Black Holes

M87 Galaxy Reconstructed in Thrilling 3D

In astronomy, we speak casually of extremely large numbers and extremely vast distances as if they’re trivial. A supermassive black hole can have several billion solar masses, a distant quasar is 500 million light-years away, etc. Objects like galaxies that are mere tens of millions of light years away start to seem familiar.

But even though our Wikipedia pages are full of data on distant objects, there’s a deceptive lack of understanding of some of their basic properties. Take Messier 87, for example, a galaxy often talked about and seen in images. It’s noteworthy for being home to the first black hole ever imaged.

It’s so far away that astronomers have no real idea what its three-dimensional shape is.

Until now.

Messier 87 is 54 million light-years away in the Virgo cluster. It’s also known as NGC 4486. French astronomer Charles Messier discovered it over 200 years ago, and with the telescopes available at the time, it looked like a nebula. In the early 20th century, things became a little clearer. It still wasn’t called a galaxy because the term galaxy was reserved for the Milky Way. Eventually, by the 1950s, the term galaxy stuck and applied to more than just our own galaxy.

M 87 is important in astronomy for a number of reasons. It served as a test bed for methods that measure the mass of supermassive black holes at galactic centers. Astronomers have also studied its abundant globular clusters (~15,000 of them) to understand metallicity relations between the clusters and the rest of the galaxy. And, of course, its SMBH was the first black hole ever imaged, thanks to the Event Horizon Telescope.

The Event Horizon Telescope (EHT) collected the first direct visual evidence of a supermassive black hole when it captured this image of Messier 87’s SMBH. Credit: Event Horizon Telescope Collaboration

But it’s only now, thanks to a trio of astronomers working with the Hubble Space Telescope and the Keck Observatory on Maunakea in Hawaii, that we’re getting a sense of M 87’s shape in three dimensions, and coinciding with that, a better idea of the mass of its black hole.

The astronomers published their results in a paper titled “Keck Integral-field Spectroscopy of M87 Reveals an Intrinsically Triaxial Galaxy and a Revised Black Hole Mass” in the Astrophysical Journal Letters. The lead author is Emily R. Liepold from the Department of Physics at UC Berkeley.

One of the problems with studying a distant object like M 87 is that astronomers have to make assumptions about its shape. They describe it simply as spherical or axisymmetric (symmetrical about an axis.) But those assumptions cause problems when it comes to determining the mass of the central black hole. The mass of the black hole is inferred from the motions of stars, and our understanding of the motions of stars is affected by our simplistic assumption about the galaxy’s shape.

M 87 is so far away that it appears flat. But astronomers can’t use stereoscopic vision to determine its depth and shape due to the vast distance that separates us. In this study, they relied on the power of the Hubble telescope and the Keck telescopes to measure its shape by discerning the motion of its stars.

The galaxy is far too distant to track individual stars. This is where one of Keck II’s instruments came into play. The Keck Cosmic Web Imager (KCWI) allowed the researchers to examine the galaxy’s central region. They used the KCWI to capture the spectra of stars in a region about 70,000 light-years across. Included in that region is an area 3,000 light-years across containing M 87’s black hole.

Near M 87’s central black hole, the strong gravitational pull makes stars swarm around the hole. The KCWI can’t resolve individual stars, but their spectra show their range of velocities. “It’s sort of like looking at a swarm of 100 billion bees,” said co-author and lead investigator Chung-Pei Ma. “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.”

By tracking the velocities and positions of the stars, the team of astronomers identified prominent rotational patterns of the stars that are quite chaotic. The kinematic axis of rotation is misaligned by 40 degrees with the photometric axis. What does this mean?

“Such misaligned and twisted velocity fields are a hallmark of triaxiality, indicating that M87 is not an axisymmetrically shaped galaxy,” the authors write.

This figure from the research explains some of the astronomers’ findings. The left panel shows line-of-sight velocities for stars in M 87, and the right shows the velocity dispersion for the same. Look at the left panel and notice the white arrow in the bottom corner. It shows the orientation of the photometric major and minor axes. Now notice the blue and red lines in the same panel. They show the kinematic axis. Clearly, the photometric axes and the kinematic axis are misaligned. (The middle panel is an HST image that drives home the point. The photometric major axis is yellow, and the kinematic axis is blue/red. Image Credit: Liepold et al. 2023.

The results show that prior assumptions about M 87’s shape being axisymmetrical are incorrect. Instead, the galaxy is triaxial.

The team’s efforts revealed more than just M 87’s shape. If the stars swarming around the black hole are bees, then the black hole is their queen. And the detailed data on the swarm of stars sheds light on the black hole’s mass. “Knowing the 3D shape of the ‘swarming bees’ enabled us to obtain a more robust dynamical measurement of the mass of the central black hole that is governing the bees’ orbiting velocities,” said Ma.

This isn’t the only recent study to try to establish parameters for the intrinsic shape of galaxies, but it’s one of only a handful. Even fewer galaxies have been observed with enough resolution, field of view, spectral coverage, and signal-to-noise to determine their intrinsic shape along with the mass of the supermassive black hole and the mass of the galaxy itself.

This is no small matter for astronomers trying to accurately measure black hole masses. It all comes down to the detailed data they gathered and the triaxial modelling it allowed. The difference between triaxial modelling and axisymmetrical modelling can vary the black hole mass by a huge degree. “When direct comparisons between axisymmetric and triaxial modelling were made on the same galaxy, the black hole mass from axisymmetric models has ranged from about 50% to 170% of the mass when triaxiality was allowed,” the authors write in their conclusion.

Back in 1995, researchers used the Hubble to measure M 87’s black hole mass at 2.4 billion solar masses. This new research arrives at a vastly different figure: 5.4 billion solar masses.

Overall, the triaxial models of galaxies in this work “… were able to match the observed stellar kinematics significantly better than axisymmetric models,” the authors conclude.

Why are more accurate black hole masses desirable? They’re at the heart of our understanding of galaxies themselves.

“More secure black hole masses could result in significant changes to the local black hole census and the shapes of the scaling relations between black holes and host galaxies, thereby impacting our understanding of black hole fueling and feedback physics, as well as binary black hole merger physics,” the authors explain.

There’s a lot we still don’t know about black holes. But in ways we don’t totally understand yet, they play a central role in the evolution of galaxies. If we ever hope to have a more complete understanding of how black holes have shaped galaxies, we better get their masses correct.

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Evan Gough

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