Echo mapping in galaxies helps measure distances in space

The echo display process, also known as reverb display, starts when the disc is hot

plasma (atoms that have lost electrons) nearthe black hole becomes brighter, sometimes even emitting short bursts of visible light (that is, the wavelengths that can be seen by the human eye). This light escapes from the disk and eventually falls into a common feature of most supermassive black hole systems: a huge donut-shaped dust cloud (also known as a torus). When a burst of light from the accretion disk reaches the inner wall of the dusty torus, the light is absorbed, causing the dust to heat up and emit infrared light. This enlightenment of the torus is a direct response or, one might say, an "echo" of the changes taking place in the disc.

At the center is a supermassive black holesurrounded by a disk of material called an accretion disk. As the disc gets brighter, there are short flashes of visible light. The blue arrows show the light from this flash, moving away from the black hole, both towards an observer on Earth and towards a huge donut-shaped structure (called a torus) made of dust. Light is absorbed, causing the dust to heat up and emit infrared light. This lightening of the dust is a direct response - or, one might say, an “echo” - of the changes taking place in the disc. The red arrows show this light moving away from the galaxy in the same direction as the original flash of visible light. Thus, the observer will first see visible light and then (with the right equipment) infrared. Credit: NASA / Caltech Jet Propulsion Laboratory

Distance from accretion disk to innerparts of the dusty torus can be huge - billions or trillions of kilometers. Even light traveling at a speed of 300,000 km per second can travel this distance in months or years. If astronomers can observe both the initial burst of visible light in the accretion disk and the subsequent infrared brightening of the torus, they can also measure the time it took for light to travel between the two structures. Since light travels at a standard speed, this information gives astronomers an idea of ​​the distance between the disk and the torus.

Scientists can then use the measurementdistances for calculating the luminosity of the disk and theoretically its distance from the Earth. How? The fact is that the temperature in the part of the disk that is closest to the black hole can reach tens of thousands of degrees. It is so high that even atoms are torn apart, and dust particles are able to form. As you move away from the black hole, the temperature of the disk gradually decreases.

Astronomers know that dust forms whenthe temperature drops to about 1,200 degrees Celsius. Thus, the more energy a disk emits, the further away dust will form. As a result, measuring the distance between the accretion disk and the torus gives an idea of ​​the disk's output energy, which is directly proportional to its luminosity.

The idea to use echo mapping for measurementThe distance from Earth to distant galaxies is not new, but a new experiment by scientists is showing significant success in demonstrating its capabilities. This is the largest study of its kind, confirming that a similar measurement works the same in all galaxies, regardless of variables such as the size of the black hole.

However, due to many factors, the authors did notthere is enough distance measurement accuracy. In particular, scientists say they need to learn more about the structure of the inner regions of the dusty torus that surrounds the black hole. This structure can influence exactly what wavelengths of infrared light the dust emits when the light first reaches it.

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