Elusive black holes: Have we found the ‘middle sibling’?

Title: A luminous X-ray outburst from an intermediate-mass black hole in an off-centre star cluster

Authors: Dacheng Lin, Jay Strader, Eleazar R. Carrasco, Dany Page, Aaron J. Romanowsky, Jeroen Homan, Jimmy A. Irwin, Ronald A. Remillard, Olivier Godet, Natalie A. Webb, Holger Baumgardt, Rudy Wijnands, Didier Barret, Pierre-Alain Duc, Jean P. Brodie, Stephen D. J. Gwyn

First Author’s Institution: Space Science Center, University of New Hampshire, Durham, NH 03824, USA

Status: Published in Nature [closed access – open access version available on arXiv]

The ‘middle sibling’ problem

In the past few years, black holes have gone from being far-fetched concepts of science fiction to being observed in many different ways by many different surveys, such as the supermassive black hole (SMBH) at the centre of the Milky Way as well as the recent pioneering observations of the SMBH at the centre of M87. These are examples of SMBHs, expected to reside in the centre of every massive galaxy, and so these are the older, wiser and more experience siblings of the black hole family.

In contrast to SMBHs which have been observed to reach masses of 40 billion solar masses, stellar mass black holes in the range of 7 to 50 solar masses have been observed by LIGO through detections of gravitational waves from the merging of black hole pairs. Whilst not young, their relatively small masses make them the toddlers of the black hole world. Stellar mass black holes are the final stage of the evolution of individual massive stars. Black holes even smaller than this are also theoretically predicted and are even suggested to be numerous enough to be a dark matter candidate. But putting all of this together poses a surprising question. 

We know that when massive stars die, they form stellar mass black holes (up to around 100 solar masses). To build up their mass, black holes merge and also accrete mass from their surrounding stars and gas. Therefore it’s expected that we should see less massive black holes in the early universe, compared to today. 

But we have a mass gap. This mass gap between stellar mass and supermassive black holes are known as intermediate-mass black holes (IMBHs), having masses from around 100 to 100,000 solar masses, but the weird thing is we don’t really observe them. These ‘middle siblings*’ are the grumpy teenagers who are either hiding somewhere or they just don’t exist.

*analogy butchered and tortured from previous astrobites here and here!

There is a well observed black hole mass-stellar mass scaling relation showing that most galaxies have a SMBH of the order of 0.1% of their stellar mass. Therefore, when considering the local Universe, the place to look for IMBHs is not in the centres of massive galaxies, but instead they could be hiding in lower mass objects such dwarf galaxies or stellar/globular clusters, which brings us on to today’s paper where the authors present strong evidence for the presence of an IMBH residing in a star cluster located in the outer region of a large lenticular galaxy.

Timeline of 3XMM J215022.4−055108

Several observations over around 15 years with different instruments allow us to create a timeline of both X-ray sources we’re studying, ‘3XMM J215022.4−055108’ and the large galaxy named ‘6dFGS gJ215022.2-055059’, both seen in Figure 1. Seeing as the names aren’t so catchy, from here on we’ll call the large galaxy in the centre ‘Gal1’ (as named on the image). 

Figure 1: The X-ray position of J2150−0551 is marked with a green circle and appears to be in a barred lenticular galaxy Gal1. The source has a faint optical counterpart, at an offset of only 0.14 arcsec from the X-ray position. The galaxy at the bottom of the image (Gal2) could be a satellite galaxy connected with Gal1 through a tidal stream (red arrow), indicating that Gal1 is rich in minor mergers. (Figure 1 from paper)

2000-2003: Optical Hubble observations

Assuming a group of stars all at a single age (so all formed at the same time), the authors create models of potential systems that match what is observed in the optical and X-ray. The model best matching the observations of the area with the suspected IMBH (within the green circle in figure 1) implies a star cluster with a stellar mass ∼107 M⊙. It could be a very massive globular cluster or more likely a remnant nucleus of a tidally stripped dwarf galaxy in a minor merger. They also observe that the luminosity of the source goes with the temperature to the power of 4, which is often observed in accreting stellar-mass black holes.

2003-2005: Optical flare outburst

In the observations taken in May-November 2005, an optical flare was detected with the source appearing much bluer and brighter in comparison to August 2000. This optical flare was not seen in 2003 and therefore the authors infer the outburst to have started at some point between September 2003 and May 2005. This optical flare is thought to have come from the disruption of a star around the black hole, occurring around mid October 2003.

2006: X-ray outburst

In 2006, the source was observed in the X-ray with XMM-Newton and this time they saw from the X-ray spectrum that the luminosity was now no longer proportional to its temperature to the power of 4. The deviation from this relation is more characteristic of a super-Eddington accretion state.

By fitting these observations, the authors can infer that the star was disrupted by an IMBH with a mass between 5×104 and 105 M⊙.

2014-2016: Observed again in the X-ray with Swift and Chandra

When observed again in the X-rays nearly 10 years later as seen in figure 2, all seems to have calmed down. The source is emitting much lower X-ray fluxes.

Figure 2: Three slits are shown here, with one going through the nucleus of Gal1 and the counterpart to J2150−0551, one for Gal2, and one for a possible background galaxy Gal3. (Supplementary Figure 1 from the paper)

Clues to where other IMBHs may be hiding

The authors suggest that observations of IMBHs are be so difficult because it’s expected that they preferentially form in very dense star clusters which don’t have much gas, and therefore the black holes are electromagnetically invisible. So the analogy of the teenager hiding in their bedroom is pretty perfect! The optical counterpart to this particular source was very faint, but a chance encounter with a star enabled us to see the black hole, emitting energetic feedback as it lunches on its companions. As more and more IMBHs are being found, perhaps this will answer the question of how we end up building up to supermassive black holes in our Universe.

About Jessica May Hislop

Doctoral Student at the Max Planck Institute of Astrophysics in Munich, Germany. Studying the formation of nuclear star clusters and intermediate mass black holes in high resolution simulations of dwarf galaxies.

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    • Thanks Nandini!


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