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The dense interiors of massive galaxies are among the most intriguingenvironments in the Universe. In this paper we ask when these dense cores wereformed and determine how galaxies gradually assembled around them. We selectgalaxies that have a stellar mass >3x10^10 Msun inside r=1 kpc out to z=2.5,using the 3D-HST survey and data at low redshift. Remarkably, the numberdensity of galaxies with dense cores appears to have decreased from z=2.5 tothe present. This decrease is probably mostly due to stellar mass loss and theresulting adiabatic expansion, with some contribution from merging. We inferthat dense cores were mostly formed at z>2.5, consistent with their largelyquiescent stellar populations. While the cores appear to form early, thegalaxies in which they reside show strong evolution: their total massesincrease by a factor of 2-3 from z=2.5 to z=0 and their effective radiiincrease by a factor of 5-6. As a result, the contribution of dense cores tothe total mass of the galaxies in which they reside decreases from ~50% atz=2.5 to ~15% at z=0. Because of their early formation, the contribution ofdense cores to the total stellar mass budget of the Universe is a strongfunction of redshift. The stars in cores with M_1kpc>3x10^10 Msun make up ~0.1%of the stellar mass density of the Universe today but 10%-20% at z~2, dependingon their IMF. The formation of these cores required the conversion of ~10^11Msun of gas into stars within ~1 kpc, while preventing significant starformation at larger radii.

Dense cores in galaxies out to z=2.5 in SDSS, UltraVISTA, and the five 3D-HST/CANDELS fields