M87*: Supermassive Black Hole in M87

• M87* is the supermassive black hole at the center of M87, notable for its resolved event-horizon-scale shadow and powerful extragalactic jet.
• EHT and VLBI imaging reveal an asymmetric ring (~42 µas) that constrains its mass and spin with about 10% accuracy, confirming Doppler beaming effects.
• Studies of M87* provide practical insights into accretion physics, jet launching mechanisms, and constraints on both dark matter models and strong-field general relativity.

M87* (commonly “M87 star”) is the supermassive black hole (SMBH) at the dynamical center of the giant elliptical galaxy M87 (NGC 4486). Located in the Virgo cluster at a luminosity distance of $D\simeq16.8$ Mpc, M87* possesses the largest apparent event-horizon-scale shadow of any known SMBH, making it the archetype for direct black hole imaging and horizon-scale jet studies. As the central engine of one of the longest and most powerful extragalactic relativistic jets, M87* provides a unique laboratory for accretion, jet launching, strong-field general relativity (GR) tests, and constraints on both baryonic and dark matter interactions in galaxy evolution.

1. Fundamental Parameters and Horizon-Scale Imaging

Multiwavelength constraints robustly fix the gravitational mass of M87*, with stellar-dynamical, gas-dynamical, and very-long-baseline interferometry (VLBI) imaging yielding $M \simeq (6.5\pm0.7)\times10^{9}\,M_\odot$ (Hada et al., 2024, Davoudiasl et al., 2019). The characteristic gravitational radius is $r_g \equiv GM/c^2 \approx 9.6\times10^{14}$ cm, or $4.6~\mu$as as seen from Earth. The viewing angle of the jet/spin axis is inclined at $\theta_{\rm view}\simeq14^\circ\!-\!20^\circ$.

Direct VLBI imaging by the Event Horizon Telescope (EHT) at 230 GHz (2017–2018) resolved an asymmetric ring of emission of angular diameter $\theta_{\rm ring}\simeq42\pm3~\mu$as, corresponding to a physical scale of $\sim5.5\,r_g$. The brightness peaks strongly on the southern side, a signature consistent with Doppler beaming in an inclined rotating (Kerr) space-time. The measured ring size directly constrains $GM/c^2$ to $\sim10$% accuracy, essentially reducing the degeneracy between mass, distance, and shadow geometry. Time-domain monitoring over multiple epochs and days shows that the diameter and centroid of the ring are stable to within $\lesssim10~\mu$as (Wielgus et al., 2020, Broderick et al., 2022), firmly establishing the ring as the signature of the photon capture region.

Recent imaging at 86 GHz with GMVA + ALMA (resolution $M \simeq (6.5\pm0.7)\times10^{9}\,M_\odot$0as) confirms the presence of a larger, co-spatial ring and resolves the limb-brightened jet base. “Resolve” and DoG-HiT, two modern Bayesian and sparse-regularized imaging algorithms, measure the 3 mm ring with $M \simeq (6.5\pm0.7)\times10^{9}\,M_\odot$1as, $M \simeq (6.5\pm0.7)\times10^{9}\,M_\odot$2–$M \simeq (6.5\pm0.7)\times10^{9}\,M_\odot$3as. The similarity of these results across RML, CLEAN, and forward modeling methods corroborates the reliability of the geometric features at the current angular resolution (Kim et al., 2024).

2. Accretion Flow Structure, Efficiency, and Environment

M87* sits at the center of a dynamically relaxed, low-density elliptical galaxy with a well-measured dark plus luminous mass profile from $M \simeq (6.5\pm0.7)\times10^{9}\,M_\odot$41 pc to $M \simeq (6.5\pm0.7)\times10^{9}\,M_\odot$5 Mpc (Laurentis et al., 2022). The Bondi radius, where the SMBH’s gravitational potential captures the ambient hot gas, is $M \simeq (6.5\pm0.7)\times10^{9}\,M_\odot$6 pc, with $M \simeq (6.5\pm0.7)\times10^{9}\,M_\odot$7 and $M \simeq (6.5\pm0.7)\times10^{9}\,M_\odot$8 keV. The Bondi accretion rate is $M \simeq (6.5\pm0.7)\times10^{9}\,M_\odot$9 yr$r_g \equiv GM/c^2 \approx 9.6\times10^{14}$0, but Faraday rotation and synchrotron constraints at $r_g \equiv GM/c^2 \approx 9.6\times10^{14}$1 indicate a strong inward suppression to $r_g \equiv GM/c^2 \approx 9.6\times10^{14}$2 yr$r_g \equiv GM/c^2 \approx 9.6\times10^{14}$3 (Hada et al., 2024, Davoudiasl et al., 2019).

