black holes

Black hole with mass of 6.6 billion suns

U. TEXAS-AUSTIN (US) — The enormous mass of the black hole at the center of galaxy M87 makes it a prime candidate for future studies to “see” a black hole for the first time.

The black hole is the most massive one in our cosmic neighborhood—equal to 6.6 billion suns. Astronomers were able to determine the mass by combining data from a giant telescope in Hawaii and a smaller telescope in Texas to probe the motions of stars around the black hole.

The results were presented at the 217th meeting of the American Astronomical Society in Seattle. Two papers detailing the results will be published soon in the Astrophysical Journal.

In order to pin down the black hole’s mass conclusively, says Karl Gebhardt of the University of Texas at Austin, one must account for all the components in the galaxy.

Studies of the central and outermost regions of a galaxy are necessary to “see” the influence of the outer reaches of the galaxy—the so-called “dark halo”—the black hole, and the stars. The dark halo is a region surrounding the galaxy filled with “dark matter,” an unknown type of mass that gives off no light but is detectable by its gravitational effect on other objects.

When all of these components are considered together, Gebhardt says, the results on the black hole are definitive, meeting what he calls the “gold standard” for accurately sizing up a black hole.

Gebhardt used the Near-Infrared Field Spectrograph on the 8-meter Gemini North telescope in Hawaii to measure the speed of the stars as they orbit the black hole. The study was improved by Gemini’s use of “adaptive optics,” a system that compensates, in real time, for shifts in the atmosphere that can blur details seen by telescopes on the ground.

Together with the telescope’s large collecting area, the adaptive optics system allowed researchers to track the stars at M87’s heart with 10 times greater resolution than previous studies.

“The result was only possible by combining the advantages of telescope size and spatial resolution at levels usually restricted to ground and space facilities, respectively,” says graduate student Joshua Adams.

Astronomer Tod Lauer of the National Optical Astronomy Observatory, which was also involved in the Gemini observations, says “our ability to obtain such a robust black hole mass for M87 bodes well for our ongoing efforts to hunt for even larger black holes in galaxies more distant than M87.”

Graduate student Jeremy Murphy used a different instrument to track the motions of stars at the outskirts of the galaxy. Studying the stars’ movements in these distant regions gives astronomers insight into what the unseen dark matter in the halo is doing. Murphy employed an innovative instrument called VIRUS-P on McDonald Observatory’s Harlan J. Smith Telescope.

Studying the distant edges of a galaxy, far from the bright center, is a tricky business, Gebhardt says.

“That has been an enormous struggle for a long time, trying to get what the dark halo is doing at the edge of the galaxy, simply because, when you look there, the stellar light is faint,” he says. “This is where the VIRUS-P data comes in, because it can observe such a huge chunk of sky at once.”

This means the instrument can add together the faint light from many dim stars and add them together to create one detailed observation. This kind of instrument is called an “integral field unit spectrograph,” and VIRUS-P is the world’s largest.

“The ability of VIRUS-P to dig deep into the outer halo of M87 and tell us how the stars are moving is impressive,” Murphy says. “It has quickly become the leading instrument for this type of work.”

The combined Gemini and McDonald data allowed the team to pinpoint the mass of M87’s black hole at 6.6 billion suns. But measuring such a massive black hole is only one step toward a greater goal.

“My ultimate goal is to understand how the stars assembled themselves in a galaxy over time,” Gebhardt says.

“How do you make a galaxy? These two datasets probe such an enormous range, in terms of what the mass is in the galaxy. That’s the first step to answering this question. It’s very hard to understand how the mass accumulates unless you know exactly what’s the distribution of mass: How much is in the black hole, how much is in the stars, how much is in the dark halo.”

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