Of all the galaxies that we observe in our universe, elliptical galaxies might be the simplest of all, which at times might make us wonder why these are called galaxies. The name for this type of galaxy was given by the Astronomer Edwin Hubble, who realized that many of the nebulae he observed were galaxies like our own and that most could be organized according to their shapes. His morphological classification is famously known as the Hubble Sequence or Hubble’s tuning fork (for its fork-like shape). Elliptical galaxies with type E0, are round while E7 could be elongated as a sausage. Astronomers have determined that the shape of a galaxy provides information on its past, in particular about how galaxies merged to ultimately form these elliptical galaxies.
One of these galaxies is M87, the largest elliptical galaxy known in our near universe. The cloudiness around the center that gives all elliptical galaxies their uncomplicated look is actually billions of stars that are far away, making them look like a cloud to us. Messier 87 is cataloged as an E0 to E1 elliptical; however, more recent observations taken with the Keck Observatory [https://www.keckobservatory.org/] reveal that this galaxy has an elongated shape like a potato. With this result, we might exclude its classification as an E0 galaxy.
M87 is also well known for its supermassive black hole (SMBH) in the center. It was the target of the first-ever image of a black hole by the Event Horizon Telescope in 2017. [https://www.cfa.harvard.edu/facilities-technology/telescopes-instruments/event-horizon-telescope-eht] Using this telescope, it was possible to measure with more precision its angular gravitational radius of the black hole [1].
Looking to determine the growth history of M87 over cosmic time, Emily R. Liepold, Chung-Pei Ma, & Jonelle L. Walsh, studied the stellar kinematics in the galaxy, from the central supermassive black hole (SMBH) to the most outer regions, allowing them to provide a visualization of the real shape of this elliptical galaxy[2]. Determining how the stars move in the galaxy required spectroscopic observations on top of the existing Hubble data. With the help of the Keck Telescope, they observed 39 pointings using an integral-field unit (IFU) spectrograph. Although using the IFU allowed them to obtain simultaneous images from different types of light and full coverage of the galaxy, the study required many observing campaigns, extending the length of this study to about two years.
Another interesting aspect of this research was that in order to come up with the 3D visualization, they did not only use observations but also numerical code developed by Chung-Pei Ma with other scientists. The code is a dynamical modeling of galaxies and Supermassive black holes called “Axisymmetry in Triaxial Schwarzschild Orbit Superposition Models[4].” This model, like many other models used by astronomers, is a numerical code developed from basic theoretical principles to help obtain this type of prediction. Using this code, they also infer that the mass of the SMBH is about 5.4 billion solar masses, which is slightly less than the predicted value by the Event Horizons Telescope, which predicts it to be about 6.5 billion solar masses. Although this might look like a small difference in our eyes, precise measurements help astronomers understand the relations between black holes and their host galaxies, how these are fueled, and how they impact the environment around them.
So this research not only provided a 3D visualization of the M87 but also a new constraint to the mass of its SMBH in the center.
References:
[1] The Event Horizon Telescope Collaboration et al 2019 ApJL 875 L6
[2] Emily R. Liepold et al 2023 ApJL 945 L35
[3] https://hubblesite.org/contents/news-releases/2023/news-2023-014
[4] Matthew E. Quenneville et al 2021 ApJS 254 25