Jellyfish and bunny ear galaxies have cosmic consequences

"This effect gets more and more severe for galaxies that move at faster speeds through the intergalactic medium, and also for environments where the intergalactic medium is denser and more particle-rich. A galaxy that moves within a galactic group (like our own Local Group) experiences both faster speeds and also denser environments than an isolated one in deep intergalactic space, which can begin to strip material out of the fastest-moving galaxies in the densest environments,"

"An isolated galaxy still moves through the abyss of deep space: where a rarified, sparsely populated sea of ions (mostly protons and electrons) still persists. There may only be about one particle per cubic meter populating the intergalactic medium, on average, but considering that galaxies are frequently 100,000 light-years across or even more, they do encounter large numbers of particles, particularly at higher speeds."

Intergalactic space contains a rarified sea of ions, mostly protons and electrons, with an average density near one particle per cubic meter. Galaxies, often tens to hundreds of thousands of light-years across, encounter many of these particles as they move, especially at high speeds. Ram pressure increases with galaxy speed and medium density, making group environments like the Local Group more hostile than deep intergalactic space. Stripping can remove gas and produce long tails of material; dense tails can collapse and form stars. Observable morphologies include jellyfish-like or bunny ear-like tails, with significant implications for galaxy evolution. Firing a bullet into vacuum, air, or fog provides an intuitive analogy for differing ram-pressure effects.

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