It all began with the discovery of Sagittarius A*, a persistent radio source located at the Galactic Center of the Milky Way that turned out to be a supermassive black hole (SMBH). This discovery was accompanied by the realization that SMBHs exist at the heart of most galaxies, which account for their energetic nature and the hypervelocity jets extending from their center. Since then, scientists have been trying to get a better look at Sag A* and its surroundings to learn more about the role SMBHs play in the formation and evolution of our galaxy.
This has been the goal of the GRAVITY collaboration, an international team of astronomers and astrophysicists that have been studying the core of the Milky Way for the past thirty years. Using the ESO’s Very Large Telescope Interferometer (VLTI), this team obtained the deepest and sharpest images to date of the region around Sag A*. These observations led to the most precise measurement yet of the black hole’s mass and revealed a never-before-seen star that orbits close to it.
The GRAVITY collaboration is made of scientists from the Max Planck Institute for Extraterrestrial Physics (MPE), the Laboratoire d’Etudes Spatiales et d’Instrumentation en Astrophysique (LESIA), the Centre national de la recherche scientifique (CNRS), the Centro de Astrofisica e Gravitação (CENTRA), the Max Planck Institute for Astronomy (MPIA), and the European Southern Observatory (ESO). The collaboration takes its name from the GRAVITY adaptive optics instrument that they developed for the VLTI.
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This unique instrument combines the light of all four 8.2-meter (~27 ft) telescopes at the Very Large Telescope’s (VLT) located at the Paranal Observatory in Chile – a technique known as interferometry. MPE director Reinhard Genzel, a member of the GRAVITY collaboration, was awarded a Nobel Prize in 2020 for his extensive research of Sagittarius A*. As he said in an ESO press release, this latest research offers new insight into the many questions astronomers have had about the SMBH at the center of our galaxy:
“We want to learn more about the black hole at the centre of the Milky Way, Sagittarius A*: How massive is it exactly? Does it rotate? Do stars around it behave exactly as we expect from Einstein’s general theory of relativity? The best way to answer these questions is to follow stars on orbits close to the supermassive black hole. And here we demonstrate that we can do that to a higher precision than ever before.”
The collaboration team also employed a machine-learning technique called Information Field Theory. This consisted of modeling how the real light sources would appear, how GRAVITY would observe them, then comparing the simulated results to the actual observations. This allowed them to acquire highly-accurate measurements of Sag A* and images of Galactic Center that were 20 times sharper than any made by the individual VLT telescopes alone.
In addition to the GRAVITY observations, the team also used data from two former VLT instruments (NACO and SINFONI) and measurements from the Keck Observatory and NOIRLab’s Gemini Observatory in the US. During their observation period, which ran from March to July 2021, the team used these instruments to make precise measurements of the stars that orbit Sag A* as they made their closest approach.
This included S29, which holds the record for making the closest and speediest approach around Sag A* ever observed. This star made its nearest pass in late May 2021, passing within 13 billion km (8 billion mi) – or 90 times the distance between the Earth and Sun (90 AU) – and achieving a velocity of 8,740 km per second (5430 mps). In addition, they found a new star (S300) that was previously undetected, demonstrating the power and effectiveness of their observations.
“The VLTI gives us this incredible spatial resolution, and with the new images, we reach deeper than ever before,” said Julia Stadler, a researcher at the MPA who led the team’s imaging efforts. “We are stunned by their amount of detail, and by the action and number of stars they reveal around the black hole.”
“Following stars on close orbits around Sagittarius A* allows us to precisely probe the gravitational field around the closest massive black hole to Earth, to test General Relativity, and to determine the properties of the black hole,” added Genzel. Originally proposed by Albert Einstein in 1916, General Relativity provides a geometric explanation of gravitation and its effect on space-time. Since then, scientists have sought opportunities to test this theory under the most extreme conditions, which SMBHs provide.
These latest observations, combined with the team’s previous data, confirmed that the stars follow paths predicted by General Relativity perfectly. From this, the team was able to constrain the mass of Sag A* to 4.3 million Solar masses, the most precise estimate of the black hole’s mass yet. Lastly, the precise nature of the images and measurements allowed the collaboration team to fine-tune the distance to Sagittarius A* – 27,000 light-years from Earth.
These latest results, which expand on thirty years of observations of our galactic center, are presented in two papers in the journal Astronomy & Astrophysics. The first paper, “Mass distribution in the Galactic Center based on interferometric astrometry of multiple stellar orbits,” details how the observations of the group of stars that orbit Sag A* serve as “precision probes” for determining the SMBH’s mass.
The second paper, “Deep Images of the Galactic Center with GRAVITY,” describes the new analysis technique the team developed to obtain the deepest and sharpest images of the region surrounding Sag A*. Further observations of the Galactic Center will be possible in the coming years as the GRAVITY instrument is upgraded with the installation of GRAVITY+. This upgrade will push the sensitivity of the VLTI even further and reveal fainter stars that orbit even closer to Sag A*.
The team aims to eventually find stars that orbit so close to Sag A* that they are subject to the gravitational effects caused by the black hole’s rotation. The ESO is also busy constructing the Extremely Large Telescope (ELT) in the Atacama Desert in northern Chile. Once it is complete (scheduled for 2027), the ELT will be the most powerful observatory in the world and allow for the most precise measurements of these stars’ velocities. It will be joined by the Giant Magellan Telescope (GMT), which is scheduled for completion by 2025.
In addition, several next-generation telescopes will be headed to space in the coming years, like the James Webb Space Telescope (JWST) that will be launching from the European Spaceport in French Guiana tomorrow! By 2027, when the ELT begins to gather light, the JWST will be joined by Hubble’s successor (and “mother”), the Nancy Grace Roman Space Telescope (RST). It’s little wonder why astrophysicists are looking to the coming years with such great excitement!