La verdadera forma 3D de la galaxia M87 ha sido revelada

Una enorme ciudad de innumerables estrellas resultó tener forma de papa

Aunque se estima que el universo contiene un billón de galaxias, solo tienen unas pocas formas básicas. El astrónomo estadounidense Edwin Hubble se dio cuenta de esto a principios del siglo XX cuando utilizó el telescopio más potente de la Tierra en ese momento para buscar en el universo. Como un niño que recoge rocas, las clasificó en formas. Muchos eran discos espirales planos de estrellas. Otras parecían bolas de algodón, por lo que se las llamó galaxias elípticas. Aunque el universo es tridimensional, las galaxias aparecen planas en el cielo. Está demasiado lejos para que los astrónomos utilicen la visión estereoscópica. Ahora, un siglo después, los astrónomos finalmente tienen las herramientas para estimar la verdadera forma de una galaxia elíptica. Eligieron una de las galaxias elípticas más cercanas a la Tierra, M87, que se encuentra a 55 millones de años luz de distancia en el corazón del vasto cúmulo de galaxias de Virgo. Siguiendo el movimiento de las estrellas alrededor del centro de M87, como abejas alrededor de una colmena, midieron que la galaxia parece tener forma de patata. No solo tienen un eje largo y uno corto, que define una elipse en una hoja de papel cuadriculado, sino que miden un tercer eje que ayuda a definir las tres dimensiones. El término de ingeniería es: triaxial.

Esta animación comienza con[{» attribute=»»>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.

Giant Galaxy Seen in 3D by NASA’s Hubble Space Telescope and Keck Observatory

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 M87 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, an international collaboration of ground-based telescopes, 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 M87 and the more precise mass of the central black hole could help astrophysicists learn the black hole’s spin rate. “Now that we know the direction of the net rotation of stars in M87 and have an updated mass of the black hole, we can combine this information with data from the Event Horizon Telescope to constrain the spin,” said Chung-Pei Ma, a UC Berkeley lead investigator on the research.

Over ten times the mass of the Milky Way, M87 probably grew from the merger of many other galaxies. That’s likely the reason M87’s central black hole is so large — it assimilated the central black holes of one or more galaxies it swallowed.

Ma, together with UC Berkeley graduate student Emily Liepold (lead author on the paper published in the Astrophysical Journal Letters) and Jonelle Walsh at Texas A&M University were able to determine the 3D shape of M87 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 M87’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,” said 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.”

The researchers took the data between 2020 and 2022, as well as earlier star brightness measurements of M87 from Hubble, and compared them to computer model predictions of how stars move around the center of the triaxial-shaped galaxy. The best fit to this data allowed them to calculate 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.

In the 1920s, astronomer Edwin Hubble first classified galaxies according to their shapes. Flat disk spiral galaxies could be viewed from various projection angles of the sky: face-on, oblique, or edge-on. But the “blobby-looking” galaxies were more problematic to characterize. Hubble came up with the term elliptical. They could only be sorted out by how great the ellipticity was. They didn’t have any apparent dust or gas inside of them to better distinguish between them. Now, a century later astronomers have a stereoscopic look at a prototypical elliptical galaxy.

Reference: “Keck Integral-field Spectroscopy of M87 Reveals an Intrinsically Triaxial Galaxy and a Revised Black Hole Mass” by Emily R. Liepold, Chung-Pei Ma and Jonelle L. Walsh, 15 March 2023, Astrophysical Journal Letters.
DOI: 10.3847/2041-8213/acbbcf

The Hubble Space Telescope is a project of international cooperation between NASA and ESA. NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble and Webb science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, in Washington, D.C.