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How a black hole grows

Researchers have developed a method for mapping the central regions around black holes, which contributes to a better understanding of their growth

The black hole's accretion disk. Illustration: depositphotos.com
The black hole's accretion disk. Illustration: depositphotos.com

Black holes were predicted in the general theory of relativity developed by Albert Einstein at the beginning of the 20th century, and there is much research evidence of their presence in the centers of galaxies - and in other places - in the universe. These are heavenly bodies with tremendous gravity that nothing can detach from. They warp the space near them so that beyond a certain limit, even light cannot escape. When they absorb matter from their environment, they become active and thus their mass increases over time. In this way they also influence their environment and especially the development of the galaxy in the center of which they are located. Thus, the study of their evolution contributes to the understanding of the formation of stars, planets and building blocks of life.

The physical process of the absorption of matter into black holes is shrouded in fog because their environment is too small to be explored with existing imaging means. According to hypotheses, in this process the adsorbed material (mostly gas) creates a structure of a rotating disk around the black hole (adsorption disk). This material heats up and turns into plasma and therefore shines with an intensity equal to that of thousands of billions of suns. The process of absorbing the gas and emitting the radiation lasted several tens of millions of years. Thus the centers of galaxies become very bright.

What is the question? How can one estimate the exact size of the accretion disk - the structure that exists around black holes?

In the last 20 years, research evidence has accumulated suggesting that the size of the adsorption disk is significantly different from theoretical predictions. Thus the understanding of the evolution of black holes throughout the life of the universe was doubted. Alternative models for the absorption of matter into black holes have been proposed in abundance, but no consensus has yet been reached in the field.

Prof. Doron Shelus, Deputy Dean for Research at the Faculty of Natural Sciences at the University of Haifa, focuses his research on how black holes gain mass and grow, thus influencing the development of galaxies and the universe. According to him, "In the past, black holes were thought to be mathematical factors and nothing else, and only in the last 20 or 30 years did they begin to understand that they are planted in the center of most galaxies. However, it is still unclear how they grow over the lifetime of the universe. We investigate the hypotheses about the size of the accretion disks and for this purpose we develop observational tools and innovative data processing methods. In these studies, we also use dedicated theoretical models that take into account not only the physics of the adsorption disc but also that of the regions adjacent to it.'

In their latest study, which won a grant from the National Science Foundation, the researchers wanted to try to estimate the size of the adsorption disc and understand if its research interpretation is accurate. For the most part, studies in the field use large telescopes where observation time is expensive. The researchers chose to use a small, dedicated telescope, and advanced statistical methods.

The light emanating from the proximity of active black holes changes in time and is reflected back towards us from matter in their surroundings

In addition, they used the echo technique, which is reminiscent of radar and can provide information about the geometric structure of distant regions of the universe as long as light is coming from them. Through precise measurement of changes in light intensity captured in the telescope's photographs, the researchers discovered that the interpretation usually given to the size of the absorption disks is incorrect. "The light emanating from the proximity of active black holes changes in time and is reflected back to us from matter in their surroundings," says Prof. Shlouch. "Thus, through daily monitoring of changes in the intensity of light in different colors, we mapped the central areas around the black hole. We found that the light signals sampled in the photographs, which reflect the size of the various regions, are not only affected by the absorption disk but also and perhaps mainly by the broad line region, which is near active black holes. In fact the study revealed that this region originates from the accretion disk but is not attached to the black hole. Its existence has been known for many years, but its effect on the observed properties of the adsorption disk was not correctly assessed, which caused a significant bias in the measurement of its size.'

These findings contribute to a more accurate estimate of the size of the accretion disks and to the understanding of how black holes gain mass and grow, influencing the evolution of the universe.

Life itself:

Prof. Doron Shelusch, 49 years old, lives in Tel Aviv, is an avid lover of music (classical and more), plays the piano, enjoys swimming, and loves to cook and eat.

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