First Ever Intermediate-Mass Black Hole Directly Observed

When you think about stars ending their life in the Universe, the movies tell us they explode in brilliant blasts that create huge shockwaves, that sweep and destroy over any planets nearby. But not all stars are going to go off with a bang when they die.

Some stars, like our Sun, will huff and puff their outer layers as they near the end of their lives, departing in a relatively simple and quiet manner. They’ll eventually pollute the surrounding region of space with all the new elements they’ve churned in their cores over their 10-billion year lifetimes, billowing out the material and allowing the next generation of stars to use it when it is their time to start the cycle over again.

A lifeless white-hot cinder core of the star that gave life and light to the humans and many other species for all those years, will be the only thing left from this time in our neighbourhood in space.  

Other stars, somewhat larger, will explode like one of those big firecrackers that tears through the night sky with an array of incendiary colours, on New Year’s Eve as the clock strikes 12 - blasting the majority of their own stellar materials out into space at violent speeds, further spreading the polluting tide of heavier elements out into the cosmos.

The cores of these ones however go the other way – they collapse inwards and form mind boggling compact objects – each no larger than a small city – yet spinning hundreds of times per second on its axis, possessing a magnetic field billions of times stronger than that of Earth’s and having densities so large, that a single teaspoon of this new remnant would weigh as much as a cube of Earth’s crust with dimensions of 800m X 800m X 800m.

Even bigger stars don’t stop there, though. Following their violent demise and destruction, their seeding of the Universe with new heavy elements, and their short, yet brilliant lives (which only span a few million years) – the cores of these big ones keep collapsing inwards.

It keeps going and going, until all the material – with a lower limit of about 3 times the mass of our Sun – is squeezed into a point so small, a point with such infinite density, that it causes the very fabric of space-time to curve in on itself, not even allowing light to escape.

A black hole is born.

Which pathway a star ends up following is dependent entirely on its mass.

Low mass stars like our Sun will end up as white dwarfs. High mass stars (with a range of 9 – 25 solar masses) will leave behind a neutron star. And extremely massive stars (which contain over 25 solar masses of materials) will form a black hole.

It’s interesting to note that the entire 25 solar masses doesn’t become the black hole – a lot of that material is blown away and only a few solar masses in the collapsed core becomes the belly of the beast.

By understanding this evolutionary model of how stars end their lives, scientists are able to then use this information (along with a variety of other data) to start to calculate the upper limits of how big stars can be based solely on the type of remnant object observed, and from this also determine the evolutionary formation of such remnant objects.

And here is where it gets really interesting – even more so beyond the fact that we can now observe black holes (an achievement upon itself that should be marvelled at).

By studying a variety of different sources of information (such as gravitational-wave data, x-ray observations, Galaxies, binary star systems, etc.), astrophysicists have been able to classify different categories of black holes: those that are the remnant product of stellar supernova explosions, and those which reside in the centre of galaxies, known as supermassive black holes.

These two distinct populations are classed based on their observed masses, and immediately showcase a fascinating question – how did supermassive blackholes, whose mass range is millions or billions of times that of our Sun, come to be?

There couldn’t have been stars this big to create them individually, so scientists have been trying to work out if they accumulated from smaller, stellar-mass sized black holes, and if so, why isn’t there a third category of objects that sits between – the Intermediate-Mass Black Holes?

Now, Australian scientists (working as part of the LIGO/VIRGO collaboration) have for the first time used gravitational waves to directly observe the birth of a black hole with a resultant mass of around 150 suns, challenging existing models of the lives of big stars before they become black holes.

“Australian researchers in OzGrav have a played a major role in this discovery and, indeed, in all gravitational wave discoveries since the first detection in 2015. We develop key components of the detectors to keep improving their sensitivity, including the recent addition of a squeezed light device," said OzGrav Chief Investigator and co-author, Prof. Susan Scott, from the Australian National University.  

“We construct search pipelines to detect different types of sources of gravitational waves and extract their parameters, and we lead follow-up programs to identify and image electromagnetic counterparts for some of our detected gravitational wave sources. It is beyond exciting to know that Australian research lies at the heart of this new era of gravitational wave discovery.”

