A new study, accepted for publication in the Astrophysical Journal (arXiv.org preprint), finds a link between the presence of supermassive black holes that power radio-signal-emitting jets and the merger history of their host galaxies.
This artist’s impression illustrates how high-speed jets from supermassive black holes would look. Image credit: ESA / Hubble / L. Calçada, ESO.
The authors of the study – Dr Marco Chiaberge of the Space Telescope Science Institute and his colleagues from Italy and the United States – used the Wide Field Camera 3 aboard the NASA/ESA Hubble Space Telescope to investigate the relationship between galaxies that have undergone mergers and the activity of the supermassive black holes at their cores.
They studied a large selection of active galactic nuclei (AGNs), thought to be the result of large quantities of heated matter circling around and being consumed by a supermassive black hole. Whilst most galaxies are thought to host a supermassive black hole, only a small percentage of them are this luminous and fewer still go one step further and form what are known as relativistic jets.
The two high-speed jets of plasma move almost with the speed of light and stream out in opposite directions at right angles to the disc of matter surrounding the black hole, extending thousands of light-years into space. The hot material within the jets is also the origin of radio waves.
Dr Chiaberge and co-authors observed five categories of galaxies for visible signs of recent or ongoing mergers – two types of galaxies with jets, two types of galaxies that had luminous cores but no jets, and a set of regular inactive galaxies.
“The galaxies that host these relativistic jets give out large amounts of radiation at radio wavelengths,” Dr Chiaberge said.
The astronomers found that almost all of the galaxies with large amounts of radio emission, implying the presence of jets, were associated with galactic mergers.
“However, it was not only the galaxies containing jets that showed evidence of mergers.”
“We found that most merger events in themselves do not actually result in the creation of AGNs with powerful radio emission. About 40 percent of the other galaxies we looked at had also experienced a merger and yet had failed to produce the spectacular radio emissions and jets of their counterparts,” said co-author Dr Roberto Gilli of the Osservatorio Astronomico di Bologna in Italy.
“It is now clear that a galactic merger is almost certainly necessary for a galaxy to host a supermassive black hole with relativistic jets,” the astronomers said.
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Marco Chiaberge et al. 2015. Radio Loud AGNs are Mergers. ApJ, accepted for publication; arXiv: 1505.07419
