Black Hole Appears, Disappears, and May Return Again

Date:20 January 2003

When working with big numbers and data from faraway places, small errors can have huge consequences. Black holes, for example, can seem to pop in and out of existence, only to possibly materialize yet again.

Just ask the Hubble Space Telescope astronomers who last September announced they'd found a mid-sized black hole in a distant star cluster. Their apparent discovery provided long-sought and compelling evidence that the most massive black holes might evolve through mergers of smaller ones.

The Hubble team based their findings on observed motions of stars in the cluster, movement that can only be explained by a lot of mass that must exist in the center of the cluster. But just how tightly packed is that mass? They used a computer model that had been developed earlier by theorists at Indiana University to answer that question.

Unfortunately, a scientific paper explaining the Indiana model had one figure labeled incorrectly.
The error threw the Hubble calculations off, leading to the announcement of a black hole thought to weigh 4,000 times more than our Sun.

It turns out there may not even be a black hole in the star cluster, called M15, a separate team of astronomers now says. Instead, a more plausible explanation is that the central region of M15 is a mass cosmic grave littered with stellar corpses. These individual neutron stars and white dwarfs are dense objects, the remains of exploded stars.

Encore?

Astronomers have gone back-and-forth on the question of a black hole in M15 for some 30 years. In fact, in the early 1990s some Hubble data was used to refute the possibility. And this latest round of the saga is far from over.

The apparently evaporated black hole may soon theoretically reappear, SPACE.com has learned.

Astronomers will meet in California next week to argue the issue while discussing other topics.



Meanwhile the turnabout, if it is one, comes in part from fresh computer simulations done by a Japanese-led team from the University of Tokyo and published in the Astrophysical Journal earlier this month.

"The [original Hubble] findings do not require a black hole," said Steve McMillan, a Drexel University researcher and part of the Japanese-led team. "There are other ways of explaining the data." McMillan was careful to point out that the new calculations do not rule out a black hole.

Researchers involved in the September M15 finding and the computer-model developers from Indiana University have independently reached a similar conclusion and published their revised findings in the same journal.

What happened

The error fed into the Hubble study involved an underestimate of the number of neutron stars thought to exist near the center of M15, which sits about 32,000 light-years away, within our Milky Way Galaxy. Put more neutron stars into the picture and the mass of the black hole is mathematically driven down and can even be disregarded.

"The horizontal axis had been labeled incorrectly," said Roeland Van Der Marel of the Space Telescope Science Institute, which operates Hubble for NASA. "Little more than a typo, but with important consequences for our analysis. The Indiana group only discovered their error after we published our work."

Van Der Marel said the mistake was "a pity for all parties involved. But in the end, as scientists we are all just interested in learning about the true nature of the universe, so we are happy that the error was caught."

The snafu is important because middleweight black holes are viewed as a possible missing link in the evolution of the universe's first stellar black holes to supermassive black holes, which weighed as much as billions of stars. Because globular clusters are typically ancient star groupings, finding middleweight black holes in them would suggest they might have long ago served as building blocks; black hole mergers might have been very important to black hole growth.

Experts are hotly debating whether middleweights exist, however.

Holding out

The Hubble team reworked their calculations of M15, too. They came up with a new, lower ceiling for the possible mass of a black hole there, but they admit that there might not be one. Here's why: Because M15 is so densely packed with stars and other matter, it could drive neutron stars or white dwarfs toward the center, which would then mimic the appearance of a black hole harboring the collective mass of all the neutron stars.

"A black hole of 2,000 solar masses continues to be a possible interpretation of the data, although not necessarily the preferred one," Van Der Marel said Friday.

Karl Gebhardt, another member of the Hubble team, is a complete holdout.

The University of Texas astronomer worked on the original M15 finding and staunchly adheres to the likelihood of the middleweight black hole's presence. He does not think the original Indiana University model, even when applied with the correct numbers, paints a proper picture of the population of neutron stars.

"I still stand by the value of the mass that we reported," Gebhardt said in a telephone interview Friday.

The whole debate will be hashed out face-to-face beginning Jan. 27, when Gebhardt and his colleagues meet McMillan and other members of his Japanese-led team. Gebhardt expects a friendly and professional airing of quite different views at the Santa Barbara gathering.

"It's going to be me against the Japanese group," he said. "It's going to be a fun meeting."

Wildcards to play

Gebhardt is holding some wildcards.

In a separate Hubble-based study, also released last September and led by Gebhardt, similar evidence for a middleweight black hole was found in a globular cluster called G1. That discovery has not been double-checked (though the Japanese team is doing so right now and may report their result in Santa Barbara, McMillan said).

Gebhardt thinks the G1 finding is safe. "The same arguments do not apply in G1," he said.

The G1 cluster is not as concentrated as M15, he explained, so it is theoretically difficult to drive as many neutron stars to the center of G1. This implies that the more likely candidate for the mass there, to sufficiently describe the observed star motions, is a middleweight black hole.

Whatever comes of next week's meeting, Gebhardt says he's not worried about the fact that "most of the scientific community does not believe that globular clusters have black holes. I have a pile of other data on other globular clusters" that may well reveal additional middleweight black holes in about two months time.