The inferred radiative efficiency is extremely low, $r_g \equiv GM/c^2 \approx 9.6\times10^{14}$4, characteristic of a radiatively inefficient accretion flow (RIAF) in the magnetically arrested disk (MAD) regime, dominating over the standard thin-disk model. Horizon-scale estimates from GRMHD+GRRT fitting yield poloidal $r_g \equiv GM/c^2 \approx 9.6\times10^{14}$5 G, $r_g \equiv GM/c^2 \approx 9.6\times10^{14}$6, and electron temperature $r_g \equiv GM/c^2 \approx 9.6\times10^{14}$7 K near the black hole (Hada et al., 2024). The plasma is highly magnetized, $r_g \equiv GM/c^2 \approx 9.6\times10^{14}$8, consistent with MAD expectations.

3. Jet Launching, Structure, and Kinematic Observations

The jet power inferred from X-ray cavities, knot energetics, and large-scale radio lobes is $r_g \equiv GM/c^2 \approx 9.6\times10^{14}$9–$4.6~\mu$0 erg s$4.6~\mu$1 (Hada et al., 2024). Near the horizon, magnetic flux threading the black hole, $4.6~\mu$2, enables the Blandford–Znajek (BZ) process to efficiently convert spin energy into Poynting-dominated outflow. For $4.6~\mu$3, the BZ power $4.6~\mu$4 erg s$4.6~\mu$5 matches the jet’s large-scale energetics.

Observationally, the collimation profile tracks a paraboloidal streamline $4.6~\mu$6 up to $4.6~\mu$7 and becomes conical beyond the Bondi radius. Kinematic studies with VLBA (43 GHz) and HST proper motions derive an acceleration profile with $4.6~\mu$8, with flow velocity increasing from subrelativistic near the core to $4.6~\mu$9 at HST-1 ($\theta_{\rm view}\simeq14^\circ\!-\!20^\circ$0 pc). The jet displays strong limb-brightening, indicative of a velocity-sheared spine-sheath structure. GMVA 86 GHz observations constrain the width of the base—a sheath with diameter $\theta_{\rm view}\simeq14^\circ\!-\!20^\circ$1 anchored near ISCO, expanding with $\theta_{\rm view}\simeq14^\circ\!-\!20^\circ$2 (Kim et al., 2018).

Long-term VLBA (43 GHz) monitoring detects persistent jet “wobble” with a characteristic $\theta_{\rm view}\simeq14^\circ\!-\!20^\circ$39 yr transverse oscillation, likely driven by disk precession (Lense–Thirring effect) or propagation of magnetohydrodynamic instabilities (Walker et al., 2016). The observed kinematic and morphological consistency across bands and years provides direct empirical support for magnetically dominated (BZ-driven) outflows.

4. Event Horizon Properties, Black Hole Spin, and Shadow Interpretation

The shadow observed by the EHT is not strictly the classical photon-sphere shadow but rather the lensed event-horizon “silhouette” defined by the last-scattering surface of inner-disk synchrotron emission. For a Kerr spacetime, the shadow diameter’s weak spin dependence at low inclination implies the observed $\theta_{\rm view}\simeq14^\circ\!-\!20^\circ$4as is only attained for $\theta_{\rm view}\simeq14^\circ\!-\!20^\circ$5 (Dokuchaev et al., 2020, Broderick et al., 2022, Dokuchaev et al., 2020). Ray-tracing studies confirm that only a moderate- to high-spin prograde hole produces an event-horizon silhouette of the observed size.

Closure amplitude and phase likelihood analysis shows that the inferred ring fractional width $\theta_{\rm view}\simeq14^\circ\!-\!20^\circ$6 is sensitive to the treatment of gain uncertainties; Bayesian modeling with variable (model-based) closure likelihoods favors ring widths $\theta_{\rm view}\simeq14^\circ\!-\!20^\circ$7–$\theta_{\rm view}\simeq14^\circ\!-\!20^\circ$8%, broadly consistent with GRMHD predictions, while “fixed” likelihoods yield artificially narrower rings ($\theta_{\rm view}\simeq14^\circ\!-\!20^\circ$9\%) (Lockhart et al., 2021). High-fidelity imaging recovers not only the primary $\theta_{\rm ring}\simeq42\pm3~\mu$0 photon ring but also diffuse emission extending along the jet launch axis, consistent with BZ-driven Poynting flux outflow (Broderick et al., 2022).

