Stars, gas, and dust can orbit black holes for long periods of time until a major disruption pushes the material in. 

Only high-energy X-rays escape telling astronomers about the accreting black holes within.

Galaxies combine and their central black holes approach each other. Thus, they push the gas and dust, surrounding it, into their respective black holes. The material releases an enormous amount of high-energy radiation as it spirals rapidly toward the hungry black hole. Thus, becoming what astronomers call an active galactic nucleus (AGN).

A study using NASA’s NuSTAR telescope shows that in the late stages of galaxy mergers, so much gas and dust falls toward a black hole that they enshroud the extremely bright AGN. The combined effect of the gravity of the two galaxies slows the rotational speeds of gas and dust that would otherwise be orbiting freely. This loss of energy makes the material fall onto the black hole.

“The further along the merger is, the more enshrouded the AGN will be,” said Claudio Ricci, lead author of the study published in the Monthly Notices Royal Astronomical Society. “Galaxies that are far along in the merging process are completely covered in a cocoon of gas and dust.”

Ricci and colleagues observed the penetrating high-energy X-ray emission from 52 galaxies. About half of them were in the later stages of merging. Because NuSTAR is very sensitive to detecting the highest-energy X-rays, it was critical in establishing how much light escapes the sphere of gas and dust covering an AGN.

The Monthly Notices of the Royal Astronomical Society published this study. Researchers compared NuSTAR observations of the galaxies with data from NASA’s Swift and Chandra and ESA’s XMM-Newton observatories. They look at lower energy components of the X-ray spectrum. If astronomers detect high-energy X-rays from a galaxy, but not low-energy X-rays, that is a sign that an AGN is heavily obscured.

What do we learn from this study?

An AGN’s black hole does most of its eating while enshrouded during the late stages of a merger. So, this study helps confirm the longstanding idea

“A supermassive black hole grows rapidly during these mergers,” Ricci said. “The results further our understanding of the mysterious origins of the relationship between a black hole and its host galaxy.”

NuSTAR is a Small Explorer mission. Caltech and NASA’s Jet Propulsion Laboratory manage it for NASA’s Science Mission Directorate in Washington. The Danish Technical University in partnership with the Italian Space Agency (ASI) developed NuSTAR. Orbital Sciences Corp., Dulles, Virginia built the spacecraft. NuSTAR’s mission operations center is at UC Berkeley. The official data archive is at NASA’s High Energy Astrophysics Science Archive Research Center. ASI provides the mission’s ground station and a mirror archive.