Alexander J. Richings

Recent observations with James Webb Space Telescope have revealed a population of UV-bright galaxies at $z\gtrsim 10$. This discovery naturally raises the question: What do such early galaxies evolve into by the present day? In this work, we address this descendant question using the new-generation COLIBRE (COLd ISM and Better REsolution) cosmological hydrodynamical simulations to trace bright galaxies selected at $z=10$ and follow their descendants to the present day. Most of the high-redshift galaxies do not survive as distinct, self-bound objects to $z=0$; instead, the majority are incorporated into more massive systems through merging or disruption. The surviving descendants span a broad range of present-day stellar masses, although they are most commonly intermediate- to high-mass, $M_\star \sim 10^{10}$–$10^{11}\, {\rm M_\odot }$. They typically reside in galaxy groups and clusters, with host halo masses, $M_{200{\rm c}}\sim 10^{13}$–$10^{14}\, {\rm M_\odot }$. The large scatter in descendant stellar mass shows that present-day outcomes retain only a weak memory of the stellar mass of the high-redshift progenitor. We show that the evolution of descendant host halo masses is consistent with the forward conditional distribution predicted by extended Press–Schechter (EPS) theory, both in the median growth and in the large scatter in descendant mass. In particular, EPS confirms that massive present-day galaxies typically do not originate from the most massive objects at high redshift. A galaxy observed at $z\gtrsim 10$ therefore cannot be interpreted as the direct progenitor of a single class of $z=0$ galaxies.

James Webb Space Telescope (JWST) has enabled the detection of galaxies in the earliest stages of cosmic history. We compare the ultraviolet luminosity functions (UVLFs) at redshifts $z=7\mathrm{-}20$ predicted by a set of new cosmological hydrodynamical simulations, colibre (COLd ISM and Better REsolution), with observations, including those from JWST. The UV luminosities of colibre galaxies are derived using the radiative transfer code skirt, which tracks stellar emission and its processing through the multiphase interstellar medium and dust distribution predicted by colibre. We find that although colibre is consistent with the observed evolution of the stellar mass function up to $z=12$, its dust-attenuated UVLFs fall systematically below the observations at the bright end: at the number density of $10^{-6}\, \mathrm{Mpc^{-3}\, mag^{-1}}$, the brightest galaxies are underluminous by $\approx 1\, \rm mag$ at $z=7$, increasing to $\approx 2.5\, \rm mag$ at $z=15$. Accounting for observational uncertainties brings the colibre UVLFs closer to the observational data, but does not fully resolve the discrepancy. Ignoring dust attenuation allows colibre to produce sufficiently bright galaxies at $7\lesssim z \lesssim 12$, but at $z=15$, colibre still underpredicts the luminosities of the brightest galaxies, indicating the need for additional physical mechanisms to boost the UV luminosities at the earliest cosmic epochs, such as a ‘top-heavy’ stellar initial mass function. We fit the colibre UVLFs with Schechter functions and calculate the evolution of the best-fitting parameters. We find that the galaxy number density decreases, the characteristic luminosity becomes fainter and the faint-end slope becomes steeper towards higher redshifts. The UV luminosity density decreases by a factor of several hundred from $z = 7$ to $15$.

Early ($z \ge 2$) Massive ($M_{\star } \gt 10^{10}\, \mathrm{{\rm M}_{\odot }}$), Quenched Galaxies (MQGs) challenge current galaxy formation models. In this series, we study these systems using the new colibre cosmological hydrodynamical simulations. Following the broad agreement between their predictions and observations found in the first paper, this second paper explores the processes driving galaxies to become massive and quenched in colibre, identifying Active Galactic Nucleus (AGN) feedback as the primary quenching mechanism in both the thermal (L200m6 simulation) and hybrid (thermal + jet, L200m7h simulation) AGN feedback models implemented. However, the two models behave differently: while the thermal model efficiently quenches massive galaxies at $z\gt 3$, the hybrid model is less effective because black holes (BHs) grow more slowly in the early Universe, and the jet component, which dominates the feedback energy, acts on longer time-scales to impact galaxies. Both models predict quasar-like MQGs (AGN with $L_{\rm bol}\gtrsim 10^{45}\, \mathrm{erg\, s^{-1}}$), with the most luminous systems associated with more recently quenched galaxies. Compared to star-forming galaxies of similar mass, MQGs host more massive BHs and exhibit higher star formation efficiencies. These differences arise primarily from their environments before quenching, particularly at local ($\rm 0.3\, cMpc$) to intermediate scales ($\rm 1.0\, cMpc$), where overdense regions are associated with enhanced gas inflows, higher BH accretion and, hence, feedback power. We find that about 54 per cent (20 per cent) of the $z=3$ MQGs survive as the main progenitors of $z=0$ galaxies, although up to 56 per cent (60 per cent) experience rejuvenation episodes at a given redshift $z\lt 3$ in L200m6 (L200m7h). Our results highlight the central role of BH growth, AGN feedback and environment in driving rapid quenching in the early Universe.

