More than 400 black holes have been found, hidden from our view by cocoons of cosmic dust

(ORDO NEWS) — Astronomers have discovered more than 400 previously hidden black holes feeding on stars and dust at the center of galaxies.

It seems that many of the new black holes found by NASA‘s Chandra X-ray Observatory have remained unknown until now, because they are buried under cocoons of dust.

Supermassive black holes, which can be millions of times heavier than the sun, live at the center of almost every galaxy in the universe.

These colossal objects emit bright beams of energy as they feed on the gas, dust and stars in their immediate vicinity, creating what are known as active galactic nuclei (AGNs), according to NASA.

AGNs are especially bright in the X-ray part of the electromagnetic spectrum, said astronomer Dong-Woo Kim of the Harvard and Smithsonian Center for Astrophysics.

They also leave telltale visible patterns in optical light that identify them as AGNs, he added.

But certain objects have been seen emitting tons of X-rays without the specific optical signatures associated with AGNs, Kim said, and they have been dubbed “X-ray bright optically normal galaxies” or “XBONGs.”

More than 400 black holes have been found hidden from our view by cocoons of cosmic dust 2
Parallel image of X-ray and optical data showing what appears to be an active black hole hidden in the center of a distant dusty galaxy

To further investigate these mysterious creatures, he and his colleagues looked through a catalog of more than 300,000 bright X-ray objects captured by Chandra.

They then took optical images of the night sky from the Sloan Digital Sky Survey (SDSS) and combined the two datasets to select X-ray bright but optically normal objects.

Researchers have identified 820 XBONGs located between 550 million and 7.8 billion light-years from Earth, Kim said, the largest specimen ever created.

“Immediately the question arose:“ What is it? ”, The scientist admitted.

Dust cocoons of black holes

More than 400 black holes have been found hidden from our view by cocoons of cosmic dust 3
X-ray (left) and optical (right) images of two galaxies with hidden black holes at their centers

X-rays penetrate dust and optical light is obscured by it, so Kim and his colleagues tried to figure out if these 820 XBONGs could be black holes surrounded by a lot of dust.

If that were the case, the lowest-energy X-ray light would be slightly absorbed by the dust, while the most powerful X-ray light would continue to shine brightly through it, Kim said.

His team saw exactly this pattern in about 50% of XBONGs, suggesting that they were AGNs hidden in dusty cocoons.

What about the other 50%? One possibility is that Chandra sees extremely distant clusters of galaxies that glow brightly in X-rays but lack the characteristic optical signature that identifies them as AGNs, the scientist suggested. Kim added that this could explain about 20% of the remaining XBONGs.

The last 30%, in his opinion, are galaxies whose optical light is especially powerful, bright enough to blur the optical signature of AGNs, which can happen when such galaxies are particularly far away.

Kim hopes Chandra’s further research will help answer the remaining questions about these strange structures.

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