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Role of SKA in detecting multiple suppermassive black hole systems

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Posted January 7, 2015

Undoubtedly, supermassive black holes (SMBHs) are peculiar and awe-inspiring objects of the Universe. Large enough to drive the evolution of entire galaxies, they are of particular interest to modern astronomers. And, despite the fact that we cannot observe them directly (and only barely – using available measures of indirect observation, such as X-ray or gamma-ray astronomy), scientists are already preparing for explorations of multiple supermassive black hole systems.

Astronomers predict that many galaxies have not one black hole (as in the illustration above), but two or more giant black holes in their centers. NASA/CXC/UCLA/Z.Li AND NRAO/VLA

Astronomers predict that many galaxies have not one black hole (as in the illustration above), but two or more giant black holes in their centers. NASA/CXC/UCLA/Z.Li AND NRAO/VLA

“Galaxies and supermassive black holes are believed to evolve through a process of hierarchical merging and accretion. Through this paradigm, multiple SMBH systems are expected to be relatively common in the Universe”, say the authors of the article published on arXiv.org this week. In this scientific paper they explore the future potential of the Square Kilometre Array (SKA) in tracing black hole evolution and formation of systems containing more than one black hole.

Currently, there are only a handful of candidate binary/dual SMBH systems. Most of them are separated in thousands of parsecs, which is quite a large distance even on astronomical scale (worth noting: the diameter of Milky Way galaxy is somewhere between 31 and 37 kpc). And, as illustrated in the figure below, there are only two sub-parsec level system candidates.

Sample of the strongest dual/binary AGN candidates as revealed by direct imaging at X-ray (squares), optical/infrared (triangles) and radio (stars) wavelengths. Image courtesy of the researchers.

Sample of the strongest dual/binary AGN candidates as revealed by direct imaging at X-ray
(squares), optical/infrared (triangles) and radio (stars) wavelengths. Image courtesy of the researchers.

Nonetheless, in the next decade it is expected that large-scale surveys with the SKA will greatly increase this number.  Scientists say that binary black hole systems are predicted to have important implications towards the evolution of galaxies. For example, gravitational perturbations of such objects could increase bulge star formation rates, surface brightness distribution, as well as various morphological parameters of galaxies. In some cases the interacting supermassive black holes could even eject tremendous amounts of matter from the galaxy bulge. Therefore, this appears to be an important consideration in astrophysical research.

Two closely orbiting black holes in a galaxy about 4.2 billion light-years from Earth emit wavy jets while the third black hole in the trio is more distant, emitting linear jets.

Two closely orbiting black holes in a galaxy about 4.2 billion light-years from Earth emit wavy jets while the third black hole in the trio is more distant, emitting linear jets.

As the authors note, SKA will provide an unprecedented combination of depth, area and angular resolution of future astronomical surveys. “Theoretical expectations suggest that 0.1-1 percent of galactic halos may host binary SMBHs, at least at intermediate to high redshift“, they explain. Considering technical parameters of SKA, the scientists expect several orders of magnitude increase in the number of known binary supermassive black holes.

And what about a possible existence of triple black hole systems? The article briefly mentions that there have been attempts to simulate the interactions between three closely separated black holes. In such systems (as well as in binary systems) SMBHs should trigger exceptionally strong radio-jets. Certainly, such phenomenon is expected to be quite rare, although even today at least eight candidate triple active galactic nuclei (AGN) candidates are known (illustrated by the figure below).

Projected separations of candidate triple active galactic nuclei. The dashed lines isolate the sub-10kpc systems, which is roughly the effective stellar radius of an elliptical galaxy. Image courtesy of the researchers.

Projected separations of candidate triple active galactic nuclei. The dashed lines isolate the sub-10kpc systems, which is roughly the effective stellar radius of an elliptical galaxy. Image courtesy of the researchers.

Two or even three interacting black holes should be detectable, considering their capability to modulate radio jets on large scale and enhanced accretion of matter within such systems. The stronger the accretion, the stronger rays could potentially be emitted, which is also a good thing speaking in terms of detection possibility.

“The preliminary evidence that radio-jet triggering is enhanced in dual and triple SMBH systems; negligible dust and gas attenuation at GHz frequencies; and unmatched sensitivity and angular resolution suggests that SKA will play a leading role in their discovery, characterization and followup”, the team concludes.

Written by Alius Noreika

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