Stars form in clusters from large clouds of gas and dust. The Milky Way forms star clusters with masses 10,000 times that of our Sun. A new study shows that other galaxies can easily form much bigger clusters, which can be millions of times the mass of our Sun. There’s a catch though: These galaxies must be merging.
As reported in a paper in the Monthly Notices of the Royal Astronomical Society, observations of six merging pairs of galaxies suggest that star clusters up to 1 million solar masses can form in these collisions. An increase in star formation for colliding galaxies was well established, but the estimate of how it is concentrated in large clusters is an important insight into galaxy evolution.
In particular, the team found that the clusters’ properties change massively and quickly, and the observations show that these are a feature that builds up over time. Galaxy mergers can take billions of years, from the first interaction of two galaxies of roughly the same mass to the relaxed shape of the final merged galaxy. The team noticed that the more advanced mergers have more massive clusters.
Given the sheer density of stars in such conglomerations, clusters are very bright. This is a great advantage for studying them. Thanks to the keen eye of the Hubble Space Telescope, the team was able to confidently estimate the total stellar mass in the clusters as well as their age, and then compare it to the star-formation rate of the entire galaxy. The combination of these three numbers paints an important picture. Galaxy mergers are the most efficient environment to form star clusters in the local universe.
The observations were part of the Hubble imaging Probe of Extreme Environments and Clusters (HiPEEC) survey, with observations taken by Hubble between 2008 and 2020. The team stresses that while the keen eye of the space telescope is invaluable in studying galaxies, some insight into these objects might be lost due to their properties. These clusters form in dense gas clouds which trap visible and UV light, the region of the electromagnetic spectrum Hubble is sensitive to.
The launch of the James Webb Space Telescope, expected in October 2021, will deliver more detailed observations and an even better understanding of these galaxies.
Every galaxy in the universe will undergo at least one major merger event over its lifetime, as it collides with a neighboring object of roughly the same size. For the Milky Way, in a few billion years, this will be Andromeda. This study is a look into the future of our galaxy.