A unified scenario for the origin of spiral and elliptical galaxy
  structural scaling laws
  Elliptical (E) and spiral (S) galaxies follow tight, but different, scaling
laws that link their stellar masses, radii, and characteristic velocities. Mass
and velocity, for example, scale tightly in spirals with little dependence on
galaxy radius (the "Tully-Fisher relation"; TFR). On the other hand,
ellipticals appear to trace a 2D surface in size-mass-velocity space (the
"Fundamental Plane"; FP). Over the years, a number of studies have attempted to
understand these empirical relations, usually in terms of variations of the
virial theorem for E galaxies and in terms of the scaling relations of dark
matter halos for spirals. We use Lambda cold dark matter (LCDM) cosmological
hydrodynamical simulations to show that the scaling relations of both
ellipticals and spirals arise as the result of (i) a tight galaxy mass-dark
halo mass relation and (ii) the self-similar mass profile of cold dark matter
halos. In this interpretation, E and S galaxies of a given stellar mass inhabit
halos of similar masses, and their different scaling laws result from the
varying amounts of dark matter enclosed within their luminous radii. This
scenario suggests a new galaxy distance indicator applicable to galaxies of all
morphologies and provides simple and intuitive explanations for long-standing
puzzles, such as why the TFR is independent of surface brightness, or what
causes the "tilt" in the FP. Our results provide strong support for the
predictions of LCDM in the strongly non-linear regime, as well as guidance for
further improvements to cosmological simulations of galaxy formation.
