of Galaxies

The smallest aggregates of galaxies are called groups. These typically consist of fewer than 50 galaxies, and have diameters ~ 2 Mpc. The total masses contained in groups is typically ~ 1013 solar masses, and the velocity dispersion (that is, the spread in velocities for individual members of the group, which is a measure of the mass of the group) is typically about 150 km/s. The mass to light ratio found in groups of galaxies is typically ~ 260 in units of solar masses divided by solar luminosities, indicating the presence of large amounts of non-luminous matter or dark matter.

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