The evolution of the sizes and angular momentum content of galaxies in the COLIBRE simulations

We analyse the sizes and specific angular momentum content of galaxies in the Colibre cosmological hydrodynamical simulations spanning two orders of magnitude in mass resolution. We compare the predicted size-mass and angular momentum-mass relations to a broad range of observational measurements spanning redshifts $z=0$ to $4$. At $z=0$, Colibre reproduces observed size-mass relations over the sampled mass range $10^8 \lesssim M_\star/{\rm M_\odot}\lesssim 10^{11.5}$, and for multiple size definitions, including two- and three-dimensional stellar half-mass radii, half-light radii across several wavelengths, as well as alternative measures such as baryonic half-mass radii and characteristic radii defined by stellar surface density thresholds. The simulations also recover the observed segregation of galaxies in the size-mass plane by morphological type and star formation rate, and reproduce the distinct, approximately parallel sequences followed by star-forming discs and quenched spheroids in the stellar specific angular momentum-mass plane. The angular momentum content of star-forming Colibre galaxies match that of observed systems out to $z\approx 1.5$. At higher redshifts, massive galaxies ($ 10^{9.5}\lesssim M_\star/{\rm M_\odot}\lesssim 10^{11}$) in the simulations are somewhat smaller than observed, and the separation between star-forming and passive populations in the size-mass plane is reduced relative to observations, while at lower masses the agreement remains good. This apparent discrepancy may reflect the effects of dust attenuation, which is neglected in our analysis and may preferentially obscure the central regions of observed systems. Overall, our findings highlight the close connection between galaxy size, angular momentum, and morphology over cosmic time.

💡 Research Summary

This paper presents a comprehensive analysis of the sizes and angular momentum of galaxies across cosmic time, using the state-of-the-art COLIBRE cosmological hydrodynamical simulations. The study compares the simulated galaxy population against a wide array of observational data from redshift z=0 to z=4, focusing on the fundamental scaling relations between stellar mass, size, and specific angular momentum.

The COLIBRE simulations model galaxy formation within the ΛCDM framework, employing the SWIFT code with the SPHENIX SPH scheme. They feature a sophisticated suite of subgrid physics for radiative cooling, star formation, stellar feedback (winds, radiation, supernovae), chemical enrichment, and two modes of AGN feedback (thermal and hybrid thermal/kinetic). A key technical advancement is the use of four times as many dark matter particles as baryonic particles, ensuring similar mass resolution across species and reducing spurious numerical heating that can artificially inflate galaxy sizes.

The analysis defines galaxy sizes using multiple metrics: three-dimensional stellar half-mass radius, half-light radii in various bands, the characteristic radius R1 (defined by a stellar surface density of 1 M⊙ pc⁻²), and the baryonic half-mass radius. Galaxies are classified as star-forming or passive based on their specific star formation rate.

The primary findings are as follows:

1. Size-Mass Relation at z=0: COLIBRE reproduces the observed size-mass relation over a broad stellar mass range (10^8 to 10^11.5 M⊙) for all size definitions tested. The simulations also successfully capture the observed segregation in this plane, where star-forming disc galaxies are systematically larger than passive spheroidal galaxies of the same mass.
2. Redshift Evolution of Sizes: The evolution of galaxy sizes from z=0 to z≈1.5 is in good agreement with observations for both star-forming and passive populations. However, at higher redshifts (z > 1.5), massive simulated galaxies (10^9.5 to 10^11 M⊙) are somewhat smaller than observed, and the size separation between the two populations is less pronounced than in the data. The authors suggest this apparent discrepancy may stem from the neglect of dust attenuation in their analysis, which can cause observed galaxies—particularly their obscured central regions—to appear larger.
3. Angular Momentum-Mass Relation: At z=0, the simulations accurately reproduce the observed distinct, nearly parallel sequences in the specific angular momentum (j⋆) vs. mass (M⋆) plane for disc and spheroid galaxies. Star-forming discs retain a significantly higher fraction of their halo’s specific angular momentum (~50-60%) compared to passive spheroids (~10-20%), matching inferences from observations and highlighting the tight link between morphology, angular momentum retention, and feedback processes.
4. Model Robustness: Comparisons between simulations using the standard thermal AGN feedback and a more complex hybrid thermal/kinetic jet model show that the main results regarding sizes and angular momentum are robust to the details of the AGN feedback implementation.

In conclusion, the study demonstrates that the COLIBRE simulations form a realistic galaxy population whose structural and dynamical properties—sizes, angular momentum, and their correlations with mass, morphology, and star formation activity—closely match observations from the present-day universe back to z≈1.5. The work underscores the maturity of modern galaxy formation models and the critical role of feedback in regulating angular momentum to produce galaxies with correct sizes and morphologies. The residual tension at high-z points toward the importance of employing more observationally-equivalent metrics, like dust-inclusive mock imaging, for future comparisons.