Astronomers found an intermediate black hole — not too big, not too small — that sheds light on how the universe was assembled in the dark.
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 Ducheng 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 center of our own Milky Way. But this X-ray signal was not coming from the center of our or any other galaxy.
Rather the X-rays, the fading Cheshire 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 Dr. 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 Dr. 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 Astronomy and Astrophysics, 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 centers 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.
“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 center: 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 center of it all.
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 Dr. Lin’s team, said in an email.
“Why would the universe only make big and little black holes, 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 center: 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 center of it all.
Intermediate-mass black holes, weighing hundreds or tens of thousands of solar masses, could be expected to anchor the centers 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 cannibalize 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