Properties of massive galaxy clusters, such as mass abundance and concentration, are sensitive to cosmology, making cluster statistics a powerful tool for cosmological studies. However, favoring a more simplified, spherically symmetric model for galaxy clusters can lead to biases in the estimates of cluster properties. In this work, we present a deep learning approach for estimating the triaxiality and orientations of massive galaxy clusters (those with masses greater than or similar to 1014 M circle dot h-1) from 2D observables. We utilize the flagship hydrodynamical volume of the suite of cosmological-hydrodynamical MillenniumTNG (MTNG) simulations as our ground truth. Our model combines the feature extracting power of a convolutional neural network and the message passing power of a graph neural network in a multimodal, fusion network. Our model is able to extract 3D geometry information from 2D idealized cluster multiwavelength images (soft X-ray, medium X-ray, hard X-ray, and tSZ effect) and mathematical graph representations of 2D cluster member observables (line-of-sight radial velocities, 2D projected positions and V-band luminosities). Our network improves cluster geometry estimation in MTNG by 30% compared to assuming spherical symmetry. We report an R2 = 0.85 regression score for estimating the major axis length of triaxial clusters and correctly classifying 71% of prolate clusters with elongated orientations along our line of sight.

Recent James Webb Space Telescope observations hint at unexpectedly intense cosmic star formation in the early Universe, often attributed to enhanced star formation efficiencies (SFEs). Here, we analyse the SFE in THESAN-ZOOM, a novel zoom-in radiation-hydrodynamic simulation campaign of high-redshift (z greater than or similar to 3) galaxies employing a state-of-the-art galaxy formation model resolving the multiphase interstellar medium (ISM). The halo-scale SFE (is an element of(halo)*) the fraction of baryons accreted by a halo that are converted to stars follows a double power-law dependence on halo mass, with a mild redshift evolution above M-halo greater than or similar to 10(95) M-circle star. The power-law slope transitions from similar to 2/3 to similar to 1/3 as halo mass increases, which hints at a transition from energy-driven to momentum-driven outflow. is an element of(halo)* is a factor of 2-3 larger than commonly assumed in empirical galaxy formation models at M-halo less than or similar to 10(95) M-circle star. On galactic (pkpc) scales, the Kennicutt-Schmidt relation of neutral gas is universal in THESAN-ZOOM, following Sigma(SFR) alpha Sigma(2)(gas), indicative of a turbulent energy balance in the ISM maintained by stellar feedback. The rise of is an element of(halo)* with halo mass can be traced primarily to increasing gas surface densities in massive galaxies. These results are is an element of(halo)* primarily increasing gas galaxies. robust against variations in numerical resolution and star formation and feedback models, depending mainly on the total feedback momentum budget. Although the increase in is an element of(halo)* with redshift is modest, it is sufficient to explain the large observed number density of UV-bright galaxies at z >= 12. However, reproducing the brightest sources at M-UV less than or similar to-21 may require extrapolating the SFE beyond the halo mass range covered by THESAN-ZOOM.

Mock galaxy catalogues are often constructed from dark-matter-only simulations based on the galaxy-halo connection. Although modern mocks can reproduce galaxy clustering to some extent, the absence of baryons affects the spatial and kinematic distributions of galaxies in ways that remain insufficiently quantified. We compare the positions and velocities of satellite galaxies in the MTNG hydrodynamic simulation - a state-of-the-art cosmological hydrodynamic run - with those in its dark-matter-only counterpart, assessing how baryonic effects influence galaxy clustering and contrasting them with the impact of galaxy selection, i.e. the dependence of clustering on sample definition. We introduce a new method to track subhaloes using the merger trees of the simulations, which enables us to match systems even when their positions at z = 0 differ. We then compute positional and velocity offsets as functions of halo mass and distance from the halo centre, and use these to construct a subhalo catalogue from the dark-matter-only simulation that reproduces the galaxy distribution in the hydrodynamic run. Satellites in the hydrodynamic simulation lie 3-4% closer to halo centres than in the dark-matter-only case, with an offset that is nearly constant with halo mass and increases towards smaller radii. Satellite velocities are also systematically higher in the dark-matter-only run, with differences that grow towards lower halo masses and radii. At scales of 0.1 h(-1) Mpc, these spatial and kinematic differences produce 10-20% variations in clustering amplitude - corresponding to 1-3 sigma assuming DESI-like errors - though the impact decreases at larger scales. We repeat the analysis in zoom-in simulations with varied physical models and find consistent trends. These baryonic effects are relevant for cosmological and lensing analyses and should be accounted for when building high-fidelity mocks. However, they remain smaller than the differences introduced by galaxy selection, which thus represents the dominant source of uncertainty when constructing mocks based on observable quantities.

