The size of galaxies in an era of ultra-deep imaging

The traditional measurement of the size of galaxies, the effective radius, is a relic of the epoch when shallow imaging was unable to capture the full extension of astronomical sources. However, current deep imaging surveys have revolutionised our view of these objects, allowing us to regard critically our own conventions. In this sense, it is time to move from a definition of the light concentration of galaxies, the effective radius, to a definition that intuitively captures the concept of the size of galaxies, such as its edge or boundary. In this contribution, we introduce a new definition of the size of a galaxy based on the gas density threshold value for star formation in galaxies. Remarkably, our new size definition not only captures what the human visual system identifies as the edge of a galaxy, but also dramatically decreases the scatter in the stellar mass - size relation by more than a factor of 2. What’s even more unique is that our size parameter unifies galaxies spanning 5 orders of magnitude in stellar mass on a single mass-size relationship. To place our discovery in the context of galaxy formation, we discuss its application in two scientific cases: the nature of ultra-diffuse galaxies and the location of the onset of the stellar halo of a galaxy.