Three supermassive black holes (SMBHs) glowing in x-ray emissions have been identified by astronomers at the center of three colliding galaxies a billion light-years away from Earth. All three black holes are active galactic nuclei(AGN), consuming material. This finding may clarify a long-standing issue in astrophysics and black hole mergers known as “final parsec problem”. The study appears in The Astrophysical Journal.
Researchers detected the three SMBHs with the data from several telescopes, Sloan Digital Sky Survey (SDSS,) the Chandra X-ray Observatory, and the Wide-field Infrared Survey Explorer (WISE)A nearly unbelievably astronomical event, the fusion of three galaxies may play a crucial role in how the most massive black holes expand over time.
Ryan Pfeifle from George Mason University in Fairfax, Virginia, the paper’s first author said that they found this incredible system through their selection technique while they were only looking for black hole pairs. He also added that this is the most powerful evidence found for such a triple system of active supermassive black holes. It is very challenging to locate triple black hole systems since they are wrapped in gas and dust. It took several telescopes functioning in different parts of the electromagnetic spectrum and also the work with researchers to detect these black holes.
Co-author Shobita Satyapal, also belonging to George Mason said that dual and triple black holes are extremely rare but such systems are actually a natural outcome of galaxy mergers, through which galaxies grow. This triple-merger was first spotted in visible light by the SDSS and only through a citizen science project named Galaxy Zoo the system of colliding galaxies was detected. The system was in a state of galaxy merger glowing in the infrared as seen by the WISE when more than one black holes were expected to be feeding.
Researchers shifted to the Chandra Observatory and the Large Binocular Telescope (LBT) for confirmation as Sloan and WISE data were fascinating clues. Chandra observations revealed bright x-ray sources in the galactic centers where SMBHs are expected to detect. Chandra and Nuclear Spectroscopic Telescope Array (NuSTAR) satellite of NASA discovered more shreds of evidence showing the presence of SMBHs and the existence of large amounts of gas and dust near one of them. It was expected in merging of black holes. Spectral evidence received by optical light data from SDSS and LBT shows that these are characteristics of the feeding SMBHs.
Christina Manzano-King, co-author from the University of California, Riverside said that optical spectra include plenty of information about a galaxy which is frequently used to detect active accreting supermassive black holes and can tell about their influence on the inhabitant galaxies. Pfeifle said that they have found a new method of identifying triple supermassive black holes using these major observatories as each telescope gives them a distinct idea about these systems. They expect to extend their work to find more triples using the same method.
The final parsec problem is a theoretical problem that is fundamental to our understanding of binary black hole mergers that states that the enormous orbital energy of two approaching black holes stops them from merging. They can get separated by a few light-years, then the merging process stables.
The hyperbolic trajectories of two initially approaching black hole carry them right past each other. The two holes catapult the stars as they interact with them in their proximity transferring a fraction of their orbital energy to a star every time. The energy of the black holes gets reduced by the emission of gravitational waves. The two black holes finally slow down and approach each other more closely shedding enough orbital energy finally getting within just a few parsecs of each other. More matter is discharged via sling-shotting as they come closer. As a result, for the black holes, no more matter is left to interact with and shed more orbital energy. The merging process halts.
Astronomers know that strong gravitational waves are responsible for black hole mergers.LIGO (Laser Interferometry Gravitational-Wave Observatory) discovers a black hole merger almost every week. The final parsec problem is about how they merge with each other finally. Researchers think that a third black hole like seen in this system could give the push needed for the black holes to get merged. Nearly 16% of supermassive black hole pairs in colliding galaxies are expected to interact with a third supermassive black hole before they merge.
The challenge is that gravitational waves produced during merging would be too low-frequency for LIGO or the VIRGO observatory to detect. Researchers may have to depend on future observatories like LISA, ESA/NASA’s Laser Interferometer Space Antenna to detect those waves. LISA is better-equipped than LIGO or VIRGO to detect merging of giant and massive black holes as it can detect lower frequency gravitational waves.
Reference: The Astrophysical Journal.