Galaxies are the largest compact objects in the Universe – enormous islands of up to hundreds of billions of stars, accompanied by huge clouds of gas and dust and, likely, an even larger mass of the still somewhat mysterious dark matter. The galaxies that we observe in the Universe around us show a wide range of properties – sizes, shapes, populations of stars and internal structures. Upon seeing this variety, the question of “how did all these galaxies ended up looking like they do” must come up on everyone’s mind. It turns out that the current state of galaxies is a result of an evolution that has been going on over most of the age of the Universe. The key to understanding galaxies today is thus in understanding their history and the history of the Universe as whole, the cosmology.
In the most widely accepted cosmological model, the so-called ΛCDM (Λ Cold Dark Matter) model, the growth of large structures in the Universe is dominated by the gravitational effects of the dark matter. This hypothetical substance has been postulated in the first half of the 20th century to explain unexpectedly high velocities of movement of various objects –stars in galaxies, as well as galaxies in galaxy clusters. While there are still open questions, the ΛCDM has demonstrated a tremendous ability to reproduce observable features in the current Universe.
In the ΛCDM model, large galaxies are formed by merging of smaller galaxies, protogalaxies, and diffuse accretion of the surrounding matter. Thus, galaxy interactions play a crucial role in the life of every galaxy. Galaxy mergers ensure the mass growth, enhance or suppress star formation, possibly trigger active galactic nuclei (AGNs), and many more. Knowing the merger history of the observed galaxies is crucial to understanding their role in the evolution of the individual galaxies as well as of the whole universe.
Galaxy mergers are slow processes, happening over millions of years and so the information about the merger history of any individual galaxy must be indirectly derived from its present-day state. Luckily, a significant portion of elliptical and lenticular galaxies show a unique type of fine structure known as stellar shells, which are created in mergers and can be used to date the last significant merger that the galaxy has undergone.
Previously, we developed methods to extract such information from individual galaxies. The aim of this project is to expand the applicability of such methods to vastly larger samples. To this end, we are developing a set of self-contained tools to identify and analyze shell galaxies in images of large sky surveys, so that the estimates of merger times can be automatically obtained. We gradually apply those tools to existing data from current surveys and prepare for their application to the unprecedentedly large data set that will be produced by the Large Survey of Space and Time (LSST) at the Vera C. Rubin Observatory, which is becoming operational as the project is concluding.
![Selected shell galaxies from legacysurvey.org. image credit: Legacy Surveys / D. Lang (Perimeter Institute)]](https://www.fzu.cz/sites/default/files/2025-12/galaxie_LS_kolaz.jpg)
Through this project, we transform shell galaxies from a position of curiosity to that of utility, allowing statistical applications using the merger data on the hundreds of shell galaxies that can be found in the existing data and later on the thousands of galaxies eventually observed by the LSST project – a huge qualitative leap from the handful of galaxies with known merger histories available before the project.
Funded by the European Union. Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or European Research Executive Agency (REA). Neither the European Union nor REA can be held responsible for them.