Jellyfish galaxies studied with the Hubble Space Telescope

The jellyfish galaxy JO201 (Image ESA/Hubble & NASA, M. Gullieuszik)
The jellyfish galaxy JO201 (Image ESA/Hubble & NASA, M. Gullieuszik)

An article in publication and an article under peer-review in “The Astrophysical Journal” report various aspects of a study of 6 so-called jellyfish galaxies. A team of researchers used various instruments to examine them and try to understand the processes taking place in the “tentacles” generated by the gas stripped from those galaxies during the passage within a galaxy cluster. In that space, there’s intergalactic plasma that generates a pressure that caused that gas loss in a process called ram pressure stripping. An image of the jellyfish galaxy cataloged as JO201 captured by the Hubble Space Telescope was published by ESA.

Galaxy clusters are dynamic formations that can merge with other clusters or engulf galaxies that get close enough to them to be drawn into them. We tend to think of the space between galaxies as an absolute vacuum but actually, there’s hot plasma, ionized gas, in that space. When a galaxy enters a galaxy cluster, it interacts with that plasma, which can generate a pressure greater than the galaxy’s gravity. The result is what is known as ram pressure stripping, with gas being stripped from the galaxy.

Interactions between a galaxy’s gas and intergalactic gas can create filamentary structures that resemble tails as well as the tentacles of a jellyfish, hence the nickname for this type of galaxy. Astronomers have long wondered what consequences those interactions could have on star formation in those filaments, and this study collected useful information to understand them.

This study is part of the GASP (GAs Stripping Phenomena in galaxies with MUSE) program which began in 2016 with the use of the MUSE instrument mounted on ESO’s VLT. Subsequently, observations with other instruments were conducted, including the Hubble Space Telescope, the focus of the articles to be published. Those observations include the galaxy JO201, in which the filaments are visible under its main mass.

The observations conducted with the MUSE instrument already suggested that star formation processes could take place in the filaments of jellyfish galaxies. The Hubble Space Telescope provided confirmation with more detailed information. The data collected indicate that the processes underway are somewhere between those typical of normal galaxies and those of starburst galaxies, where there’s considerable star formation. They also indicate that the disks of jellyfish galaxies have higher star formation than the disks of normal galaxies.

The study of star formation in these galactic filaments is complex, so the researchers haven’t yet drawn conclusions on the mechanisms in place. This isn’t just an astronomical curiosity because the environment far from galactic disks directly exposed to the hot and turbulent plasma of the cluster is a condition that is believed to be similar to that of primordial galaxies. For this reason, the studies of jellyfish galaxies will continue with other examinations of the known ones.

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