A black hole of roughly 20 million suns, kicked out of its galaxy, leaving a 200,000 light-year trail of newborn stars. Scrub the timeline to watch the wake form.
Pieter van Dokkum's team found a 62 kpc linear feature pointing at a compact galaxy, ending in a bright [O III] knot. Star colours suggested ages increasing away from the tip. They proposed a three-body slingshot about 39 Myr earlier at ~1,600 km/s, with a binary black hole recoiling the other way to make a fainter counter-feature.
Sánchez Almeida and colleagues argued the feature could be a thin galaxy seen edge-on. The later deep data show the light dropping by over 40× right at the tip, which a galaxy disk does not do.
An integral-field map shows a ~600 km/s velocity jump across the tip, the signature of a supersonic bow shock. The derived speed is 954 km/s at 29° inclination. Deep HST/UVIS images show no counter-wake on the opposite side.
The host galaxy, GX, has a stellar velocity dispersion of only ~60 km/s, which makes three-body ejections unlikely. With no second runaway in sight, the best fit is a gravitational-wave recoil from two black holes merging ~70 Myr earlier (mass ratio ~3, spinning, precessing).
Toggle between them in the viewer. Each uses the numbers from its own paper. Neither is drawn to physical scale for the black hole or the gravitational waves.
| Merger recoil | Three-body slingshot | |
|---|---|---|
| Status | Favoured 2025–26 | Proposed 2023 |
| Black holes | Two merge; the merged remnant is RBH-1 | Three interact; the lightest is flung out, the binary survives |
| Kick | Uneven gravitational-wave emission at merger | Gravitational slingshot |
| Speed | 954 km/s now | ~1,600 km/s |
| Age | ~70 Myr | ~39 Myr |
| Counter-wake | None expected, none seen | Expected from the recoiling binary |
How the stars ignite: the black hole compresses ~10⁶ K halo gas in a bow shock. Behind it, turbulent mixing with cooler gas makes the gas radiate and cool quickly. The cooled gas condenses into knots that form stars. The viewer models each knot igniting a few Myr after the black hole passes, so the youngest stars sit nearest the tip.
One Python script builds the whole animation with no add-ons: the host galaxy, the black holes, the bow shock, the growing wake and about 400 star-forming knots that ignite on schedule. Choose either scenario at the top of the script. It is tested on Blender 5.1 and written to work on 4.x.
blender --python rbh1_scene.py -- --scenario gw
The script is in the project's blender/ folder (rbh1_scene.py).
The kinematics in the viewer are illustrative. For the recoil scenario the speed eases from ~1,030 km/s to 954 km/s, so the path covers 71 kpc in 70 Myr and projects to 62 kpc at 29°. Knot ignition delays (3–15 Myr) and the stellar-mass counter are scaled to the published totals, not taken from a simulation.