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The (extended) Local Group of galaxies

Galaxy Groups

Most galaxies form groups of galaxies - with a few to a few tens of galaxies - and clusters of galaxies, which contain from a few tens to several hundreds of galaxies. Within these groups the distances between galaxies are typically from tens to thousands of kiloparsecs. While galaxy groups are often far from spherical, one can say that the diameter of them ranges from one to twenty megaparsecs. The few galaxies that do not belong to groups are between groups and are called field galaxies. Our own galaxy, the Milky Way, belongs to the Local Group of galaxies (often abbreviated as LG).

Galaxy groups stay together as groups, and are defined as groups, due to the gravitational interaction, i.e. dynamics, they impose on each other. Usually a group has two or three massive galaxies that dominate the dynamics of the group and a variety of smaller galaxies which more or less orbit the massive ones or are exchanged between them, or in some cases are flung out of the system altogether when they fly close to a massive galaxy. It is also possible that the massive one devours a dwarf galaxy that comes too close.

The Local Group of Galaxies

The Local Group consists of two giant spiral galaxies; the Milky Way and the Andromeda Galaxy; a few medium sized galaxies; M33, Large Magellanic Cloud, and Small Magellanic Cloud; and approximately forty dwarf galaxies. The total number is not known because some small dwarf galaxies may be so faint that they have not been detected yet, but mainly because a large part of the sky is covered by our own galaxy and there may be a number of galaxies, even large ones, lurking behind the dust and gas clouds of the Milky Way.

The Milky Way - Andromeda Galaxy pair dominates the Local Group dynamics. Most of the smaller Local Group galaxies orbit either one. Some dwarf galaxies have clearly been flung out of the Local Group by one of these giants. There are some field galaxies nearby, which could eventually be pulled into the Local Group by its combined gravitational pull.

The Extended Local Group of Galaxies

Sometimes other nearby groups are included in the Local Group and then we talk about the Extended Local Group of Galaxies. These other groups may have played an important rôle in the Local Group dynamics, or may still do so.

The most prominant such group is Maffei Group, also known as IC342/Maffei Group or sometimes IC342 Group. The Maffei Group lies behind the central regions of the Milky Way and was therefore found only in the late 1960's when Italian astronomer Paolo Maffei discovered two giant galaxies, Maffei 1 and Maffei 2. It took a decade or two before astronomers realised that there is a whole group out there and it is quite nearby.

Another group that is often included in the Extended Local Group of galaxies is Sculptor Group. The Sculptor Group is not hidden behind dust and gas in the Milky Way around us but it is quite sparce and therefore not easily described. An old description includes galaxies from a wide range, from about one megaparsec to six megaparsecs. There seems to be a clear break in the middle, so later the group has been divided into two subgroups, so called B7a and B7b in some catalogues. The nearer one clearly has some importance to the Local Group and one or two galaxies traditionally included in the Sculptor group may, in fact, be members of our own Local Group.

The Extended Local Group Dynamics

Dynamics of galaxies, i.e. gravitational effects caused and experienced by bodies in galaxies, is studied in order to understand how galaxies are formed and how they develop. The objective is to learn to predict their behaviour in the future. Objective is the same for groups of galaxies. We believe groups have formed mainly from fluctuations in the matter distribution created by the big bang. A totally homogeneous matter distribution could not evolve into the highly structured and complex universe as ours. On the other hand it is interesting to study the development of galaxy groups: why groups are what they are, why do they form filamentary structures in the universe, why do they stick together, how do they function.

Studying group dynamics, especially of our own Local Group of galaxies, we can fathom the structure, workings, and history of the group and estimate the age of the universe independent from expansion of the universe. The best method to study interactions between galaxies is the use of computer simulations. All known properties are fed into simulations, along with an estimate of the properties less well known. Simulations use a model of galaxy movements following laws of physics programmed into the model. The most important of these laws is Newton's theory of gravity, which the computer uses to move the galaxies around.

In my own simulations I assumed the universe was formed in the big bang, the matter later known as the Local Group has stayed in a uniform collection of matter ever since, and galaxies formed with time without disturbing matter flows in the group and orbited each other ending up in their current locations. In the simulation, galaxy movements are followed from present time backward almost to the big bang. In order to get a sensible result, poorly known properties of galaxies must be fine-tuned until all galaxies merge within a suitable time (in this backward running time frame).