It is during rare merging events that galaxies undergo dramatic changes in their appearance and in their stellar content. These systems are excellent laboratories to trace the formation of star clusters under extreme physical conditions.
The Milky Way typically forms star clusters with masses that are 10 thousand times the mass of our Sun. This doesn’t compare to the masses of the star clusters forming in colliding galaxies, which can reach millions of times the mass of our Sun.
These dense stellar systems are also very luminous. Even after the collision, when the resulting galactic system begins to fade into a more quiescent phase, these very massive star clusters will shine throughout their host galaxy, as long-lasting witnesses of past merging events.
By studying the six galaxy mergers shown here, the Hubble imaging Probe of Extreme Environments and Clusters (HiPEEC) survey has investigated how star clusters are affected during collisions by the rapid changes that drastically increase the rate at which new stars are formed in these galaxies.
– For me the most interesting result was that the two minor merger systems (i.e., the collision between one massive galaxy and a less massive galaxy) in the study have by far the highest cluster formation efficiencies, says Sabine König, one of two Chalmers astronomers involved in the study.
– So the conditions in these two galaxies support that more of the stellar mass in a galaxy is located in stellar clusters, than what can be observed in the other, major mergers of the study (collisions between two galaxies with equal masses). One reason for the difference between the cluster formation efficiencies in this study could be that for the two minor mergers we have found that the stellar clusters are not located where the molecular gas is found, as one would normally expect it to be.
–That's why I find it particularly important to not only study major galaxy mergers and their importance for how galaxies and the Universe evolve, but indeed the minor mergers as well, which occur much more often than the major mergers, says Sabine König.
Hubble’s capabilities have made it possible to resolve large star-forming “knots” into numerous compact young star clusters. Hubble’s ultraviolet and near-infrared observations of these systems have been used to derive star cluster ages, masses, and extinctions and to analyse the star formation rate within these six merging galaxies. The HiPEEC study reveals that the star cluster populations undergo large and rapid variations in their properties, with the most massive clusters formed towards the end of the merger phase.
Each of the merging systems shown here has been previously published by Hubble.
The Hubble Space Telescope
is a project of international cooperation between NASA, National Aeronautics and Space Administration and ESA, the European Space Agency.
The HiPEEC survey was led by Angela Adamo, Stockholm University. Susanne Aalto and Sabine König, Chalmers Univsersity of Technology, the Department of Space, Earth and Environment were part of the international team of astronomers in this study.