Every self-respecting galaxy boasts a supermassive black hole in its heart — one that is either active or inactive. For instance, the black hole at the centre of Milky Way — Sagittarius A*— is roughly four million times the mass of the sun, and has been more or less sedate for about six million years.

This is usually the norm for several other galaxies in the observable universe. Supermassive black holes can remain surrounded with a thick, orbiting disk of gas, dust and debris for millions of years without being gobbled up. However, when a disruptive event — say two galaxies colliding and merging — occurs, the orbiting material rapidly spirals toward the ravenous black hole, which turns into an active galactic nucleus (AGN).

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An AGN, once it’s formed, releases massive bursts of high-energy X-ray, ultraviolet and optical radiation.

A team of astronomers using NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR) has now discovered, after studying 52 AGNs of galaxies in different stages of mergers, that in the late stages of galactic mergers, so much gas and dust falls toward a black hole that the extremely bright AGN is almost completely enshrouded.

Their observations have been detailed in a study published in the latest edition of the Monthly Notices of the Royal Astronomical Society.

“A supermassive black hole grows rapidly during these mergers,” study lead author Claudio Ricci from the Kavli Institute for Astronomy and Astrophysics at Peking University in China, said in a statement. “The results further our understanding of the mysterious origins of the relationship between a black hole and its host galaxy.”

Roughly half of the galaxies Ricci and his colleagues studied were in the later stages of merging. This allowed them to establish how much light was escaping the sphere of gas and dust enshrouding the AGNs. When they compared the observations made using NuSTAR with those made using the European Space Agency’s XMM-Newton observatories — which looked at lower-energy X-ray spectrum — the researchers found that if high-energy X-rays are detected from a galaxy, but low-energy X-rays are not, it is a sign that an AGN is heavily obscured.

“The further along the merger is, the more enshrouded the AGN will be,” Ricci said. “Galaxies that are far along in the merging process are completely covered in a cocoon of gas and dust.”

As a result, while a supermassive black hole in a normal galaxy is surrounded by a donut-shaped cloud of gas and dust, the one in a merging galaxy is almost completely covered with a sphere of debris.

The observations confirm the theory that an AGN's black hole does most of its feeding during the later stages of galactic mergers, when it’s heavily obscured.

“These observations show that the material is most effectively funnelled from the galactic scale to the inner tens of parsecs during the late stages of galaxy mergers, and that the close environment of SMBHs [supermassive black holes] in advanced mergers is richer in gas and dust with respect to that of SMBHs in isolated galaxies, and cannot be explained by the classical AGN unification model in which the torus is responsible for the obscuration,” the researchers concluded in the study.