Supermassive black holes can cloak themselves in a cocoon of dust, making them invisible even when they should be bright quasars


Quasars are the most powerful sources of light in the universe, but sometimes they’re hard to find. A team of astronomers used the Chandra X-ray Space Telescope to find some diamonds in the rough.

When gas finds itself in the center of a galaxy, it must come toe-to-toe with a truly terrible monster: a supermassive black hole. These black holes can devour entire stars whole, and they don’t mess around. As material falls towards the event horizon, it crams together into a small volume. The friction of that trek generates heat and light. When this happens, the galaxy is known as a quasar.

To give you some perspective, a single quasar can outshine millions of galaxies for millions of years.

Like I said, the supermassive black holes don’t mess around.

But that insane brightness doesn’t mean that they’re easy to spot. For one, quasars are incredibly far away – billions of light-years far away. And they can also be obscured.

If too mush gas and dust enters the core at once, the black hole can still feast and the quasar can still light up, but the thick envelopes of dust can block it from our view, masking the quasar as something more benign, like a slowly-feeding black hole or even just a normal galaxy.

Recently a team of astronomers conducted a survey of the deep universe in several wavelengths, including X-rays with the Chandra Observatory. Through the other observations, the team was able to spot 28 quasars that had previously been hidden in plain sight.

By comparing the expected X-ray emission from these quasars to the actual X-ray emission observed with Chandra, the astronomers concluded that the black holes were actively feeding, but were swaddled in immense layers of dust – up to ten times as much dust as we had previously thought.

The study was a part of a much larger campaign, called the Chandra Deep Field-South, the deepest X-ray image ever taken.

Astronomers are keen to understand the number and sizes of giant black holes in the universe, as their activity is a part of the growth and evolution of galaxies. When galaxies get big, lots of material winds its way to the core. But once the quasar lights up, it floods its host with intense radiation, shutting off star formation and expelling extra material.

But the details of that relationship are poorly understood, and surveys like the Deep Field-South, and its uncovering of a hidden treasure trove of quasars, certainly help.

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