A new class of black holes found

 Astronomers find a new class of black holes and there may be billions or even trillions more

Astronomers have finally confirmed the existence of a group of black holes which are too hefty to be born from normal stars, yet too slim to anchor galaxies. Black holes that have been obscured by clouds of dust still emit infrared light, enabling astronomers to spot them for the very first time. Astronomers have discovered hundreds of hidden supermassive black holes lurking in the universe, and there may be billions or even trillions more out there that we still haven't found. The researchers identified these giant black holes by peering through clouds of dust and gas in infrared light. The finds could help astronomers refine their theories of how galaxies evolve, the researchers say. According to Vanderbilt University astrophysicist Krystal Ruiz-Rocha, these bodies have masses of 100 to 300 times that of the Sun, and emerged in data from the third observing run of the gravitational‑wave detectors LIGO and Virgo. Most known black holes sit either below 50 solar masses or soar into the millions, leaving a puzzling middle zone. Finding occupants of this zone matters because their masses preserve clues about how the first heavy stars lived and died. Hunting for black holes is difficult work. They are the darkest objects in the universe, as not even light can escape their gravitational pull. Scientists can sometimes "see" black holes when they devour matter around them; the surrounding material accelerates so fast it starts to glow. But not all black holes have a bright visible ring, so finding them takes a bit more creativity.

LIGO and Virgo register ripples in space time which last less than a half‑second, yet from that faint murmur scientists can compute the masses and spins of the colliding objects. During the 2019–2020 run, the network logged eleven mergers whose combined detector‑frame masses exceeded 100 Suns, hinting at a fresh category of “lite” intermediate objects. “Black holes are the ultimate cosmic fossils,” said Karan Jani, lead scientist of Vanderbilt’s Lunar Labs Initiative. His team used a Bayesian code named RIFT to test each event with three waveform families and confidently identified five mergers that left behind remnants of between 110 and 350 solar masses. Astronomers believe there are billions, or perhaps even trillions, of supermassive black holes, black holes with a mass at least 100,000 times that of our sun, in the universe. One probably lurks at the centre of every large galaxy. But it is impossible for scientists to count every single supermassive black hole. Instead, they need to take surveys of nearby galaxies to estimate the number of these black holes hiding in our corner of the cosmos.

Intermediate-mass black holes are hard to identify because their signals fall into the low-frequency range, where ground-based detectors struggle with noise. That makes it difficult to separate genuine cosmic events from background static, especially for faint or distant mergers. To overcome this, the researchers used three different waveform models to compare the shape of the observed gravitational waves. The experts also applied a Bayesian inference tool to estimate the black holes’ masses and spins with greater precision, even when the signals were weak or short. Scientists followed a mysterious signal and found 2 black holes gorging on something like never before. There's just one problem: While some black holes are fairly obvious thanks to the bright halo of matter surrounding them, others fly under the radar. This could be because they are obscured by clouds of gas and dust that haven't yet accelerated enough to become incandescent, or because we are viewing them at the wrong angle. A paper in the Astrophysical Journal estimates that around 35% of supermassive black holes are hidden in this way. This is a dramatic increase from the previous estimate of 15%, though the paper's authors think the true number could be closer to 50%.

Stellar theory predicts that a pair‑instability supernova blasts apart stars whose cores grow too hot, forbidding black holes between roughly 60 and 120 solar masses. Yet at least one pre‑merger object in the new sample lands inside that gap, and two more perch above it, supporting the idea that repeated mergers in dense clusters can sidestep the supernova ceiling. Hierarchical smash‑ups let smaller black holes build larger ones inside young star clusters, where escape speeds are high and fresh companions are plentiful. Such stacking predicts spins that sometimes point opposite the orbital motion. This is a trait seen in several of the lite candidates, strengthening the case for a dynamical origin. However, astronomers are coming up with ways to locate them. The clouds around obscured black holes still emit some light,  just in infrared, rather than in the visible spectrum. The researchers used data from two instruments to detect these infrared emissions. The first was NASA's Infrared Astronomical Satellite (IRAS), which operated for just 10 months in 1983 and was the first space telescope to peer into the infrared range. The second was the Nuclear Spectroscopic Telescope Array (NuSTAR), a space-based telescope that is run by NASA's Jet Propulsion Laboratory in Pasadena, and can detect the high-energy X-rays emitted by the superheated matter swirling around black holes.

Space‑based detectors will catch the same binaries years before they collide on Earth, tracing their slow in spiral and mapping their birthplace. NASA and ESA’s Laser Interferometer Space Antenna (LISA) is slated for launch in the mid‑2030s and will measure low‑frequency waves that are impossible to hear from the ground. “Access to lower frequencies from the lunar surface could allow us to identify the environments these black holes live in,” noted post‑doctoral fellow Anjali Yelikar, whose group studies detector concepts for the Moon. Combining Earth, space, and lunar observatories would stretch the observable waveform from minutes to years, and offer a full biography of each merger. NASA’s NuSTAR X-ray telescope has helped astronomers get a better sense of how many supermassive black holes are hidden from view by thick clouds of gas and dust that surround them. Using archival data from IRAS, the researchers identified hundreds of probable hidden black holes. Then, they used ground-based visible light telescopes and NuSTAR to rule out some candidates and confirm others. A few turned out to be galaxies in the process of forming lots of stars, but many were obscured black holes.

Across the sample, the farthest event travelled roughly 37 billion light‑years to reach us, while the nearest sat a scant 2.5 billion light‑years away. Five of the mergers have over a 95 % chance of producing a remnant above the 100‑solar‑mass threshold, making them the heaviest gravitational‑wave sources on record. Lite intermediate-mass black holes could be the missing link between the well-known populations of stellar and supermassive black holes. Understanding how these middleweights form and where they live helps astrophysicists refine models for galaxy growth, star cluster dynamics, and the history of matter in the universe. "It amazes me how useful IRAS and NuSTAR were for this project, especially despite IRAS being operational over 40 years ago," study co-author Peter Boorman, an astrophysicist at Caltech, said. This technique may help astronomers determine how common supermassive black holes are in the universe, and what role they play in galaxy formation. For instance, these giant tears in space-time may help limit a galaxy's size by drawing it towards a gravitational centre or consuming vast quantities of star-forming dust. The technique may even help scientists learn more about the heart of our own Milky Way.

Some theories suggest that intermediate black holes may grow into the supermassive black holes that sit at galactic centres. Others may be relics from the first generation of stars, and may offer a glimpse into a time before the first galaxies formed. “We hope this research strengthens the case for intermediate‑mass black holes as the most exciting source across the detector network,” said Ruiz‑Rocha. With every new detection, astronomers refine their census of black holes and inch closer to uncovering the first generation of stars which seeded today’s galaxies. "If we didn't have a supermassive black hole in our Milky Way galaxy, there might be many more stars in the sky," study co-author Poshak Gandhi, a professor of astrophysics at the University of Southampton in the UK, said.