Researchers at the University of Pretoria have played an important role in the international science consortium that captured the first-ever real images of a Black Hole. That consortium is the Event Horizon Telescope (EHT), which earlier this month announced its achievement at six simultaneous press conferences in four languages (English, Mandarin Chinese, Japanese and Spanish) around the world. The object successfully imaged by the EHT is the supermassive Black Hole in the centre of the Messier 87 (M87) galaxy, 55-million light years from earth.
The EHT is composed of an array of eight ground-based radio telescopes around the world. The observations were conducted at millimetre wavelengths, which can be blocked by water vapour in the earth’s atmosphere, so all eight of the EHT telescopes are located at high altitudes in dry places. The M87 Black Hole was chosen for observation because theoretical predications indicated it was one of the biggest Black Holes that could be observed from earth.
Although no South African telescope forms part of the EHT, University of Pretoria (UP) astrophysicist Professor Roger Deane is a member of the EHT consortium. Deane heads the University’s Radio Astronomy Research Group. The university hosted the live-streaming feed from the EHT press conference in Brussels, Belgium, which was one of those announcing and releasing the images, which total four.
Deane, who hails from the Free State province, started working with the EHT team (which totals some 200 scientists from all around the world) some four years ago. He joined UP in January last year. At UP, Deane and his team created an extremely realistic simulation of the eight telescopes working together as a single array. This allowed the determination of the array’s limits, including the effects that the instrument itself could have on the data it was collecting, and the testing of the algorithms that were used to recover the images.
“We built a simulation package that physically modelled a number of nondesirable effects that prevent one from seeing any sort of Black Hole shadow feature,” he explained. “The accurate simulation of the telescope enables astronomers to better understand the real observations, discriminate between theoretical Black Hole shadow models, and gain insights into the characteristics and performance of the telescope itself.”
Although the existence of Black Holes was predicted by Albert Einstein in his Theory of General Relativity, originally published in 1915, the EHT images are the first direct evidence that they exist. Previously, their existence had to be inferred from their effects on other matter that was close to them.
“Don’t let the name fool you: a Black Hole is anything but empty space,” cautions the US National Aeronautics and Space Administration in its explanation of the phenomenon. “Rather, it is a great amount of matter packed into a very small area – think of a star ten times more massive than the sun squeezed into a sphere approximately the diameter of New York City. The result is a gravitational field so strong that nothing, not even light, can escape.” An Event Horizon is the boundary of a Black Hole.
By definition, a Black Hole is invisible. “You cannot see a Black Hole,” noted EHT Science Council chairperson Professor Heino Falcke. “But you can see its shadow.” The “shadow”, or outline, being formed by the material surrounding the Black Hole is clearly visible in the images.
This ring of material is known as the accretion disc. As the size of the disc is determined by the mass of the Black Hole, the astronomers have been able to calculate the mass of the M87 Black Hole. It is 6.5-billion times the mass of the sun. The fact that, in the images, the accretion disc is brighter at the bottom than at the top shows that it is rotating. The images are not sharp enough to determine the actual velocity of the rotation but it has been established that the rotation is clockwise. Theoretical models indicate that material in the accretion disk would be travelling close to the speed of light. The data obtained from the images of M87’s Black Hole fit the theoretical predictions extremely well.
“It is like looking at the gates of hell –” said Falcke, “the end of space and time.”