An article published in the journal “Monthly Notices of the Royal Astronomical Society” describes the effects that a merger between two galaxies can have on a supermassive black hole at the center of a galaxy involved in that process. A team of researchers led by Claudio Ricci used especially NASA’s NuSTAR space telescope to study how in the last stages of galactic merger gas and dust fall towards a black hole enshrouding it and generating an active galactic nucleus.
When two galaxies merge, the combinations of gravitational forces involved cause disruption within them. In the final stages of this process, one of the consequences is that the combined gravitational effects slow down the rotation speed of dust and gas disks surrounding the black holes at the center of the galaxies involved. The consequence is that the existing balance is broken and these materials and start falling towards these black holes, wrapping them in spheres and causing them to grow rapidly.
The enormous energy generated by the heating of dust and gas falling into a supermassive black hole causes the emission of very strong electromagnetic radiation giving rise to what is called in jargon an active galactic nucleus. Despite its energy levels, those emissions can be obscured by the amount of dust and gas, so the NuSTAR telescope is ideal for this research as it can detect the high-energy X-rays that pass through the thick envelope.
The NuSTAR (Nuclear Spectroscopic Telescope Array) space telescope, launched in June 2012, was designed to detect high-energy X-rays. In this research, this was crucial to determine how much light escapes the dust and gas envelope surrounding the active galactic nuclei of 52 galaxies. These observations were compared to those conducted with NASA’s Swift and Chandra and ESA’s XMM-Newton space telescopes, which can detect X-rays at lower energies.
The result is that in some cases high-energy X-rays were detected observing a galaxy but no lower-energy X-rays. It’s a sign that the active galactic nucleus is heavily obscured by the gas and dust that surround it. The galaxies observed are in different stages of their merger and the detections indicate that the period of the supermassive black holes’ greatest activity corresponds to the last stages, when it’s most enshrouded withing gas and dust.
The image illustrates the difference between the growth of a supermassive black hole in a normal situation, as much as the term “normal” can make sense talking about black holes, and the growth towards the end of a galactic merger. Normally a supermassive black hole is surrounded by a ring of gas and dust slowly falling into it. In a galaxy involved in a merger, gas and dust form a sphere around the black hole obscuring it.
The activity of a supermassive black hole has a major influence on the galaxy that hosts it but during a galactic merger the influences are reciprocal. This type of research can provide more information on the relationship between galaxies and supermassive black holes. In the far future the Milky Way and Andromeda will start merging so their supermassive black holes will probably go through a period as active galactic nuclei as well.