A link between supermassive black holes and jellyfish galaxies

The jellyfish galaxy JO204 (mage ESO/GASP collaboration)
The jellyfish galaxy JO204 (mage ESO/GASP collaboration)

An article published in the journal “Nature” describes a research that shows a new function of the “tentacles” of the so-called jellyfish galaxies. An international team of astronomers led by Bianca Poggianti of the INAF-Astronomical Observatory of Padua in Italy used the observations conducted during ESO’s GASP program with the MUSE instrument installed on the Very Large Telescope (VLT), discovering that the mechanism that generates those tentacles is the same that powers the supermassive black holes at the center of those galaxies.

Jellyfish galaxies are called that way precisely because they have formations that look like tentacles that are actually composed of stars and gas. Their length of many light years makes these tentacles visible from the Earth and the mechanism that produces them is a kind of interstellar wind called in jargon “ram pressure”.

Simply put, this mechanism acts within galactic clusters because of the force of gravity of the galaxies within it. The various galaxies within a cluster attract each other, tend to move to the center of the cluster and this produces a kind of hot and dense wind. In jellyfish galaxies, it tends to push the gas of the tentacles out of them.

The result is that the tentacles’ gas contributes in short-term to the generation of new stars but in the long run that mechanism is supposed to subtract gas from the galaxy ending up to cause the end of star formation. Trying to understand that kind of galactic evolution is the main reason why jellyfish galaxies are being studied and in 2015 the GASP (GAs Stripping Phenomena in galaxies with MUSE) program was started, led by Bianca Poggianti.

For this new research, 94 jellyfish galaxies were studied among those observed with the MUSE spectrograph and the 7 ones with the longest tentacles were selected for further examination. At the center of 6 of those galaxies, an active galactic nucleus was found, created by a supermassive black hole surrounded by huge amounts of gas and dust.

This result was a surprise for Bianca Poggianti’s team as a different result, if not the opposite, was expected. It’s normal for a galaxy to have a supermassive blck hole but on average less than 1/10 of the galaxies has an active galactic nucleus and the ram pressure phenomenon is supposed to bring a lot of gas away from the galactic center so no one would expect jellyfish galaxies to have an active galactic nucleus.

Bianca Poggianti pointed out that such a strong link between jellyfish galaxies and active galactic nuclei wasn’t anticipated and was never found in simulations. That suggests that a part of the gas ends up in the galactic center instead of being pushed away by the ram pressure.

Clarifying this mechanism could help solve a wider problem concerning the reason why only a small percentage of supermassive black holes form an active galactic nucleus. In general, it could help to better understand the possible processes that determine the evolution of galaxies, one of the ultimate goals of astronomical research.

The jellyfish galaxy JW100 (Image ESO/GASP collaboration)
The jellyfish galaxy JW100 (Image ESO/GASP collaboration)

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