The myth of Icarus in groups of galaxies

Just as planets and comets in the solar system orbit the center of mass where the Sun is located, so do galaxies in groups of galaxies, orbiting around the center of mass of their group, where a giant galaxy is often found. A study carried out by an international team calculated the orbital characteristics of a sample of over 5,000 galaxies in about a hundred groups of galaxies.

The study found that when the giant galaxy in the center of the group is very dominant - that is, much brighter than any other galaxy in the group - the orbits of the other galaxies are very radial, similar to the orbits of comets in the solar system. Their orbital pericenter is therefore very close to the dominant galaxy. The orbits of galaxies in groups characterized by less dominant central galaxies are instead more isotropic, neither too circular, nor too radial.

This discovery confirms a theory on the evolution of giant galaxies, according to which they grow by incorporating other galaxies of the group when they get too close to the group center, as do galaxies on radial orbits. Daring to get too close to the giant galaxy, these galaxies have a bad end, like Icarus who dared to approach the Sun too closely.

This study is based on the FOGO spectroscopic survey (FOssil Group Origin, p.i. J.A.L. Aguerri) and it will be published in the journal Astronomy & Astrophysics, and among its authors are two researchers from the INAF-Trieste Astronomical Observatory and the Trieste University.


Group of galaxies with a dominant central galaxy (credit: ESO) and Icarus (from a painting by J.P. Gowy)