M87 in 3D: New view of galaxy helps pin down mass of the black hole at its core

Seen from Earth, the giant elliptical galaxy M87 is just a two-dimensional blob, though one that appears perfectly symmetrical and thus a favored target of amateur astronomers.

Yet, a new, highly detailed analysis of the motion of stars around its central supermassive black hole — the first black hole to be imaged by the Event Horizon Telescope (EHT) in 2019 — reveals that it's not as perfect as it looks.

In fact, M87 is highly asymmetrical, like a russet potato. The galaxy's shortest axis is about three-fourths (72.2%) the length of its long axis, while the intermediate axis is about seven-eighths (84.5%) that of the long axis.

Knowing this, University of California, Berkeley, astronomers were able to determine the mass of the supermassive black hole at the galaxy's core to a high precision, estimating it at 5.37 billion times the mass of the sun. By comparison, our own Milky Way has at its center a massive black hole only 4 million times the mass of the sun.

They also were able to measure the rotation of the galaxy, which is a relatively sedate 25 kilometers per second. Interestingly, it is not rotating around any of the galaxy's major axes, but instead about an axis that is 40 degrees away from the long axis of its 2D image as observed by the Hubble Space Telescope.

The stereo reconstruction of the M87 galaxy and the more precise figure for the mass of the central black hole could help astrophysicists learn about a characteristic of the black hole they've had no way to determine before for any black hole: its spin.

"Now that we know the direction of the net rotation of stars in M87 and have an updated mass of the black hole, we can combine this information with the amazing data from the EHT team to constrain the spin," said Chung-Pei Ma, a UC Berkeley professor of astronomy and of physics who led the research. "This may point toward a certain direction and range of spin for the black hole, which would be remarkable. We are working on this.”

Further analyses to determine the true shape of giant elliptical galaxies — the galaxies with the largest black holes at their cores — will help astronomers understand better how large galaxies and large black holes form and could help astronomers better interpret gravitational wave signals. Ma leads a long-term study of supermassive black holes that is dubbed MASSIVE.

The results were published online March 15 in The Astrophysical Journal Letters (ApJ Letters).

Determining a galaxy's 3D shape

While spiral galaxies tend to be small, rotate quickly and have a well-recognized pancake shape, giant elliptical galaxies rotate slowly and have a blobby appearance, their 3D shape difficult to discern. Like M87, the largest galaxy in the massive Virgo Cluster of galaxies, giant elliptical galaxies have grown from the merger of many other galaxies. That's likely the reason M87's central black hole is so large — it assimilated the central black holes of all the galaxies it swallowed. In all, the galaxy contains about 100 billion stars, 10 times larger than the Milky Way.

Ma, UC Berkeley graduate student and lead author Emily Liepold, and Jonelle Walsh at Texas A&M University in College Station were able to determine the 3D shape of M87 thanks to a relatively new precision instrument mounted on the Keck II Telescope, one of the twin 10-meter Keck telescopes atop Mauna Kea, a volcano in Hawai'i. Called the Keck Cosmic Web Imager (KCWI), the integral field spectrometer allowed Ma and her team to measure the spectra of stars in the center of the galaxy.

They pointed the telescope at 62 adjacent locations in the galaxy, completely covering a region about 70,000 light-years across, and recorded the spectra of stars within that region. The observations span the central region — about 3,000 light-years across — where gravity is largely dominated by the supermassive black hole, as well as the outer part dominated by dark matter. Though the telescope cannot resolve individual stars — M87 lies about 53 million light- years from Earth — the spectra can reveal the range of velocities within each pixel of each image, enough information to calculate the gravitational mass they're orbiting.

"It's sort of like looking at a swarm of 100 billion bees that are going around in their own happy orbits," said Ma, the Judy Chandler Webb Professor in the Physical Sciences. "Though we are looking at them from a distance and can’t discern individual bees, we are getting very detailed information about their collective velocities. It's really the superb sensitivity of this spectrograph that allowed us to map out M87 so comprehensively."

This is the first time KCWI has been used to reconstruct the geometry of a distant galaxy, and M87 is one of only a handful of giant elliptical galaxies whose 3D structure has been determined. Ma’s team had previously determined the 3D structure of two other giant elliptical galaxies, NGC 1453 and NGC 2693, both harboring smaller black holes than M87.

The researchers took the data obtained during four nights of Keck observations between 2020 and 2022, along with earlier photometric data for M87 from NASA's Hubble Space Telescope, and compared them to computer model predictions of how stars move around the center of a triaxial galaxy. The best fit to the data — axial ratios of 1 to 0.84 to 0.72 — then allowed them to calculate the black hole mass.

"The Keck data are so good that we can measure the intrinsic shape of M87 along with the black hole at the same time," Ma said. "We made the first measurement of the actual 3D shape of the galaxy. And since we allowed the swarm of bees to have a more general shape than just a sphere or disk, we have a more robust dynamical measurement of the mass of the central black hole that is governing the bees’ orbiting velocities."

The authors dedicated their manuscript to the late astronomer Wallace "Wal" Sargent, who first suggested that a supermassive black hole lurked at the center of M87 and calculated its mass to be about 5 billion solar masses.

"His number is a twiddle with our error bars, which is very interesting to see after decades of work," said Ma, who credits Sargent with being a mentor when she was a postdoctoral fellow at the California Institute of Technology.

The previous estimate of the mass of the supermassive black hole in M87, published in 2011, was based on a similar analysis of the dynamical movement of stars around the black hole, though that study assumed the galaxy was axisymmetric. The number, 6.14 billion solar masses, is within error bars of the new, more precise estimate. When imaging the black hole four years ago, the EHT scientists estimated the black hole mass to be 6.5 billion solar masses, 21% higher than the new number.

Interestingly, the dark matter within the volume of the galaxy they analyzed is much higher than that of the black hole — about 388 billion solar masses, or 67% of the entire mass of M87. Though the identity of dark matter is still a mystery, it makes up about 85% of the mass of the universe.

Jonelle Walsh is with the George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy at Texas A&M. The work was funded by the National Science Foundation (AST-1817100, AST-2206307), the Heising-Simons Foundation and the Miller Institute for Basic Research in Science.