Astronomers Identify Largest Known Ultramassive Black Hole at 36 Billion Solar Masses

Astronomers have recently detected what is believed to be the largest ultramassive black hole (UMBH) ever identified, weighing approximately 36 billion times the mass of the Sun. This extraordinary cosmic entity resides at the center of the galaxy LRG 3-757, located nearly six billion light-years away. The groundbreaking discovery was made possible through the unique properties of a rare gravitational lensing system known as the Cosmic Horseshoe, first documented in 2007.

The research, led by Dr. Carlos R. Melo-Carneiro from the Institute of Physics at the Federal University of Rio Grande do Sul, utilized advanced gravitational lens modeling techniques and detailed analyses of stellar dynamics to accurately measure the black hole's mass. This innovative approach combined observations from the Multi Unit Spectroscopic Explorer (MUSE) instrument on the Very Large Telescope and high-resolution imaging from the Hubble Space Telescope, allowing researchers to derive a precise mass estimate for the black hole at the heart of LRG 3-757.

The Cosmic Horseshoe exemplifies gravitational lensing, a phenomenon where the gravity of an intervening massive galaxy distorts the light from a more distant background galaxy. This results in the background galaxy appearing as a luminous arc surrounding the lensing galaxy, effectively forming what is known as an Einstein Ring. LRG 3-757, classified as a luminous red galaxy, is significantly larger than the Milky Way, boasting a mass approximately 100 times greater.

Determining the mass of black holes, particularly those of significant size, presents substantial challenges for astronomers. Typically, mass estimates rely on the motion of nearby stars and gas to gauge the gravitational influence of the black hole. However, for LRG 3-757, located over five billion light-years away, direct observations are particularly fraught with difficulty. Dr. Melo-Carneiro's team innovated by integrating both the gravitational lensing effects observed in the background light and the kinematics of stars within the lensing galaxy, leading to the remarkable finding of a mass of 36 billion solar masses.

Historically, supermassive black holes have been known to reside at the centers of most large galaxies, with their masses corresponding to the dynamics of stars in their vicinity—a relationship referred to as the MBH–σe relation, with σe denoting the effective stellar velocity dispersion. Notably, the black hole identified in LRG 3-757 significantly deviates from this established trend, being approximately 1.5 standard deviations above expectations based on its stellar velocity dispersion of 366 km/s. This anomaly suggests that the galaxy may have experienced unusual dynamics, potentially including a tumultuous history of mergers that could have inflated the black hole's mass without correspondingly increasing the average stellar speeds.

Several hypotheses have been proposed to account for the exceptional mass of this black hole. These include the possibility that LRG 3-757 has undergone multiple mergers, leading to the amalgamation of central black holes and the expulsion of surrounding stars—a process known as scouring. Another theory posits that active galactic nucleus (AGN) feedback, wherein the black hole injects energy into its surroundings, may disrupt star formation and alter the internal structure of the galaxy. Furthermore, there is speculation that this black hole could represent a remnant of an ancient quasar, signifying a period of rapid growth in the early universe.

This discovery joins a growing list of ultramassive black holes that defy conventional scaling laws between galaxies and their central black holes. Its significance is amplified not only by its mass but also by its remarkable distance, as the Cosmic Horseshoe allows astronomers to observe it as it existed over four billion years ago—much earlier than most direct measurements of supermassive black holes to date.

With advanced astronomical tools like the upcoming Extremely Large Telescope (ELT) and ongoing surveys such as the Euclid mission, researchers anticipate the identification of numerous new gravitational lenses in the coming years. Many of these may host similarly massive black holes, posing profound implications for our understanding of galactic evolution and the interplay between baryonic and dark matter components. Dr. Melo-Carneiro and his colleagues concluded, "This new era of discovery promises to deepen our understanding of galaxy evolution and the intricate dynamics governing the universe." The full study has been published in the online journal Arxiv.