Colossal primordial stars may have seeded supermassive black holes

The supermassive black holes lurking at the center of most galaxies, including our own, are believed to have grown so big by gobbling up matter over billions of years. But a new study suggests they may have taken a shortcut, starting life as a hypothetical, primordial supermassive star far bigger than any around today, that exploded in a colossal supernova. And we might soon be able to detect the leftovers.

Black holes fall mostly into two distinct classes, based on their mass. There are the stellar mass black holes, with masses between about five and a few dozen times that of the Sun. These form when certain stars exhaust their fuel supply and collapse in on themselves. Up the other end of the scale there are supermassive black holes, with masses of millions or even billions of Suns.

It’s generally thought that the former grows into the latter over billions of years, as they gobble up dust, gas, stars, planets, other black holes, and anything else that ventures too close. That hypothesis is supported by increasing evidence of a new class called intermediate-mass black holes, with masses between 100 and 10,000 times that of the Sun.

Unfortunately the story isn’t quite that simple. Early stars seem to top out between 100 and 200 solar masses, and if they collapsed into black holes, they would need to swallow huge amounts of matter over a long time to grow into supermassive monsters. In many cases there just isn’t enough “food” around. Plus, observations have shown that these gigantic black holes appeared fairly early in the universe’s lifespan, so there literally shouldn’t have been enough time for them to grow that big through this method.

The origin story of supermassive black holes is under heavy investigation by astronomers, but one intriguing possibility is that they took a shortcut, being born from “supermassive” stars with masses in the realm of tens of thousands of Suns. These absolute beasts would be far more massive than any star we’ve ever seen, and since the physics might not add up, their existence is debatable. They would, however, be a convenient explanation.

"There may be a small number of the first stars in the early universe with tens of thousands of solar masses,” says Ke-Jung Chen, an author of the study. “They are likely to be the progenitors of supermassive black holes in galaxies, because the more massive of the black hole seed, the more efficient it is to swallow the surrounding matter. The black holes don’t need to maintain a high accretion rate to grow quickly!”

The tricky part is finding direct evidence for them, because if they did exist they’d likely have all collapsed into black holes billions of years ago. Most proponents of the supermassive star theory predict that they would collapse directly, without going supernova – but Chen disagrees.

In a 2014 paper, Chen outlined a model for how a primordial supermassive star of around 55,000 solar masses could undergo a supernova caused by explosive helium burning. In the new study, Chen and his team simulated the radiation dynamics of this kind of event, and found that they should give off a luminous glow for decades after the event. This would appear extremely red thanks to the phenomenon of redshift, where the expansion of the universe stretches wavelengths of light towards the red end of the spectrum.

Most interestingly, this should be visible in near-infrared surveys like the upcoming James Webb Space Telescope. This, the team says, means we could soon detect the afterglow of these supermassive star supernovae, and validate the hypothesis.

Or perhaps we won’t find anything, and can cross them off the list. Either way, it’s another reason to be excited about the launch of the long-awaited successor to Hubble.

The research was published in the journal Monthly Notices of the Royal Astronomical Society.

Source: Academia Sinica Institute of Astronomy and Astrophysics