Galaxy mergers in the era of large surveys

As galaxies are formed, to a large extent, by merging of smaller galaxies, knowing the merger history of the observed galaxies is crucial to understand their evolution as well as the evolution 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. We have already developed methods to extract such information from individual galaxies. Now we will expand the applicability of such methods to vastly larger samples. To this end, we will develop 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 will apply those tools on 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 will become operational around the time when the project concludes. This will 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 will be identified 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 today. We will pioneer the use of this data by applying it for the benefit of LSST itself, where it will help optimize the observation time in search of transients by exploring the correlation of their occurrence with the merger histories of the host galaxies.