May 10 2017
This illustration compares growing supermassive black holes in two different kinds of galaxies. A growing supermassive black hole in a normal galaxy would have a donut-shaped structure of gas and dust around it (left). In a merging galaxy, a sphere of material obscures the black hole (right). Credit: National Astronomical Observatory of Japan
In popular culture, black holes get a bad rap for swallowing everything existing in their environments. In reality, dust, gas and starts are capable of orbiting black holes for prolonged time periods, until the material is pushed in by a major disruption.
One such disruption refers to a merger of two galaxies. Dust and gas in the surrounding area are pushed onto their respective black holes, as the galaxies merge and their central black holes come close to each other. A huge amount of high-energy radiation is discharged as material spirals in an extremely fast manner toward the hungry black hole, which turns into an active galactic nucleus (AGN), according to astronomers.
A study using NASA's NuSTAR telescope demonstrates that in the late stages of galaxy mergers, huge amounts of dust and gas fall toward a black hole, thus enshrouding the extremely bright AGN. The collective effort of the gravity of the two galaxies slows the rotational speeds of dust and gas that would otherwise keep orbiting freely. This loss of energy results in the material falling onto the black hole.
The further along the merger is, the more enshrouded the AGN will be. Galaxies that are far along in the merging process are completely covered in a cocoon of gas and dust.
Claudio Ricci, NASA
The penetrating high-energy X-ray emission from 52 galaxies was observed by Ricci and colleagues. They discovered that about half of them were in the later stages of merging. The concept of establishing how much light escapes the sphere of dust and gas covering an AGN is considered to be important as NuSTAR is extremely sensitive to detecting the highest-energy X-rays.
The research was featured in the Monthly Notices of the Royal Astronomical Society. NuSTAR observations of the galaxies were compared with data from NASA's Swift and Chandra and ESA's XMM-Newton observatories, which focus on the X-ray spectrum’s lower energy components. An AGN is studied to be heavily obscured if high-energy X-rays are detected from a galaxy and low-energy X-rays are not.
This study enables confirming the very old idea that an AGN's black hole performs most of its eating while being enshrouded during the late stages of a merger.
A supermassive black hole grows rapidly during these mergers. The results further our understanding of the mysterious origins of the relationship between a black hole and its host galaxy.
Claudio Ricci, NASA
NuSTAR is a Small Explorer mission managed by NASA's Jet Propulsion Laboratory for NASA's Science Mission Directorate in Washington and headed by Caltech. NuSTAR was developed in collaboration with the Italian Space Agency (ASI) and the Danish Technical University. Orbital Sciences Corp., Dulles, Virginia, built the spacecraft. NuSTAR's official data archive is at NASA's High Energy Astrophysics Science Archive Research Center, and the mission operations center is at UC Berkeley. A mirror archive and the mission's ground station are provided by ASI. JPL is handled by Caltech for NASA.