An Etymological Dictionary of Astronomy and Astrophysics
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The pressure exerted on a body moving through a → fluid medium. For example, a → meteor traveling through the Earth's atmosphere produces a → shock wave generated by the extremely rapid → compression of air in front of the → meteoroid. It is primarily this ram pressure (rather than → friction) that heats the air which in turn heats the meteoroid as it flows around the meteoroid. The ram pressure increases with → velocity according to the relation P = (1/2)ρv², where ρ is the density of the medium and v the relative velocity between the body and the medium. Similarly, → ram pressure stripping produces → jellyfish galaxies. Same as → dynamic pressure.

→ ram; → pressure.

A process proposed to explain the observed absence of gas-rich galaxies in → galaxy clusters whereby a galaxy loses its gas when it falls into a cluster. There is a tremendous amount of hot (~ 10⁷ K) and tenuous (~ 10^-4 cm^-3) gas (several 10¹³ → solar masses) in the → intracluster medium (ICM). Ram pressure stripping was first proposed by Gunn & Gott (1972) who noted that galaxies falling into clusters feel an ICM wind. If this wind can overcome the → gravitational attraction between the stellar and gas disks, then the gas disk will be blown away. The mapping of the gas content of spiral galaxies in the → Virgo cluster showed that the → neutral hydrogen (H I) disks of cluster spiral galaxies are disturbed and considerably reduced. Their molecular gas, more bound to the galaxy, is less perturbed, but still may be swept out in case of very strong ram pressure. These observational results indicate that the gas removal due to the rapid motion of the galaxy within the intracluster medium is responsible for the H I deficiency and the disturbed gas disks of the cluster spirals (e.g., J. A. Hester, 2006, ApJ 647:910).

→ ram; → pressure; → strip.