First Image of the Milky Way’s Supermassive Black Hole

In 2019, the Event Horizon Telescope collaboration published an image of the supermassive black hole at the center of the galaxy Messier 87 — the first image of its kind ever created. Now, the long-awaited first image of the Milky Way’s central supermassive black hole has been released. The results of this international endeavor are summarized in a new Focus Issue of The Astrophysical Journal Letters.

A Black Hole Bonanza

Over the past decade, gravitational-wave observatories, telescope arrays, and detailed computer models have advanced our understanding of black holes throughout the universe. Now, our knowledge of the black hole at the center of our galaxy — Sgr A*, pronounced “sadge-ay-star” — is getting a huge boost thanks to years of work by the Event Horizon Telescope (EHT) collaboration.

The EHT is a global network of radio dishes stretching from Antarctica to Spain that can be combined to make a planet-size telescope. In April 2017, eight telescopes in six locations around the world collected the faint radio emission from Sgr A*, Messier 87’s central black hole (M87*), and a handful of other objects. The images of M87* released in 2019 helped us to determine this black hole’s mass, spin, and orientation, and they provided a valuable test of how massive objects bend spacetime.

Although M87* and Sgr A* appear around the same size from Earth — Sgr A* is roughly the scale of a donut placed on the surface of the Moon, as viewed from Earth — the two supermassive black holes are vastly different. M87* is known to be active, shooting out a jet of particles that stretches 5,000 light-years from the galaxy’s center, while our own supermassive black hole is relatively subdued. M87* is 2,000 times farther away than Sgr A*, but it’s also 1,500 times more massive and far more luminous. M87* likely feeds on massive reservoirs of gas while Sgr A* sips from the stellar winds of a few dozen stars in its neighborhood.

Sgr A*’s size and location create some unique observational challenges. Because Sgr A* is much smaller than M87*, our black hole’s appearance can change on much faster timescales, leading to hourly brightness variations that made processing the EHT data challenging. On top of that variability, there’s the difficulty of viewing angle: to observe Sgr A*, the researchers had to peer through 27,000 light-years of gas and dust between Earth and the galactic center. How did the EHT collaboration handle these challenges, and what do the resulting images tell us?

Inspecting Images

Unlike M87*, which looked similar each day it was observed, Sgr A*’s brightness varied during a single day’s observations. Building on the programs used to create the image of M87*, the team devised new algorithms to process their data, accounting for Sgr A*’s rapid variation and removing the effects of scattering by interstellar gas and dust.

The articles published today focus on the best set of observations, which were taken on April 7, 2017. An image created from these data reveals a bright, asymmetrical ring of emission from the superheated gas that orbits the black hole. At the center of the ring is a dark region called the shadow, which contains the black hole’s event horizon — the surface beyond which nothing, not even light, can escape the black hole’s grasp.

These stunning images provide the first visual confirmation that the massive object at the center of the Milky Way is indeed a black hole. The diameter of the ring — 51.8 microarcseconds, encircling a shadow with a diameter of 48.7 microarcseconds — precisely matches the predictions of the general theory of relativity for a black hole four million times as massive as the Sun, providing yet another validation of this leading theory of gravity.

Modeling a Massive Black Hole

After constructing the images, the team used models to explore conditions close to Sgr A*, determine the black hole’s properties, and test theories of gravity. Overall, the simulations match the observations beautifully, with models that incorporate strong magnetic fields, involve a moderate rate of spin, and place Sgr A* at a slight angle relative to our line of sight matching the observations best.

However, none of the models passed all of the team’s stringent tests. In particular, the models were unable to match Sgr A*’s level of variability, which may mean that they don’t fully capture the behavior of the hot ionized gas in the black hole’s vicinity.

The team also used the decrease in brightness at the center of the ring to constrain the nature of Sgr A*. The brightness decrease is consistent with the presence of a black hole with an event horizon and specifically rules out several exotic black-hole alternatives like naked singularities, boson stars, and certain types of wormholes.

Looking Ahead

Combining the EHT results for Sgr A* and M87* with insights gleaned from gravitational-wave measurements of colliding stellar-mass black holes, we have validated the predictions of Einstein’s general theory of relativity over an impressive range of masses and distances.

Though the EHT’s results give definitive answers to many questions about the Milky Way’s central black hole, many questions about supermassive black holes remain. How do supermassive black holes like M87* create their particle jets? Do supermassive black holes hinder star formation in the galaxies they inhabit, or do they spur it onward? And how and when did supermassive black holes form in the first place?

Future analysis of the EHT observations will delve into the structure of the magnetic field near Sgr A*, as well as investigate structural changes that may have accompanied a flare that was seen at X-ray wavelengths on the final day of EHT observations.

While the existing observations of Sgr A* already promise a wealth of future discoveries, there are even more exciting things to come. More detectors and recording bandwidth have been added to the EHT since 2017, greatly enhancing the capabilities of this planet-spanning telescope and potentially enabling the first video of hot gas circling a black hole. It’s a thrilling time to be studying black holes!

For more information, check out the full ApJL Focus Issue here: Focus Issue on First Sgr A* Results from the Event Horizon Telescope

Citation

“First Sagittarius A* Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole in the Center of the Milky Way,” EHT Collaboration et al 2022 ApJL 930 L12. doi:10.3847/2041-8213/ac6674