= Galaxy group
A <galaxy group> is a gravitationally associated collection of <galaxies>, usually less massive and less richly populated than a <galaxy cluster>. Its physical system can include a shared <dark matter halo> and diffuse hot gas as well as its visible galaxies. A collection close together on the sky need not be a bound group: distance and velocity information are essential.
Spectroscopy measures line-of-sight velocities, imaging estimates projected separations, and diffuse <X-ray> emission can trace hot gas in the common gravitational potential. Under equilibrium assumptions, the scale $M\sim R\sigma^2/G$ relates a characteristic group size $R$ and velocity dispersion $\sigma$ to total mass. For the illustrative values $R=0.3$ Mpc and $\sigma=200$ km/s, this scale is about $2.8\times10^{12}$ solar masses, before geometry-dependent factors. It is an inference model, not a direct mass measurement.
The ROSAT group catalogue combines optical group selection with extended X-ray measurements, illustrating why complementary observations distinguish a physical group from a projected association. Dynamically disturbed groups need not satisfy simple equilibrium mass estimates.<RASSCALS Galaxy Groups Catalog by NASA HEASARC>[Catalogue description and X-ray luminosities of optically selected groups]{ref}
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