M87* and Sagittarius A*: Photographing a Black Hole

In April 2019, a global collaboration called the Event Horizon Telescope (EHT) released the first image of a black hole's shadow. The subject was **M87***, the supermassive black hole at the center of the galaxy Messier 87, about 55 million light-years from Earth. It's approximately 6.5 billion solar masses.

The image isn't a photograph in the ordinary sense. The EHT is a network of eight radio observatories spread across Earth, from Hawaii to Antarctica, synchronized to function as a single Earth-sized dish. The technique is called **very long baseline interferometry (VLBI)**. The data collected at each site was combined using atomic clocks accurate to one second every 100 million years, then processed by algorithms including one developed by MIT's Katie Bouman.

What the image shows is a bright ring of radio emission — the photon sphere and accretion disk — surrounding a dark central region: the black hole's shadow, the absence of light from behind the horizon. The ring is brighter on one side due to relativistic beaming: material moving toward Earth in the accretion disk appears brighter.

Three years later, in May 2022, the EHT released an image of **Sagittarius A\*** (Sgr A*) — the supermassive black hole at the center of our own Milky Way, approximately 27,000 light-years away. It's about 4 million solar masses. Imaging Sgr A* was harder than M87* despite being closer: Sgr A* is far smaller, so the material in its accretion disk orbits in minutes rather than days, meaning the source was actively changing during observations. The team had to develop new imaging algorithms to handle the variability.

Both images closely match general relativity's predictions for what the shadow of a rotating black hole should look like.

The role of supermassive black holes in galaxy formation is one of astronomy's active research fronts. Black holes at galactic centers correlate tightly with the properties of their host galaxies — particularly with the **bulge mass** and **velocity dispersion of stars** in the galactic bulge. The relationship is known as the M-sigma relation. Why should a black hole with a gravitational sphere of influence far smaller than the galaxy as a whole co-evolve so tightly with its host galaxy?

The leading hypothesis is **AGN feedback**: when a supermassive black hole is actively accreting (becoming an Active Galactic Nucleus), its jets and radiation can heat and eject gas across the entire galaxy, suppressing star formation. The black hole, in effect, regulates how large the galaxy can become. Whether this feedback drives the M-sigma relation or whether both arise from a common formation process is still being worked out.

M87* and Sagittarius A*: Photographing a Black Hole

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