M87*

M87* denotes the massive compact object at the center of the giant elliptical galaxy Messier 87. Stellar and gas dynamics place a billion-solar-mass-scale object in the nucleus; millimeter interferometry resolves a ring-like emission structure; and a relativistic jet links the central engine to kiloparsec and larger scales. Together these observations support a massive black hole. The inferred horizon, spin, and detailed Kerr geometry remain conditional on dynamical, plasma, and imaging models.

M87* and its jet-launching nucleus

Property	Working statement
Object class	Named observed system
Designation	M87*
Host	Messier 87 in the Virgo Cluster
Physical status	High-confidence massive-black-hole identification with a relativistic jet
Direct measurements	Stellar and gas spectra, surface brightness, jet images and proper motions, millimeter visibilities and polarimetric correlations
Model-dependent posteriors	Central mass, distance-dependent angular scale, stellar anisotropy, disk geometry, magnetic state, inclination, and spacetime parameters
Evidence cutoff	20 August 2026

M87* is not selected because it is typical. Its enormous mass and nearby host make the angular gravitational scale accessible, while its low radiative output and bright jet make the conversion from emitted light to accretion rate particularly model dependent.

Measured parameters

The three principal mass channels do not share one likelihood. Stellar dynamics, gas dynamics, and horizon-scale interferometry use different data and nuisance models, while all require a distance scale.

Quantity and dataset	Reported value	Measurement or inference status
Stellar-dynamical mass	$M_\bullet=(6.6\pm0.4)\times10^9\,M_\odot,\quad D=17.9\ \mathrm{Mpc}$	Orbit-superposition posterior using the NIFS-only central kinematics; conditional on axisymmetry, stellar anisotropy, mass-to-light ratio, and the adopted distance (Gebhardt et al. 2011, Abstract and §§3–5)
Gas-dynamical mass	$M_\bullet=3.5^{+0.9}_{-0.7}\times10^9\,M_\odot,\quad D=17.9\ \mathrm{Mpc}$	68% interval from a rotating-disk model; the error budget includes the tested pressure-support correction but remains conditional on disk geometry and nongravitational forces (Walsh et al. 2013, Abstract and §§6–7)
2017 EHT crescent diameter	$d=42\pm3\ \mu\mathrm{as}$	Multi-day, multi-band image/visibility-model result; an image-domain emission scale, not a horizon diameter (Event Horizon Telescope Collaboration 2019, Paper VI, Abstract and §10)
EHT angular gravitational radius	$\theta_g=3.8\pm0.4\ \mu\mathrm{as}$	Median and 68% interval after calibrating emission size with a Kerr-GRMHD library; the dominant uncertainty is that calibration (Event Horizon Telescope Collaboration 2019, Paper VI, Table 7 and §10)
EHT distance prior and inferred mass	$D=16.8^{+0.8}_{-0.7}\ \mathrm{Mpc},\qquad M_\bullet=(6.5\pm0.2_{\rm stat}\pm0.7_{\rm sys})\times10^9\,M_\odot$	External distance synthesis combined with the calibrated angular-scale posterior; the systematic term is dominated by the GRMHD size calibration (Event Horizon Telescope Collaboration 2019, Paper VI, Abstract and §§8–10)

The two dynamical masses should not be averaged as independent Gaussian estimates: their distance dependence is shared, while their dominant modeling systematics differ. The EHT mass is likewise conditional on the Kerr interpretation and on the simulation library used to map an emission diameter to $GM/(Dc^2)$.

Dynamical mass before horizon-scale imaging

Stellar-dynamical models fit the line-of-sight velocity distribution and surface brightness after convolution with the instrumental point-spread function. The inferred point mass is covariant with stellar orbital anisotropy, mass-to-light ratio, dark matter, triaxiality, template spectra, and the adopted distance. Gas-dynamical fits use emission-line velocities and widths but add disk inclination, turbulence or pressure support, warps, and possible nongravitational forces.

The stellar-dynamical analysis of Gebhardt et al. 2011, §§2–4 and 6.1 and gas-dynamical work such as Walsh et al. 2013, §§2–4 and 7 illustrate why quoted masses are method-conditional posteriors. Their differences are scientific information about nuisance models, not a reason to average best fits without covariance.

For a posterior over $M$ and distance $D$, the predicted angular scale is

$$\theta_g=\frac{GM}{c^2D}.$$

Propagating the full $M/D$ distribution is the clean comparison with imaging. Fixing either variable at a preferred value understates uncertainty.

