Discriminating Between the Physical Processes that Drive Spheroid Size
  Evolution
  Massive galaxies at high-z have smaller effective radii than those today, but
similar central densities. Their size growth therefore relates primarily to the
evolving abundance of low-density material. Various models have been proposed
to explain this evolution, which have different implications for galaxy, star,
and BH formation. We compile observations of spheroid properties as a function
of redshift and use them to test proposed models. Evolution in progenitor
gas-richness with redshift gives rise to initial formation of smaller spheroids
at high-z. These systems can then evolve in apparent or physical size via
several channels: (1) equal-density 'dry' mergers, (2) later major or minor
'dry' mergers with less-dense galaxies, (3) adiabatic expansion, (4) evolution
in stellar populations & mass-to-light-ratio gradients, (5) age-dependent bias
in stellar mass estimators, (6) observational fitting/selection effects. If any
one of these is tuned to explain observed size evolution, they make distinct
predictions for evolution in other galaxy properties. Only model (2) is
consistent with observations as a dominant effect. It is the only model which
allows for an increase in M_BH/M_bulge with redshift. Still, the amount of
merging needed is larger than that observed or predicted. We therefore compare
cosmologically motivated simulations, in which all these effects occur, & show
they are consistent with all the observational constraints. Effect (2), which
builds up an extended low-density envelope, does dominate the evolution, but
effects 1,3,4, & 6 each contribute ~20% to the size evolution (a net factor
~2). This naturally also predicts evolution in M_BH-sigma similar to that
observed.
