Galaxy group

A galaxy group is a collection of galaxies held together by gravity. These groups are the most common structures in our universe. Scientists estimate that at least 50% of all galaxies live in these groups. Each member of a group is quite large and bright. Specifically, every galaxy in a group is at least as luminous as the Milky Way. The Milky Way is our home galaxy, and it belongs to a collection called the Local Group.

Galaxy groups are much smaller than galaxy clusters. While clusters are massive, groups are considered the smallest aggregates of galaxies. A typical group contains about 50 or fewer members. These groups usually span a diameter of 1 to 2 megaparsecs (Mpc). The total mass of a group is approximately 10 to the 13th power of solar masses. Within these groups, individual galaxies move at a velocity spread of about 150 km/s.

Many groups show special signs of energy in the space between galaxies. In the local universe, about half of all groups exhibit diffuse X-ray emissions. This radiation comes from the intracluster media, which is the gas located between galaxies. These X-ray emissions usually appear within the inner 10% to 50% of a group's virial radius. This area typically covers a distance of 50 to 500 kiloparsecs (kpc). Groups that show these X-rays often contain early-type galaxies as their members.

Some groups are much more crowded than others. These are known as compact groups. A compact group usually contains only about five galaxies. These galaxies stay in close proximity and are relatively isolated from other large formations. Astronomers first discovered a compact group in 1877, known as Stephan's Quintet. This specific group actually consists of four galaxies and one unassociated foreground galaxy. In 1982, an astronomer named Paul Hickson created a famous catalogue of these groups called the Hickson Compact Groups.

Compact groups are very useful for studying the invisible parts of our universe. They provide clear evidence of dark matter. Scientists know dark matter exists because the visible mass in these groups is too low. Without dark matter, there would not be enough gravity to hold the galaxies together. Furthermore, compact groups are not dynamically stable over a Hubble time. This means they change over the age of the universe. This instability shows that galaxies evolve by merging with one another.

When galaxies merge over a very long time, they can create a fossil group. This happens through a physical process called dynamical friction. As galaxies interact, they eventually merge into a single, large central galaxy. This leaves behind an X-ray halo from the original group. Fossil groups are considered old, undisturbed systems. They have not seen many new large galaxies arrive since their initial collapse. One example is the elliptical galaxy NGC 6482. It is located in the constellation Hercules, about 180 million light-years away.

There are also groups that are just beginning to exist. These are called proto-groups. They are the smaller versions of protoclusters. Proto-groups consist of galaxies and protogalaxies that are still forming. These early structures are embedded in dark matter haloes. Over time, these haloes fuse together to create singular, larger dark matter structures. Studying these helps scientists understand how the large-scale structures of the universe are built.