Hubble finds missing Intermediate Mass Black Holes

Using the Hubble telescope astronomers have finally spotted an example of an intermediate-mass black hole — the ‘missing link’ between stellar-mass and supermassive black holes.

This artist’s impression depicts a star being torn apart by an intermediate-mass black hole (IMBH), surrounded by an accretion disc. This thin, rotating disc of material consists of the leftovers of a star which was ripped apart by the tidal forces of the black hole. (ESA/Hubble, M. Kornmesser)

Whilst cosmologist’s knowledge of black holes and their locations throughout the cosmos has increased exponentially over the past few decades, one thing frustratingly still eludes researchers — intermediate-mass black holes (IMBHs). That was until the Hubble Space Telescope spotted one of these elusive objects lurking in a dense star cluster.

IMBHs are the long-sought ‘missing link’ between smaller stellar-mass black holes and supermassive black holes, and as such, are a key element in the understanding of the evolution of black holes, in general.

The first example, discovered by Dacheng Lin of the University of New Hampshire, principal investigator of the study published in the Astrophysical Journal Letters, weighs in at a massive 50,000 times that of the Sun and is located in the J2150−0551 region.

“Intermediate-mass black holes are very elusive objects, and so it is critical to carefully consider and rule out alternative explanations for each candidate,” says Lin. “That is what Hubble has allowed us to do for our candidate.”

Hubble discovers Black Holes in unexpected places. (NASA/ESA and G. Bacon (STScI))

The team used Hubble to follow up on observations made with NASA’s Chandra X-ray Observatory and the European Space Agency’s XMM-Newton, the latter of which carries three X-ray telescopes and an optical monitor to so that it can take long uninterrupted exposures and deliver highly sensitive observations.

“Adding further X-ray observations allowed us to understand the total energy output,” says team member Natalie Webb of the Université de Toulouse in France. “This helps us to understand the type of star that was disrupted by the black hole.”

The previous observations consisted of a powerful X-ray flare — named 3XMM J215022.4−055108 — detected in 2006. At the time researchers were unable to determine if the emission had originated from inside or outside the Milky Way. They suspected that the emission was the result of a star being torn apart after straying too close to a massive compact object with a powerful gravitational influence— most likely a black hole.

Speculation that this black hole could be a IMBH initially arose from the fact that the emission had not come from the centre of a galaxy — where supermassive black holes reside. Yet, before a conclusion could be drawn, researchers had to first rule out another object notorious for its strong X-ray emissions, a neutron star.

The emission could well have originated from one of these ultra-dense stellar remnants lurking within our galaxy as it cooled off from being super-heated. Thus, Hubble was pointed towards X-ray source in order to determine the location of the emission and reveal if the culprit was a local neutron star, or indeed, the first example of an IMBH.

This Hubble Space Telescope image identified the location of an intermediate-mass black hole (IMBH), weighing over 50 000 times the mass of our Sun (making it much smaller than the supermassive black holes found in the centres of galaxies). The black hole, named 3XMM J215022.4−055108, is indicated by the white circle. This elusive type of black hole was first identified via a telltale burst of X-rays emitted by hot gas from a star as it was captured and destroyed by the black hole. Hubble was needed to pinpoint the black hole’s location in visible light. Hubble’s deep, high-resolution imaging shows that the black hole resides inside a dense cluster of stars that is far beyond our Milky Way galaxy. The star cluster is in the vicinity of the galaxy at the centre of the image. Much smaller images of distant background galaxies appear sprinkled around the image, including a face-on spiral just above the central foreground galaxy. This photo was taken with Hubble’s Advanced Camera for Surveys. (NASA, ESA, and D. Lin (University of New Hampshire))

Deep, high-resolution imaging confirmed that the X-rays emanated not from an isolated source in our galaxy, but rather from a distant, dense star cluster on the outskirts of another galaxy. Exactly the kind of location astronomers expect to find evidence of IMBHs.

Previous Hubble research has shown a correlation between the size of the galaxy, and the mass of its black hole. Thus, these new results suggest that the star cluster that is home to 3XMM J215022.4−055108 could well be the stripped-down core of a lower-mass dwarf galaxy which has been gravitationally disrupted by close interactions with its a larger galactic host.

The reason IMBHs have been considered difficult to find is that they are smaller and less active than supermassive black holes, without an accessible of fuel, or a gravitational pull that is strong enough for them to be constantly drawing in stars and other cosmic material. This means that they are usually unable to produce the tell-tale X-ray glow associated with their more massive counterparts.

This means that to spot an IMBH, astronomers must catch it as it consumes a star. To do this, Lin and his colleagues combed through the XMM-Newton data archive, searching hundreds of thousands of sources to find strong evidence for particular IMBH candidate.

Once found this tell-tale signal, the team was able to analyse the X-ray glow from the star as it was ripped apart to estimate the black hole’s mass. Confirming the first sighting of an IMBH.

This is an animation of a rare and exotic intermediate-mass black hole at the centre of a star cluster, similar to the one thought to be at the centre of globular cluster Messier 15. Studying these unusual black holes could tell us about how such objects grow and evolve within both star clusters and galaxies. (NASA, ESA, and M. Kornmesser)

For Lin and his team, confirming one IMBH suggests many more remain undetected only to be revealed when an unfortunate star passes too close. Lin intends to continue using the methods his team has proved successful, scouring through data to find another signal.

One of the questions that the researchers will be seeking to answer do supermassive black holes evolve from IMBHs? Currently, cosmology has a time-scale problem, in that these objects seem to have existed at an early stage in the Universe’s history. A point so early, in fact, that it would not leave them enough time to gather the required mass to reach supermassive status.

There are also questions to be asked about the conditions that favour IMBHs and the galaxies in which they are most likely to be found.

Wide-field image around the field of J2150−0551 (ground-based view), in which an intermediate-mass black hole named 3XMM J215022.4−055108 has been detected. (NASA, ESA, Digitized Sky Survey 2. Acknowledgement: Davide De Martin)

In addition to this, black holes can act as testing grounds for Einstein’s theory of general relativity — the geometric theory of gravity that suggest that objects of great mass curve spacetime much like a bowling ball would curve a stretched rubber sheet.

“Studying the origin and evolution of the intermediate-mass black holes will finally give an answer as to how the supermassive black holes that we find in the centres of massive galaxies came to exist,” concludes Webb.