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