The published EHT central values pass a direct unit-conversion check:

$$M_\bullet =\frac{c^2D\theta_g}{G} =6.47\times10^9\,M_\odot \quad \left(D=16.8\ \mathrm{Mpc},\ \theta_g=3.8\ \mu\mathrm{as}\right).$$

This reproduces the quoted $6.5\times10^9\,M_\odot$ after rounding. It is an integrity check, not an independent mass measurement: $\theta_g$ is already the GRMHD-calibrated EHT inference and $D$ is an external prior. The separate $0.2\times10^9\,M_\odot$ statistical and $0.7\times10^9\,M_\odot$ systematic components cannot be recovered by naive Gaussian propagation of the two displayed central inputs.

What the EHT measures

At 1.3 mm, the Event Horizon Telescope samples complex Fourier components of the sky brightness. Atmospheric phase corruption motivates closure quantities; amplitude calibration, station gains, scattering, and sparse baseline coverage remain explicit. Independent imaging and geometric-model pipelines applied to the 2017 data recovered a persistent asymmetric ring-like structure (Event Horizon Telescope Collaboration 2019, Paper IV, §§3–8). The source is less rapidly variable during one observing scan than Sgr A*, but finite-epoch variability and jet-base structure still matter.

The measured emission-ring diameter is not identical to the Kerr shadow or critical curve. Relating them uses ray tracing and a library of emitting plasma models. The collaboration found an angular scale consistent with the high stellar-dynamical mass and used simulation-calibrated relations to infer a mass (Event Horizon Telescope Collaboration 2019, Paper VI, §§3–6 and 8). The data establish horizon-scale structure; the mass result remains a posterior over calibration, geometry, distance, and model-library uncertainty.

Polarimetric visibilities constrain ordered polarization around the ring. After leakage, absolute electric-vector position angle, Faraday effects, and imaging choices are propagated, comparisons favor strongly magnetized flow models within the tested library (Event Horizon Telescope Collaboration 2021, Paper VII, §§2–7). Projected polarization ticks are not direct three-dimensional magnetic-field lines, and the favored library class is not a unique reconstruction of magnetic flux at the horizon.

Jet connection and controlled comparisons

M87’s jet supplies scale-dependent observables: collimation profile, apparent motions, synchrotron spectrum, polarization, variability, and mechanical interaction with surrounding gas. Turning radio luminosity or cavity energetics into instantaneous jet power requires particle content, magnetic field, radiative efficiency, age, and environment. A horizon-scale rotation-powered model also requires magnetic-flux supply and mass loading; a jet does not measure spin by itself.

Useful controlled comparisons include:

• stellar versus gas dynamics under the same distance and point-spread function;
• image-domain ring fits versus direct visibility-domain likelihoods;
• alternative imaging regularizers tested on the same synthetic visibility data;
• weakly and strongly magnetized plasma libraries with matched electron-heating prescriptions;
• jet-power budgets that distinguish radiation, bulk kinetic energy, magnetic flux, and work on the host gas.

The broader energy and environment connection belongs with Jets, Feedback, and Host Environments.

Kerr tests and limitations

Agreement between the dynamical angular scale and a ring-like feature of the expected size is a nontrivial consistency test. It does not independently establish a horizon boundary condition or measure multiple Kerr multipoles. A credible metric test must define a deformation, predict both photon propagation and emitting matter, check pathologies and stability, and compare in visibility space while marginalizing mass, distance, calibration, and plasma structure. See Horizon-Scale Imaging Tests for the stable framework and Horizon-Scale Imaging Tests of Kerr for dated assessments.

Dominant systematics include distance, stellar anisotropy and mass profile, gas pressure and disk geometry, millimeter calibration, sparse Fourier coverage, imaging regularization, electron thermodynamics, magnetic topology, nonthermal particles, and epoch variability. Spin and inclination posteriors are more library dependent than the existence of a compact central mass. M87* is therefore a powerful consistency laboratory, not a model-independent photograph of Kerr geometry.

References

• Event Horizon Telescope Collaboration. “First M87 Event Horizon Telescope Results. IV. Imaging the Central Supermassive Black Hole.” Astrophysical Journal Letters 875 (2019): L4. DOI.
• Event Horizon Telescope Collaboration. “First M87 Event Horizon Telescope Results. VI. The Shadow and Mass of the Central Black Hole.” Astrophysical Journal Letters 875 (2019): L6. DOI.
• Event Horizon Telescope Collaboration. “First M87 Event Horizon Telescope Results. VII. Polarization of the Ring.” Astrophysical Journal Letters 910 (2021): L12. DOI.
• Gebhardt, Karl, et al. “The Black Hole Mass in M87 from Gemini/NIFS Adaptive Optics Observations.” Astrophysical Journal 729 (2011): 119. DOI.
• Walsh, Jonelle L., Aaron J. Barth, Luis C. Ho, and Marc Sarzi. “The M87 Black Hole Mass from Gas-Dynamical Models of Space Telescope Imaging Spectrograph Observations.” Astrophysical Journal 770 (2013): 86. DOI.