Galactic mergers and radio wave emissions

Artistic concept of a galaxy generatic relativistic jets with radio waves coming from its supermassive black hole (Image ESA/Hubble, L. Calçada (ESO))
Artistic concept of a galaxy generatic relativistic jets with radio waves coming from its supermassive black hole (Image ESA/Hubble, L. Calçada (ESO))

An article published in the journal “Astrophysical Journal” describes a study that established a link between the presence of supermassive black holes that emit jets of materials to nearly the speed of light but also radio waves and galaxy mergers. An international team of astronomers led by Italian INAF researcher Marco Chiaberge used the Hubble Space Telescope in the most extensive survey of the kind ever conducted.

This survey was based on images taken by the Hubble Space Telescope’s Wide Field Room 3 (WFC3) to study a wide selection of galaxies that have extremely luminous nuclei. They’re called Active Galactic Nuclei (AGN) and are believed to be the result of large amounts of heated materials revolving around a supermassive black hole which eventually get swallowed by it.

It’s our current knowledge that most galaxies have a supermassive black hole at its center but only a small percentage of them is of the AGN type and even less emit relativistic jets, meaning jets of material that travel at speeds approaching that of light.

When there are relativistic jets, they’re originated from the poles of the supermassive black hole and extend in opposite directions up and down from the galactic plane. These jets can extend for thousands of light years from the galaxy of origin and the gases contained in them are very hot and ionized becoming plasma.

Due to the ionization, the gas is electrically charged. The interaction between the particles of these gases and the magnetic field inside the jets generates intense emission of radio waves. These jets are the subject of this research to establish the connection with galaxy merger.

By studying a number of galaxies in which that kind of interaction is ongoing or has just ended, the researchers found that in almost all the galaxies that have large amounts of radio emissions there have been mergers. However, only a part of those cases of galactic mergers generated AGNs with strong radio wave emission.

In essence, the merger is a necessary condition but not sufficient to generate relativistic jets. It may be necessary also the merger between two black holes to obtain one with a fast spin. The spin acceleration may be the factor that generates relativistic jets.

It will take further observations with the Hubble Space Telescope but also with ALMA (Atacama Large Millimeter/submillimeter Array) to expand the survey. The observation of other galaxies will allow to better understand better these phenomena of enormous size and power that in the distant future could also concern the galaxy which will be formed from the merger of the Milky Way and Andromeda.

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