In April 2019, the Event Horizon Telescope (EHT) collaboration revealed the first image of the candidate supermassive black hole (SMBH) at the center of the giant elliptical galaxy Messier 87 (M87). This event horizon-scale image shows a ring of glowing plasma with a dark patch at the center, which is interpreted as the shadow of the black hole. This breakthrough result, which represents a powerful confirmation of Einstein’s theory of gravity, or general relativity (GR), was made possible by assembling a global network of radio telescopes operating at mm-wavelengths that for the first time included the Atacama Large Millimeter/submillimeter Array (ALMA). The addition of ALMA as an anchor station has enabled a giant leap forward by increasing the sensitivity limits of the EHT by an order of magnitude, effectively turning it into an imaging array. The published image demonstrates that it is now possible to directly study the event horizon shadows of SMBHs via electromagnetic radiation, thereby transforming this elusive frontier from a mathematical concept into an astrophysical entity. Expansion of the array in the next few years, including new stations on different continents and eventually satellites in space, will provide progressively sharper and higher-fidelity images of SMBH candidates, and potentially even movies of the hot plasma orbiting around SMBHs. These improvements will shed light on processes of black hole accretion and jet formation on event horizon scales, thereby enabling more precise tests of GR in the truly strong-field regime.


The centre of the giant elliptical galaxy M87 seen at spatial resolution scales spanning six orders of magnitude. The detailed structure of the relativistic jet is revealed by observations at different radio wavelengths using several interferometric facilities, zooming into the supermassive black hole imaged by the EHT collaboration.

Full article published in C. Goddi et al. 2019, The Messenger, 177, 25