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 $2.8\times 10^{12}$ 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.^[ref]