A series of images constructed from observational data and mathematical modelling show the evolution of the black hole at the centre of the M87 galaxy from 2009 to 2017.Credit: Event Horizon Telescope Collaboration; gif compiled by Nature.

Images created from old observations show a stormy evolution around that dark void over the past ten years.

The historic first image of a black hole, released last year, has now been turned into a movie. The short sequence of frames shows the changes in the black hole’s environment over several years as gravity churns the surrounding material in a permanent vortex.

The images show an uneven patch of light moving in the eddy surrounding the supermassive black hole at the center of the galaxy M87. To create them, a group of researchers from different institutions (the “collaboration” that runs the planetary network of observatories that makes up the Event Horizon Telescope) has unearthed old data relating to the black hole and combined it with a mathematical model, which is based on the image published in April 2019, to show how the environment of the hole has evolved in the last eight years. Although partly dependent on guesswork, the result gives astronomers many ideas about the behavior of black holes, whose intense gravity pulls matter and light into their vicinity.

“The flow of matter falling onto the black hole is turbulent, and so we see that the ring wavers over time,” says lead author Maciek Wielgus, a radio astronomer at Harvard University in Cambridge, Massachusetts.

The work, released Sept. 23 in The Astrophysical Journal, offers a sneak peek at what the team will be able to do in the near future as their techniques improve. “In a few years it could start to look like a real movie,” according to Wielgus.

A wavering ring

The image of the black hole that the EHT collaboration released last year made the front pages of newspapers around the world. It portrayed M87 *, the supermassive black hole located in the center of the M87 galaxy, some 17 megaparsecs away (55 million light years). The researchers constructed the image by combining radio frequency signals captured by observatories around the Earth on two nights in April 2017. This achievement has been compared to being able to distinguish from Earth the shape of a donut on the surface of the Moon.

Although blurry, the image was consistent with Albert Einstein’s predictions of the theory of general relativity regarding the appearance of the vicinity of a black hole. In particular, he gave researchers the first direct evidence of the “shadow” of an event horizon, the non-returnable surface that separates a black hole from its surroundings. That dark disk was defined by the ring of light emitted by superheated matter just beyond the event horizon.

It is striking that one side of the ring is brighter. This is what was expected, as a consequence of a combination of effects on the complex dynamics that occurs around a black hole. In particular, matter falling toward this “nothing” spirals at high speed outside the black hole’s equator, thereby forming an accretion disk, as astronomers call it. The uneven appearance is partly due to the Doppler effect: on the side of the disk that rotates towards the observer, the movement of matter intensifies the radiation, which then appears brighter; on the side that rotates away from the viewer, the opposite occurs.

Go back to the old data

Based on those results, Wielgus wanted to go back and look at older data also obtained by the EHT telescopes to see if he could reinterpret it by taking the 2017 image as a guide. As the EHT network added more observatories, the quality of the observations improved. In 2017, the collaboration comprised eight telescopes scattered around the world, from Hawaii and Chile to Europe, and reached the level necessary for the EHT to produce a true image.

The old data consisted of four batches, corresponding to 2009, 2011, 2012 and 2013; two of them were not published. “They had been forgotten, to some extent, by how excited everyone was with the 2017 data,” says Wielgus. With a group of other EHT researchers he reanalyzed the data and found that it matched well with the results of the 2017 campaign, including the dark disk and a bright ring. And while the 2009-2013 batch data alone did not have the resolution to create images, the team was able to generate synthetic images for each of the years by combining the limited data available with a mathematical model of the black hole based on in the 2017 data.

And it turned out that what was thus obtained contained more information than Wielgus expected. Like the 2017 image, these other synthetics showed that one side of the ring shone more than the other, but the shiny part was moving. The reason may be that different regions of the accretion disk became brighter or dimmer, thus exaggerating or even nullifying the Doppler brightening.

Dynamic disk

This was not something unexpected, say the authors: Although the black hole M87 * itself does not change from year to year, its surroundings do. On a scale of a few weeks, strong magnetic fields could shake the accretion disk and produce hotter spots that will then orbit the black hole. In 2018, another team tested a mass of hot gas that swirled around Sagittarius A *, the Milky Way’s central black hole, for about an hour. Since M87 *, whose mass is 6.5 billion times that of the Sun, is more than a thousand times the size of Sagittarius A *, dynamics are slower in its environment.

The EHT collaboration attempts to observe M87 * and Sagittarius A * every year, in late March or early April. These are the times when the weather conditions are most likely to be good at the same time at the many sites on that network. The 2020 campaign had to be suspended due to restrictions imposed by COVID-19, but the team hopes to have another chance in 2021. If all goes well, more observatories (in Greenland, in France) will join the effort.

The team also hopes that its first global observations with shorter wavelength radiation will be made in the campaign next year; Seeing this through the Earth’s atmosphere is more difficult, but that would improve the resolution of the EHT images. “We would get even closer to that black hole shadow and get sharper images,” explains Sara Issaoun of the EHT and radio astronomer at Radboud University of Nijmegen, the Netherlands.

Davide Castelvecchi / Nature News

Article translated and adapted by Research and Science with permission from Nature Research Group.

Reference: “Monitoring the Morphology of M87 * in 2009–2017 with the Event Horizon Telescope”, by Maciek Wielgus et al.,in The Astrophysical Journal, Volume 901, Number 1.