JWST-Keck Kinematics Fix M87 Black Hole Mass at 5.38 Billion Solar Masses with Distance Uncertainty Dominant

The combined dataset maps stellar kinematics across three radial decades with ~5000 constraints in 618 bins. NIRSpec spectra inside 0.45 arcsec required custom wiggle subtraction via the hyphen pipeline to remove detector artifacts before TriOS triaxial Schwarzschild modeling. The inner 30,000 Schwarzschild radii resolve the sphere of influence directly, halving statistical error relative to prior KCWI-only work while exposing distance as the leading systematic.

This result bridges stellar-dynamical and horizon-scale measurements. The derived θ_g prior directly informs upcoming EHT multi-epoch analyses and tests whether the 2019 shadow diameter is consistent with the same central mass under general relativity. It also constrains M87's merger history by anchoring the M_BH–σ and M_BH–M_* relations at the high-mass end with reduced scatter.

Future space-based integral-field spectrographs will extend this approach to more distant galaxies where ground-based resolution fails. The current overlap with EHT validates independent mass ladders but highlights the need for a sub-1% geometric distance to M87 to make statistical and systematic errors comparable.

Next steps include joint EHT-stellar-dynamical modeling and Gaia DR4 proper-motion constraints on the outer halo to further isolate distance systematics.