In a paper published in Nature Astronomy, Christian Wolf of the Australian National University and his colleagues from Australia and Europe called the object at the centre of a newly discovered quasar known as J0529-4351 “the fastest-growing black hole in the universe”.

According to their estimates, this black hole tipped the scales as one of the most massive black holes ever found: 17 billion times as massive as the sun.

However, other astrophysicists cast doubt on the result, questioning the methods by which the mass and luminosity of the new quasar had been estimated. They said the calculations were too uncertain to be conclusive.

“They may have the right value, but I don’t think other observers would be shocked if it turned out the true mass was somewhat less,” said Daniel Holz, a theoretical astrophysicist at the University of Chicago, US.

“It does seem like an extreme object,” he said. But, he added, “I would be shocked if this turned out to be the most luminous quasar in the sky.”

Jenny Greene, a professor of astrophysical sciences at Princeton University, US, called the result “cute”.

“It’s nice to pick out the brightest of something,” she said.

Still, she agreed with Holz: “I don’t think the luminosity difference between this and other quasars is that big. And given the historical variability of quasars, it is not clear this object even really is more luminous than the others.”

Chung-Pei Ma, an astrophysicist at the University of California, Berkeley, US, weighed in, saying that the estimate of these black hole masses could be off by a factor of two or three, “too large to make me lose sleep over the viability of prevailing cosmological models”.

This is a story of mind-bending big numbers, no matter how it comes out.

“There’s this weird game we play in astronomy where we’re always looking for the biggest, the brightest, the youngest, the oldest, etc.,” Holz said in an email. “Record-breaking objects are an efficient way to learn about the universe. Extremes help illuminate the contours of a problem, and help push our theories up to (or past) their breaking points.”

So it is with quasars and black holes. Quasars are distant objects that look like stars in the sky. In the 1960s, they were discovered to be emitting improbable torrents of energy, outshining all the stars in the galaxy in which they were embedded.

Astronomers have since concluded that all this energy is produced by matter falling into giant black holes. Just as a bathtub can’t drain in an instant, matter can only disappear down the cosmic drain at a rate called the Eddington limit, depending on the black hole’s size. The rest is trapped in a sort of turnstile of doom, a
swirling, sparking disc radiating energy. Which makes black holes, despite their name, the brightest objects in the universe.

Because they look like stars, quasars are hard to find in the sky. Wolf, who is a dedicated quasar hunter, said in an email that he relished the hunt. “It makes me feel like a kid again,” he wrote.

In this case, the quasar was hiding in plain sight in the database of the European Space Agency’s Gaia spacecraft, which has mapped the locations and properties of billions of stars since it was launched way back in 2013.

Wolf and his team identified it as a quasar after observing it with a telescope at Siding Spring Observatory in Australia. Follow-up spectrographic measurements with the Very Large Telescope operated by the European Southern Observatory at La Silla in Chile, allowed them to estimate the size of the accretion disc and the speed of the gas within it.

That in turn let them conclude that the black hole was some 17 billion solar masses and was accreting mass as fast as it could, at the Eddington limit, given its size or mass.

“In this process its accretion disc alone releases a radiative energy that is equivalent to the output from between 365 and 640 trillion suns,” the astronomers wrote in their paper. They hope to do better soon with an upgraded version of a new high-resolution
instrument, called Gravity on the Very Large Telescope, and the upcoming Euro-
pean Extremely Large Telescope now under construction in Chile.

Acknowledging that all estimates of these distant early universe black hole masses were indeed uncertain by a large margin, Wolf said that the new instruments should be able to get a really well-defined image of the rotating storm disc leading to an accurate black hole mass.

“This will check the scale that we are using right at the highest and most extreme end, and it may help to settle the debate on all these extrapolations that we currently rely on,” he said. “This will definitely be an important step for
cosmology.”

By comparison, the black hole at the centre of the Milky Way is only 4 million times as massive as the sun, and the black hole imaged at the centre of the giant galaxy M87 in Virgo is 6.5 billion times as massive as the sun.

NYTNS