Abstract
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 μas, 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) × 109 M ⊙. 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.
Highlights
Black holes are a fundamental prediction of the theory of general relativity (GR; Einstein 1915)
The Event Horizon Telescope (EHT) collaboration was established to assemble a global very long baseline interferometery (VLBI) array operating at a wavelength of 1.3 mm with the required angular resolution, sensitivity, and baseline coverage to image the shadows in M87* and Sgr A*
Motivated by the asymmetric emission ring structures seen in the reconstructed images (Section 5) and the similar emission structures seen in the images from general-relativistic magnetohydrodynamics (GRMHD) simulations (Section 6), we developed a family of geometric crescent models to compare directly to the visibility data
Summary
Black holes are a fundamental prediction of the theory of general relativity (GR; Einstein 1915). Modern general-relativistic simulations of accretion flows and radiative transfer produce realistic images of black hole shadows and crescents for a wide range of near-horizon emission models (Broderick & Loeb 2006; Mościbrodzka et al 2009; Dexter et al 2012; Dibi et al 2012; Chan et al 2015; Mościbrodzka et al 2016; Porth et al 2017; Chael et al 2018a; Ryan et al 2018; Davelaar et al 2019) These images can be used to test basic properties of black holes as predicted in GR (Johannsen & Psaltis 2010; Broderick et al 2014; Psaltis et al 2015), or in alternative theories of gravity (Grenzebach et al 2014; Younsi et al 2016; Mizuno et al 2018). The accompanying papers give a more extensive description of the instrument (EHT Collaboration et al 2019a, Paper II), data reduction (EHT Collaoration et al 2019b, hereafter Paper III), imaging of the M87 shadow (EHT Collaboration et al 2019c, hereafter Paper IV), theoretical models (EHT Collaboration et al 2019d, hereafter Paper V), and the black hole mass estimate (EHT Collaboration et al 2019e, hereafter Paper VI)
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