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Quasars are powered by some of the most extreme black holes in the universe. These black holes swallow material and release huge amounts of radiation in the process, and now astronomers have tested a new method to work out just how heavy these supermassive black holes really are.
Precise measurements of their properties help us to understand these objects better, as well as improving our understanding of the Universe itself. Quasars are so bright that they can be seen even from incredible distances. With this new method – called spectroastrometry – researchers can learn about how big a black hole is and how gas moves around it.
Key to that is the determination of the “broad emission-line region” (BLR). The gas around a black hole is very energetic and it’s moving at speeds of several thousand kilometers per second. All of this creates specific signals in the BLR. The team demonstrated that it is possible to estimate the whole structure from a single emission line, the so-called Hydrogen alpha (Hα) line.
As reported in The Astrophysical Journal, the team measured the Hα line for the quasar J2123-0050 located in the constellation of Aquarius. The light of this quasar dates back to when the cosmos was just 2.9 billion years old. The method estimates that the supermassive black hole weighs at most 1.8 billion times the mass of our Sun. That's almost 400 times heavier than the supermassive black hole at the center of our own galaxy.
“The exact mass determination was not yet the main goal of these first observations at all,” co-author Jörg-Uwe Pott, head of the “Black Holes and Accretion Mechanisms” working group at Max Planck Institute for Astronomy, said in a statement. “Instead, we wanted to show that the spectroastrometry method can in principle detect the kinematic signature of the central quasar masses using the 8-metre telescopes already available today.”
The fact that such a measurement was possible heralds an exciting new way to study the largest black holes, and it can be used in conjunction with other methods. The next-generation observatories will be able to employ this method with even better results.
“With the significantly increased sensitivity of the James Webb Space Telescope (JWST) and the Extremely Large Telescope (ELT with a primary mirror diameter of 39 metres) currently under construction, we will soon be able to determine quasar masses at the highest redshifts,” Joe Hennawi added.
The team is now working on planning observation campaigns to be attempted with the ELT when it starts observing in 2027.
“We will use the ELT to astrometrically measure numerous quasars at different distances in a single night, allowing us to observe the cosmological evolution of black hole masses directly," lead author Felix Bosco commented.
