Galactic Morpholography: Linking present-day galaxy compactness to assembly history and relic formation in the COLIBRE simulations

Abstract

Compactness is widely used to identify candidate relic galaxies, the largely untouched survivors of the first compact massive galaxies. Yet relicness is an evolutionary rather than structural property. Using the COLIBRE cosmological hydrodynamical simulations, we test whether present-day structure traces galaxy formation and assembly histories. Six complementary tracers (stellar age, metallicity, [Mg/Fe], sSFR, velocity dispersion and ex-situ mass fraction) vary coherently across the stellar mass-size plane and independently identify a characteristic compactness scale of $\log_{10}(Σ_{1.5,\mathrm{crit}}/(\mathrm{M}_\odot\mathrm{kpc}^{-1.5}))=9.75\pm0.10$. We term this mapping between present-day structure and assembly history morpholography. Combining this threshold with the Degree of Relicness ($\mathrm{DoR}>0.6$), which quantifies early and rapid star formation, we distinguish compact from ancient galaxies and identify 267 relic candidates at their intersection. Ancient galaxies span nearly the full ex-situ-fraction range, from predominantly in-situ formation to substantial merger-driven growth. Relic candidates are concentrated at stellar masses of $\approx 10^{10}$-$10^{11}$ solar masses, host black holes that lie $\approx 0.25$-$2.5$ dex above the median black-hole-stellar-mass relation, and are predominantly satellites (76%), preferentially in cluster environments. Only $\approx 3$% show evidence of strong tidal stripping, suggesting that environment primarily preserves rather than creates relic-like systems. Approximately one third (87/267) are direct descendants of the $z=2$ massive quiescent galaxies identified in COLIBRE. Present-day structure thus retains a statistical fossil record of assembly history, supporting morpholography as a link between nearby galaxies and the early Universe.