How to weigh a black hole with the Webb Space Telescope

At first glance, the galaxy NGC 4151 looks like an average spiral. Examine its center more closely, though, and you can spot a bright smudge that stands out from the softer glow around it. That point of light marks the location of a supermassive black hole weighing about 40 million times as much as our Sun.

Astronomers will use NASA's James Webb Space Telescope to measure that black hole's mass. The result might seem like a piece of trivia, but its mass determines how a black hole feeds and affects the surrounding galaxy. And since most galaxies contain a supermassive black hole, learning about this nearby galaxy will improve our understanding of many galaxies across the cosmos.

"Some central questions in astrophysics are: How does a galaxy's central black hole grow with time; how does the galaxy itself grow with time; and how do they affect each other? This project is a step toward answering those questions," explained Misty Bentz of Georgia State University, Atlanta, the principal investigator of the project.

Probing a galaxy's core
There are several methods of weighing supermassive black holes. One technique relies on measuring the motions of stars in the galaxy's core. The heavier the black hole, the faster nearby stars will move under its gravitational influence.

NGC 4151 represents a challenging target, because it contains a particularly active black hole that is feeding voraciously. As a result, the material swirling around the black hole, known as an accretion disk, shines brightly. The light from the accretion disk threatens to overwhelm the fainter light from stars in the region.

"With Webb's beautifully shaped mirrors and sharp 'vision,' we should be able to probe closer to the galaxy's center even though there's a really bright accretion disk there," said Bentz.

The team expects to be able to investigate the central 1,000 light-years of NGC 4151, and be able to resolve stellar motions on a scale of about 15 light-years.

A thousand spectra at once
To achieve this feat, the team will use Webb's Near-Infrared Spectrograph (NIRSpec) integral field unit, or IFU. It will be the first IFU flown in space, and it has a unique capability.

Webb's IFU takes the light from every location in an image and splits it into a rainbow spectrum. To do this it employs almost 100 mirrors, each of them precision crafted to a specific shape, all squeezed into an instrument the size of a shoebox. Those mirrors effectively slice a small square of the sky into strips, then spread the light from those strips out both spatially and in wavelength.

In this way a single image yields 1,000 spectra. Each spectrum tells astronomers not only about the elements that make up the stars and gas at that exact point of the sky, but also about their relative motions.

Despite Webb's exquisite resolution, the team won't be able to measure the motions of individual stars. Instead, they will get information about groups of stars very close to the center of the galaxy. They will then apply computer models to determine the gravitational field affecting the stars, which depends on the size of the black hole.

"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 Monica Valluri of the University of Michigan, a co-investigator on the project.

The result of this technique will be compared with a second one that focuses on the gas at the galaxy's center, rather than the stars.

"We should get the same answer, no matter what technique we use, if we're looking at the same black hole," said Bentz. "NGC 4151 is one of the best targets for making that comparison."

These observations will be taken as part of the Director's Discretionary-Early Release Science program. The DD-ERS program provides time to selected projects enabling the astronomical community to quickly learn how best to use Webb's capabilities, while also yielding robust science.

Related Links
James Webb Space Telescope
Understanding Time and Space

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