“We want to understand why only a small fraction of supermassive black holes are active,” said Benedetta Vulcani, an astronomer from the University of Melbourne, in a statement. She was part of a team of scientists using the MUSE (Multi-Unit Spectroscopic Explorer) instrument on the Very Large Telescope to study AGNs. “Supermassive black holes are present in almost all galaxies,” she noted, “so why are only a few accreting matter and shining brightly?”
The team wrote in their paper that “a number of physical processes have been proposed to account for the funneling of gas towards the galaxy centers to feed the AGN. There are also several physical processes that can remove (strip) gas from a galaxy, and one of them is ram pressure stripping.”
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Ram pressure stripping takes place when gas from within the galaxy is “blown off.” Galaxy clusters are filled with hot, X-ray emitting gas known as the intra-cluster medium. As individual galaxies move within a galaxy cluster, they experience this intra-cluster gas as a “wind” — like the wind you feel when you ride a bike or motorcycle. Ram pressure stripping occurs if this wind is strong enough to overcome the gravitational pull of the galaxy, and the gas contained within it is “peeled off.”
The jellyfish galaxies are one of the spectacular and beautiful examples of galaxies undergoing gas stripping by ram pressure, and so an observational program known as GASP (GAs Stripping Phenomena in galaxies with MUSE) has been specifically targeting jellyfish galaxies to study where, how and why gas can be removed from galaxies. (Their logo even includes a cute likeness of a jellyfish).
The team focused on seven jellyfish galaxies, and amazingly, six out of the seven jellyfish galaxies were found to host an active supermassive black hole at the center, feeding on the surrounding gas. They said this fraction is surprisingly high, as among galaxies in general the fraction is less than one in ten. Their research shows that the ram pressure funnels the gas towards the galaxy center, feeding the black hole.
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“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 center.”
Ram pressure stripping has been studied previously, and other research found this process to have profound effects on the evolution of galaxies, but in a different way. One study showed that the gas being stripped out included the cool, denser gas that is the source of continued star formation and so the galaxies appear to be dying from a lack of new stars being formed. Another study of galaxies moving through galaxy clusters found that the ram pressure did not strip the galaxies of gas but instead compressed the gas, thereby accelerating star formation.
The research team said they will continue to study ram pressure stripping to better understand the process and its consequences. The GASP team has an ongoing study, obtaining deep, detailed data for 114 jellyfish galaxies in various environments (out of about 400 known). They hope that they can provide more pieces in the puzzle of the poorly understood connections between supermassive black holes and their host galaxies.
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