  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.
