Houjun Mo

Dark matter sets the gravitational environment in which galaxies form and evolve, but cannot be observed directly. We present a conditional diffusion model that reconstructs the projected dark matter density field from the galaxy stellar-mass density field for direct application to galaxy surveys. The model is trained on CAMELS and validated on the independent IllustrisTNG300-1 simulation. Halo masses inferred from the reconstructed projected-aperture measurements agree well with the corresponding true values, with a scatter below 0.2 dex. On 100 kpc scales, reconstructed surface densities show a typical scatter of ∼0.3 dex in the regime most relevant for observations. We apply the model to Sloan Digital Sky Survey (SDSS) galaxies with M _⋆ ≥ 10 ⁹ M _⊙ in a contiguous low-redshift region. Averaging over 100 stochastic realizations, we reconstruct and publicly release a projected dark matter field covering 90 × 90 ( h − 1 Mpc ) 2 with a pixel size of 0.097 h ⁻¹ Mpc. This pixel area corresponds to the characteristic projected area of halos with masses of ∼10 ^10.6 h ⁻¹ M _⊙ . The map reveals the multiscale projected cosmic web, including cluster-scale overdensities, filaments, and voids. Projected-aperture masses are statistically consistent with SDSS group-catalog masses, while the derived halo mass function broadly matches mock-catalog expectations. The reconstructed projected potential places Coma in one of the deepest wells and near a convergence region of the inferred projected acceleration field, suggesting that the reconstruction retains both local overdensities and coherent large-scale projected gravitational structure. This work shows that diffusion-based dark matter reconstruction can be applied to real galaxy surveys, enabling halo-mass- and spatially resolved dark-matter-environment-based studies of galaxy evolution in SDSS and future wide-area surveys.

One-halo galactic conformity is the tendency for satellites in halos with quenched centrals to have lower star formation activity than those in halos with star-forming centrals at fixed halo mass. It is an important probe of the galaxy–halo connection and halo-wide quenching processes that may couple central and satellite evolution. However, its existence remains controversial, because conformity must be measured at fixed halo mass, while halo masses are difficult to estimate accurately. In this Letter, we measure one-halo conformity in the Sloan Digital Sky Survey (SDSS) using five stellar-mass-complete samples and three halo-mass estimates: a machine learning (ML) estimate whose star-forming and quenched stellar mass–halo mass relations agree with independent weak-lensing constraints, and two conventional abundance-matching (AM) estimates. We quantify conformity as the difference in median log ( sSFR ) between satellites of star-forming and quenched centrals, using both satellite-level and halo-level statistics. The two AM estimates produce strong positive conformity signals, consistent with previous AM-based measurements, but these signals are not reproduced with the ML halo masses. For the halo-level statistic, the representative AM-based signals are +0.38 ± 0.04 dex and +0.23 ± 0.04 dex for the luminosity-ranking and mass-ranking AM halo masses, detected relative to no conformity at about 10 σ and 6 σ , respectively. In contrast, the ML result is consistent with no conformity, +0.00 ± 0.03 dex; the satellite-level statistic gives a similar result. Thus, with halo-mass estimates consistent with weak-lensing constraints, we find no detectable one-halo conformity signal in the present SDSS sample, suggesting that the strong AM-based signal is largely driven by halo-mass estimation biases.

In a recent paper, Chen et al. developed a framework for modeling the seeding and growth of supermassive black holes (BHs) in the context of ΛCDM cosmogony. Here, we use a set of physically motivated criteria to select a population of predicted BHs and link them to little red dots (LRDs) discovered by JWST. We show that the LRD population at high redshift ( z ) emerges naturally from a subset of BHs with super-Eddington accretion during nuclear bursts. The model suggests that the observed LRDs are the “tip of the iceberg” of a much larger population of less luminous BHs in the same subset. The model makes specific predictions for the LRD population, such as the mass distributions of their BHs and host galaxies/halos, and the piece-wise redshift evolution of their number density. The cosmological context of the model also allows us to link the observed LRD population to their progenitors (their BH seeds) and lower- z descendant BHs, galaxies, and halos. Most LRDs at z ∼ 5 are seeded at z ≳ 20 through direct-collapse BHs or pair-instability supernovae from Pop-III stars and have grown to M _BH ≈ 10 ⁵ –10 ⁷ M _⊙ through nuclear bursts by their observed redshift. LRDs are predicted to have diverse descendants, ranging from compact dwarf galaxies to brightest cluster galaxies (BCGs) at z = 0. These predictions are consistent with current observations and can be further tested. The success of this model indicates that the results presented here provide a robust foundation for building detailed models of the LRD population.

We investigate how galactic disk structures connect to the detailed properties of their host dark-matter halos using the TNG50 simulation. From the hydrodynamic and matched dark-matter-only runs, we measure a comprehensive list of halo properties describing density structure, angular momentum, shape, assembly history, and environment. Using the morphological decomposition developed in Paper I, we quantify the sizes, scale heights, and mass fractions of the disk components for galaxies at 0 ≤ z ≤ 4. A random forest regression shows that halo properties alone predict disk size and thickness with high accuracy, while a symbolic regression (SR) provides compact empirical relations with slightly lower accuracy. Disk height is consistently easier to predict than disk size, and lower-mass halos yield higher accuracy than massive halos. Predictions based on halo properties measured in the hydro simulations outperform those based on halos matched in the dark-matter-only simulation, reflecting the imprint of baryonic restructuring on the inner halo. The SHAP analysis reveals the most informative halo parameters include concentration, Einasto shape, global and inner spin, and recent mass accretion, although their importance varies across disk properties. We show correlations between disk size and the density–profile shape arise primarily from a disk-induced modification of the inner halo, rather than a primordial connection. Finally, we point out that disks become more extended with respect to their host halos at higher redshift in low-mass halos, while massive high-redshift halos show the opposite trend. We provide SR-based prescriptions that accurately map halo properties to disk structures, offering practical tools for galaxy–halo modeling.

