Largest known black holes come to light
by Shirali Pandya
It doesn’t get much bigger than this. Astronomers at the University of California, Berkeley have recently found two black holes that dwarf all others. Weighing in at 10 and 21 billion times the mass of our Sun, respectively, these behemoths trounce the former heavyweight, which has a mass 6.3 billion times larger than our Sun. These newly discovered celestial objects aren’t simply the biggest black holes found to date—they are also making a large impact on our understanding of the Universe.
A team led by Professor Chung-Pei Ma found the supermassive black holes residing at the centers of their namesake galaxies, NGC3842 and NGC4889, which are in turn part of two galactic clusters over 330 million light years away. Using measurements from the Gemini and Keck telescopes in Hawaii and the Hubble Space Telescope, astronomy graduate student Nicholas J. McConnell calculated the mass of these black holes by using the speed and distance of the stars orbiting around them.
Black holes are the mind-bending prediction of Einstein’s theory of relativity—extraordinarily heavy objects that severely distort the space-time continuum. These cosmic bodies are so dense that the escape velocity required to break away from their gravitational field exceeds the speed of light. While the mass of the black hole itself is concentrated at an infinitesimally small point, nothing within a certain radial distance from this point—defining a boundary known as the event horizon—can escape its pull. Beyond the event horizon, the black hole exerts a gradually decreasing (but still powerful) gravitational force on objects within its sphere of influence.
The motivation for the UC Berkeley group’s research and the import of the findings extend far beyond their record-shattering dimensions. “We went into the project not really to break the record or to purposely look for the biggest [black hole],” explains Ma. “We really just [wanted] to understand how they evolve and how they become so big.” Indeed, the new findings are changing astronomers’ theories of how black holes and their host galaxies co-evolve by providing a missing piece of an outstanding scientific mystery.
Scientists have predicted the existence of dormant black holes as large as 10 billion solar masses based on measurements of their younger, actively growing relatives, luminous objects known as quasars. A quasar is believed to be a primordial black hole engaged in a gaseous feeding frenzy. As gas spirals into the center of a black hole, it emits light due to rapid acceleration. The more massive the black hole, the faster the gas accelerates toward the center, and the brighter the quasar. Quasars were ubiquitous in the younger days of the Universe; at that time, black hole nuclei grew rapidly by devouring, or “accreting,” the plentifully available gases in the early cosmic soup. Fast-forward a few billion years, and fuel is depleted. The black holes have now stopped accreting and have become sleeping giants.
NGC3842 and NGC4889 are examples of this type of mature black hole, lying in galaxies that have been devoid of gas for eight–to–nine billion years. These dormant giants are counterparts to the colossal black holes found at the centers of distant quasars. “Not only are these the most massive black holes ever known in the Universe today,” says McConnell, “but they are also massive enough to compare to the most massive ones we’ve seen from the young Universe.” Now, with snapshots of both nascent and more mature versions of supermassive black holes, as well as their individual cosmic milieux, scientists can begin piecing together the puzzle of the formation of supermassive black holes and the galaxies that host them.
The discoveries have yielded an interesting surprise for astronomers, suggesting that the growth of the most massive black holes occurs by different processes than that of more ordinary ones. The scientists focused their quest on the centers of large galaxies, because it was thought that the mass of a black hole scales with the size of its host galaxy. However, McConnell and his colleagues found that these supermassive black holes are about twice as large as this trend would suggest. In addition to growth by gas accretion, these supermassive black holes may have grown rapidly when smaller black holes combined in galaxy merger events. Scientists believe that galaxy mergers were frequent in the early Universe, and the new discoveries will provide insight into just how frequently such events occurred. As such, it will be important to determine whether these supermassive black holes are simply outliers from the standard predictions of black hole size, or if astronomers need to tweak their current models at the higher end.
Ma, who is primarily a theoretical astrophysicist, began the research project based on some clues from her group’s work that current models of black hole size may not apply for the largest supermassive black holes. She credits the collaborative spirit and rich resources at UC Berkeley with enabling the data collection: “An environment like Berkeley was important for making this project feasible. Having these facilities, resources and smart people, you’re not held back. If the current instruments can do it, let’s do it.”