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An astrophysical robbery

This Picture of the Week shows a snippet of the Hydra I cluster, which contains hundreds of galaxies. Each has its own quirks and history — but today, we focus on the story behind the leaky galaxy NGC 3312, which is the largest spiral galaxy known in the cluster.

This spiral galaxy, right at the centre of this image, looks almost smudged across the screen, spilling its contents into the cosmos around it. This is NGC 3312, falling victim to an astrophysical robbery: ram pressure stripping.

This happens when a galaxy moves through a dense fluid, like the hot gas suspended between galaxies in a cluster. This hot gas drags against the colder gas on the outer shell of the galaxy, ‘pulling’ it off of the galaxy and causing it to leak into the cosmos. This cold gas is the raw material out of which stars form, meaning galaxies losing gas this way risk a dwindling stellar population. Affected galaxies — usually those falling into the centre of clusters — tend to eventually form long tendrils of gas trailing behind them, leading to their nickname: jellyfish galaxies.

This is just one of the many astronomical processes that make pictures of the Universe so varied and captivating. What other stories are waiting to be told about the hundreds of blips in this image?