Media releaseFrom: ARC Centre of Excellence for Gravitational Wave Discovery (OzGRav)
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.
INFORMATION The ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) is funded by the Australian Government through the Australian Research Council Centres of Excellence funding scheme. OzGrav is a partnership between Swinburne University of Technology (host of OzGrav headquarters), the Australian National University, Monash University, University of Adelaide, University of Melbourne, and University of Western Australia, along with other collaborating organisations in Australia and overseas.
LIGO is funded by NSF and operated by Caltech and MIT, which conceived of LIGO and led the Initial and Advanced LIGO projects. Financial support for the Advanced LIGO project was led by the NSF with Germany (Max Planck Society), the U.K. (Science and Technology Facilities Council) and Australia (Australian Research Council-OzGrav) making significant commitments and contributions to the project. Nearly 1300 scientists from around the world participate in the effort through the LIGO Scientific Collaboration, which includes the GEO Collaboration.
A list of additional partners is available at https://my.ligo.org/census.php. The Virgo Collaboration is currently composed of approximately 350 scientists, engineers, and technicians from about 70 institutes from Belgium, France, Germany, Hungary, Italy, the Netherlands, Poland, and Spain. The European Gravitational Observatory (EGO) hosts the Virgo detector near Pisa in Italy and is funded by Centre National de la Recherche Scientifique (CNRS) in France, the Istituto Nazionale di Fisica Nucleare (INFN) in Italy, and Nikhef in the Netherlands.
A list of the Virgo Collaboration members can be found at http://public.virgo-gw.eu/the-virgo-collaboration/. The Kamioka Gravitational Wave Detector (KAGRA), formerly the Large Scale Cryogenic Gravitational Wave Telescope (LCGT), is a project of the gravitational wave studies group at the Institute for Cosmic Ray Research (ICRR) of the University of Tokyo. It will be the world's first gravitational wave observatory in Asia, built underground, and whose detector uses cryogenic mirrors. The design calls for an operational sensitivity equal to, or greater, than LIGO. The project is led by Nobelist Takaaki Kajita who had a major role in getting the project funded and constructed.