Messier 87, a massive elliptical galaxy located approximately 53 million light-years away in the constellation of Virgo, turns out to be triaxial, or potato-shaped, and hosts a supermassive black hole about 5.4 billion times the mass of the Sun, according to new observations by the NASA/ESA Hubble Space Telescope and the W.M. Keck Observatory.
A photo of the huge elliptical galaxy Messier 87 (left) is compared to its 3D shape as gleaned from meticulous observations made with Hubble and Keck telescopes (right). Image credit: NASA / ESA / Joseph Olmsted, STScI / Frank Summers, STScI / Chung-Pei Ma, UC Berkeley.
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 elliptical galaxies look like blobs in pictures.
Determining the true shape of such galaxies will help astronomers understand better how large galaxies and their central large black holes form.
University of California, Berkeley astronomer Chung-Pei Ma and her colleagues made the 3D plot by measuring the motions of stars that swarm around M87*, the supermassive black hole in the center of Messier 87.
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 the astronomers to build a 3D view of the galaxy.
They were able to determine the mass of M87* to a high precision, estimating it at 5.4 billion times the mass of the Sun.
Hubble observations in 1995 first measured the black hole as being 2.4 billion solar masses, which astronomers deduced by clocking the speed of the gas swirling around the black hole.
When the Event Horizon Telescope (EHT) Collaboration released the first-ever image of the same black hole in 2019, the size of its pitch-black event horizon allowed researchers to calculate a mass of 6.5 billion solar masses using Einstein’s theory of general relativity.
The stereo model of Messier 87 and the more precise mass of the central black hole could help astronomers learn the black hole’s spin rate.
“Now that we know the direction of the net rotation of stars in Messier 87 and have an updated mass of the black hole, we can combine this information with data from the EHT to constrain the spin,” Professor Ma said.
Over ten times the mass of the Milky Way, Messier 87 probably grew from the merger of many other galaxies.
That’s likely the reason M87* is so large — it assimilated the central black holes of one or more galaxies it swallowed.
The authors were able to determine the 3D shape of Messier 87 thanks to a new precision instrument mounted on the Keck II Telescope.
They pointed Keck at 62 adjacent locations of the galaxy, mapping out the spectra of stars over a region about 70,000 light-years across.
This region spans the central 3,000 light-years where gravity is largely dominated by the supermassive black hole.
Though the telescope cannot resolve individual stars because of Messier 87’s great distance, the spectra can reveal the range of velocities to calculate mass of the object they’re orbiting.
“It’s sort of like looking at a swarm of 100 billion bees,” Professor Ma said.
“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.”
“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.”
The findings appear in the Astrophysical Journal Letters.
_____
Emily R. Liepold et al. 2023. Keck Integral-field Spectroscopy of M87 Reveals an Intrinsically Triaxial Galaxy and a Revised Black Hole Mass. ApJL 945, L35; doi: 10.3847/2041-8213/acbbcf
