A Novel Approach to Constrain the Mass Ratio of Minor Mergers in
  Elliptical Galaxies: Application to NGC 4889, the Brightest Cluster Galaxy in
  Coma
  Minor mergers are thought to be important for the build-up and structural
evolution of massive elliptical galaxies. In this work, we report the discovery
of a system of four shell features in NGC 4889, one of the brightest members of
the Coma cluster, using optical images taken with the Hubble Space Telescope
and the Sloan Digital Sky Survey. The shells are well aligned with the major
axis of the host and are likely to have been formed by the accretion of a small
satellite galaxy. We have performed a detailed two-dimensional photometric
decomposition of NGC 4889 and of the many overlapping nearby galaxies in its
vicinity. This comprehensive model allows us not only to firmly detect the
low-surface brightness shells, but, crucially, also to accurately measure their
luminosities and colors. The shells are bluer than the underlying stars at the
same radius in the main galaxy. We make use of the colors of the shells and the
color-magnitude relation of the Coma cluster to infer the luminosity (or mass)
of the progenitor galaxy. The shells in NGC 4889 appear to have been produced
by the minor merger of a moderate-luminosity (M_I ~ -18.7 mag) disk (S0 or
spiral) galaxy with a luminosity (mass) ratio of ~ 90:1 with respect to the
primary galaxy. The novel methodology presented in this work can be exploited
to decode the fossil record imprinted in the photometric substructure of other
nearby early-type galaxies.
