universe, a mere three billion years after the Big Bang, during an era called the Cosmic Noon. They were bright because they were producing stars at an incredible rate; more recently they've been producing debate among Earthly scientists.

Their star formation rate is so intense that it actually challenged conventional models of how galaxies form when these SMGs were first discovered a decade ago. The debate over the issue still hasn’t been totally settled, but a new paper provides some significant support for one of the competing hypotheses.

Some researchers argue that the SMGs might have been formed through the collision of gas-rich galaxies in a model known as the "merger-starburst" hypothesis. In this model, the intense brightness of the SMGs is the result of a relatively brief—only about 100 million years long—period of star formation that flared up after a collision and would shortly calm down. This is consistent with the fact that in the modern Universe, the brightest galaxies we see are those resulting from two merged galaxies.

Other researchers have instead argued for the "smooth accretion" hypothesis, which has more massive galaxies slowly being fed by gas over a longer period (about a billion years). Star formation would thus happen continuously over a much longer period.

This is just a simulation

One of the reasons we can't choose between these two models is that SMGs are notoriously hard to simulate. Computer simulations are an important tool for astrophysicists, as they allow them to put their ideas to a test. If a scientific model explains how, say, galaxies form, then it should produce a functioning computer model. Researchers should be able to plug in the details like initial conditions and have the laws of physics produce the galaxies we observe.

In other words, they don’t specifically tell the simulations to make those galaxies; they just let it fly and, if their hypothesis was right, out pop the galaxies, looking just like they do in reality. This isn’t the same thing as observational evidence, but it has an important place in astrophysics.

So if either of the competing hypotheses are correct, they should be able to reproduce those galaxies in computer simulations. Until now, efforts to do so have failed for both of them. They can sometimes create galaxies that are bright enough, but they haven’t been able to match all of the SMGs’ characteristics (star formation rates, masses, relative gas quantities, and environments) at the same time.

This was something of a problem for current models, as SMGs are thought to be the progenitors of modern-day elliptical galaxies. A failure to explain the SMGs is also a failure to explain why the Universe looks like it does. But a new study has at last successfully simulated the SMGs and has turned up some interesting details in the process.

The new simulation succeeds where others have failed in part because the researchers employed a sort of “zoom” technique. Handling what’s going on in a system like a galaxy takes a lot of processing power. In a simulation that covers a wider picture of the Universe, researchers typically have to sacrifice some of that fine detail in order to capture what’s going on at larger scales. The bigger the picture, the lower the resolution, in other words.

The researchers couldn’t avoid this problem by only simulating a single galaxy because they required the larger structure in the galaxy's neighborhood to show how the SMGs arise in the first place. So they made a compromise: the simulation did include the larger area, but they chose one particular galaxy to simulate at a higher resolution. That way they would have the best of both worlds: the detail they needed within the galaxy and the context of the larger scales.

Those fine details are important in a galaxy’s evolution. Supernova explosions, for example, can drive star-forming gas out of the galaxy, slowing star formation. The researchers can also predict the galaxies’ energy output in more detail than before. This allows the researchers to synthesize a more detailed model of galaxy evolution that incorporates these key factors.

Results

The simulation worked with groups of galaxies resting in massive halos of dark matter. These will have increasing rates of star formation, peaking at a whopping 500 to 1,000 times the mass of the Sun being put into stars forming each year, at just about the right time in the Universe’s history—the Cosmic Noon.

The key to that timing was the interstellar space becoming sufficiently enriched with metals by then. In astronomical terms, "metals" refers to elements beyond hydrogen and helium on the periodic table. These are formed in the cores of stars and are released to space at the end of the stars’ lifetimes, so it takes time for the space between stars to become sufficiently filled with them.

This material is then drawn into a galaxy over a long period of time, about a billion years, just as the smooth accretion hypothesis suggests. That is a strong indicator that this is what’s really happening in the Universe.

Part of the reason it lasts so long is a feedback process. Some gas is ejected from the galaxy by supernovas, but that gas doesn’t fully escape; it’s pulled back in by the SMGs’ intense gravity—SMGs are among the most massive galaxies as well. This ebb and flow of gas draws out the star formation phase, allowing it to continue steadily for a billion years.

“Our picture for the formation of SMGs suggests that they are not transient events, but rather natural long-lived phases in the evolution of massive haloes,” the authors write in their paper. This doesn’t mean that merger-starbursts don't happen or can't create SMGs. But this work strongly suggests they’re not the primary mechanism that leads to the creation of these incredible galaxies)

Additionally, since SMGs seem to exist in groups, other nearby galaxies may contribute material to the star-forming furnace of an SMG, allowing a “perfect storm” of factors to combine in these supremely bright, supremely massive galaxies.

This marks a departure. A recent influential paper had provided some support for a version of the merger-starburst scenario involving the collision of multiple smaller galaxies. This work makes a strong case for the smooth accretion model, though it’s not the final word on the matter. The simulation makes specific predictions that can be tested in future observations of actual SMGs, though, which may help provide some clarity.

Nonetheless, the work is undoubtedly a milestone: the first simulation that successfully produced the SMGs in all their splendor.

Nature, 2015. DOI: 10.1038/nature15383 (About DOIs)