Many supermassive black holes can't be seen with current technology

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 better estimate for the ratio of hidden to visible black holes, thus shedding more light on the enigmatic structures that lie at the heart of most galaxies.

Searching for supermassive black holes

Supermassive black holes can weigh billions of times more than the Sun. However, many of these cosmic giants are hidden behind dense clouds of gas and dust, making them difficult to detect.

Researchers estimate that every large galaxy harbors a supermassive black hole at its center.

Yet, identifying the total population of these behemoths is challenging. Instead, scientists study smaller samples and extrapolate to the larger population.

The latest study, published in the Astrophysical Journal, revealed 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 earlier surveys that identified less than 15% as obscured. Scientists believe the true ratio may approach 50/50, a finding that could reshape theories about how galaxies and black holes grow together.

Bright monsters in the dark

Supermassive black holes are completely dark because no light can escape their immense gravitational pull. However, the matter swirling around a black hole glows intensely.

As gas spirals closer to the black hole, the huge 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.”

The torus resembles a doughnut, with the bright center at its core. Whether we can see the black hole or not depends on the angle of this torus.

If we are looking straight into the doughnut hole, the bright emissions are visible. If we see it edge-on, the torus blocks most of the light, which means that 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 Astronomical Satellite (IRAS). The IRAS detected infrared light, which is 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 that can penetrate the dense gas and dust.

Combining data from both telescopes allowed scientists to locate and study hidden black holes with much greater precision, and offered a clearer understanding of these cosmic giants.

Black holes and galactic evolution

Supermassive black holes play a huge role in shaping how galaxies grow and change over time. When matter gets too close to a supermassive black hole, it spirals in and heats up, creating bright energy emissions that can influence the entire galaxy.

This intense energy can regulate the formation of new stars by either triggering it through shockwaves or suppressing it by blowing away the gas needed for star creation.

Essentially, the black hole acts like a cosmic governor, controlling how busy or quiet the galaxy is in terms of star production.

Beyond just star formation, supermassive black holes also help shape the overall structure and behavior of their host galaxies.

They can launch powerful jets of particles and radiation that stretch far beyond the galaxy’s core, impacting the surrounding environment.

These jets can heat up the gas in the galaxy, preventing it from cooling down and forming new stars, which influences the galaxy’s size and brightness.

Additionally, the interaction between the black hole and the galaxy can lead to the growth of the galaxy’s central bulge, making it more rounded and stable.

“If we didn’t have black holes, galaxies would be much larger,” noted Poshak Gandhi, a coauthor of the study. “Without a supermassive black hole in the Milky Way, there might be many more stars in the sky.”

Why does any of this matter?

This study demonstrates the lasting importance of data collected by telescopes, even decades after their initial operation.

By combining observations from multiple instruments that work across different wavelengths of light, scientists can uncover details that a single telescope alone might miss.

“It amazes me how useful IRAS and NuSTAR were for this project, especially despite IRAS being operational over 40 years ago,” said study lead author Peter Boorman of Caltech.

The IRAS provided critical infrared data, while the more modern NuSTAR telescope added complementary X-ray observations. Together, these tools created a fuller picture of hidden supermassive black holes.

This study is not just about uncovering hidden black holes; it’s a testament to the ingenuity and innovation that drive astronomical research.

By leveraging these tools and techniques, scientists gain deeper insights into how hidden black holes influence the growth of galaxies and, ultimately, into the universe’s structure.

The study is published in the Astrophysical Journal. 

Video Credit: NASA/JPL-Caltech

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