Galaxy interactions and mergers are central to hierarchical structure formation, yet their long-term impacts on galactic structures and satellite populations remain debated. In this talk, I will show how extragalactic archaeology, using resolved stellar populations as fossil records, allows us to reconstruct how past accretion events reshape halos, disks, bulges, and satellite populations. Because stellar halos are typically dominated by a galaxy's single most massive progenitor, they provide a direct path to inferring the mass, metallicity, and timing of past major mergers on an individual system basis. Using resolved-star data from ground and space observatories, we find that galactic disks are remarkably resilient to these interactions; rather than being destroyed, many disks survive major accretions while undergoing kinematic tilts and varied star formation responses. Furthermore, host bulge properties show little correlation with the most massive accreted progenitor, challenging models where bulges are built predominantly from accreted debris. Finally, I will discuss evidence that satellite galaxies are delivered to host environments via group infall alongside the primary merger partner. Star formation histories of both isolated ultra-faint dwarfs and M31 ultra-faint satellites show star formation continuing past reionization, ruling out models of ultra-faint galaxies as simple, pristine reionization fossils. Together, these results point away from merger-driven destruction and toward a framework of structural resilience and complex satellite delivery.

Speaker: Eric Bell (University of Michigan)

The 2026 Fall Colloquium talks are held on Wednesdays at 3:00 PM. This colloquium is hosted by STScI and will be held as an in-person and virtual event.

You may join in person at STScI’s John N. Bahcall Auditorium or virtually on the STScI Research YouTube channel.

Please direct questions or comments to contact above. The 2026 Fall Colloquium Committee members are: Gagandeep Anand (STScI), Nimisha Kumari (STScI), Chris Nagele (JHU), Kevin Schlaufman (JHU), Greg Sloan (STScI), and Ethan Vishniac (JHU).