Astronomers have a new way to measure the mass of supermassive black holes

Even the most supermassive black holes are not very large, making it extremely difficult to measure their size. However, astronomers recently developed a new technique that can estimate the mass of a black hole based on the movement of hot gas around them – even when the black hole itself is smaller than a single pixel.

Supermassive black holes are surrounded by tons of superheated plasma. This plasma swirls around the back hole, forming a torus and an accretion disk that continuously feeds material into the black hole. Due to the extreme gravity, this gas travels incredibly fast and glows fiercely. It is this light that we identify as a quasar, which can be seen from all over the universe.

While quasars are relatively easy to spot, it is much more difficult to quantify the properties of the central black hole. Now Felix Bosco, in close collaboration with Jörg-Uwe Pott, both of the Max Planck Institute for Astronomy (MPIA) in Heidelberg, and former MPIA researchers Jonathan Stern (now Tel Aviv University, Israel) and Joseph Hennawi (now UC Santa Barbara; USA and Leiden University, The Netherlands), succeeded for the first time in demonstrating the feasibility of directly determining the mass of a quasar using a technique called spectroastrometry.

Spectroastrometry is based on the observation of the area around the black hole. As the gas swirls around it, some will move in our direction and others will move away. The part of the gas moving towards us will be shifted toward blue, and the part moving away will shift more toward red. Even though the central black hole and accretion disk are too small to resolve, the technique can still be applied to more distant regions and, through modeling, researchers can estimate a mass.

“By separating the spectral and spatial information in the collected light, as well as statistically modeling the measured data, we can derive distances much less than an image pixel from the center of the accretion disk,” explained Bosco.

The team successfully applied this technique to J2123-0050, a quasar active when the universe was only 2.9 billion years old. They discovered that the central black hole weighed 1.8 billion solar masses. However, taking this technique to the next level and targeting the first quasars will require new telescopes.

Joe Hennawi adds: “With the dramatically increased sensitivity of the James Webb Space Telescope (JWST) and Extremely Large Telescope (ELT, with a primary mirror diameter of 39 meters) currently under construction, we will soon be able to determine the masses of quasars at the highest redshifts. Jörg-Uwe Pott, who also leads Heidelberg’s contributions to ELT’s first near infrared camera, MICADO, adds: “The now published feasibility study is helping us define and prepare our planned ELT research programs.