Astronomers capture orbital motion around black hole with new clarity

Illustration: NRAO/AUI/NSF

An international team of astronomers that includes Texas A&M University’s Benjamin Boizelle and Jonelle Walsh has obtained the clearest view to date of cold gas rotation around a supermassive black hole.

Using the National Radio Astronomy Observatory’s Atacama Large Millimeter/submillimeter Array (ALMA)—a huge, highly sophisticated radio telescope array situated at an altitude of 16,500 feet in the high desert of Chile—the team zeroed in on a cosmic behemoth at the center of the giant elliptical galaxy NGC 3258 located about 100 million light-years from Earth. They then calculated the mass of this supermassive black hole—the largest ever measured with ALMA—to be a whopping 2.25 billion times the mass of our Sun.

“Observing the orbital motion of material as close as possible to a black hole is vitally important when accurately determining the black hole’s mass.” said Boizelle, a postdoctoral researcher in Texas A&M’s George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy and lead author on the team’s study, published Aug. 7 in The Astrophysical Journal and also available via arXiv. “These new observations of NGC 3258 demonstrate ALMA’s amazing power to map the rotation of cold gaseous disks around supermassive black holes in stunning detail.”

Walsh is an assistant professor in the Department of Physics & Astronomy, College of Science.

According to the official press release, ALMA has pioneered a new method to study black holes in giant elliptical galaxies during the past few years. About 10 percent of elliptical galaxies contain regularly rotating disks of cold, dense gas at their centers. By measuring the Doppler shift of emission from molecules like carbon monoxide (CO) with millimeter-wavelength radio telescopes, astronomers can trace the motion of orbiting gas clouds. Given the nature of its configuration and capabilities, ALMA is uniquely suited to resolve rapidly rotating gas at the centers of galaxies.

Images taken by NASA’s Hubble Space Telescope reveal a dusty disk at the center of NGC 3258 that is nearly 1,000 light-years across, making this galaxy a promising target for ALMA observations. As a part of his doctoral dissertation at the University of California, Irvine, Boizelle obtained ALMA CO observations of NGC 3258 in August 2017 that showed the gas is in nearly perfect rotation from the outer edge of the disk down to just 65 light-years from the supermassive black hole. By carefully modeling the gas rotation speed, he was able to measure the black hole’s mass with unprecedented precision.

“Previous studies have demonstrated ALMA’s ability to detect rotating gas disks near a black hole and even to measure black hole masses in a few cases,” Boizelle said. “For the first time, however, these observations highly resolve cold gas rotation within what is called the sphere of influence — the innermost region of the galaxy where a black hole’s gravity is the dominant force.”

While ALMA observations have traced out cold gas rotation in dusty disks at the centers of other massive galaxies, Boizelle notes they do not frequently reveal rapid gas rotation within the sphere of influence. He describes NGC 3258 as an extraordinarily uncommon case and says that finding more examples like this one will help astronomers better understand the growth of black holes and galaxies over cosmic time.

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