Some of the First Galaxies Were Big Babies
New research shows early galaxies were much bigger—some 10 times the mass of the Milky Way—than astronomers expected. What does that mean about the origins of the Universe?
A long time ago, there were no galaxies—not even galaxies far far away. One of the big challenges in astronomy involves determining when the first galaxies formed, and what they looked like. According to conventional wisdom, small galaxies were born first, then clumped together to make larger and larger galaxies, until everything settled into the Universe we see today.
However, there’s a growing realization that some early galaxies were much bigger than we expected. A survey of 3,398 galaxies dating to 900 million years after the Big Bang showed many massive specimens, some 100 times the mass of the Milky Way. That’s more than expected at such an early period in the Universe’s history, leaving astronomers with the challenge of explaining why that should be.
Because the Universe is about 13.8 billion years old, and because light takes time to travel across space, any astronomical observation we do is looking back in time. Even our view of the Sun in the sky is eight minutes old. That means with sufficiently powerful telescopes, we should be able to see the first galaxies in the cosmos—though we have to cope with the fact that very little light from them reaches us, and that other light sources closer by can be confounding.
As a result, astronomers are still hunting for the earliest galaxies. One such effort is the Spitzer Large Area Survey with Hyper-Suprime-Cam (SPLASH, which despite appearances is not the most ridiculous acronym in cosmology), using the Spitzer infrared space telescope and the Subaru telescope in Hawaii. In a new paper published in Astrophysical Journal Letters, Charles Steinhardt and colleagues measured the masses of very distant galaxies, along with the rate at which they were forming stars.
According to the hierarchical model of galaxy formation, the first galaxies were built out of smaller collections of matter. In that view, galaxies formed stars in bursts, with each wave of birth corresponding to individual events in the galaxy’s history. Those events might be mergers between or munching of smaller galaxies, but in any case, each galaxy should be governed by its own history. Not least, the earliest specimens should be small, with bigger galaxies coming along later.
The new research discovered something different. The authors found a substantial number of very massive galaxies—more than 10 times the mass of the Milky Way—at a much earlier time than expected. (I have to say that the image accompanying that press release is among the more … embarrassingly dorky examples I’ve seen.) Not only that, these galaxies were busily making new stars, with bigger galaxies giving birth faster than those with lower mass.
That’s consistent with what astronomers have seen for closer galaxies. But early galaxies—at least very high-mass ones—should drop off in star formation at some point. That expectation depends on galaxies merging from smaller chunks, and depleting some of their available star-making fuel.
Instead, the SPLASH data indicate at least some very massive galaxies may have formed in the same way, producing stars as fast as they possibly could. If that’s true, they might be a small exception to the hierarchical rule: rather than forming out of smaller galaxies, maybe they were born big.
The weasel words are necessary, though. Due to the limitations of the current survey sample, SPLASH can see big, bright galaxies very well at large distances, but those might be anomalies. If there are big galaxies where star formation stopped, or smaller galaxies that behave more as theory predicts, SPLASH won’t contain those: they would be too faint.
But this is hardly the end of the story. The next set of results from the Subaru Hyper-Suprime-Cam will extend the SPLASH survey in important ways. This sample will include lower mass galaxies, big galaxies making stars at a slower rate, and—perhaps most importantly—galaxies at an even earlier era, back to 630 million years after the Big Bang.
The larger survey should tell us how typical the big SPLASH galaxies are. If other galaxies behave more like theory predicts, then maybe we’re just seeing the weird exceptions right now. Additionally, by looking earlier in time, there should be a point before any large galaxies existed.
If there were such massive galaxies with rapid star-formation much earlier than in the current catalog, then we might need to rethink our models for galaxy birth. However, it’s too soon to do that. By peeking into that period, astronomers will hopefully learn whether the big babies were born that huge, or if they merged out of smaller galaxies.