Compact High-Redshift Galaxies Are the Cores of the Most Massive
  Present-Day Spheroids
  Observations suggest that effective radii of high-z massive spheroids are as
much as a factor ~6 smaller than low-z galaxies of comparable mass. Given the
apparent absence of low-z counterparts, this has often been interpreted as
indicating that the high density, compact red galaxies must be 'puffed up' by
some mechanism. We compare the ensemble of high-z observations with large
samples of well-observed low-z ellipticals. At the same physical radii, the
stellar surface mass densities of low and high-z systems are comparable.
Moreover, the abundance of high surface density material at low redshift is
comparable to or larger than that observed at z>1-2, consistent with the
continuous buildup of spheroids over this time. The entire population of
compact, high-z red galaxies may be the progenitors of the high-density cores
of present-day ellipticals, with no need for a decrease in stellar density from
z=2 to z=0. The primary difference between low and high-z systems is thus the
observed low-density material at large radii in low-z spheroids (rather than
the high-density material in high-z spheroids). Such low-density material may
either (1) assemble at z<2 or (2) be present, but not yet detected, at z>2.
Mock observations of low-z massive systems show that the high-z observations do
not yet probe sufficiently low surface brightness material to detect the low
surface density 'wings' (if present). Thus, if the high-z galaxies resemble the
most massive systems today, their inferred effective radii could be
under-estimated by factors ~2-4. This difference arises because massive systems
at low redshift are not well-fit by single Sersic profiles. We discuss
implications of our results for physical models of galaxy evolution.
