While you may be familiar with black holes from sci-fi and many artists’ renditions of the phenomenon, our first actual view of the event horizon of the supermassive black hole which is at the centre of a neighbouring galaxy, Messier 87, was released 4 weeks ago to the day. What we have seen in the picture, obtained using the event horizon telescope, is the shadow of the event horizon (the limit of the black hole’s gravitational pull, beyond which nothing can escape from it) and jets of plasma shooting out into space, seeming to glow in the X-ray image. While the famous image shows a blurry ‘doughnut’, other images were taken with other telescopes that show the spectacular jets of superheated particles which extend more than 1000 light years (5,879,000,000,000,000 miles) out from the centre of M87.
At the Armagh Planetarium the release of the EHT image was shown live in the Copernicus hall. Here I have compiled some reactions to the black hole, both immediate and within the 4 weeks proceeding the photograph’s release.
Marc Sarzi, Head of Research
“My reaction was surprise, not because of how the new image looked, but because of how close it is to what I and other astronomers would have expected it to be. Indeed, as a scientist, no matter how much I understand current theories and the physics behind it, I am also mindful that our views of the Universe could always change in the light of fresh, ground-breaking observations. So yes, I was surprised and relieved that all seems still to work fine.
On the other hand, I was a bit less pleased with the value for the black-hole mass found by EHT, which is the ultimate measurement now. For a long time that mass had been debated between two camps: those who measure black-hole masses looking at how gas orbits around central supermassive black holes, and those who follow the motions of stars around them. Each technique has its pros and cons, and unfortunately, I belong to the camp that lost here since EHT found a mass coinciding with that found by scientists looking at stars, whereas I, along with many others, used the gas.
But hey, this means I can now look at my gas measurements and models in a different way and by knowing the black hole mass in M87, I can understand better how the gas moves around it. In the meantime, I’ve sent already a congratulation message to my friend and colleague on the other camp Karl Gebhardt, who led the 2011 paper that obtained a black-hole mass of 6.6 billion Sun, just 0.1 billion above the one found today by EHT.”
Simon Jeffery, Senior Research Astronomer
“Astonishing technical and human achievement. Very good to know that reality matches theory; a triumph for physics. But what really blows my mind is the idea that, at the heart of M87, 6 billion suns have been pulverised into a sizeless ‘dot’.”
Michael Burton, Director
“My immediate reactions are two-fold.
The first was that I was expecting the story to be about the black hole in the centre of our Galaxy, not that in M87?! Given pre-publicity had featured ALMA and the South Pole Telescope, I was assuming it would be a southern hemisphere source that would the focus of the story, not a northern one. So this favoured the centre of our Galaxy, which passes virtually overhead at the ALMA site in Chile (and so the best place to study it). Not M87, which is the heart of the Virgo cluster in the northern skies.
The Event Horizon Telescope has been a massive project, a supreme technological achievement connecting all the world’s high frequency radio telescopes, aimed at imaging 2 black holes, that in M87 and that in the centre of our own Galaxy. It turns out that they are equally difficult to observe. Or perhaps I should say, they were expected to be the two easiest black holes to measure, but with the same degree of supreme difficulty. The angular size of the black hole shadow is directly proportional to its mass, and inversely proportional to its distance away from us. Our galactic centre black hole is about a million times the mass of the Sun, that in M87 a billion times the solar mass, so 1,000 times more massive. But M87 is also 1,000 times further away than the Galactic centre, so its black hole size on the sky would be about the very same size as our Galaxy’s central engine! This realisation, and the fact that a telescope the size of the Earth would (just) be able to resolve the anticipated black hole shadow and so image it, drove the entire concept of the Event Horizon Telescope over the past decade.
My second surprise was hearing, via Marc, that we didn’t actually have the mass of M87’s black hole tightly tied down. I thought my extra-galactic colleagues had this aspect of galaxies under control, it was how galaxies form and evolve they didn’t understand? But no, even basic parameters which can, in principle, be measured in several different ways, remain uncertain. If I’d known of the two discrepant methods, i.e. stellar motions vs gas motions, I’ve also have bet that the gas method would most likely be the correct too, i.e. in Marc’s camp. After all, it’s much easier to interpret the spectrum of a gas cloud than that of a star. But no, it seems that the method of stellar motions has provided the most accurate set of scales. Which will force everyone to think again about what gas measurements are actually telling us about distant galaxies! Maybe the extra-galactic astronomers should take a look as what the galactic astronomers know about the interstellar medium of our own Galaxy?!”
Nick Parke, Education Officer
“In ‘seeing’ a black hole, and such a spectacularly supermassive one at 100bn km in diameter – astronomy and astrophysics has at long last past its own Event Horizon, evidencing in a tangible way for humanity one of the most remarkable objects in space ever to be conjectured.”
Courtney Allison, Education Officer
“To me the image of the black hole being released to the world really highlighted the disparity between the general public’s perception of what is possible in space-tech and what is actually not. Immediate complaints about how blurry the picture was were really jarring considering its 55 million lightyear distance from us, though I don’t think it detracted from the scientific community’s obvious delight and wonder. I hope this photo and the continuing discussions about what this means for our understanding of the universe increase interest in science and encourage children to go into STEM fields.
Furthermore, the controversy with which of the people involved in the EHT project deserved the most credit was only indicative of the current direction of public discourse. Immediate attempts by some to discredit the work of Dr Katie Bouman couldn’t even be thwarted by her fellow computer scientists and astrophysicists rushing to her defence. I thought that a black hole 55 million lightyears from us couldn’t be drawn into sexist discourse but I was wrong! It is so important for STEM fields to recognise the women that contribute to the ever expanding wealth of knowledge that we now possess; I hope that going forward this discourse or anything like it won’t be replicated or given the legitimacy that it currently has.”
Since its initial press release on 10th April 2019, Messier 87 has since been renamed Powehi; a name Hawaiian in origin which be translated to “the adorned fathomless dark creation”. This is a fitting tribute to its discovery given the two Hawaiian telescopes used in the EHT project.
Written by Anna Taylor, Education Officer