Multi-messenger constraints utilize the $\theta_{\rm ring}\simeq42\pm3~\mu$1 yr precession period of the jet base as a probe of the strong-gravity regime, parametrizing its coupling to black hole spin $\theta_{\rm ring}\simeq42\pm3~\mu$2 and any hypothetical electric charge $\theta_{\rm ring}\simeq42\pm3~\mu$3 in a Kerr–Newman metric. Solving for Lense–Thirring nodal precession as a function of the warp radius $\theta_{\rm ring}\simeq42\pm3~\mu$4 yields direct interrelations among $\theta_{\rm ring}\simeq42\pm3~\mu$5; an independent measurement of $\theta_{\rm ring}\simeq42\pm3~\mu$6 could therefore constrain $\theta_{\rm ring}\simeq42\pm3~\mu$7 to $\theta_{\rm ring}\simeq42\pm3~\mu$8 (Meng et al., 2024).

5. Galaxy Dynamics, SMBH Displacement, and Growth History

Sub-arcsecond HST imaging reveals that the nuclear point source (M87*) is spatially offset by $\theta_{\rm ring}\simeq42\pm3~\mu$9 pc from the photometric center of the stellar bulge, aligned in the counter-jet direction (Batcheldor et al., 2010). Among candidate mechanisms—orbital binary motion, random Brownian motion, continuous acceleration by a one-sided jet, or gravitational recoil (“kick”) from SMBH coalescence—the latter two are favored. A moderate ($\sim5.5\,r_g$0 km s$\sim5.5\,r_g$1) kick in the last $\sim5.5\,r_g$21 Myr can explain the offset, associated nuclear disk shocks, and kinematic disturbances. This suggests a recent SMBH merger event, providing rare dynamical evidence for post-coalescence settling of an SMBH in a giant elliptical.

Dynamical modeling combining EHT mass, stellar and tracer kinematics, and dark/luminous mass decomposition (using Nuker plus Burkert halo fits) finds that M87’s dark matter core displays a central flattening ($\sim5.5\,r_g$3 kpc, $\sim5.5\,r_g$4 g cm$\sim5.5\,r_g$5), with the EHT-inferred $\sim5.5\,r_g$6 far exceeding the $\sim5.5\,r_g$7–$\sim5.5\,r_g$8 relation at the high-mass end (Laurentis et al., 2022). This suggests an “exotic” growth history in which the SMBH may have gained mass from dark matter infall or non-standard baryonic processes.

6. Implications for Dark Matter and Fundamental Physics

The mass, spin, and ring diameter measurements from EHT imaging allow M87* to act as a probe of ultralight (“fuzzy”) dark matter via the mechanism of black hole superradiance. For $\sim5.5\,r_g$9 and $GM/c^2$0, superradiant exclusion limits rule out boson masses in the range $GM/c^2$1 for scalars, and $GM/c^2$2 for vectors, substantially constraining fuzzy dark matter scenarios on kpc scales (Davoudiasl et al., 2019). Complementary constraints on the dark halo structure indicate that neither baryonic feedback nor fuzzy DM models fully account for the $GM/c^2$3 kpc core, further motivating searches for self-interacting or non-standard dark matter paradigms (Laurentis et al., 2022).

7. Prospects and Open Questions

M87* stands at the forefront of horizon-scale astrophysics. Forthcoming upgrades—next-generation EHT with more stations, higher frequencies (345 GHz), and space-VLBI—will refine spin, inclination, and “horizon-proximate” structure, and may resolve sub-microarcsecond lensing features. Coordinated jet and shadow imaging will directly probe jet-launching physics, Poynting flux, and magnetic field structure from $GM/c^2$4 to kpc. Multiwavelength (radio–TeV) and time-domain campaigns address variability, reconnection physics, and particle acceleration at the base of the jet. Comparative studies of M87*, Sgr A*, and high-luminosity blazars will map accretion state, jet efficiency, and black hole population properties across orders of magnitude in $GM/c^2$5 and $GM/c^2$6 (Hada et al., 2024).

Unresolved questions persist regarding the jet’s spine composition, the role of disk precession versus instabilities, the impact of environment and galaxy assembly history on SMBH–DM entanglement, and the true width and substructure of the photon ring. Advances in both direct imaging and theoretical modeling are expected to leverage the unique properties of M87* to constrain fundamental physics, astrophysical black hole processes, and the nature of dark matter.