An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

A limbless cold-blooded vertebrate animal with gills and fins living wholly in water (OxfordDictionaries.com).

M.E. fis(c)h, fyssh, O.E. fisc; cognate with Du. vis, Ger. Fisch, O.Norse fiskr, Goth. fisks; akin to L. piscis; PIE root *pisk- "a fish."

Mâhi "fish," from Mid.Pers. mâhik; Av. masya-; cf. Skt. matsya-, Pali maccha-.

Any of various marine coelenterates of a soft, gelatinous structure, especially one with an umbrella like body and long, trailing tentacles; medusa (dictionary.com).

→ jelly; → fish.

Medusâ, from Gk. Medousa, literally "guardian," from medein "to protect, rule over."

A type of galaxy exhibiting "tentacles" (tails) of material that appear to be stripped from the main body of the galaxy, making it resemble a jellyfish. Such type of galaxies occur in → galaxy clusters and are produced by a process called → ram pressure stripping. The mutual → gravitational attraction between galaxies causes them to fall at high speed into the clusters, where they encounter a hot → intracluster medium (ICM) with dense gas. The falling galaxy feels a powerful wind, forcing tails of gas out of the galaxy's disk and triggering → starbursts within it. Jellyfish galaxies have mainly been observed in nearby clusters (e.g., Virgo, Coma, A1367, A3627, Shapley). A few examples have been identified in clusters at → redshifts z ~ 0.2-0.4, and there is accumulating evidence for a correlation between the efficiency of the stripping phenomenon and the presence of shocks and strong gradients in the X-ray → intergalactic medium (Poggianti et al., 2016, AJ 151, 78).

→ jellyfish; → galaxy.

An observed correlation between the luminosity of a spiral galaxy and its rate of rotation (measured from its 21 cm hydrogen line). This means that more luminous galaxies have stars that are moving faster. Knowing the rotational velocity of a spiral galaxy, this relation provides its absolute magnitude and then its distance. → Faber-Jackson relation.

Named after R. B. Tully and J. R. Fisher who first derived this relationship (1977, A&A 54, 661); → relation.