The new announcement, a discovery detected with gravitational wave observatories, is that of a remnant monster which was birthed from the merger of two smaller (yet still very large) black holes and our first direct observation of an Intermediate-Mass Black Hole (IMBH).

The extraordinary event, known as GW190521 (as it was a gravitational wave event observed on 21 May 2019) occurred long ago, at a time when the Universe was roughly seven billion years old, though the ripples in the fabric of space-time only swept past the Earth in 2019.

“Intermediate mass black holes (IMBHs) have masses in the approximate range of 100 to 100,000 solar masses, heavier than stellar mass black holes but lighter than supermassive black holes often located at the centres of galaxies.  Although a population of IMBHs is thought to exist throughout the Universe, they have proved very elusive to observe,” said Prof. Susan Scott.

“One of the exciting firsts associated with GW190521 is that, at 142 solar masses, the newly created black hole is the first direct observation of an IMBH. And we saw how it formed! As we accumulate further observations of IMBHs in binary black hole collisions, we hope to study this population and unlock the mystery of how very large black holes grow and evolve with time.” 

The ripples themselves, impossibly tiny distortions detected by some of the most advanced instruments in the world – were the product of eight solar masses being converted into energy as an output of the massive merger event that occurred between the two progenitor black holes.

An array of researchers from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) – a partnership between Swinburne University, Australian National University, Monash University, Adelaide University, University of Melbourne, and the University of Western Australia – all contributed to this exciting scientific finding, utilising the powerful computing resources at the new Gravitational-Wave Data Centre as part of the studies that inferred the masses of the merging black holes.

“This is the first time we’ve observed an intermediate-mass black hole, almost twice as heavy as any other black hole ever observed with gravitational-waves. For this reason, the detected signal is much shorter than those previously observed. In fact, it’s so short that we can barely observe the black hole collision, we can only see its result,” said Juan Calderón Bustillo—co-author and OzGrav postdoctoral researcher at Monash University.

Researchers from the University of Western Australia’s online detection team picked up on the event only seconds after the data became available, as part of the online team’s work, helping generate the alerts for the gravitational wave astronomy (and wider astronomy) community.

“We were among the fastest detection programs to report GW190521. Such a heavy system has never been observed before. It’s exciting to be among the first few to identify it in real-time,” said OzGrav PhD student and co-author Manoj Kovalam.

The GW190521 signal has scientists very excited, as it was both shorter in its duration and peaked in a lower frequency, as observed by the interferometers.

By noting the duration of the signal from merger events, gravitational-wave scientists can determine the total mass of the binary system. The shorter the signal period, the higher the mass of the overall binary pair pre-merger, and for GW190521, it lasted for approximately 0.1 seconds.

Similarly, the frequency at which the signal peaks is also proportional to its mass – with lower frequency signals indicating a higher mass – with lower frequency signals indicating a higher mass. For GW190521, the maximum frequency reached was about 60 Hz (for comparison, the first ever gravitational wave merger event (GW150914) of two smaller black holes peaked at 150 Hz – a much higher frequency).

The interferometers are also sensitive and allow scientists to observe different parts of the gravitational wave signal – the pre-merger, merger and post-merger of the event – which in turn tells us a bit more about the masses of the progenitor and the resultant objects.

For example, gravitational wave signals that are generated from low-mass progenitor black holes merging, present more obviously in the in-spiral pre-merger, or merger part of the event, whereas signals from larger progenitor black holes, much like GW190521, are better observed during the post-merger and ring-down phase of the gravitational wave signal.

“We had to use extremely precise and complex models to analyse these heavier black holes compared to previous models used by LIGO for gravitational waves,” said OzGrav Postdoctoral researcher and LVC member from the University of Melbourne, Meg Millhouse.

A number of tests were once again applied to GW190521 (as they are against all gravitational-wave events) to confirm if Einstein’s General Theory of Relativity – written in the early 1900s –  still stood the test of time, with the data from these latest results adding further confirmation of Einstein’s beautiful theory. In fact, several additional tests were performed to explore alternate theories and hypotheses of gravity, but all failed to contradict the results produced by Einstein’s equations.  

Black holes have always been mysterious objects to us – for the very reason that the laws of physics (and common sense) break down when we consider them. How can the entire mass of a star be squeezed into a point smaller than the sharp end of a pin? How do they have so much gravity – that light can’t even escape, and time starts to break down near them? And are they really gateways to distant locations across the Universe?

