Dynamical masses and non-homology of massive elliptical galaxies grown
  by dry mergers
  We study whether dry merger-driven size growth of massive elliptical galaxies
depends on their initial structural concentration, and analyse the validity of
the homology hypothesis for virial mass determination in massive ellipticals
grown by dry mergers. High-resolution simulations of a few realistic merger
trees, starting with compact progenitors of different structural concentrations
(S\'ersic indices n), show that galaxy growth has little dependence on the
initial S\'ersic index (larger n leads to slightly larger size growth), and
depends more on other particulars of the merger history. We show that the
deposition of accreted matter in the outer parts leads to a systematic and
predictable breaking of the homology between remnants and progenitors, which we
characterize through the evolution, during the course of the merger history, of
virial coefficients K = GM/Re \sigma^2 associated to the most commonly-used
dynamical and stellar mass parameters. The virial coefficient for the luminous
mass, K , is about 50 per cent larger at the z = 2 start of the merger
evolution than in z = 0 remnants. Ignoring virial evolution leads to biased
virial mass estimates. We provide K corresponding to a variety of dynamical and
stellar mass parameters, and provide recipes for the dynamical determination of
galaxy masses. For massive, non-compact ellipticals, the popular expression M =
5 Re \sigma^2 / G underestimates the dynamical mass within the luminous body by
factors of up to 4; it instead provides an approximation to the total stellar
mass with smaller uncertainty than current stellar population models.
