The Arp 274 trio of spiral galaxies. Image: Hubble

The universe is a big place, and we’ve only recently begun unlocking its secrets. Among the still unexplained mysteries is how massive galaxies formed in the early universe that already were dead in terms of star formation. Current formation theory suggests that young galaxies should be hotbeds of new stars, and that massive galaxies shouldn’t die so quickly. New research published in the Astrophysical Journal has an answer: these enormous galaxies were created when other galaxies collided not long after the Big Bang.

Galaxy formation is a huge area of study, but the general concept looks something like this: Matter, perhaps produced by ancient supernovae, dispersed throughout the universe in the wake of the Big Bang, and about a billion years later this matter started clumping together. Once these clumps reached a certain mass, their own gravity caused them to collapse on themselves. These collapsed clumps became protogalaxies.

Astronomers suspect that these protogalaxies gained mass through collisions with other small protogalaxies, gradually coalescing into the large galaxies we see today. With the right amount of material at the right temperature, these young galaxies would see rapid star formation, eventually cooling and settling into a comparatively less dynamic large galaxy.

Our Milky Way, for example, contains more than 200 billion stars and is about 100,000 light years across. That’s a pretty big galaxy, but it’s about 13.2 billion years old so it’s had plenty of time to amass all the material it needed to become such a large galaxy.

So shouldn’t the young, distant galaxies we can observe be small, massive, and teeming with star formation? If we look at the Milky Way as a model, they should be. But if all galaxies take a long time to mature, we shouldn’t be seeing massive young galaxies that appear to have already lived out their star-bearing years. Astronomers have seen galaxies as young as three billion years old that are not only more massive than we expect them to be at that age, but inactive as well.

The growth of massive elliptical galaxies takes billions of years. Image: NASA, ESA, S. Toft og A. Feild

The research team, led by Sune Toft, a professor at the Dark Cosmology Centre at the University of Copenhagen’s Niels Bohr Institute, decided to look even further back in time. The team theorized that the early universe was marked with galactic fusion, a process wherein galaxies smashed into one another, combining their material to become larger and more massive.

Astronomers already suspect galactic collisions are one explanation behind the different shapes of galaxies we see, those stretched them into elliptical shapes or pulled apart to develop spiral arms. Fusion between galaxies could account for the larger mass of these young galaxies, but it can’t account for the inactivity. There must have been something extreme in these galactic collisions that left them dead so early in their lives.

To answer the question of galactic inactivity, Toft and team turned to the behaviour of the gas in these early galaxies when they collided. Toft has studied submillimeter galaxies (SMGs), a class of galaxies marked by intense star formation that hidden beneath a thick layer of dust.

According to the research, when two gas-rich galaxies collide and merge, the gas from both is driven into the middle of the system where it begins a rapid period of star formation. The rapid creation of so many stars leaves the galaxy dense and starved for more gas from which to make new stars. The result is a larger young galaxy devoid of star formation.

Toft and team’s explanation might account some of the stranger galactic observations, but it’s possible another discovery or observation months or years from now will throw this latest idea for a loop. We’ll have to wait and see what kind of curveball our universe throws us next.

Correction 2/5: An earlier version of this post said “universes” in the headline, when the story is clearly about galaxies. This happened because the editor was listening to Powerman 5000 while writing. We regret the error.