These extreme objects (which first appeared as an outcome in Einstein’s equations of gravity), have captivated our imagination for over 100 years. With the advancement in technology, we’ve now also been able to observe them indirectly and directly, far out in space.

“This is a huge step towards understanding the link between the smaller black holes that have been seen by gravitational-wave detectors and the massive black holes that are found in the centre of galaxies,” said OzGrav Chief Investigator and co-author David Ottaway, from University of Adelaide – reflecting on the results of GW190521.

There are four main categories of black holes, based on their masses and how they were formed:

“These ‘impossibly’ massive black holes may be made of two smaller black holes which previously merged. If true, we have a big black hole made of smaller black holes, with even smaller black holes inside them—like Russian Dolls,” said OzGrav postdoctoral researcher and LVC member Simon Stevenson, from Swinburne University of Technology, whilst considering the implications of finding IMBHs.

“We are witnessing the birth of an intermediate mass black hole: a black hole more than 100 times as heavy as the Sun, almost twice as heavy as any black hole previously observed with gravitational-waves. These intermediate mass black holes could be the seeds that grow into the supermassive black holes that reside in the centres of galaxies,” he said.

Gravitational waves are likely passing through you, the Earth, our Solar system and more right now. The emission sources of gravitational waves are accelerating masses – so technically, a human being doing squats next to the detectors would generate gravitational waves.

We don’t experience them as part of our everyday reality, because these distortions are so tiny – they just flow past us. To detect them, scientists had to build observatories that have arms 4 kilometres in length, and place several of them around the world at different locations.

The first two detectors (the Laser Interferometer Gravitational-wave Observatories or LIGO) were built in the USA, in two locations (Hanford, Washington and Livingston, Louisiana) – separated by roughly 3,000 kilometres.

These aren’t like regular observatories or telescopes – there are no big domes or dish antennas – instead, these are L-shaped vacuum tubes that stretch out in perpendicular directions.

Lasers are fired within each of the tubes and hit mirrors (called test masses) at the very ends before being bounced back and merged in sensitive photodetectors. This causes the laser light to ‘interfere’ with itself creating certain patterns. It’s a similar process and experience that occurs when looking at the rainbow patterns of a small amount of spilt oil on the road after it has rained.

As gravitational waves roll past the Earth, they squeeze and stretch the perpendicular arms of the interferometers (along with everything, including the Earth) which causes the laser light to be affected, and as such the resulting inference pattern changes on the detectors as well.

Whilst this all sounds Earth-shattering, the scale of this movement is tiny – measuring only 10^-18 metres, roughly 1/1000^th the diameter of a proton – a vast testament to the engineering genius in the development of these instruments.

So how would scientists know that their observations are legitimate gravitational waves, and not some Raven having a laugh by bouncing up and down on one the detector’s arms?

Here’s where it gets really clever. The observatories are located at a large, measurable distance from each other – and as such, if the exact signal repeats itself at the other location – then it can’t be a localised, random event (noise) – it is seen at the exact expected time, thousands of kilometres away. As an added bonus of this set up, by studying the delay in the signal arrival time from each location, scientists are able to figure out where in the sky the signal is coming from – known as ‘localisation’.

For this reason, new gravitational-wave interferometer observatories have been opening up around the world – with the VIRGO observatory located in Pisa, Italy and the newer KARAGA observatory, in Japan. An additional observatory is also planed for India, and scientists from Australia have also proposed an observatory on our home soil – the first of its kind in the southern hemisphere.

To better understand how different sized stars form into black holes, we must first consider their masses. To begin with, stellar-mass black holes are usually the result of stars whose original masses are roughly above 20-25 solar masses – below this, the collapsing core is likely to form a neutron star, pulsar or magnetar.

However, our current theoretical understanding of the internal dynamics of massive stars tells us that this process should only occur in stars whose masses are less than roughly 65 solar masses. Above this, and up to about 130 solar masses it is thought that the stars would blow themselves up, before they could have a chance to become black holes.

