Interacting Galaxies & Mergers

Galaxy interaction and merging is seen throughout the universe, and it is believed to be an important mechanism in galaxy formation and evolution; large elliptical galaxies may be the remnants of past mergers. The effect of interaction and merging can be seen in distorted and asymmetric galaxy morphologies and in the presence of tidal tails. Accompanying these morphological effects is also increased star formation and starburst activity (see: Starburst galaxies), fostered by the shocks driven into the interstellar medium by the galaxy collision.

The prototypical galaxy merger is the Antennae galaxy, a merger of the two galaxies NGC4038 and NGC4039, which was observed several times with Chandra for a total exposure of nearly a week over a two-year period. These Chandra observations revealed a population of 120 X-ray sources, of which 14 are variable ULXs (see: X-ray sources in galaxies). The Antennae are filled with hot interstellar medium (see: Hot Interstellar Medium), which has a variety of temperatures in different parts of the galaxies and is enriched in metals (Ne, Mg, Si, Fe); the abundances in certain regions are much larger than in our own Solar neighborhood, and the relative amounts of these elements are consistent with production in the SNII explosions typical of a young stellar population rich in massive stars. Two galaxy-size loops of hot gas are also seen in the Antennae, embedded in a more tenuous diffuse hot halo. While the geometry of these features is yet to be explained, their presence suggest outflows that may disperse the metals in the inter-galactic space.