Hubble's best evidence yet for 'missing link' of black holes may solve 14-year-old space mystery

The Hubble Space Telescope is tracking down a suspected black hole that shredded a wayward star that came too close for comfort.

The famed space observatory made the find while hunting down the source of a powerful burst of X-rays caught in 2006 by two other cosmic telescopes: NASA's Chandra X-ray Observatory and the European Space Agency's X-ray Multi-Mirror Mission (XMM-Newton).

At the time, astronomers weren't sure if the X-rays had come from inside or outside of the Milky Way galaxy, but new high-resolution photography by Hubble shows that the X-ray source (known as 3XMM J215022.4−055108) is located in a star cluster at the edge of another galaxy. That's exactly where an intermediate-sized black hole (IMBH) may lurk — at least, according to theory. 

Video: Elusive midsize black hole found using Hubble
Related: The missing link: Where are medium-size black holes?

This Hubble Space Telescope image shows the location of an intermediate-mass black hole named 3XMM J215022.4−055108 (in the white circle). Hubble's high-resolution imagery revealed that the black hole resides inside a dense star cluster far beyond the Milky Way galaxy, in the outskirts of another galaxy pictured at the center of this image. (Image credit: NASA/ESA/D. Lin (University of New Hampshire))

The team's work further suggests the star cluster may have been the core of a small dwarf galaxy disrupted long ago, when the dwarf galaxy strayed too close to the larger galaxy that currently hosts the star cluster. Gravitational interactions with the larger galaxy may have ripped the dwarf galaxy apart, leaving only a small cluster of stars in its wake. In much the same way, astronomers believe that the gravitational pull of the black hole inside this cluster shredded a star that got too close, thereby producing the X-ray flare detected in 2006. 

If the X-rays indeed came from a black hole, astronomers suspect the object is about 50,000 times the mass of our sun. That's a featherweight compared with the supermassive black hole that lies in the center of our Milky Way galaxy, which has a mass four million times greater than that of the sun.

"IMBHs have been particularly difficult to find because they are smaller and less active than supermassive black holes; they do not have readily available sources of fuel, nor as strong a gravitational pull to draw stars and other cosmic material which would produce telltale X-ray glows," NASA officials said in a statement. "Astronomers essentially have to catch an IMBH red-handed in the act of gobbling up a star."

Related: Rarely seen middleweight black hole gobbles star 

An artist's impression of an intermediate-mass black hole tearing apart a wayward star that passed too close. (Image credit: NASA/ESA/D. Player (STScI))

IMBHs are believed to be a "missing link" in our understanding of how black holes evolve. Astronomers have seen many examples of small black holes that are similar in size to a star, and other examples of much larger black holes that typically reside in the centers of galaxies. 

But IMBHs remain difficult to confirm as astronomers struggle to understand how supermassive black holes got so darn big, compared to stellar-sized black holes. Hubble and other observatories have found other IMBH candidates in the past, but this newest observation is considered the strongest evidence yet for this class of black hole, according to NASA.

"Intermediate-mass black holes are very elusive objects, and so it is critical to carefully consider and rule out alternative explanations for each candidate. That is what Hubble has allowed us to do for our candidate," Dacheng Lin, a research assistant professor at the University of New Hampshire and principal investigator of the new study, said in the statement.

One other theory for the X-rays was that a neutron star — the dense remnants of an exploded supernova star — may have been responsible. Based on the X-ray glow of the chewed-up star, however, astronomers calculated that the black hole's mass was 50,000 solar masses, which is far more massive than a neutron star (typically about the size of a small city). The mass was calculated using both the X-ray luminosity — the inherent brightness of the X-rays — and the shape of the spectrum.

Hubble also helped track down another possible IMBH in 2009. The object, called HLX-1, was spotted on the edge of a galaxy known as ESO 243-49 — and also resides in a star cluster that could have been a dwarf galaxy in the ancient past. The X-rays spotted in the case of HLX-1, however, likely came from the accretion disk surrounding the black hole. The accretion disk is considered the point of no return for light or other objects to escape a black hole's gravity.

"The main difference is that our object is tearing a star apart, providing strong evidence that it is a massive black hole, instead of a stellar-mass black hole as people often worry about for previous candidates including HLX-1," Lin said.

A paper based on the study was published March 31 in Astrophysical Journal Letters.

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Elizabeth Howell
Staff Writer, Spaceflight

Elizabeth Howell (she/her), Ph.D., is a staff writer in the spaceflight channel since 2022 covering diversity, education and gaming as well. She was contributing writer for for 10 years before joining full-time. Elizabeth's reporting includes multiple exclusives with the White House and Office of the Vice-President of the United States, an exclusive conversation with aspiring space tourist (and NSYNC bassist) Lance Bass, speaking several times with the International Space Station, witnessing five human spaceflight launches on two continents, flying parabolic, working inside a spacesuit, and participating in a simulated Mars mission. Her latest book, "Why Am I Taller?", is co-written with astronaut Dave Williams. Elizabeth holds a Ph.D. and M.Sc. in Space Studies from the University of North Dakota, a Bachelor of Journalism from Canada's Carleton University and a Bachelor of History from Canada's Athabasca University. Elizabeth is also a post-secondary instructor in communications and science at several institutions since 2015; her experience includes developing and teaching an astronomy course at Canada's Algonquin College (with Indigenous content as well) to more than 1,000 students since 2020. Elizabeth first got interested in space after watching the movie Apollo 13 in 1996, and still wants to be an astronaut someday. Mastodon:

  • Jason K
    Just a quick observation. In the section of the article discussing an initial competing theory, of this X-ray burst possibly emanating from a neutron star, it's observed that the calculated mass of the X-ray emitter is at least 50,000 solar masses, which is much more massive than a neutron star. It then parenthetically mentions that neutron stars are about the size of a city.

    While the parenthetical note is completely correct about the physical size of an average neutron star, it does not aid the argument at all, which is discussing mass differences between IMBH and neutron stars. Mass and physical size are two independent measurements, and knowing one doesn't suggest anything about the other. I think giving the average range of neutron star masses (1.4 - 3 solar masses) would have been much more effective to illustrate that they are nowhere near the calculated 50,000 solar masses of the X-ray burst's progenitor.