Milky Way Supermassive Black Hole: The Event Horizon Telescope (EHT) has not made observations in distant galaxies but in our neighborhood. Still, it isn’t very complicated.
The Event Horizon Telescopes (EHT) merger has captured images of the glowing hot gas masses around the black hole for the first time. The observational data are from 2017 – the same year that the black hole at the center of galaxy M87, dubbed M87, was observed. Ironically, 55 million light-years away, this object was more accessible to spot than the black hole Sagittarius A (Sgr A*) in our Milky Way, which is almost around the corner at 27,000 light-years.
It has been known since 1974 that there is a heavy object in the center of our galaxy that emits radio waves. By observing the motion of stars in this region of the Sagittarius constellation, the center of our Milky Way, the object’s mass could be calculated from the orbits of nearby stars. It became clear that there must be about four million solar masses in a tiny space – probably a black hole.
However, this was much harder to observe than M87, whose image went worldwide in 2019. One of the reasons for this is that M87 is enormously much larger. At 6 billion solar masses, it is 1,500 times as heavy. According to the laws of gravitation, it also has 1,500 times the diameter. As a result, such a black hole is much larger than it should be intuitive. It is 3.4 billion times the volume of the black hole at the center of our Milky Way. It’s huge. At the same time, it has a density of just 590 grams per cubic meter – less than the density of air.
Smaller, Weaker, And More Fidgety In The Center of the Milky Way
What’s more, M87‘s black hole falls on between 100,000 and 1 million times as much material as in the Milky Way, making it more luminous. We also need to look through the Milky Way’s galactic disc to the center, where dense bands of dust and gas absorb much light and radio waves. When observing M87, on the other hand, we look straight out of the disk of the Milky Way and can see right into the center of the galaxy M87.
The small size of the black hole Sgr A* (Sagittarius A*) has another disadvantage when observing: we see hot gas in orbit around the black hole. For M87, one rotation lasts from five days to about a month. It is not a problem that the image data is obtained using a complicated technique that depends on its orbit. But with Sgr A, the orbital times are only 4 to 30 minutes, so its appearance changes significantly during the measurement.
Image by Wikipedia Creative commons
So the observation of Sgr A* at the center of the Milky Way was on the verge of what the Earth-spanning Event Horizon Telescope used to measure in 2017 was capable of. At that time, the observations were made with eight radio telescopes. Eleven telescopes are now part of the network, which have already observed Sgr A* in 2022. However, the data evaluation will take significantly more than just one month, which has passed since the measurement.
Image by Phys.org
Obtaining an image is much more complicated than simply taking a picture.
Lots of Data But Few Details (of Black Holes)
The Event Horizon Telescope doesn’t take photos but instead takes measurements to identify unique patterns an image might have. Ultimately, simulations and statistical evaluations must be used to decide which images are most likely to match the measured data. The process is more difficult with weaker signals and a rapidly changing signal from the hot gas in orbit around the black hole.
Care should be taken, especially with weaker details in the black hole images, such as the distribution of brighter areas on the ring. There are many possible images with different spots, all consistent with the measured data. The general shape and dimensions of the ring are different.
From the shape, we know that we are looking at the gas clouds orbiting the black hole counterclockwise at a right angle – both of which were previously unknown. The diameter of the dark inner surface, at 50 microarcseconds, is precisely what was expected from the mass determination and from previous observations with only two or three radio telescopes, which were too little data for any form of imaging.
This diameter is minimal. To see the ring around Sgr A* from Earth’s surface with an ordinary telescope, it would be 10,000 times larger. Even the James Webb Telescope has no chance with its 6.5-meter mirror. For a picture as good as that from the Event Horizon Telescope, the mirror would have to be more than 6.5 kilometers in size.
Observation Is Part of Everyday Scientific Life
Overall, there is little new or surprising in the six papers with the published scientific results. However, some possibilities for physically modeling the plasma clouds around the black hole can now be ruled out based on the data. Due to the problematic observation conditions, the measuring accuracy is insufficient for many investigations. What was presented is everyday science, not a scientific sensation.
The new observation campaign with the EHT, which was only completed a month ago, should provide better data in a few years, which should also make temporal changes around the black hole observable and thus perhaps provide explanations for the flares of the black hole that occur again and again, i.e., briefly happening higher brightness. The work on evaluating the data from 2017 forms the indispensable basis for evaluating these and other observations with radio telescopes distributed worldwide.