The discovery of RBH-1, a supermassive black hole weighing at least ten million Suns, has captivated astronomers and sparked a debate about the nature of black hole ejection. This black hole appears to have been flung from its galaxy and is now racing through space at an astonishing speed, leaving a 200,000-light-year wake of newborn stars. The James Webb Space Telescope's NIRSpec integral field unit measured the motion of gas at the tip, revealing a velocity change of about 600 kilometres per second across roughly one kiloparsec. This data, published in The Astrophysical Journal Letters, provides direct evidence of a fast, massive body ploughing through thin gas, creating a bow shock. The mass of RBH-1 is estimated to be at least ten million solar masses, with a separate estimate pointing closer to twenty million. The wake, first noticed in a Hubble image in 2023, has been interpreted as a possible black hole wake on circumstantial grounds. The new data confirms this interpretation, with emission line ratios pointing to a supersonic bow shock. The paper describes the case for a supersonic bow shock as very strong, bordering on overwhelming. The trail of young stars is real, but the exact mechanism behind their formation is still under investigation. The paper does not show the black hole itself, only the shocked gas, and the object remains unseen. The authors suggest that RBH-1 may have escaped through a three-body slingshot or the recoil from a merger that radiates gravitational waves unevenly. The similarity between the escaped mass and the central black hole of the former host points towards recoil. The discovery raises the question of whether RBH-1 is one of many ejected black holes, and counting them could help constrain how often galaxies fling their central black holes out. Ground-based surveys tend to smear away the thin wake, making wide-field space telescopes like Euclid and the Nancy Grace Roman Space Telescope crucial for finding a second example.
