The Origin of the Mass-Radius Relation of Early-Type Galaxies
  Early-type galaxies obey a narrow relation traced by their stellar content
between the mass and size (Mass- Radius relation). The wealth of recently
acquired observational data essentially confirms the classical relations found
by Burstein, Bender, Faber, and Nolthenius, i.e. log(R_1/2) \propto
log(Ms)\simeq 0.54 for high mass galaxies and log(R_1/2) \propto log(Ms) \simeq
0.3 for dwarf systems (shallower slope), where R_1/2 and Ms are the half-light
radius and total mass in stars, respectively. Why do galaxies follow these
characteristic trends? What can they tell us about the process of galaxy
formation? We investigate the mechanisms which concur to shape the Mass-Radius
relation, in order to cast light on the physical origin of its slope, its
tightness, and its zero point. We perform a theoretical analysis, and couple it
with the results of numerical hydrodynamical (NB-TSPH) simulations of galaxy
formation, and with a simulation of the Mass-Radius plane itself. We propose a
novel interpretation of the Mass-Radius relation, which we claim to be the
result of two complementary mechanisms: on one hand, the result of local
physical processes, which fixes the ratio between masses and radii of
individual objects; on the other hand, the action of cosmological global,
statistical principles, which shape the distribution of objects in the plane.
We reproduce the Mass-Radius relation with a simple numerical technique based
on this view.
