Interactions and Evolution (3) ...

The following figure shows a set of infrared observations with the Hubble Space Telescope of galaxies belonging to a class called ultraluminous infrared galaxies. As we noted in the earlier module of this chapter concerning starbursts, luminous infrared galaxies were first detected by the IRAS satellite in the early 1980s. They emit very strongly in the IR because of enormous starbursts that heat the dust surrounding the young stars. Ultraluminous infrared galaxies are the brightest of these kinds of galaxies. They can be 100 times more luminous than a normal galaxy like our own.

The observations illustrated in the above figure suggest, in some cases rather strongly, evidence for three or more galaxies in the process of merging. The study looked at 130 ultraluminous IR galaxies located within 3 billion light years of Earth. Of these, 30 percent showed possible evidence for multiple mergers. Such multiple mergers were presumably very common in the early Universe when matter was more dense. However, the galaxies exhibiting multiple mergers in this example are found at relatively modest redshifts (less than z ~ 0.3). They appear to lie in dense local nests of galaxies where multiple collisions can occur with high probability. We may assume that the enormous infrared luminosity of such galaxies is caused by these strong collisions triggering huge starbursts.