A new study has developed an innovative method to detect colliding supermassive black holes in our Universe. The study has just been published in the Astrophysical Journal and was led by postdoctoral researcher Xingjiang Zhu from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), at Monash University.
At the centre of every galaxy in our Universe lives a supermassive black hole—a black hole that’s millions to billions times the mass of our Sun. Big galaxies are assembled from smaller galaxies merging together, so collisions of supermassive black holes are expected to be common in the cosmos. But merging supermassive black holes remain elusive: no conclusive evidence of their existence has been found so far.
One way to look for these mergers is through their emission of gravitational waves—ripples in the fabric of space and time. A distant merging pair of supermassive black holes emit gravitational waves as they spiral in around each other. Since the black holes are so large, each wave takes many years to pass by Earth. Astronomers use a technique known as pulsar timing array to catch gravitational waves from supermassive binary black holes—so far to no avail.
In parallel, astronomers have been looking for the collision of supermassive black holes with light. A number of candidate sources have been identified by looking for regular fluctuations in the brightness of distant galaxies called “quasars”. Quasars are extremely bright, believed to be powered by the accumulation of gas clouds onto supermassive black holes.
If the centre of a quasar contains two black holes orbiting around each other (instead of a single black hole), the orbital motion might change the gas cloud accumulation and lead to periodic variation in its brightness. Hundreds of candidates have been identified through such searches, but astronomers are yet to find the smoking-gun signal.
‘If we can find a pair of merging supermassive black holes, it will not only tell us how galaxies evolved, but also reveal the expected gravitational-wave signal strength for pulsar watchers,’ says Zhu.
The OzGrav study seeks to settle the debate, determining if any of the identified quasars are likely to be powered by colliding black holes. The verdict? Probably not.
“We’ve developed a new method allowing us to search for a periodic signal and measure quasar noise properties at the same time,” says Zhu. “Therefore, it should produce a reliable estimate of the detected signal’s statistical significance.”
Applying this method to one of the most prominent candidate sources, called PG1302-102, the researchers found strong evidence for periodic variability; however, they argued that the signal is likely to be more complicated than current models.
“The commonly assumed model for quasar noise is wrong,” adds Zhu. “The data reveal additional features in the random fluctuations of gas accumulation onto supermassive black holes.”
“Our results are showing that quasars are complicated,” says collaborator and OzGrav Chief Investigator Eric Thrane. “We’ll need to improve our models if we are going to use them to identify supermassive binary black holes.”
This artist's impression of different mass stars; from the smallest “red dwarfs”, weighing in at about 0.1 solar masses, to massive “blue” stars weighing around 10 to 100 solar masses. While red dwarfs are the most abundant stars in the Universe, it’s the massive blue stars that contribute the most to the evolution of stars clusters and galaxies. Credit: ESO/M. Kornmesser
Massive stars are larger than about 10 times the mass of the Sun and are born far less often than their low mass counterparts. However, they contribute the most to the evolution of stars clusters and galaxies. From enriching their surroundings in supernova explosions, to altering the dynamics of their systems, massive stars are the precursors of many vivid and energetic phenomena in the Universe.
The best tool to study massive stars are ‘detailed stellar evolution codes': computer programs which can calculate both the interior structure and the evolution of these stars. Unfortunately, detailed codes are computationally expensive and time-consuming—it can take several hours to compute the evolution of just a single star. For this reason, it’s impractical to use these codes for modelling stars in complex systems, such as globular star clusters, which can contain millions of interacting stars.
To address this problem, a team of scientists led by the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) developed a stellar evolution code called METhod of Interpolation for Single Star Evolution (METISSE). Interpolation is a method for estimating a quantity based on nearby values, such as estimating the size of a star based on the sizes of stars with similar masses. METISSE uses interpolation to quickly calculate the properties of a star at any instant by using selected stellar models computed with detailed stellar evolution codes.
