Jellyfish aren’t actually floating out there, but some galaxies look eerily like them, with trails of glowing “tentacles” appearing to sway behind them as if they were swimming in a cosmic ocean. And supermassive black holes like to eat them.
The tentacles, which drift off for thousands of light-years beyond their galactic discs, are the result of the ram pressure stripping process that occurs in galaxy clusters. Galaxies fall into these clusters at warp speed when the cluster is attracted to the galaxy, and vice versa, because of mutual gravitational attraction. X-ray-emitting gas within the clusters, aka the intra-cluster medium, moves like wind as an individual galaxy passes through. The tendrils come into being when powerful gusts of this dense, scorching gas blast them outward from the disc, and set off starbursts.
Most of these galaxies have also been found to host a gargantuan gas-guzzling parasite in the middle—an active and ravenous supermassive black hole.
"This strong link between ram pressure stripping and active black holes was not predicted and has never been reported before," said Bianca Poggianti, who lead a team of Italian astronomers using the MUSE (Multi-Unit Spectroscopic Explorer) instrument on the Very Large Telescope (VLT) to observe extreme examples of these types of galaxies. "It seems that the central black hole is being fed because some of the gas, rather than being removed, reaches the galaxy center."
Why most supermassive black holes lurking in galactic centers, with the exception of those parasitically feeding off the jellyfish galaxies, are inactive remained a mystery until this study illuminated another way the live ones satisfy their cravings. MUSE observations revealed a previously undiscovered mechanism by which gas is funneled toward the vicinity of the black hole. This mechanism has shed new light on how astronomers understand the relationship between galaxies and their supermassive black holes.
These strange sea-creature galaxies also tell us more about how a galaxy evolves, even if it doesn’t have tentacles. The way they seem to mutate like some astral alien spawn is more than just inspiration for the next Ridley Scott movie. The MUSE team continues to observe them as part of a survey that will tell us not only how many galaxies that experience an increase in core activity, but which ones.
“Jellyfish galaxies are a key to understanding galaxy evolution as they are galaxies caught in the middle of a dramatic transformation," said Poggianti.
Paging Ridley Scott.