THE RELATION BETWEEN COMPACT, QUIESCENT HIGH-REDSHIFT GALAXIES AND MASSIVE NEARBY ELLIPTICAL GALAXIES: EVIDENCE FOR HIERARCHICAL, INSIDE-OUT GROWTH

Abstract

Recent studies have shown that massive quiescent galaxies at high redshift\nare much more compact than present-day galaxies of the same mass. Here we\ncompare the radial stellar density profiles and the number density of a sample\nof massive galaxies at z ~ 2.3 to nearby massive elliptical galaxies. We\nconfirm that the average stellar densities of the z ~ 2.3 galaxies within the\neffective radius, rho(<r_e), are two orders of magnitude higher than those of\nlocal elliptical galaxies of the same stellar mass. However, we also find that\nthe densities measured within a constant physical radius of 1 kpc, rho(<1 kpc),\nare higher by a factor of 2-3 only. This suggests that inside-out growth\nscenarios are plausible, in which the compact high redshift galaxies make up\nthe centers of normal nearby ellipticals. The compact galaxies are common at\nhigh redshift, which enables us to further constrain their evolution by\nrequiring that the number density of their descendants does not exceed\nconstraints imposed by the z=0 galaxy mass function. We infer that size growth\nmust be efficient, with (r_{1+2}/r_1) ~ (M_{1+2}/M_1)^2. A simple model where\ncompact galaxies with masses ~ 10^11 Msun primarily grow through minor mergers\nproduces descendants with the approximate sizes, stellar densities, and number\ndensity of elliptical galaxies with masses 2-3 x10^11 Msun in the local\nUniverse. We note that this model also predicts evolution in the M_BH - sigma\nrelation, such that the progenitors of elliptical galaxies have lower black\nhole masses at fixed velocity dispersion. The main observational uncertainty is\nthe conversion from light to mass; measurements of kinematics are needed to\ncalibrate the masses and stellar densities of the high redshift galaxies.\n