We present predictions from the colibre cosmological hydrodynamical simulations of galaxy formation for the present-day galaxy luminosity functions (LFs) at wavelengths ranging from the far-ultraviolet (FUV) to the submillimetre. The simulations are post-processed with the radiative transfer code skirt, accounting for dust attenuation and emission using the distribution and properties of dust grains predicted directly by colibre. Results from simulations varying in mass resolution by a factor of $\sim 10^2$ ($\sim 10^5\text {-} 10^7\, \mathrm{{\rm M}_{\odot }}$) show very good convergence over most luminosity ranges. The colibre-skirt LFs match the data remarkably well from the FUV to $3.4\, \rm{\mu m}$ and also in the far-infrared and submillimetre wavelength range ($70\text {-}850\, \rm{\mu m}$). In the mid-infrared (MIR; $8\text {-}24\, \rm{\mu m}$), colibre-skirt matches the data well at low luminosities but significantly underpredicts the luminosities of MIR-bright galaxies, with the discrepancy increasing towards longer wavelengths. The total infrared LF, obtained by integrating the spectral energy distributions over $8\text {-}1000\, \rm{\mu m}$, also matches observations well at the faint end but underpredicts the number of very bright galaxies. The unprecedented agreement at all other wavelengths indicates that colibre, coupled with this calibration-free skirt post-processing framework, successfully predicts the properties of stellar populations at the present day and the amount and distribution of interstellar dust.

The diversity of galaxy morphologies and their relations with galaxy and halo properties is fundamental to understanding galaxy formation. Cosmological simulations of representative volumes can help disentangle the origin of observed correlations, but most suffer from two main limitations that affect morphologies: an overpressurized interstellar medium and spurious interactions between stellar and dark matter particles. We present an overview of galaxy morphologies in the COLIBRE simulations, which address these limitations and reproduce many observed galaxy scaling relations. To quantify galaxy morphology, we use four (strongly correlated) theory-space metrics, three kinematic and one spatial. We explore how different choices and limitations affect these indicators, including luminosity- versus mass-weighting, aperture size and shot noise. Overall, we find good convergence in present-day morphologies across two orders of magnitude in mass resolution. COLIBRE predicts that kinematic morphology correlates strongly with stellar mass and colour, and that galaxies with stellar masses of $\approx (1-2)\times 10^{10}\, \, \mathrm{M}_{\odot }$ tend to be the most rotationally dominated. At fixed stellar mass, the morphology of central galaxies correlates weakly with the properties of their host halo. Morphology correlates more strongly with internal galaxy properties, with more discy galaxies being more gas-rich, having higher star formation rates and exhibiting younger and more extended stellar populations. Other properties, like the mass of the most massive black hole, the fraction of stars that are accreted and stellar metallicity, also correlate with morphology, but with correlation strengths sensitive to the stellar mass of the galaxy and whether it is a central or satellite.

The James Webb Space Telescope has uncovered a substantial population of Massive ($M_{\star } \gt 10^{10}\, \mathrm{M_{\odot }}$), Quenched Galaxies (MQGs) in the early Universe ($z \ge 2$), whose properties challenge current galaxy formation models. In this series, we examine this population of MQGs within the new colibre cosmological hydrodynamical simulations, which introduce key innovations in their sub-grid physics. In this first paper, we find a dependence of MQG number densities on both mass resolution and the Active Galactic Nucleus feedback implementation, as well as a significant impact from potential observational uncertainties. Using the fiducial $(200\, \rm cMpc)^3$ volume L200m6 simulation, which provides adequate volume, mass, and spatial resolution to study these systems, we report number densities and stellar mass functions in broad agreement with the latest observations. The predicted quenching and formation time-scales are qualitatively consistent with observational inferences, indicating extended formation (medians $t_{50}\approx 0.5{-}1.5\, \mathrm{Gyr}$) followed by rapid quenching (medians $t_{\mathrm{q}}\lesssim 0.6\, \mathrm{Gyr}$) with strong starburst episodes. Leveraging the state-of-the-art physics in colibre, the model predicts that MQGs have dust and $\rm H_{2}$ fractions more than 1 dex lower than their massive star-forming counterparts; generally consistent with the (scarce) observational estimates. MQGs and massive star-forming systems show broadly similar stellar sizes and kinematics, suggesting that size or morphological transformations occur after quenching in colibre. Our results provide robust predictions for MQGs and show that tensions with observations are reduced when an effective observational uncertainty is forward-modelled.

