A monumental moment for science and astronomy.
Learning more about new or old cosmic places is always a pleasure and always a significant moment for science and astronomy. Whether it is a new discovery about water, ice, planetary conditions, or anything else – each new fact puts a piece in the endless puzzle of mysteries about the universe. However, I feel like nothing beats the feeling when you actually see something that you’ve heard about throughout your entire life and the perfect example is the breathtaking first black hole image from last year.
In April 2019, an international team of astronomers unveiled the first results from the Event Horizon Telescope, a network of eight radio observatories located across the earth. Scientists were able to synchronize data from these telescopes and obtain a direct image of the vicinity of the black hole of the galaxy M87.
What is an Event Horizon Telescope and how does it work?
Even supermassive black holes found at the centers of many galaxies, including our Milky Way, are relatively small objects, which until now made it impossible to observe them directly. No terrestrial telescope has sufficient resolution to see regions of this size. Recall that the resolution depends on the ratio λ / D, where λ is the wavelength of the received radiation, and D is the size of the telescope. The shorter the wavelength and the larger the telescope, the better the angular resolution, the finer details it can see.
The Event Horizon Telescope (EHT) is designed specifically for imaging black holes. It is a system of several ground-based radio telescopes located in different places on the Earth. Using the method of interferometry with a very long base and rotation of our planet allows them to be combined into a single giant telescope the size of the globe.
Thanks to state-of-the-art data processing algorithms, EHT has achieved an angular resolution of about 20 microseconds, which is equal to the ability to read headlines on the moon. For comparison, the resolution of the Hubble telescope with a diameter of 2.4 meters is about 0.05 arc seconds, which is 2500 times worse.
The creation of the EHT was a technical challenge of the greatest complexity, the solution of which required the organization and debugging of a worldwide network of telescopes. Although the telescopes are not physically connected to each other, the observational data they received had to be very accurately synchronized using an atomic clock. The preparatory work took 10 years and $ 290 million.
The EHT project is not only telescopes but also an international team of more than 200 astronomers from 60 research organizations in Europe, Asia, Africa, North and South America. To obtain an image of a black hole on the basis of observations, theoretical and simulation studies, the development of data processing algorithms were required.
Obtaining the first black hole image
From April 5-11, 2017, the EHT observed M87 for four days.
The observations were carried out at a wavelength of 1.3 mm. This is practically the minimum wavelength at which it is possible to observe space objects in the radio range on Earth. The fact is that the Earth’s atmosphere is not transparent for all wavelengths of electromagnetic radiation.
Radio astronomy works in the atmospheric transparency window from 1 mm to about 30 m. Smaller wavelengths are almost completely absorbed by atmospheric gas molecules, primarily water vapor, and large ones are reflected back into space by the ionosphere. Recall that a short wavelength is needed to obtain high resolution.
Working at such short wavelengths is associated with many problems: increased noise in electronics, absorption of radiation in the atmosphere, increased phase fluctuations caused by atmospheric turbulence.
Each EHT telescope received an enormous amount of data during the campaign: 350 terabytes per day. They were recorded on high-performance hard disks, which were sent for processing on specialized supercomputers – correlators installed at the Max Planck Institute for Radio Astronomy (Germany) and the Haystack Observatory (MIT, USA).
After sophisticated procedures using the latest computational methods developed by the project participants, this data was converted into images. It took two and a half years to process several petabytes of data from all the telescopes. By the way, this amount of music recorded in mp3 format would have had to be listened to for thousands of years.
For objectivity in 2018, the team was divided into four groups, each of which processed data independently of the others, using different methods. To protect against bias, the groups had no contact with each other. All groups received similar results, which speaks of their reliability.
Note that in the radio range, where the wavelength is large enough, it is impossible to obtain a photograph of an object in the usual sense. Information about individual image fragments is complexly encrypted in the interferometer data. Through complex calculations, this information is extracted and an image is obtained from the fragments.
However, those who say that these are not real images are wrong. Recall that in magnetic resonance imaging (MRI), images are also formed using computer data processing, but they objectively reflect the real state of the body and are successfully used in medicine for diagnostics.
Why M87?
It is assumed that in any galaxy there are many black holes with masses close to the mass of stars, but their sizes are too small for observation. Supermassive black holes at the centers of galaxies are much larger, but they are located much further away. Currently, two supermassive black holes are available for observation: one in the center of our Galaxy (Sgr A*), the other in the giant elliptical galaxy M87 from the Virgo constellation.
The black hole in the center of the galaxy M87 is 54 million light-years from Earth – two thousand times farther than Sgr A*, but by astronomical standards, it is very close. The size of the event horizon of a black hole is proportional to its mass. The black hole in M87 has a mass of 6.5 billion solar masses, 1500 times that of Sagittarius A*.
