The academic article's abstract explains the research better: > We present the results from an observing campaign to confirm the peculiar motion of the supermassive black hole (SMBH) in J0437+2456 first reported in Pesce et al. Deep observations with the Arecibo Observatory have yielded a detection of neutral hydrogen (H i) emission, from which we measure a recession velocity of 4910 km s−1 for the galaxy as a whole. We have also obtained near-infrared integral field spectroscopic observations of the galactic nucleus with the Gemini North telescope, yielding spatially resolved stellar and gas kinematics with a central velocity at the innermost radii (0farcs1 ≈ 34 pc) of 4860 km s−1. Both measurements differ significantly from the ~4810 km s−1 H2O megamaser velocity of the SMBH, supporting the prior indications of a velocity offset between the SMBH and its host galaxy. However, the two measurements also differ significantly from one another, and the galaxy as a whole exhibits a complex velocity structure that implies that the system has recently been dynamically disturbed. These results make it clear that the SMBH is not at rest with respect to the systemic velocity of the galaxy, though the specific nature of the mobile SMBH—i.e., whether it traces an ongoing galaxy merger, a binary black hole system, or a gravitational-wave recoil event—remains unclear. [1]

Specifically, I was interested in how they measured the galactic velocity vs the SMBH velocity. It turns out that they have 3 velocity measures:

- galaxy as a whole recession velocity of 4910 km s−1

- center of the galaxy (specifically central velocity at the innermost radii (0farcs1 ≈ 34 pc)) of 4860 km s−1

- SMBH velocity: ~4810 km s−1

These 3 numbers differ enough from each other that it opens more questions than it answers. As the authors note, "system has recently been dynamically disturbed" but it could be explained by several possibilities including:

- an ongoing galaxy merger

- a binary black hole system

- a gravitational-wave recoil event

I love how as we get more precise measurements of all these moving bodies / systems, we will be able to piece together exactly the causes of things.

[1] https://iopscience.iop.org/article/10.3847/1538-4357/abde3d