Utilizing Shell Galaxies

Stellar shells are low surface brightness features in the form of open, concentric arcs, formed in close-to-radial collisions of galaxies. They occur in at least 10% of early-type galaxies and a small portion of spirals and their unique kinematics carry valuable information about the host galaxies. We discuss a method using measurements of the number and distribution of shells to estimate the mass distribution of the galaxies and the time since the merger. The method is applied on the shells of NGC 4993 - a galaxy hosting the electromagnetic counterpart of the gravitational wave event GW170817, to estimate the probable time since the galactic merger. We used analytical calculations and particle simulations to show that, in special cases, when kinematic data are available, further constraints on mass distribution and merger time can be derived. Applying the methods to the rapidly growing sample of known shell galaxies will constrain the dark-matter content in the galaxies and reveal detailed information on the recent merger history of the Universe.