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Recent studies have shown that massive quiescent galaxies at high redshiftare much more compact than present-day galaxies of the same mass. Here wecompare the radial stellar density profiles and the number density of a sampleof massive galaxies at z ~ 2.3 to nearby massive elliptical galaxies. Weconfirm that the average stellar densities of the z ~ 2.3 galaxies within theeffective radius, rho(<r_e), are two orders of magnitude higher than those oflocal elliptical galaxies of the same stellar mass. However, we also find thatthe densities measured within a constant physical radius of 1 kpc, rho(<1 kpc),are higher by a factor of 2-3 only. This suggests that inside-out growthscenarios are plausible, in which the compact high redshift galaxies make upthe centers of normal nearby ellipticals. The compact galaxies are common athigh redshift, which enables us to further constrain their evolution byrequiring that the number density of their descendants does not exceedconstraints imposed by the z=0 galaxy mass function. We infer that size growthmust be efficient, with (r_{1+2}/r_1) ~ (M_{1+2}/M_1)^2. A simple model wherecompact galaxies with masses ~ 10^11 Msun primarily grow through minor mergersproduces descendants with the approximate sizes, stellar densities, and numberdensity of elliptical galaxies with masses 2-3 x10^11 Msun in the localUniverse. We note that this model also predicts evolution in the M_BH - sigmarelation, such that the progenitors of elliptical galaxies have lower blackhole masses at fixed velocity dispersion. The main observational uncertainty isthe conversion from light to mass; measurements of kinematics are needed tocalibrate the masses and stellar densities of the high redshift galaxies.

The Relation Between Compact, Quiescent High Redshift Galaxies and Massive Nearby Elliptical Galaxies: Evidence for Hierarchical, Inside-Out Growth