First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole

TLDR

Black holes surrounded by transparent emission regions are expected to cast a dark shadow due to gravitational light bending, and the observed brightness asymmetry in M87’s ring can be attributed to relativistic beaming of plasma rotating near light speed. The study aims to image and analyze the black hole shadow by deploying the Event Horizon Telescope at 1.3 mm. Using the EHT’s 1.3 mm VLBI array, the team reconstructed horizon‑scale images of M87 and compared them to a library of ray‑traced GRMHD simulations to estimate the black hole mass. The observations resolved a 42 ± 3 μas bright, asymmetric ring with a central depression, matching the predicted Kerr black‑hole shadow and yielding a mass of (6.5 ± 0.7) × 10⁹ M⊙, thereby confirming the presence of a supermassive black hole and providing a new probe of strong‑field gravity.

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.