Exploring the Three Dimensional Shape of the Elliptical Galaxy M87

Monday - April 24 2023, 18:26 UTC - 1 year ago

Astronomers have used Hubble and Keck telescope's to measure the three-dimensional shape of the elliptical galaxy M87 located 55 million light-years away, determining that it is triaxial, or potato-shaped. This is the first time the true shape of a huge and close elliptical galaxy has been measured, and they have also measured the mass of the black hole at the galaxy's core to a high precision.

Though it’s estimated that the universe contains 1 trillion galaxies, they come in just a few basic shapes. American astronomer Edwin Hubble realized this in the early 20th century when he used the most powerful telescope on Earth at the time to peer across the universe. Like a kid collecting rocks, he sorted them into shapes. Many were flattened spiral disks of stars. Others looked like cotton balls, which he called elliptical galaxies .

Though the universe is three-dimensional, galaxies look flat on the sky. They are too far away for astronomers to employ stereoscopic vision. Now, a century later, astronomers at last have the tools to estimate the true shape of an elliptical galaxy. They picked one of the nearest elliptical galaxies to Earth, M87, located 55 million light-years away in the heart of the vast Virgo cluster of galaxies .

By following the motion of stars around the center of M87, like bees around a hive, they’ve measured that the galaxy looks potato-shaped. It not only has a long and short axis, which defines an ellipse on a piece of graph paper, but they measured a third axis which helps define the three-dimensionality. The geometric term is: triaxial. This animation begins with a Hubble Space Telescope photo of the huge elliptical galaxy M87 .

It then fades to a computer model of M87. A grid is overlaid to trace out its three-dimensional shape, made more evident by rotating the model and grid. This shape was gleaned from meticulous observations made with the Hubble and Keck telescopes. Because the galaxy is too far away for astronomers to employ stereoscopic vision, they instead followed the motion of stars around the center of M87, like bees around a hive .

This created a three-dimensional view of how stars are distributed within the galaxy that informed the model. Though we live in a vast three-dimensional universe, celestial objects seen through a telescope look flat because everything is so far away. Now for the first time, astronomers have measured the three-dimensional shape of one of the biggest and closest elliptical galaxies to us, M87. This galaxy turns out to be "triaxial," or potato-shaped .

This stereo vision was made possible by combining the power of NASA’s Hubble Space Telescope and the ground-based W. M. Keck Observatory on Maunakea, Hawaii. In most cases, astronomers must use their intuition to figure out the true shapes of deep-space objects. For example, the whole class of huge galaxies called "ellipticals" look like blobs in pictures. Determining the true shape of giant elliptical galaxies will help astronomers understand better how large galaxies and their central large black holes form .

Scientists made the 3D plot by measuring the motions of stars that swarm around the galaxy’s supermassive central black hole. The stellar motion was used to provide new insights into the shape of the galaxy and its rotation, and it also yielded a new measurement of the black hole’s mass. Tracking the stellar speeds and position allowed researchers to build a three-dimensional view of the galaxy. Astronomers at the University of California, Berkeley, were able to determine the mass of the black hole at the galaxy’s core to a high precision, estimating it at 5 .

4 billion times the mass of the Sun. Hubble observations in 1995 first measured the rate at which stars were orbiting the black hole, leading to an initial estimate of its size.