Deep in the cosmic forest, a black hole goldilocks might like

Seven hundred and forty million years ago, a star disappeared in a shriek of X-rays.

In 2006 a pair of satellites, NASA’s Chandra X-ray Observatory and the European Space Agency’s X-ray Multi-Mission (XMM-Newton, for short), detected that shriek as a faint spot of radiation coming from a far-off corner of the Milky Way.

To Dacheng Lin, an astronomer at the University of New Hampshire who hunts black holes, those signals were the trademark remains of a star that had been swallowed by a black hole: an arc of leftover fire, like drool on the lips of the ultimate cosmic maw.

Such events tend to be perpetrated by supermassive black holes like the one that occupies the centre of our own Milky Way. But this X-ray signal was not coming from the centre of our or any other galaxy.

Rather the X-rays, the fading Cheshire cat smile of a black hole, perhaps were coming from the edge of a disk-shaped galaxy about 740 million light years from Earth, in the direction of Aquarius but far beyond the stars that make up that constellation.

That meant that Lin had every reason to suspect that he had hooked one of the rarest and most-sought creatures in the cosmic bestiary — an intermediate-mass black hole.

The word “intermediate” might be a misnomer. If Lin was right, he (or, more to the point, that unlucky star) had stumbled upon an invisible sinkhole with the gravitational suction of 50,000 suns. He is the lead author of a paper, published in March in Astrophysical Journal Letters, that describes a cosmic ambulance chase.

Black holes are the unwelcome consequence of Albert Einstein’s general theory of relativity, which explains gravity as the warping of space-time by mass and energy, much as a heavy sleeper sags a mattress. Too much mass in one place causes space-time to sag beyond its limit, trapping even light on a one-way tunnel to eternity.

Einstein disliked the idea, but astronomers have discovered that the universe is littered with black holes. Many are the remains of massive stars that collapsed after burning through their thermonuclear trust funds. Sometimes they collide, rippling space-time and rattling antennas like the LIGO gravitational wave detectors. These holes — the stellar survivors — tend to tip the scales at a few times the mass of the sun.

At the other extreme of cosmic extremities are supermassive black holes — weighing in at millions of billions of solar masses — squatting in the centres of galaxies. Their belches produce the fireworks we call quasars.

Nobody knows where these holes came from or how they get so big. Two years ago Australian astronomers discovered a black hole that was 20 billion times more massive than the sun, gorging itself back when the universe was only a couple billion years old.

Astronomers for years have sought the “missing link” in this line of mythological-sounding monsters: black holes “only” thousands or hundreds of thousands of times more massive than the sun.

“Intermediate mass black holes are indeed fascinating, and in some sense these are becoming the frontier of black hole studies,” Daniel Holz, a University of Chicago astrophysicist who was not part of Lin’s team, said in an email.

“Why would the universe only make big and little black holes, and not ones in between? Goldilocks would not be pleased. What makes this particularly troubling for astronomers has to do with our origin stories.”

There is a suggestive correlation between the mass of a galaxy and the mass of the black hole in its centre: The bigger the galaxy, the bigger its hole. This has led astronomers to a rough theory of how the universe gets built in the dark: Small galaxies with their “small” holes accrete into bigger and bigger assemblages of stars, with ever-bigger black holes at the centre of it all.

Intermediate-mass black holes, weighing hundreds or tens of thousands of solar masses, could be expected to anchor the centres of smaller dwarf galaxies. But as such they would be hard to find.

We only notice black holes when they feed. Stellar-size black holes call attention to themselves as they cannibalise their companions in double star systems. Their supergiant cousins feed at troughs at the centers of big galaxies. But intermediate black holes living in dwarf galaxies would normally find little to eat.

“We could only find them when gas and dust fall onto them,” said Natalie Webb, an astronomer at the Institut de Recherche en Astrophysique et Planetologie in Toulouse, France, a member of the XMM team and a co-author of the paper. “When this happens, they shine less brightly than the supermassive black holes, but they are usually just as far away (if not further), so they are usually too faint for our observatories.”