“Black holes form when massive stars die, both exploding in a supernova and imploding at the same time. But, when the star has a core mass in a specific range—between approximately 65 and 135 times the mass of the Sun—it usually just blows itself apart, so there’s no leftover black hole. Because of this, we don’t expect to see black holes in this solar mass range, unless some other mechanism is producing them,” said OzGrav PhD student and co-author Isobel Romero-Shaw, from Monash University.

This is all related to the amount of high-energy radiation (gamma-rays) being produced within the internal structure of the star, which in turn is a derivative of the mass of the star – since more mass = more gravitational force and more gravitational force = higher-valued energy production.

Increasing amounts of gamma radiation inside stars result in more collisions between these high-energy photons, producing positron/electron pairs, and reducing the amount of radiation pressure inside stars. Too much of this, and the star’s outward pressure is no match for the the inward crushing force of gravity, and so – the star literally blows itself up. This is known as a pair-instability supernova.

“These ‘impossible’ black holes have ‘forbidden’ masses according to what we currently understand about the lives of massive stars,” said OzGrav postdoctoral researcher Vaishali Adya from Australian National University.

“Stars that are massive enough to make black holes this heavy should blow themselves apart in a dramatic ‘pair instability supernova’. Events like this are now in range due to the improved sensitivity of the instruments compared to the first-generation detectors.”

For progenitor stars whose core mass is above 130 solar masses, it’s the opposite. The star is so massive, that its gravitational force is unmatched – and the star collapses into a black hole, not even leaving a supernova behind to tell the tale. In this case, the entire massive star literally disappears.

Not only does GW190521 shed a little light on the potentially missing link between stellar mass black holes and IMBHs, it also occurred at a time when the Universe was about half its present age – roughly seven billion years ago.

Since then, the Universe has been expanding and as a result, the calculated distance to the event places it over the edge of the boundary of our observable Universe – that is to say, these gravitational waves came from over 15 billion light years away.

Whilst this value is a product of an expanding Universe (and does not violate any framework of a Universe which is only 14 billion years old!), it does showcase that gravitational wave events can be used as probes to further explore the ancient history of the cosmos – an epoch when the star formation rate in galaxies was about 10 times higher than the present day value.

And its not just the IMBHs that Australian researchers have got their eye fixed on, when it comes to the emerging field of gravitational wave astronomy.

“One of the upcoming exciting projects which Australian researchers have a longstanding involvement in, is the search for continuous gravitational waves from isolated neutron stars,” said Prof. Susan Scott.

“The production of gravitational waves requires asymmetry. A mountain on a neutron star of a few millimetres’ height will produce a continuous stream of gravitational waves as it swings around with the star’s rotation. The waves are not very strong, so we are looking to increased sensitivity of the interferometers to have a good shot at detecting this source.”

“It’s exciting though as it starts to get within reach. We are very keen to probe the secrets of the nature of the matter that makes up neutron stars.”

The direct observation of an IMBH from GW190521 does tell us that there is still much to learn about our Universe, and how objects within it have evolved over time. It also raises exciting new questions for scientists and researchers to delve into, building a more accurate model of our cosmos in the process.

For example, these findings provide further support for General Relativity – so why can’t we work out gravity on the quantum scale? And we now have a potential model showcasing that smaller black holes can accumulate into larger black holes and then again, into even larger black holes – so are IMBHs the missing link on how supermassive black holes get so big?

We also stop, and reflect on deep time - considering the environments that events like GW190521 are occurring in – was it the early Universe only, or do these types of mergers occur in densely populated stellar regions today, like the cores of globular clusters?

With further enhancements planned for all gravitational-wave detectors, and new observatories coming online – in addition to faster (or even predicted) event notification, we should expect to see some of these questions answered, and a new range of questions asked.

Maybe the Sci-Fi movies did get it right after all, or at least, some variation of it.

When stars die there is always drama, chaos, and destruction. Sometimes we are lucky enough to catch this in action, while at other times – it’s simply a passing distortion of our reality, a memory of an ancient event that contributed to the current state of order in the cosmos.

In all cases, the advancements made in engineering, technology, and science continue to challenge the status quo and knowledge of our Universe, leaving us to look further and deeper into space, and reflecting on our own presence here on this planet, at this time.

Discoveries we once thought of as impossible, now border on the very achievable, and the fiction of today, might surprisingly be the facts of tomorrow.  

Video credit: The Australian Academy of Science.