hide and seek

Bizarre black hole goes missing, confusing scientists

"It’s there, it’s just being very hard to find."

Astronomers are on the hunt for a missing cosmic creature. The quarry in question is dark, ruthless, and clocks in at about 100 billion times the mass of the Sun.

For years, scientists have observed a large galaxy that lies 2.7 billion light years away from Earth. From every data point they have gathered, there is clear evidence to suggest there is a massive black hole lurking at the center of the galaxy, much like in our own Milky Way. But while scientists are adamant the black hole is there, they haven't been able to find it. Not a trace.

But they are getting closer to solving this mega-sized mystery. Using a new set of data, a team of researchers has been able to home in on the missing black hole's potential whereabouts, allowing them to construct a couple of scenarios to explain where this behemoth may be hiding.

Kayhan Gultekin, a professor at the University of Michigan's department of astronomy, is the lead author on a new study detailing these scenarios. He tells Inverse he is absolutely sure the black hole is there — ad he is determined to find it.

"It’s there, it’s just being very hard to find," Gultekin says.

Gultekin and his colleagues' work is detailed in a study published to online preprint server arXiv. Although not peer-reviewed, the study has been accepted for publication in the American Astronomical Society journals.

The giant elliptical galaxy in the center of this image, taken by NASA's Hubble Space Telescope, is the most massive, central, and brightest member of the galaxy cluster Abell 2261.NASA, ESA, M. Postman (STScI), T. Lauer (NOAO), and the CLASH team

The mystery began in 1999 — Using NASA's Chandra X-ray Observatory, astronomers gathered observations on a galaxy cluster dubbed Abell 2261. At the center of the cluster is a galaxy which outshines the rest: Abell 2261-BCG. This galaxy is some ten times larger than our own Milky Way.

It is well-established that galaxies the same size as the Milky Way or larger contain a massive black hole at their center. The supermassive black hole at the center of the Milky Way is 3.6 million times the mass of the Sun.

But the observations of Abell 2261-BCG did not reveal a black hole at the large galaxy's center.

Astronomers have been scouring the galaxy in search of a missing black hole.NASA/CXC, NASA/STScI, NAOJ/Subaru, NSF/NRAO/VLA

Astronomers conducted follow up observations in 2004 using NASA's Chandra, and later collected images of the galaxy with NASA's Hubble Space Telescope and the Subaru Telescope, as well as radio emissions measurements using the National Science Foundation's Karl G. Jansky Very Large Array telescope.

And yet: Still no black hole to be found.

Astronomers were undeterred. In 2018, scientists tried Chandra once again, taking further X-ray observations of the galaxy. By imaging the entire galaxy cluster surrounding Abell 2261-BCG, they were able to do a deeper search for the black hole than ever before.

Technically all black holes are hard to find since they're dark — light cannot escape from their depths.

Astronomers usually locate black holes from the motion of the stars next to them, or when they spy certain bright sparks from the black holes which result from the material they draw in towards them, forming a band of dust, gas, and other objects known as their accretion disk.

But some of these cosmic beasts can be shy and quiet despite their size, which may be the case with this missing black hole, Gultekin says.

In the new study, he and his colleagues have narrowed this mystery down to two strong possible explanations:

The first scenario suggests the black hole at the center of the galaxy is not gobbling down on much material from its surroundings, and therefore not accreting as much matter.

"So we can't see the telltale signs of the secretion of the matter," Gultekin says. "If it were really bright it would be easy."

The second scenario is rather out there, literally.

The researchers suggest the black hole may have been expelled from the galaxy after its host merged with another galaxy. The two black holes of the respective galaxies would have merged together, creating a binary black hole.

As two black holes merge, they release a powerful ripple through space and time known as a gravitational wave. The momentum created by the gravitational waves would be strong enough to kick the original black hole out of the galaxy entirely.

This process is known as a recoiling black hole. Astronomers have theorized about these black holes before, but have never observed one.

If it got kicked out Abell 2261-BCG, then the missing black hole is possibly floating aimlessly through space, perhaps surrounded by a few stars.

If this is the case, then the missing black hole is essentially impossible to find. It will remain a mystery to us forever.

Abstract: We use Chandra X-ray observations to look for evidence of a recoiling black hole from the brightest cluster galaxy in Abell 2261 (A2261-BCG). A2261-BCG is a strong candidate for a recoiling black hole because of its large, flat stellar core, revealed by Hubble Space Telescope imaging observations. We took 100-ksec observations with Chandra and combined it with 35 ksec of archival observations to look for low-level accretion onto a black hole of expected mass M∼1010 M⊙ that could possibly be located in one of four off-center stellar knots near the galaxy's center or else in the optical center of the galaxy or in the location of radio emission. We found no X-ray emission arising from a point source in excess of the cluster gas and can place limits on the accretion of any black hole in the central region to a 2-7 keV flux below 4.3×10−16 erg s−1 cm−2, corresponding to a bolometric Eddington fraction of about 10−6. Thus there is either no 1010 M⊙ black hole in the core of A2261-BCG, or it is accreting at a low level. We also discuss the morphology of the X-ray emitting gas in the cluster and how its asymmetry is consistent with a large dynamic event.
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