Astronomers from the University of Michigan and their collaborators at Georgia State University (PI Misty Bentz) and Cambridge University will be amongst the first to use NASA’s James Webb Space Telescope (JWST) to weigh a supermassive black hole. Determining a black hole’s mass is critical to understanding how it feeds and affects the surrounding galaxy. UM astronomer Monica Valluri is a co-investigator on a project that was awarded observing time for Early Release Science which will be done shortly after JWST begins operation.
The black hole in question is located at the center of galaxy NGC 4151, and is observable as a bright smudge that stands out from the softer light that surrounds it. The bright smudge (called an “Active Galactic Nucleus” or AGN for short) arises from a hot, swirling disk of gas that glows brightly as it falls into the deep well created by the black hole’s extreme gravity. It is estimated that this supermassive black hole weights about 40 million times as much as our sun, but this estimate is quite uncertain. Since most galaxies contain a black hole, having a better understanding of the black hole in galaxy NGC 4151 will improve astronomer’s understanding of galaxies across the universe. It will also bring them one step closer to answering questions related to how a galaxy’s central black hole grows with time, how stars form in galaxies, and how the two affect each other.
There are several methods astronomers can use to weigh supermassive black holes. The technique that Valluri uses involves measuring the motion of the stars very close to the galaxy’s center where the gravity of the black hole dominates. The faster the stars are moving, the heavier the black hole.
In order to measure the motion of the stars, the research team will utilize Webb’s Near-Infrared integral field spectrograph, which takes in light from each part of the central region of the galaxy and splits in into a rainbow (spectrum). Each spectrum relays important information about the elements that stars are composed of and their relative motions. NASA has released a video demonstrating how this spectrograph works and why it is so unique. The research team will gather information about groups of stars and use computer models to determine the gravitational field affecting the stars.
“Our computer code generates a bunch of mock stars – tens of thousands of stars, mimicking the motions of real stars in the galaxy. We put in a variety of different black holes and see what matches the observations the best,” said Valluri.
The results of this measuring technique will be compared with a second technique that is focused on how gas clouds close to the central black hole respond to changes in the brightness of the AGN when material falls into the black hole. This second technique (“reverberation mapping”) currently has large uncertainties that Valluri and her collaborators want to reduce, since reverberation mapping can be used to weigh black holes in much more distant galaxies, and will be an important tool for understanding how galaxies and their black holes evolved.
Contact: Monica Valluri