Dormant 36-billion-solar-mass black hole found in Cosmic Horseshoe

Astronomers have identified a black hole weighing about 36 billion solar masses at the heart of the Cosmic Horseshoe galaxy system, a finding that places it among the most massive black holes ever confirmed. Located roughly five to six billion light-years away, the dormant behemoth was measured not by its glow but by the gravity it exerts on its surroundings, using a combination of gravitational lensing and detailed stellar motions. The discovery, described in a peer‑reviewed study, underscores how galaxies and their central black holes grow in tandem and offers a rare glimpse into the end state of galaxy formation.

The Cosmic Horseshoe is formed by a foreground galaxy whose gravity distorts and magnifies the light from a more distant galaxy, producing a near-complete ring shape around the brighter foreground object. The ultramassive black hole sits at the center of the foreground galaxy, known as LRG 3757, which is itself one of the most massive galaxies observed. Crucially, the black hole is not actively accreting material or emitting intense radiation; instead, scientists inferred its mass from the way the galaxy’s stars move in the core and from how the foreground galaxy’s...