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.
Authors 14
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Affiliation as printed
Sub-Dep. of Astrophysics , Dep. of Physics , University of Oxford , Denys Wilkinson Building , Keble Road , Oxford OX1 3RH , United Kingdom
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The University of Western Australia · International Centre for Radio Astronomy Research
Affiliation as printed
Cosmic Dawn Center (DAWN) , Denmark
International Centre for Radio Astronomy Research (ICRAR) , M468 , University of Western Australia , 35 Stirling Hwy , Crawley , WA 6009 , Australia
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Affiliation as printed
Sub-Dep. of Astrophysics , Dep. of Physics , University of Oxford , Denys Wilkinson Building , Keble Road , Oxford OX1 3RH , United Kingdom
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The University of Western Australia · International Centre for Radio Astronomy Research
Affiliation as printed
International Centre for Radio Astronomy Research (ICRAR) , M468 , University of Western Australia , 35 Stirling Hwy , Crawley , WA 6009 , Australia
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Stockholm University · AlbaNova
Affiliation as printed
The Oskar Klein Centre , Department of Physics , Stockholm University , AlbaNova University Center , 106 91 Stockholm , Sweden
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Leiden University · Durham University · Leiden Observatory
Affiliation as printed
Institute for Computational Cosmology , Department of Physics , University of Durham , South Road , Durham , DH1 3LE , UK
Leiden Observatory , Leiden University , PO Box 9513 , 2300 RA Leiden , the Netherlands
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Leiden University · Leiden Observatory
Affiliation as printed
Leiden Observatory , Leiden University , PO Box 9513 , 2300 RA Leiden , the Netherlands
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Affiliation as printed
Department of Astrophysics , University of Vienna , Türkenschanzstrasse 17 , A-1180 Vienna , Austria
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Affiliation as printed
Centre for Data Science , Artificial Intelligence and Modelling , University of Hull , Cottingham Road , Hull , HU6 7RX , UK
E. A. Milne Centre for Astrophysics , University of Hull , Cottingham Road , Hull , HU6 7RX , UK
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Affiliation as printed
Department of Astronomy , University of Geneva , Chemin Pegasi 51 , 1290 Versoix , Switzerland
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Leiden University · Leiden Observatory
Affiliation as printed
Leiden Observatory , Leiden University , PO Box 9513 , 2300 RA Leiden , the Netherlands
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Affiliation as printed
Centre for Astrophysics Research , Department of Physics, Astronomy and Mathematics , University of Hertfordshire , College Lane , Hatfield , AL10 9AB , UK