The Galactic Systems Family
From the intergalactic medium Family we now move to the Family of systems of galaxies, ranging from binary galaxies to clusters, superclusters, and filaments and voids, the largest gravitationally bound structures in the universe. A binary galaxy, sometimes called a double galaxy, consists of two galaxies either orbiting one another in bound orbits or involved in close encounters. They are distinguished from chance alignments of galaxies in the line-of-sight. Binary galaxies are of interest because, analogous to binary stars, they can in principle be used to determine the masses of the two galaxies involved, as well as the masses of halos of dark matter (G 12). Studies have shown that binary galaxies constitute about 10% of all galaxies not in clusters, and that a large fraction of these have disturbed morphologies and enhanced optical and far infrared emission, indicating they are interacting galaxies (G 20). The famous Whirlpool Galaxy M51 (Fig. 14.7) is an example of an interacting binary. Simulations of interacting binary galaxies have shown similarities to the process of mass transfer in semi-detached binary stars.