Astrophysicists used the NuSTAR X-ray telescope to study 52 supermassive black holes (SMBHs) in nearby merging galaxies to improve our understanding of the relationship between a black hole and the galaxy it lives in.
When galaxies collide, the material (gas and dust) that they contain gets disrupted and some of it moves towards the SMBH that resides in the centre of the merged system. As this material travels through the galaxy, it loses energy and eventually falls onto the black hole, creating an accretion disc around it. The black hole then becomes active, creating what is called an Active Galactic Nucleus (AGN). As the material falls into the SMBH is creates a shroud, known as torus in astronomy, that is so thick that blocks all the photons that try to penetrate it apart from the most energetic ones, such as high-energy X-rays. By comparing the number of high- and low-energy photons that manage to escape this shroud, scientists can estimate how thick this shroud is. This process is very common in the universe but scientists still try to understand what exactly turns on this activity and how it evolves.
In an attempt to uncover the details of this process, astronomers used the NuSTAR X-ray observatory to study 52 merging systems. NuSTAR has high sensitivity to high-energy X-rays compared to other X-ray telescopes such as the Chandra and XMM-Newton X-ray Observatories and the Swift mission. Their analysis revealed that further along the merger is, the more enshrouded the AGN will be. Actually, systems that are far along in the merging process are completely covered in a cocoon of gas and dust. This shroud is also much thicker in the merging systems compared with isolated galaxies that aren’t undergoing mergers or have just begun to merge. This indicates that the galaxy merger has caused large amounts of gas and dust to travel towards the centre of the galaxy. These results help us improve our understanding regarding the co-evolution of a SMBH and its host galaxy.
Publication: Ricci et al. 2017