Many supermassive black holes are hidden by gas and dust

15.01.2025/17/30 XNUMX:XNUMX    215


Astronomers have taken another step forward in their search for the universe's most elusive titans: supermassive black holes. Using multiple NASA telescopes, scientists have found a more precise estimate of the ratio of hidden to visible black holes, shedding more light on the mysterious structures that lie at the heart of most galaxies.

The search for supermassive black holes

Supermassive black holes can weigh billions of times more than the Sun. But many of these cosmic giants are hidden behind dense clouds of gas and dust, making them difficult to detect. Researchers have estimated that there is a supermassive black hole at the center of every large galaxy. But determining the total population of these giants is difficult. Instead, scientists study smaller samples and extrapolate to the larger population. The latest study, published in Astrophysical Journal, has shown that about 35% of supermassive black holes are obscured by dense gas and dust that blocks even low-energy X-ray light. This contrasts with previous surveys that identified less than 15% as unknown. Scientists believe the true ratio may be closer to 50/50, a discovery that could change theories about how galaxies and black holes grow together.

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Bright monsters in the dark

Supermassive black holes are completely dark because no light can escape their immense gravitational pull. However, the matter orbiting the black hole glows intensely. As gas spirals closer to the black hole, the immense gravitational force heats it to extremely high temperatures. This causes the gas to shine so brightly that it can outshine all the stars in the galaxy. Despite this brightness, the glow can be hidden if the black hole is surrounded by a thick ring of gas and dust called a “torus.” A torus resembles a doughnut with a bright center at its base. Whether we can see the black hole or not depends on the angle of the torus. If we look directly into the hole of the doughnut, we can see bright emissions. If we look at it edge-on, the torus blocks most of the light, meaning the black hole is hidden.

Infrared and X-ray telescopes

To study these hidden black holes, NASA used data collected in 1983 by the Infrared Astronomy Satellite (IRAS). IRAS detected infrared light emitted by the heated gas in the torus, even when the black hole itself was obscured. NASA's NuSTAR telescope complemented this data by detecting high-energy X-rays, which can penetrate dense gas and dust. Combining data from both telescopes has allowed scientists to detect and study hidden black holes with much greater precision and has offered a clearer understanding of these cosmic giants.

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Black holes and galactic evolution

Understanding hidden supermassive black holes helps scientists understand how these objects grow and affect their galaxies. Supermassive black holes consume vast amounts of material, but if they swallow too much, they eject some of it back into the galaxy. These outflows can suppress star formation by dispersing clouds of gas needed for new stars.




"If we didn't have black holes, galaxies would be much larger," said Poshak Gandhi, a co-author of the study. "Without the supermassive black hole in the Milky Way, there could be many more stars in the sky."

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Innovations in astronomical research

This study demonstrates the enduring importance of data collected by telescopes, even decades after their initial operation. By combining observations from multiple instruments operating at different wavelengths of light, scientists can uncover details that a single telescope might miss.

“I am amazed at how useful IRAS and NuSTAR were to this project, especially considering that IRAS was operational over 40 years ago,” said lead author Peter Boorman of the California Institute of Technology.

IRAS provided important infrared data, and the more advanced NuSTAR telescope added additional X-ray observations. Together, these instruments have created a more complete picture of hidden supermassive black holes. This research is not just about discovering hidden black holes; it is a testament to the ingenuity and innovation that drives astronomical research. Using these tools and methods, scientists are gaining a deeper understanding of how hidden black holes affect the growth of galaxies and, ultimately, the structure of the universe. The study is published in Astrophysical Journal. 


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