Due to its enormous mass and relative proximity to Earth, the black hole at the center of galaxy M87 is one of the largest in terms of its angular dimensions for the terrestrial observer, which made it an ideal target for research. Its event horizon is 22 microseconds, only slightly smaller than Sgr A* at 53 microseconds. It is comparable to the angular size of a matchbox placed on the moon.
Another reason for choosing M87 is that it is visible from both the Northern and Southern hemispheres of the Earth. Therefore, it can be observed by a large number of ground-based telescopes, which, in turn, allows increasing the resolution of the images obtained.
It should be noted that, due to its large mass, the black hole in M87 is less volatile than Sgr A* (the characteristic time of variability is days versus minutes). Variability interferes with observations because it limits the time it takes to receive a stable signal. In addition, Sgr A * lies for us in the galactic plane and is hidden by gas and dust clouds. Researchers will still have to solve these problems in order to obtain the Sgr A * image.
Why is the image of the black hole this blurry?
This is primarily due to the fact that the resolution is still not high enough, it is comparable to the size of the black hole itself. Imagine a small painting painted with a thick brush. However, high resolution, in this case, does not mean high image quality.
The fact is that the EHT collected information from the black hole using a small number of telescopes that worked for a fairly short time. These telescopes are busy with many other studies. With each measurement, information was obtained only about a small part of the studied area.
Moreover, with interferometry, a high-resolution image is obtained only in the direction of the straight line connecting the two telescopes in use. Since the measurements were not enough to explore the entire area, there were many unexplored places between the obtained fragments.
So the researchers then had to reconstruct the complete image by filling in the blanks. It looks like a partially crumbling mosaic picture on the wall, from which only a few separate fragments remain, and now the restorers need to restore the original image from them. The developed visualization algorithms fill in these gaps, forming an image of a black hole.
Of course, it is impossible to get real details of the image that fall into the filled area, because, in fact, it is simply painted over in a certain way. Naturally, the image is blurry, devoid of small details.
By the way, similar algorithms can be found in computer programs that work with photographs. When you enlarge a photo, the program expands its pixels, filling in the gaps between them according to a certain algorithm. It is easy to see that the photo loses its sharpness and becomes blurry.
But then the question arises: to what extent does the reconstructed image correspond to reality, because many possible pictures can be created from the fragments of the mosaic? Here, scientists come to the aid of modeling, which allows from all possible images to select those that look the most reasonable.
Another problem is the suboptimal location of existing telescopes for using them to study a given object by interferometry.
But the successful solution of this problem gives hope that other telescopes will join the research and that sufficient time will be allocated for measurements to obtain a clear and detailed image of the black hole.
Why is this event important?
Astrophysicists have long doubted the existence of black holes, but until now it was only a model that very well described a number of astrophysical phenomena: radiation from galactic nuclei, double X-ray systems, etc. Yes, without it it is difficult to explain the observed phenomena, but it was still a model.
But now we have seen the black hole with our own eyes, this is an observable fact. In addition, experimental confirmation of the rotation of black holes has been obtained for the first time.
The work of the EHT as a whole did not bring new results. Many properties of the resulting image are even surprisingly well consistent with theoretical concepts. But, on the other hand, it gives confidence in the correctness of methods for measuring and interpreting the results, including estimates of the mass of a black hole.
But in the future, a modified method and larger-scale observations, possibly with the participation of a space telescope, will make it possible to observe in detail the processes around a black hole, which have also been only a model until now. Thanks to this, astrophysicists will be able to “sort out” the questions on the strong gravitational effects expected near a black hole, on the behavior of matter near a black hole, including the mechanism for the appearance of jets.
The observations can be used to test general relativity and various alternative theories of gravity that predict, for example, different shapes of the “shadow”. Thus, general relativity predicts that the “shadow” of a black hole will be round, while other theories suggest that it is compressed along different axes and has a complex shape. But in order to see the differences, you need to get a clearer image of it.
One of the further goals of the EHT is to understand why, unlike other galaxies, the supermassive black hole at the center of the Milky Way is a relatively dim object – its brightness is only several hundred times that of the Sun.
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Sources:
• Castelvecchi, D. (2020, September 23). The first-ever image of a black hole is now a movie.
• Loff, S. (2019, April 10). Black Hole Image Makes History; NASA Telescopes Coordinate Observation.
• Lutz, O. (2019, April 19). How Scientists Captured the First Image of a Black Hole – Teachable Moments.
• Tafreshi, P. (2019, April 10). First-ever picture of a black hole unveiled.
