Free Astronomy Magazine May-June 2024

35 MAY-JUNE 2024 ASTRO PUBLISHING funded by the U.S. National Science Foundation, to analyze a supermas- sive black hole binary located within the elliptical galaxy B2 0402+379. This is the only supermassive black hole binary ever resolved in enough detail to see both objects sepa- rately, and it holds the record for having the smallest separation ever directly measured — a mere 24 light-years. While this close separation foretells a powerful merger, further study revealed that the pair has been stalled at this distance for over three billion years, begging the question; what’s the hold-up? To better understand the dynamics of this sys- tem and its halted merger the t e am looked to archival data from Gemini North’s Gemini Multi-Object Spectro- graph (GMOS), which allowed them to de- termine the speed of the stars within the vicinity of the black holes. “The excellent sensitivity of GMOS allowed us to map the stars’ in- creasing velocities as one looks closer to the galaxy’s center,” said Roger Romani, Stanford University physics professor and co-author of the paper. “With that, we were able to infer the total mass of the black holes residing there.” The team estimates the binary’s mass to be a whopping 28 billion times that of the Sun, qualifying the pair as the heaviest binary black hole ever measured. Not only does this measurement give valuable context to the formation of the binary system and the history of its host galaxy, but it sup- ports the long- standing theory t h a t the T he merging of two supermassive black holes is a phenomenon that has long been predicted, though never directly observed. One theory put forth by astronomers is that these systems are so massive that they deplete their host galaxy of the stellar material needed to drive their merger. The illustration in the back- ground describes this scenario. [NOIR- Lab/NSF/AURA/J. daSilva/M. Zamani]