Galactic jets are the mysterious party poppers of the universe. An energetic galaxy will occasionally release long, beautiful streamers of dust and matter from its core. But astronomers never knew exactly what prompted these astro parties—that is, until now.
Scientists have conducted the world's most extensive survey of galactic jets ever and feel confident to conclude that they only appear from the core of galaxies that are either undergoing or have recently undergone a galaxy merger: a chaotic joining of two galaxies (think Red Wedding but galactic). This involves the combination of the two black holes at the center of each galaxy. Not every merging galaxy pair spits out jets though, so there are still some questions left unsolved.
It is logical that highly energetic, merging galaxies are the ones that emit these so-called relativistic jets. There is an abundance of energy shooting around the combining cores; it is not surprising that these central black holes would want to release some of the energy build-up in the form of jets.
The hot matter within the galactic jets emits radio waves. This is the marker that astronomers look for when searching for these sorts of galaxies. "By using Hubble's WFC3 camera we found that almost all of the galaxies with large amounts of radio emission, implying the presence of jets, were associated with mergers," explains Marco Chiaberge from the Space Telescope Science Institute. "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," added co-author Roberto Gilli from Osservatorio Astronomico di Bologna. "About 40% 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."
This observation will hopefully start to unravel some of the mysteries around galactic jets as these phenomena are currently poorly understood. Even though it seems to be certain that a galaxy merger is essential for hosting a supermassive black hole with relativistic jets, scientists sensibly propose that there are probably other criteria that we are still unaware of. Colin Norman, co-author of the paper, has some theories.
"There are two ways in which mergers are likely to affect the central black hole. The first would be an increase in the amount of gas being driven towards the galaxy's centre, adding mass to both the black hole and the disc of matter around it," he explains. "But this process should affect black holes in all merging galaxies, and yet not all merging galaxies with black holes end up with jets, so it is not enough to explain how these jets come about. The other possibility is that a merger between two massive galaxies causes two black holes of a similar mass to also merge. It could be that a particular breed of merger between two black holes produces a single spinning supermassive black hole, accounting for the production of jets."