Mapping the velocities of stars around a giant elliptical galaxy reveals its asymmetrical nature. Researchers have discovered that the galaxy M87, which was previously thought to be symmetrical, is actually asymmetrical. They determined that the supermassive black hole has a mass 5.37 billion times that of the Sun, which could help them learn about the black hole’s spin.

Viewed from Earth, the giant elliptical galaxy M87 is a two-dimensional blob that appears perfectly symmetrical, making it a favorite target for amateur astronomers.

But a new, highly detailed analysis of stellar movement around the central supermassive black hole, the first black hole imaged by the Event Horizon Telescope (EHT) in 2019, shows it’s not as perfect as it sounds. In fact, the M87 is rather asymmetrical, like a red potato. The galaxy’s minor axis is about three-quarters (72.2%) of its long axis, and its intermediate axis is about eight-sevenths (84.5%) of its long axis.

Knowing this, astronomers at the University of California at Berkeley were able to determine with high precision the mass of the supermassive black hole in the core of the galaxy, estimating it at 5.37 billion solar masses. For comparison, at the center of the Milky Way is a massive black hole that is only 4 million times the mass of the Sun.

They were also able to measure a relatively slow rotation of the galaxy at 25 kilometers per second. Interestingly, the galaxy does not revolve around any of its major axes, but around an axis that is 40 degrees away from the long axis of its two-dimensional image observed using the Hubble Space Telescope. A stereo reconstruction of the M87 galaxy and a more accurate estimation of the central black hole’s mass could help astrophysicists learn about a feature of a black hole they hadn’t been able to detect before for any black hole: its spin.

“Now that we know the exact rotation direction of the stars in M87 and have an updated mass of the black hole, we can combine this information with surprising data from the EHT team to limit the spin,” said Chung-Pei Ma. , professor of astronomy and physics at the University of California, Berkeley, who led the research team. “It could show a specific direction and rotation range of the black hole, which would be great. We’re working on it.”

Further analysis to determine the true shape of giant elliptical galaxies (the galaxies with the largest black holes in their cores) will help astronomers better understand how large galaxies and massive black holes form, and can help astronomers better interpret gravitational wave signals. Ma is leading a long-term study of supermassive black holes, called MASSIVE.

Spiral galaxies are typically small, rapidly rotating and have a well-recognized flat shape, while giant elliptical galaxies have a slow rotating and fluffy appearance, making their three-dimensional shape difficult to distinguish. Like M87, the largest galaxy in the vast Virgo galaxy cluster, giant elliptical galaxies arose from the merger of many other galaxies. This is probably why M87’s central black hole is so large – it has absorbed the central black holes of all the galaxies it has absorbed. In total, the galaxy contains about 100 billion stars, which is 10 times more massive than the Milky Way.

UC Berkeley graduate student Ma and lead authors Emily Lippold and Johnelle Walsh of Texas A&M University in College Station were able to determine the three-dimensional shape of M87 thanks to a relatively new precision instrument mounted on one of the twin telescopes, the Keck II telescope. 10 telescopes. Keck meter telescopes atop Mauna Kea, a volcano in Hawaii. An integrated field spectrometer called the Keck Cosmic Web Imager (KCWI) allowed Ma and his team to measure the spectra of stars at the center of the galaxy.

They pointed the telescope at 62 contiguous locations in the galaxy, which completely covered a region about 70,000 light-years in diameter, and recorded the spectra of stars in that region. The observations cover the central region, where gravity is largely dominated by the supermassive black hole, as well as the outer region dominated by dark matter. While the telescope can’t distinguish individual stars – M87 lies about 53 million light-years from Earth – the spectra can reveal the range of speeds in each pixel of each image. This is enough information to calculate the gravitational mass around which they revolve.

“It’s like watching a swarm of 100 billion bees go around their happy orbits,” said Ma, the Judy Chandler Webb Professor of Physical Sciences. “Even though we cannot distinguish individual bees from afar, we get very detailed information about their collective velocity. Indeed, the excellent sensitivity of this spectrograph allowed us to map M87 precisely.”