Technically Speaking: Velocity Dispersion and Mass-to-Light Ratios

In discussions of galaxy groupings, it is useful to introduce two technical terms: The velocity dispersion measures the spread in velocities for the members of the group of galaxies. As we shall elaborate later, it is important because it indicates the amount of mass in the grouping of galaxies.

The mass-to-light ratio is a quantity measured in units of the mass of the Sun divided by the luminosity of the Sun that indicates how much unseen or dark matter is present. If this ratio is around one, the amount of light being produced by the mass is what we commonly see associated with our Sun. If it is much larger than one, there is a large amount of dark matter present.

Groups of Galaxies

Galaxies are enormous, but they are not the largest structures in the Universe. We find that the galaxies themselves form into groups and clusters of galaxies held together by mutual gravitational attraction, that clusters of galaxies are themselves associated with even larger superclusters of galaxies that are bound gravitationally, and that there may be even larger structures that correlate groups of superclusters.

Groups
The smallest aggregates of galaxies are called groups. Groups are bound gravitationally and typically consist of fewer than 50 galaxies. They have characteristic diameters of about 2 Mpc (6-7 million light years). The total mass contained in a group is of order 10 trillion solar masses, which is several times the mass of the Milky Way, and the velocity dispersion (that is, the range of velocities for individual members of the group, see the adjacent box) is typically about 150 km/s or less. The mass-to-light ratio found in groups of galaxies is about 250 in units of solar masses divided by solar luminosities (see the box). This large mass-to-light ratio indicates that a significant amount of nonluminous or dark matter is present in groups of galaxies.

The Local Group of Galaxies

The Milky Way is a member of a group of galaxies termed the Local Group that contains approximately 20 bright galaxies and 30 galaxies total in a volume of space about a megaparsec across. The largest galaxies in the local group are the spirals Andromeda (M31) and the Milky Way. Two of the closest galaxies to the Milky Way are the Magellanic Clouds that we have already cited as examples of irregular galaxies. They are members of the Local Group at a distance of a little less than 200,000 light years. Although visible only in the Southern Hemisphere, they can be seen easily by the naked-eye and their brightest stars can be seen with binoculars.