In effect they are only visible when they swallow a star, an event that occurs only once every 10,000 years in any particular galaxy, Webb said.

So Lin may have been lucky indeed. The new source, which his team named 3XMM J215022.4-055108, would be only the second good candidate known.

But there was a possibility that he had been unlucky, and merely detected an outburst on a dense neutron star left over from a supernova explosion in our own galaxy.

Using the Hubble Space Telescope and the XMM for more observations, Lin and his team traced the X-ray emanations to a dense knot of stars about 80 light-years wide that was far past the Milky Way. It was on the outskirts of a faraway galaxy named Gal1.

By coincidence, astronomers using the Canada-France-Hawaii Telescope on Mauna Kea had recorded an outburst of light from that same spot in 2005. That was perhaps the first fatal bite.

Moreover, the knot of stars resembled precisely what astronomers thought the core of a small galaxy would look like if it had been swallowed by bigger one. It fit the notion that galaxies are assembled by mergers.

“This is good news, as it was thought that it is likely that intermediate-mass black holes are found in dwarf galaxies,” Webb said.

Back in the day, the black hole had been the centre of its own little dwarf galaxy. Now it was an empty-nester, most of its stars gone. And it was on the way to an eventual marriage with the bigger black hole at the centre of Gal1.

“Therefore, the new observations confirm the source as one of the best intermediate-mass black hole candidates,” Lin wrote in the recent paper.

This is only one of a few good candidates for the missing link black holes. Another one, HLX-1, was discovered in 2009 — by many of the same astronomers — on the edge of a distant galaxy called ESO 243-49. It, too, is in a small cluster of stars that looks like the remains of a dwarf core, and weighs in at about 20,000 solar masses.

In the case of HLX-1, however, the X-rays seem to be coming from an accretion disk, the doughnut of hot, doomed material swirling outside the edge of a black hole — material that is periodically ripped from a star orbiting the black hole.

Webb, who was the lead discoverer of HLX-1, said, “The star keeps coming back to a similar position and a bit more mass is ripped off and falls onto the black hole,” she said. “We have now seen eight X-ray flares from HLX-1 and have observed it with many different types of telescope.”

The main difference with the new missing link candidate, “is that our object is tearing a star apart, providing strong evidence that it is a massive black hole,” Lin said in a statement released by the Space Telescope Science Institute.

Left unanswered is where such gigantic vortices of hungry nothing come from. The universe seems to come with some assembly required, and black holes are key, but astronomers are still struggling to put the parts list together.

Astronomers have a pretty good sense of how “ordinary” black holes, three to 100 times more massive than the sun, result from the collapse and explosions of massive stars. But there has not been enough time in the history of the universe for such black holes to grow millions or billions of times bigger into the supermassive black holes we see today.

“They must have formed from something else,” Webb said, namely, intermediate mass black holes. Such holes would be dragged together as their home galaxies coalesced into ever bigger galaxies.

Lin agreed that it was “popular“ to believe that supermassive black holes can form from intermediate mass black holes, dragged together as their home galaxies collide and merge.

Less certain is where these medium-class black holes — too massive to result from the collapse of stars as we know them today — came from. One possibility, Lin said, was that they were created by runaway mergers of massive stars in star clusters.

Another idea, Webb said, is that they are left over from the first generation of stars in the universe. Astronomers have calculated that such stars, composed only of primordial hydrogen and helium fresh from the ovens of the Big Bang, could have grown much more massive than stars today and produced giant black holes capable of growing into the missing-link intermediate-mass black holes.

Indeed, some astronomers theorise, hugely dense clouds of primordial gas or dark matter could have collapsed directly into black holes, bypassing the star stage altogether.

Regardless, intermediate-mass black holes “are the missing link between stellar and supermassive black holes,” Holtz of Chicago said. “If we confidently detect this population, it will provide insights into how the universe makes all of its black holes.”

If not, he added, “theorists will need to work even harder to explain how a baby universe makes monster black holes.”

© 2020 New York Times News Service