There’s a supermassive black hole at the center of almost every galaxy. Each of these cosmic objects weighs millions, if not billions, of times the mass of our Sun. But their formation is an unsolved mystery. Now, astronomers have spotted a unique object that they think is a precursor to a supermassive black hole.

As reported in the journal Nature, combining Hubble archival data, the object – named GNz7q – was spotted in a galaxy from the very early universe and they think it could be the evolutionary "missing link" between star-forming galaxies and the earliest supermassive black holes. Ultraviolet and infrared observations of the galaxy show that a black hole is rapidly growing inside it, its light traveling all the way from just 750 million years after the Big Bang.

The finding is a boon. Astronomers have struggled to explain the existence of already huge black holes so early in the universe. So far the most distant objects spotted are either galaxies that have huge amounts of star formation, known as starburst galaxies, or galaxies with an incredibly active supermassive black hole outshining the light of all its surrounding stars, known as quasars.

GNz7q is one of these starburst galaxies, but the observations suggest that its central black hole is gearing up to take over in brightness and change into a quasar.

"Our analysis suggests that GNz7q is the first example of a rapidly growing black hole in the dusty core of a starburst galaxy at an epoch close to the earliest supermassive black hole known in the universe," lead author Seiji Fujimoto, of the Niels Bohr Institute of the University of Copenhagen, said in a statement.

"The object's properties across the electromagnetic spectrum are in excellent agreement with predictions from theoretical simulations."

The galaxy is forming stars at a rate of 1,600 solar masses per year, thousands more than our own galaxy, the Milky Way. And while it is emitting UV light consistent with a disk of material accreting around a black hole – like a quasar – it hasn’t any X-ray emission visible yet. That comes from the innermost part of an accretion disk. This suggests that the nascent supermassive black hole is shrouded in dust and gas. Rapidly growing black holes in dusty early galaxies had been predicted by computer simulations but not seen until now.

The team thinks this object could be the missing link between starburst galaxies and quasars.

"GNz7q provides a direct connection between these two rare populations and provides a new avenue toward understanding the rapid growth of supermassive black holes in the early days of the universe," said Fujimoto. "Our discovery provides an example of precursors to the supermassive black holes we observe at later epochs."

The Hubble observations were combined with data from a well-studied region by the Great Observatories Origins Deep Survey-North (GOODS-North). Going through archived data and finding this object suggests there may be more of these objects out there.

"GNz7q is a unique discovery that was found just at the center of a famous, well-studied sky field — it shows that big discoveries can often be hidden just in front of you," said co-author Gabriel Brammer, also from the Niels Bohr Institute. "It's unlikely that discovering GNz7q within the relatively small GOODS-North survey area was just 'dumb luck,' but rather that the prevalence of such sources may in fact be significantly higher than previously thought."

The team now hopes to study this object further with the JWST once science operations begin in a few months.