Observations of 'Jellyfish galaxies' with ESO's Very Large Telescope have revealed a previously unknown way to fuel supermassive black holes. It seems the mechanism that produces the tentacles of gas and newborn stars that give these galaxies their nickname also makes it possible for the gas to reach the central regions of the galaxies, feeding the black hole that lurks in each of them and causing it to shine brilliantly.This picture of one of the galaxies, nicknamed JO204, from the MUSE instrument on ESO's Very Large Telescope in Chile, shows clearly how material is streaming out of the galaxy in long tendrils to the lower-left. Red shows the glow from ionised hydrogen gas and the whiter regions are where most of the stars in the galaxy are located. Some more distant galaxies are also visible. Credit: ESO/GASP collaboration

An Italian-led team of astronomers used the MUSE (Multi-Unit Spectroscopic Explorer) instrument on the Very Large Telescope (VLT) at ESO's Paranal Observatory in Chile to study how gas can be stripped from galaxies. They focused on extreme examples of jellyfish galaxies in nearby galaxy clusters, named after the remarkable long "tentacles" of material that extend for tens of thousands of light-years beyond their galactic discs.

The tentacles of jellyfish galaxies are produced in galaxy clusters by a process called ram pressure stripping. Their mutual gravitational attraction causes galaxies to fall at high speed into galaxy clusters, where they encounter a hot, dense gas which acts like a powerful wind, forcing tails of gas out of the galaxy's disc and triggering starbursts within it.

Six out of the seven jellyfish galaxies in the study were found to host a supermassive black hole at the centre, feeding on the surrounding gas. This fraction is unexpectedly high—among galaxies in general the fraction is less than one in ten.

"This strong link between ram pressure stripping and active black holes was not predicted and has never been reported before," said team leader Bianca Poggianti from the INAF-Astronomical Observatory of Padova in Italy. "It seems that the central black hole is being fed because some of the gas, rather than being removed, reaches the galaxy centre."

A long-standing question is why only a small fraction of supermassive black holes at the centres of galaxies are active. Supermassive black holes are present in almost all galaxies, so why are only a few accreting matter and shining brightly? These results reveal a previously unknown mechanism by which the black holes can be fed.

This video shows a 3D visualisation of the Jellyfish galaxy JO194. The three axes are: X and Y on the sky and Z the velocity along the line of sight. The red corresponds to Hydrogen emission, blue shows Nitrogen emission and green shows Oxygen emission. The white component shows the stars in the galaxy. The Hydrogen gas, which is an indicator of ongoing star formation, can be seen being drawn into tails by the ram-pressure and is significantly stripped away from the stars. The Oxygen traces intense shocks and reveals the bright AGN in the centre of the disk. Credit: Callum Bellhouse and the GASP collaboration

Yara Jaffe, an ESO fellow who contributed to the paper explains the significance: "These MUSE observations suggest a novel mechanism for gas to be funnelled towards the black hole's neighbourhood. This result is important because it provides a new piece in the puzzle of the poorly understood connections between supermassive black holes and their host galaxies."

The current observations are part of a much more extensive study of many more jellyfish galaxies that is currently in progress.

"This survey, when completed, will reveal how many, and which, gas-rich galaxies entering clusters go through a period of increased activity at their cores," concludes Poggianti. "A long-standing puzzle in astronomy has been to understand how galaxies form and change in our expanding and evolving Universe. Jellyfish galaxies are a key to understanding galaxy evolution as they are galaxies caught in the middle of a dramatic transformation."

This video shows a 3D visualisation of the Hydrogen alpha gas in the Jellyfish galaxy JO201, reconstructed from the MUSE data. The gas can be seen being stripped by the ram pressure into tails which stretch out from the disk of the galaxy and from its trailing blobs, not only along the plane of the sky, but also along the line of sight (in the velocity axis, which corresponds to the side view). Credit: Callum Bellhouse and the GASP collaboration

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More information: Bianca M. Poggianti et al. Ram-pressure feeding of supermassive black holes, Nature (2017). Journal information: Nature Bianca M. Poggianti et al. Ram-pressure feeding of supermassive black holes,(2017). DOI: 10.1038/nature23462