We investigate the co-evolution of the stellar and dark matter mass of satellite galaxies using the colibre cosmological hydrodynamical simulations with subhaloes resolved by the history-based hbt-herons subhalo finder. We identify a universal tidal track connecting stellar mass-loss to subhalo mass-loss characterized by two distinct phases, which can be well described by the two-parameter model. The initial phase consists primarily of dark matter stripping, whereas stellar stripping becomes significant only after the subhalo total bound mass fraction drops below a critical value ($\sim 0.057$). We find a bimodal mass-loss rate distribution of subhaloes. In satellites with modest mass-loss rates, the stellar mass is largely frozen. By contrast, the galaxy quickly becomes unresolved, along with the dark matter component for the extreme mass-loss population, naturally explaining the lack of ‘orphan’ galaxies in previous hydrodynamical simulations. Our model also predicts the formation condition for dark-matter-deficient galaxies (DMDGs), whose abundance peaks at $m_{*}\sim 10^{9.5}\, \rm {M}_{\odot }$. The abundance of DMDGs can be very sensitive to numerical effects, with colibre resolving a much larger DMDG population than previous hydrodynamical simulations. We also estimate the influence of artificial disruption on the satellite stellar mass function, which can amount to 20 (50) per cent at $m_* \sim 10^{9} (10^{8}) \, \rm M_\odot$, given a baryonic mass resolution of $\sim 10^{6}\, \rm {M}_{\odot }$.

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 matches 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.

We introduce a suite of Lambda cold dark matter ($\Lambda$CDM) cosmological, hydrodynamical simulations that track the evolution of a large population of dwarf galaxies. The suite comprises zoom-in simulations of 25 spherical, underdense regions of $r=5\, \rm {cMpc}$, selected to span ${\approx} 1.5$ dex in mean enclosed density, covering voids to filamentary structures, whilst excluding haloes of Milky Way-mass or larger. The simulations achieve a mass resolution of ${\sim} 10^5\rm {M_\odot }$ with a galaxy formation model including cold, dense interstellar gas and whose subgrid stellar feedback efficiency reproduces the $z=0$ galaxy stellar mass function. We investigate the impact of the cosmic environment on dwarf galaxy formation and evolution. We find that the $5\, \rm {cMpc}$ environment influences the normalization of the halo and galaxy mass functions, but does not significantly affect the stellar mass–halo mass (SMHM) relation and halo occupation fraction for galaxies with $M_{\star }=10^6{-}10^9 \rm {M_\odot }$. Instead, host halo concentration, estimated from DM-only counterparts, is more important: both the fraction of haloes hosting a resolved galaxy and the scatter about the SMHM relation correlate positively with concentration. Owing to halo assembly bias, concentration also influences galaxy formation times, such that at fixed halo mass more concentrated haloes host galaxies that are both older and more massive. The offset from the mean SMHM relation also anticorrelates with $t_{90}$, the time at which 90 per cent of a galaxy’s stellar mass has assembled. These correlations between halo properties and galaxy star formation histories present testable predictions for forthcoming observational surveys.

We investigate the correlation between star formation rate (SFR) surface density and gas surface density (known as the Kennicutt-Schmidt, KS, relation) at kiloparsec (kpc) scales across cosmic time (0 ≤ z ≤ 8) for galaxies with stellar masses $>10^9\, \rm M_{\odot }$, using the COLIBRE state-of-the-art cosmological hydrodynamical simulations. These simulations feature on-the-fly non-equilibrium chemistry coupled to dust grain evolution and detailed radiative cooling down to ≈10 K, enabling direct predictions for the atomic (H i) and molecular (H2) KS relations. At z ≈ 0, COLIBRE reproduces the observed (spatially-resolved) KS relations for H i and H2, including the associated scatter, which we predict to be significantly correlated with stellar surface density, local specific SFR (sSFR), and gas metallicity. We show that the H i KS relation steepens for lower-mass galaxies, while the H2 KS relation shifts to higher normalization in galaxies with higher sSFRs. The H2 depletion time decreases by a factor of ≈20 from z = 0 to z = 8, primarily due to the decreasing gas-phase metallicity. This results in less H2 and more H i being associated with a given SFR at higher redshift. We also find that galaxies with higher sSFRs have a larger molecular gas content and higher star formation efficiency per unit gas mass on kpc scales. The predicted evolution of the H2 depletion time and its correlation with a galaxy’s sSFR agree remarkably well with observations in a wide redshift range, 0 ≤ z ≤ 5.