We investigate the rest-frame optical size–stellar mass relation of galaxies at 0.5 < z < 6.0 using deep JWST/NIRCam and MIRI imaging from the PRIMER survey. We find that star-forming galaxies (SFGs) exhibit a broken power-law relation at all redshifts, with a nearly constant pivot mass ( M _p ) of ∼10 ¹⁰ M _⊙ and a slope flattening above M _p . This highlights the prevalence of a population of compact, massive SFGs that was underrepresented in previous studies. The size distribution of quiescent galaxies (QGs) is well described by a mixture power-law model, with a pivot mass that increases from M _p ∼ 10 ^10.0 M _⊙ at z = 0.75 to M _p ∼ 10 ^10.5 M _⊙ at z = 2.6, suggesting that the minimum halo mass required to quench high-mass galaxies increases with redshift. The bending in the size–mass relation of SFGs supports two distinct size growth modes. At M _⋆ < M _p , size growth is closely coupled to halo growth, while at M _⋆ > M _p , an increasing fraction of SFGs decouple from halo growth and become compact, likely associated with rapid bulge (and black hole) growth in M _h ≳ 10 ¹² M _⊙ halos. These compact SFGs are promising progenitors of massive QGs, as evidenced by their similar masses, surface brightness profiles, and morphologies. Their high number densities can account for the observed buildup of massive QGs at z > 2, suggesting that the compaction pathway, rather than major mergers of extended SFGs, dominates the formation of high- z massive QGs.

We employ the high-redshift suite of FIRE-2 cosmological hydrodynamic zoom-in simulations to investigate the evolution of gas-phase metallicity radial gradients in galaxies in the epoch of reionization (EOR). Our sample consists of 22 galaxies spanning the redshift range z ∼ 10–5. We find that galaxies at z ∼ 10 exhibit a median metallicity gradient of −0.15 dex kpc ⁻¹ with substantial scatter, which gradually flattens to −0.1 dex kpc ⁻¹ at z ∼ 6, accompanied by a reduction in scatter. In the EOR, metallicity gradients correlate positively with stellar mass: more massive galaxies display flatter gradients with smaller scatter, broadly consistent with recent JWST observations. At fixed stellar mass, galaxies with a higher star formation rate (SFR) exhibit steeper negative gradients, while the specific SFR shows a strong anticorrelation with gradient slope. Because EOR galaxies in FIRE-2 generally lack significant rotational support, we adopt the ratio of the peak-to-peak velocity shear to twice the velocity dispersion (Δ v /2 σ ) as a proxy for the strength of gas flows. We find a strong positive correlation between metallicity gradients and Δ v /2 σ : galaxies with a lower Δ v /2 σ (i.e., weaker gas flows) tend to exhibit steeper negative gradients. Furthermore, galaxies with a steeper gradient display higher central SFR surface densities, suggesting localized star formation with inefficient interstellar medium mixing that drives inside-out chemical enrichment in galaxy evolution in the early Universe.

We use FIRE-2 cosmological zoom-in hydrodynamic simulations to investigate the co-evolution between Milky Way-size galaxies and their host dark matter haloes. We find that the formation of these galaxies follows a two-phase pattern, with an early phase featured by hot dynamics, bulge-dominated structure and bursty star formation, and a later phase featured by cold dynamics, disc-dominated structure and steady star formation. The transition times of these galaxy properties are correlated with the time when the host halo transits from fast to slow accretion, indicating the two-phase assembly of haloes as a potential mechanism that drives the two-phase formation of galaxies. The physical origin of dynamical hotness can be summarized into two modes of star formation: a scattered mode in which stars form at large radii within cold gas streams associated with fast assembly of haloes, and a concentrated mode in which stars form at small radii through violent fragmentation from globally self-gravitated gas when halo assembly is about to slow down. Cold gaseous and stellar discs can form when the conditions of the two modes are removed by the stall of fast halo assembly and the reduction of gas by feedback processes. The two modes of star formation leave distinct imprints on the structural properties of high-redshift galaxies, providing implications to be tested by James Webb Space Telescope and future observations.

Globular clusters (GCs) are some of the oldest bound structures in the Universe, holding clues to the earliest epochs of star formation and galaxy assembly. However, accurate age measurements of ancient clusters are challenging due to the age–metallicity degeneracy. Here, we report the discovery of 36 compact stellar systems within the “Relic,” a massive, quiescent galaxy at z = 2.53. The Relic resides in an overdensity behind the Abell 2744 cluster, with a prominent tidal tail extending towards two low-mass companions. Using deep data from the UNCOVER/MegaScience JWST Surveys, we find that clusters formed in age intervals ranging from 8 Myr up to ∼2 Gyr, suggesting a rich formation history starting at z ∼ 10. While the cluster-based star formation history is broadly consistent with the high past star formation rates derived from the diffuse host galaxy light, one potential discrepancy is a tentative ∼2–3× higher rate in the cluster population for the past Gyr. Taken together with the spatial distribution and low inferred metallicities of these young-to-intermediate age clusters, we may be seeing direct evidence for the accretion of star clusters in addition to their early in situ formation. The cluster masses are high, ∼10 ⁶ –10 ⁷ M _⊙ , which may explain why we are able to detect them around this likely post-merger galaxy. Overall, the Relic clusters are consistent with being precursors of the most-massive present-day GCs. This unique laboratory enables the first connection between long-lived, high-redshift clusters and local stellar populations, offering insights into the early stages of GC evolution and the broader processes of galaxy assembly.