First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole (1906.11238v1)

Abstract: When surrounded by a transparent emission region, black holes are expected to reveal a dark shadow caused by gravitational light bending and photon capture at the event horizon. To image and study this phenomenon, we have assembled the Event Horizon Telescope, a global very long baseline interferometry array observing at a wavelength of 1.3 mm. This allows us to reconstruct event-horizon-scale images of the supermassive black hole candidate in the center of the giant elliptical galaxy M87. We have resolved the central compact radio source as an asymmetric bright emission ring with a diameter of 42+/-3 micro-as, which is circular and encompasses a central depression in brightness with a flux ratio ~10:1. The emission ring is recovered using different calibration and imaging schemes, with its diameter and width remaining stable over four different observations carried out in different days. Overall, the observed image is consistent with expectations for the shadow of a Kerr black hole as predicted by general relativity. The asymmetry in brightness in the ring can be explained in terms of relativistic beaming of the emission from a plasma rotating close to the speed of light around a black hole. We compare our images to an extensive library of ray-traced general-relativistic magnetohydrodynamic simulations of black holes and derive a central mass of M = (6.5+/-0.7) x 10^9 Msun. Our radio-wave observations thus provide powerful evidence for the presence of supermassive black holes in centers of galaxies and as the central engines of active galactic nuclei. They also present a new tool to explore gravity in its most extreme limit and on a mass scale that was so far not accessible.

• The paper presents the first direct image of M87’s supermassive black hole shadow, measuring a 42±3 microarcsecond bright emission ring.
• The paper applies a global VLBI array operating at 230 GHz to achieve an exceptional 25 microarcsecond resolution for detailed imaging.
• The paper validates general relativity by comparing observations with GRMHD simulations, constraining the black hole's mass and spin properties.

The First M87 Event Horizon Telescope Results: Imaging the Black Hole Shadow

"The Shadow of the Supermassive Black Hole" presented by the Event Horizon Telescope Collaboration captures the inaugural results from the Event Horizon Telescope (EHT), focusing on the supermassive black hole at the center of the galaxy M87. This paper profoundly extends the frontiers of black hole astrophysics by showcasing the first event-horizon-scale image of a supermassive black hole.

Summary of Methodology

This research harnesses the global network capabilities of the EHT—a very long baseline interferometry (VLBI) array operating at a high frequency of 230 GHz (1.3 mm wavelength). The array’s configuration, involving eight stations distributed across the globe, achieves an exceptional angular resolution of approximately 25 microarcseconds. This enhancement in resolution is pivotal in observing the black hole shadow and associated structures.

The interferometric techniques employed by the EHT utilize the Earth’s rotation to increase the effective aperture, achieving sufficient baseline coverage and sensitivity to resolve the shadow of M87’s central black hole. The paper outlines an intensive observation campaign conducted over several days in April 2017, followed by intricate processes of correlation, calibration, and imaging to translate raw data into coherent images of M87’s core.

Observational Findings

Central to the paper is the detection of an asymmetric bright emission ring surrounding a darker region—the shadow. The diameter of the emission ring is measured at 42±3 microarcseconds, with a notable brightness contrast exceeding 10:1 within the center, aligned with the theoretical predictions for a Kerr black hole. This observed ring is consistent across multiple observational days, attesting to the stability of the features.

A comprehensive comparison is made between the observed data and general-relativistic magnetohydrodynamics (GRMHD) simulations, which simulate magnetized accretion flows onto black holes. This comparison aids in characterizing the properties of the black hole, including an estimated mass of $6.5 \pm 0.7 \times 10^9$ solar masses.

Numerical Results and Implications

The numerical robustness of the paper lies in the stability and consistency of the shadow and emission ring measurements against GR predictions. The approximately circular nature of the observed ring, with a measured axial ratio below 4:3, provides constraints on the orientation and spin of the black hole’s accretion flow and its alignment with the jet observed in M87.

This first direct image of a supermassive black hole serves as empirical evidence validating the existence of event horizons as predicted by general relativity. This research bolsters the paradigm that supermassive black holes, such as that in M87, are indeed responsible for the dynamics observed in active galactic nuclei.

Future Directions

The paper notes potential future avenues for research and observational improvements. Enhancements could involve utilizing higher frequencies and adding more VLBI sites to improve resolution and sensitivity. Additionally, polarimetric studies are anticipated to provide deeper insights into magnetic fields at these extreme scales, further unveiling the accretion physics and potential relativistic jet formation mechanisms.

Another promising frontier is the observation of Sgr A*—the supermassive black hole at the center of our galaxy—though this presents unique challenges due to its rapid variability and interstellar scattering effects. Advances in computational methods will be critical in handling the challenges posed by its shorter dynamical timescales.

Conclusion

This paper marks a seminal achievement in observational astrophysics. By transcending theoretical abstraction, the EHT’s data opens new methodologies to test gravitational theories in regimes previously unreachable. As we continue to gather more precise data and hone our interpretive models, the potential to explore gravity’s extreme limits holds significant promise for further theoretical and practical breakthroughs in understanding our universe.