Lightning fast, METISSE can evolve 10,000 stars in less than 3 minutes. Most importantly, it can use different sets of stellar models to predict the properties of stars—this is extremely important for massive stars. Massive stars are rare, and their complex and short lives make it difficult to accurately determine their properties. Consequently, detailed stellar evolution codes often have to make various assumptions while computing the evolution of these stars. The differences in the assumptions used by the different stellar evolution codes can significantly impact their predictions about the lives and the properties of the massive stars.
In their recently published study, the OzGrav researchers used METISSE with two different sets of state-of-the-art stellar models: one computed by the Modules for Experiments in Stellar Astrophysics (MESA), and the other by the Bonn Evolutionary Code (BEC).
Poojan Agrawal—OzGrav researcher and the study’s lead author—explains: ‘We interpolated stars that were between 9 and 100 times the mass of the Sun and compared the predictions for the final fates of these stars. For most massive stars in our set, we found that the masses of the stellar remnants (neutron stars or black holes) can vary by up to 20 times the mass of our Sun’.
When the stellar remnants merge, they create gravitational waves—ripples in space and time—that scientists can detect. Therefore, this study’s results will have a huge impact on future predictions in gravitational-wave astronomy.
Agrawal adds: ‘METISSE is just the first step in uncovering the part massive stars play in stellar systems such as star clusters, and already the results are very exciting.’
Black holes are massive, right? The first pair of black holes detected were each about 30 times more massive than the Sun. When they merged, the resulting ‘remnant’ was a black hole that was a whopping 60 times more massive than the Sun.
Today astronomers from the LIGO and Virgo Scientific Collaboration (LVC) have reported the first ever direct observation of the most massive black hole merger to date. Two monster black holes collided to form an even more massive object—an intermediate-mass black hole, about 150 times as heavy as the Sun.
Researchers from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) contributed to the detection and used the computing resources of the new Gravitational-Wave Data Centre to infer the masses of the merging black holes.
Juan Calderón Bustillo—co-author and OzGrav postdoctoral researcher at Monash University—reports: ‘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’.
The online detection team at the University of Western Australia detected the event, GW190521, seconds after the gravitational-wave data were available, and helped generate public alerts for the LIGO Scientific Collaboration.
OzGrav PhD student and co-author Manoj Kovalam: ‘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’.
These ‘impossible’ black holes have ‘forbidden’ masses according to what we currently understand about the lives of massive stars. OzGrav postdoctoral researcher Vaishali Adya from Australian National University explains: ‘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.’
The rare event has prompted researchers to question how the black hole formed, its origins and how the two black holes found each other in the first place.
OzGrav PhD student and co-author Isobel Romero-Shaw, from Monash University, comments on the perplexing masses: ‘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.’
Since gravitational waves directly measure the masses of the colliding black holes, this measurement should be much more robust than the similar mass black hole previously reported by Liu et al. (published in the journal Nature last year). That measurement was based on an interpretation of the spectrum of light from the Galactic star system LB-1, which has since been refuted. Based on this current study’s mass measurements, researchers found that this kind of black hole couldn’t have formed from a collapsing star—instead, it may have formed from a previous black hole collision.
OzGrav postdoctoral researcher and LVC member Simon Stevenson, from Swinburne University of Technology, says: ‘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.’
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.
Meg Millhouse, OzGrav Postdoctoral researcher and LVC member from the University of Melbourne, was involved in the discovery paper’s analysis. Millhouse says: ‘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’.
OzGrav Chief Investigator and co-author David Ottaway, from University of Adelaide, says: ‘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’.
These gravitational waves came from over 15 billion light years away! But isn't the Universe only around 14 billion years old, you ask? It turns out that the Universe was actually around 7 billion years old when these two black holes collided. As the gravitational waves rippled out through the Universe, the Universe was expanding.
Consequently, the measured distance to this collision is now further than the product of the speed of light and the time travelled—mind (and space) bending stuff! This shows gravitational waves are able to probe the ancient history of the Universe, when galaxies were forming stars at a rate around 10 times higher than the present day.