Earth's Closest Supermassive Black holes Observed in action

Using one of the world’s most powerful optical telescopes, an international team of scientists - including an Australian researcher - have uncovered the closest pair of supermassive black holes to Earth ever observed. The pair is undergoing a merger event, providing scientists with a wealth of information and insight (at relatively close proximity) on the processes that take place when such behemoths spiral together and collide. 

Located in the constellation of Aquarius at a distance of approximately 89 million light-years away, the pair of supermassive black holes can be found inside their host galaxy, NGC 7727, which is considered a peculiar galaxy due to its unusual plumes and streams of irregular shapes. Even more interesting, is that unlike most other galaxies (such as our own Milky Way), NGC 7727 features a double nucleus - that is, two supermassive black holes (SMBH). 

The first of the two, located at the heart of the galaxy, contains 154 million solar masses, whilst the second - a little offset from the centre - weighs in at 6.3 million solar masses. Not only does the close proximity to this duelling pair make them interesting, as to does the relatively small distance between them both - a mere 1,600 light-years (which in astronomical terms, is fairly close). 

“It’s the first time we find two supermassive black holes that are this close to each other, less than half the separation of the previous record holder,” said lead author and astronomer Karina Voggel from Strasbourg Observatory in France. 

To make detailed measurements of the two massive objects, Voggel and her team used the European Southern Observatory’s VLT (located at the Paranal Observatory in Chile), and in particular, a powerful spectrograph known as MUSE (Multi-Unit Spectroscopic Explorer). 

MUSE was able to measure the forceful pull of each object by its gravitational influence on surrounding stars - confirming that indeed, these were supermassive black holes. Associate Professor Holger Baumgardt from the University of Queensland was also involved as co-author of the paper, indicating what will likely be the future outcome of this system.

“The small separation and velocity of the two black holes indicate that they will merge into one monster black hole, probably within the next 250 million years,” he said. 

Supermassive black holes have an important role to play in the Universe, given that nearly every galaxy has one residing in its centre. Even our own Milky Way Galaxy has one (called “Sgr A*) which weighs in at 4.3 million solar masses. In some cases, these objects (and their mass) have a direct correlation and impact on the galaxy’s evolution, star formation and other considerations. 

Unlike their stellar-mass counterparts, what makes SMBHs special is their enormous mass, which ranges in the millions or even up to the billions in solar masses. It’s very unlikely that a single object (of many billion masses) collapsed to form SMBHs, so many scientists have concluded that to form these structures, smaller and smaller mass black holes are needed to merge. 

Since 2015, when gravitational waves were detected for the first time using LIGO’s interferometers, scientists have observed stellar-mass black holes merging and forming larger structures. The largest of these post-merger objects so far detected is just under 200 solar masses, whereas the electromagnetically observed black hole is slightly over 20 solar masses. This data tells us that black holes can grow in size through mergers with other black holes.

But is there an upper limit to this growth? And are there also regions of space that are so overpopulated in black holes that there is enough of them to grow into billions of times the mass of the Sun. Inversely, and importantly - has enough time passed in the Universe’s history for this to have happened yet? For example, there are a number of extremely distant SMBHs which have been observed in high redshift, which present problems for the accreting smaller black holes model - because not enough time would have passed from the birth of the Universe through to the observed SMBH (an example of this is an object known as J0313-1806) at this distance. 

These are all important questions that are active areas of research by scientists across the world to help us better understand how SMBH form over time. 

One way of learning about merging SMBHs is to study them not just in the visual bands, but also across the gravitational spectrum - similar to how current gravitational wave detectors observe the mergers of stellar-mass black holes. 

However, the current detectors we have (like the giant interferometers of LIGO, Virgo and KAGRA) are specifically tuned into detecting stellar-mass events (i.e. merging stellar black holes and neutron stars). When it comes to supermassive black holes, a whole new type of detector is needed. 

Any accelerating mass produces gravitational waves - normally, these are extremely feeble and unable to be detected. But as you increase the mass of an object (say, to something as compact and heavy as a neutron star or black hole), then this really distorts the fabric of space-time. 

But these objects merge in minutes and seconds, so the frequency of the merger signal (known as the gravitational wave strain) is considered in the higher frequency bands. But if we’re talking about even bigger masses, moving in inspiral orbits that are lasting millions of years - then this frequency is much lower - and our detector needs to be much bigger. 

One clever technique that astronomers have come up with is to use pulsars located around the galaxy, as giant arms of an interferometer in all directions. By studying the extreme regularity of ticking pulsar signals, and then analysis this data, scientists can establish if there is a correlated signal amongst all pulsars that would be indicative of a low-frequency gravitational wave signal - the exact kind generated by colliding supermassive black hole binaries. 

Another method would be to study these objects using future space-based interferometers, such as the LISA mission - an ambitious project that has three spacecraft orbiting in configuration with lasers beaming between each (and forming a triangle), sensitive enough to be perturbed by passing low-frequency gravitational waves. 

These types of studies, combined with electromagnetic observations - such as those outlined in this paper - will help enlighten our understanding of SMBH, their growth via galaxy mergers, and their impact on galaxy formation and evolution.

Historically, other double SMBH binary candidates have been at such large distances, that resolving data to fortify that dual SMBH systems exist within the one galaxy has been a challenge - with greater distance comes a higher degree of effort to attempt to ‘see’ both objects.

“Merging of supermassive black holes is expected to happen if our understanding of the way how galaxies form and evolve is correct. So it is important to look out for black holes that are on their way of merging and this system is the closest one that we have found so far,” said Associate Prof. Baumgardt. 

These latest findings, which have now been published in the journal Astronomy & Astrophysics, outline that NGC 7727 contains a double nucleus featuring two supermassive black holes, which are currently spiralling towards each other. It’s a viable candidate for such a system and thankfully for us, it’s relatively nearby in astronomical terms - which means scientists can study it in greater detail. 

“Stars move around a black hole and they will move faster the more massive the black hole is. So by measuring the velocities of stars, one can determine the mass of the central black hole which is making them move,” he added. 

The post morphology structures and dynamics of this system are also indicative that both SMBHs are thought to have recently experienced interactivity and a merger event - where the second nucleus is reported to be the former centre of the galaxy that merged with NGC 7727 - one that is thought to have been gas-rich with an estimated mass of 5 billion Suns. Even still, this former merger event is considered to be minor, not completely disrupting NGC 7727. 

Whilst these results are intriguing, especially given the historical detective-like analysis on this particular galaxy’s history and evolution, Baumgardt did express that the findings come with some caution of uncertainty. 

“There are relations that connect the mass of the central nucleus with the properties of the Galaxies that host them So by measuring how massive the nucleus is one can deduce the mass of the former galaxy that was hosting it,” he said. 

In Addition to the usage of the MUSE instrument with the VLT, archival data from the Hubble Space Telescope was also incorporated in determining the results. “Hubble is important since with Hubble one can measure how light is distributed in the galaxy and the two nuclei. The light distribution is, together with the kinematics, an important ingredient in the dynamical modelling of the nuclei and the derivation of the masses of the two black holes.”

According to Baumgardt, the next steps could potentially be resolving the structures in multi-wavelength or finding other galaxies that feature orbiting supermassive black hole binaries. 

“One could use radio or X-ray telescopes to look for signs of gas accretion onto the black holes. Apart from that, the next step would be to find more systems like this one. Finding one galaxy like this still does not give very strong constraints on galaxy evolution models, but if we can find more of this type then we could start to constrain these models, especially on those aspects of the models that predict the number and masses of supermassive black holes,” he concluded.