On April 10, 2019, scientists released the first-ever image of the supermassive black hole at the heart of galaxy M87. Sitting 55 million light-years from Earth, the supermassive black hole at the centre of galaxy Messier 87 is estimated to be about 6.5 billion times the mass of our sun.
The first images released revealed a bright ring-like structure with a dark central region – the black hole's shadow.
Since that release, the Event Horizon Telescope (EHT) team delved deeper into the data collected, which was used to reveal the first image. Using the EHT, an international team of astronomers measured the polarisation for the first time, close to the edge of the black hole.
The team further discovered that a significant fraction of light around the black hole is polarised.
Study co-author from University College London, Dr Ziri Younsi, stated that, during the first study, scientists looked at visible light and its intensity. However, this time, scientists are delving deeper into the polarization of that light. Light becomes polarized when it goes through certain filters. Polarisation allows astronomers to map the magnetic field lines present at the inner edge of the black hole, as can be seen in the new image, across the bottom of the disc.
Finally, the supermassive black hole at the heart of galaxy M87 has been photographed in polarised light, revealing the magnetic field. These include its interactions at the edge of this massive stellar phenomenon for the first time.
The new photos of the black hole also revealed more details about the dense object and the powerful jets of energy bursting from its centre.
Andrew Chael, a NASA Hubble Fellow at Princeton, stated that, "The newly published polarised images are key to understanding how the magnetic field allows the black hole to 'eat' matter and launch powerful jets".
The bright jets of energy and matter that emerge from M87's core can extend at least 5,000 light-years from its centre.
Most of the matter, which is lying close to the edge of a black hole, falls inwards. Some of the surrounding particles escape into space as they shoot out in the form of jets only moments before capture.
To better understand this process, astronomers had to rely on the different models of how matter behaves near the black hole. This discovery allows both current and future astronomers to understand the process involved in producing these energetic jets from the core.
Even with the breakthrough, astronomers still don't know exactly how jets larger than the galaxy are launched from its centre, or how matter falls into the black hole. However, astronomers managed for the first time to look into the region just outside the black hole where this interplay between matter flowing in and being launched out happens, with the use of the new EHT image of the black hole and its shadow in polarised light.
Younsi added that "We looked at how that light is composed. It can be described in terms of four components – like when you wear sunglasses – it has a polarizing filter to filter out the stronger intensity components of the light, and those different components give you information on how light is produced and the source itself that produced it.
"Depending on whether more of the light is linearity polarised, more of the intensity is focused to the line of sight. It is more circularly, or more diffuse, you can get a lot more information about the black hole itself."
Along with this breakthrough, one of the biggest discoveries made was gathering information on how the magnetic field surrounding the black hole is structured. Scientists were able to conclude that it is responsible for tapping the rotational energy of the black hole and powering these spinning jets coming from the centre of the stellar object.
They were also able to discover that the jets are "collimated". They're maintaining a consistent and coherent shape over vast distances, being launched from the very heart of galaxies, and are powered by black holes.
"The jets are coupled to the black hole and the magnetic field controls how the matter flows," said Younsi, and added that you can see the matter being locked in on the event horizon.
"Before we had no idea, it was very speculative. But now we have very strong evidence to indicate how the magnetic field is organised, and it is very organised, very structured, which is surprising. We are starting to get a much better picture of how black holes feed energy back into the universe, we always think of them as objects that suck, but they do play a role in feeding back energy."
Jason Dexter, Assistant Professor at the University of Colorado Boulder explained that: "The observations suggest that the magnetic fields at the black hole's edge are strong enough to push back on the hot gas and help it resist gravity's pull. Only the gas that slips through the field can spiral inwards to the event horizon".
By using the EHT setup, it allowed the team to directly observe the black hole shadow and the ring of light around it, discovering that the ring is magnetised.
According to the images, scientists were able to see that everything is rotating in an anti-clockwise direction, with Younsi saying that it tells us that matter is spinning around the black hole.
"If you were near the edge of the event horizon, and had a light source shining on you from all directions, you would see an image of yourself and the back of your head as the gravitational field warps the light," said Younsi.
The Event Horizon Telescope team's next big project is to publish an image of the black hole at the centre of the Milky Way Galaxy, named Sagittarius A, by the end of 2021. However, this is not an easy task. It is very difficult as you have an 'interstellar scattering' of stars and light sources between the Earth and the black hole, all moving at different rates, making it harder to directly image the black hole.
"The other trickier issue is that the black hole is 1,000-times smaller than M87, and 1,000-times closer. So, on the plane, it is about the same size as seen from Earth, but it is also much quicker."
In the case of the black hole in M87, things change and move on a time scale of days and weeks. "What you see is matter reconfiguring itself so quickly that it is like taking a photograph of a fast moving car. So you have to create techniques to interpolate between multiple photographs of the same image."