Which black hole was the first to be photographed, in 2019?

Answer

The Event Horizon Telescope linked radio dishes around the world to image the glowing ring around M87*.

Context

The image released on 10 April 2019 shows the shadow of the supermassive black hole at the center of the giant elliptical galaxy M87, about 55 million light-years away. Its mass is roughly 6.5 billion times that of the Sun. It was produced by the Event Horizon Telescope, a network of radio dishes around the world that works as one Earth-sized telescope through a technique called very long baseline interferometry.

The picture does not show the black hole itself, which emits no light. It shows a bright ring of hot gas, lit by matter swirling toward the hole, surrounding a dark central region, the shadow, that is caused by gravity bending light and by the event horizon swallowing light. The ring is brighter on one side because material moving toward us is boosted by relativistic beaming. Its size matched the prediction from Einstein's general relativity for a black hole of that mass.

Obtaining the image took years. The telescopes recorded data in April 2017, and the signals, amounting to petabytes, were flown on hard drives to central processing sites, then combined with algorithms that fill in gaps between the dishes. In 2022 the collaboration published a similar image of Sagittarius A*, the black hole at the center of the Milky Way, which is much smaller and changes faster, making it harder to image. The 2019 picture is thus the first, but not the only, direct image of a black hole's shadow.