Merger of ancient galaxies could explain the origin of today’s giants

The elliptical galaxy NGC 4150. Observation of a merger between two galaxies early in the life of the Universe could explain the origin of large elliptical galaxies.

NASA, ESA, R.M. Crockett (University of Oxford, U.K.), S. Kaviraj (Imperial College London and University of Oxford, U.K.), J. Silk (University of Oxford), M. Mutchler (Space Telescope Science Institute, Baltimore), R. O'Connell (University of Virginia, Charlottesville), and the WFC3 Scientific Oversight Committee

The largest galaxies in the Universe aren't beautiful spirals like our Milky Way; they are enormous egg-shaped structures known as giant elliptical galaxies. We don't know how they formed, but observations of very distant and bright galaxies revealed information about the formation of smaller elliptical galaxies. The giants remained mysterious.

Where one galaxy is insufficient, two may do instead. A new set of observations caught two bright elliptical galaxies right before the act of merging into one that would have a combined mass large enough to make the equivalent of 400 billion Suns. Hai Fu and colleagues determined that these galaxies collided more than 10 billion years ago and that the merger was driving extremely rapid star formation, at least ten times the rate seen in ordinary galaxies. Based on these observations, the researchers concluded that such collisions could be responsible for the birth of the largest galaxies, allowing for most of them to finish forming by 9.5 billion years ago.

Nearby elliptical galaxies contain virtually no young stars and are poor in the raw ingredients of star formation—gas and dust. However, based on observation, those stars must have formed fairly rapidly as a group about 10 billion years ago. Such aggressive star formation would pump a lot of light out, leading to extremely bright galaxies.