You probably have already seen the fabulous image of the black hole in the center of our Milky Way galaxy, published on May 12th 2022. The Event Horizon telescope team achieved an unbelievable milestone in observational astronomy one again, the first one being the picture of the black hole at the center of galaxy M87. It is terrifying to think that in the center of our cosmic home lies a monster that might eventually devour everything around it. We have already looked at black holes in detail in another post, so instead it might be more insightful to focus on the imaging techniques used to take this fantastic picture.
What can we see in this image? The orange/red rings are the accretion disk around the black hole. It is impossible to “see” a black hole directly, as by definition not even light can escape its gravitational pull, hence astronomers rely on the stars and gas orbiting around it to show that there is a black hole. The image looks distorted not only due to the tremendous distance that separates us from the center of the galaxy, but also because the gas rotates around at a significant fraction of the speed of light. With such an immense gravitational pull, gravitational lensing becomes significant. In fact, because we look at the black hole from an angle, we also see the back side of the accretion disk and the event horizon. Light bends around the event horizon and falls back around it, creating a dizzying display of Einstein’s general relativity. Check our this Veritasium video for a more visual explanation. Looking at a black hole accretion disk is not the same as looking at the rings of Saturn.
Sagittarius A* is massive, with a diameter of 51.8 million kilometers, but it appears tiny to us because it is located 27,000 light years away. We are seeing it how it was back when the first ceramics were created by humans. In fact, the angular diameter is 51.8 micro arc-seconds, which is 1.4*10-8 degrees. How is it possible to produce an image of something so tiny? The equation for the angular resolution of a circular telescope is
The smaller the value the better, as smaller objects can be resolved using the Rayleigh criterion. You might think that the easy solution is reducing the wavelength of light that is being measured. However, due to the tremendous amount of gas near the center of the galaxy, visible light wavelengths get almost completely absorbed. Hence, astronomers have to use radio waves as they can more easily get through the light-years of gas clouds. Another way to decrease the angular resolution is to increase the size of the telescope dish. Doing this with a single telescope would require a dish the size of the Earth, so instead the EHT collaboration uses multiple telescopes stationed around the globe to produce a similar effect. They use interference patterns of radio waves to sync up the images and make an effectively huge telescope our of many smaller ones.
The amount of data generated was so huge that it took the team 5 years to process it, as the initial measurements were gathered over five nights in 2017. They had to manually transport the hard drives since it would be inefficient to transport it over the internet. They then used supercomputers to combine the images using a mathematical model that produces pictures consistent with laws of physics. This is another reason why the superimposed image looks distorted, but the team has made significant progress since the M87 black hole image in 2019.
The insane data processing that went into this image suggests once again the importance of programming and computer science skills for physicists. So much of current research relies on coding and understanding of processing power that computational physics should be one of the core topics covered in physics curricula.
What’s the big deal with this image? Before 2019 we only had indirect evidence of the existence of black holes, based on the movement of stars. However, now we are able to directly see the edges of the event horizon. The more data (images) we have, the stronger the hypothesis of supermassive black holes existing in centers of almost all big galaxies is. Furthermore, taking more pictures of Sagittarius A* may allow us to track how exactly the black hole consumes matter around it, which can teach astronomers more about its properties. It is definitely an exciting time for black hole research.
Impey, C. (2022, May 22). Say hello to sagittarius A*, the black hole at the center of the milky way galaxy. Space.com. Retrieved May 24, 2022, from https://www.space.com/meet-milky-way-black-hole-sagittarius-a
Khadilkar, D. (2022, May 22). Snapping sagittarius A*: How scientists captured the first image of our galaxy’s Black Hole. RFI. Retrieved May 24, 2022, from https://www.rfi.fr/en/science-and-technology/20220522-snapping-sagittarius-a-how-scientists-captured-the-first-image-of-our-galaxy-s-black-hole
O’Callaghan, J. (2022, May 12). Black Hole image reveals sagittarius A* . Quanta Magazine. Retrieved May 24, 2022, from https://www.quantamagazine.org/black-hole-image-reveals-sagittarius-a-20220512/
Yazgin, E. (2022, May 20). Black Hole Sagittarius A* imaged for the first time. Cosmos. Retrieved May 24, 2022, from https://cosmosmagazine.com/space/sagittarius-a-black-hole-image-nasa/