ABSTRACT Early James Webb Space Telescope (JWST) observations have revealed a high-redshift universe more vibrant than predicted by canonical galaxy-formation models within Lambda cold dark matter ($\Lambda$CDM), showing an excess of ultraviolet(UV)-bright, massive, and morphologically mature galaxies. Departures from $\Lambda$CDM prior to recombination can imprint signatures on non-linear structure formation at high redshift. In this paper, we investigate one such scenario – Early Dark Energy, originally proposed to resolve the Hubble tension – and its implications for these high-redshift challenges. We present the first large-scale cosmological hydrodynamic simulations of these models. Modifications to the pre-recombination expansion history accelerate early structure formation and produce UV luminosity and stellar mass functions in excellent agreement with JWST measurements, requiring essentially no additional calibrations. Predictions converge to $\Lambda$CDM at lower redshifts ($z \lesssim 3$), thereby preserving all successes of $\Lambda$CDM. This model also accelerates the emergence of stellar and gaseous discs, increasing their number densities by $\sim 0.5$ dex at $z\simeq 6$–7, primarily due to the higher abundance of massive galaxies. Taken together, these results demonstrate how early-universe physics can simultaneously reconcile multiple high-redshift challenges and the Hubble tension while retaining the core achievements of $\Lambda$CDM. This opens a pathway to constraining a broad class of beyond-$\Lambda$CDM models with forthcoming observations.

Line intensity mapping (LIM) has garnered attention as a powerful cosmological probe, with current-generation instruments such as SPHEREx (Spectro-Photometer for the History of the Universe, Epoch of Reionization and Ices Explorer) capable of mapping the evolution of large-scale structure during the epoch of reionization (EoR). Lyman-alpha (Ly alpha) emission in the EoR is strongly shaped by resonant absorption from neutral hydrogen in the diffuse intergalactic medium (IGM), which transforms galactic sources into a low surface-brightness background. In this work, we leverage the state-of-the-art THESAN cosmological simulations to produce high-resolution theoretical predictions for future Ly alpha LIM studies, constructing continuous light-cones for line-of-sight cosmological integrations. We assess the contributions of recombination, collisional excitation, and unresolved HII regions to the total Ly alpha spectral intensity. In addition, we explore the IGM in absorption at different redshifts using damping wing analysis. We produce channel maps exploring spatial fluctuations across redshift bands probe-able by LIM instruments. We find that the slope of the absorption-included Ly alpha fluctuation power spectrum at smaller scales (k greater than or similar to 2 & times; 10(-2) arcsec(-1)) steepens toward lower redshift, and that our emission-only Ly alpha power spectrum lies above the SPHEREx sensitivity, whereas the absorption-included signal is similar to 7-8 orders of magnitude lower-providing a conservative lower limit on inhomogeneity signatures and highlighting the importance of including resonant scattering in the future. We also find that including outflows in a simple toy model boosts power by four orders of magnitude for a channel map spanning z is an element of [5.5, 6.5]. We identify limitations in our analysis and propose next steps, including incorporating the effects of resonant Ly alpha scattering and line interlopers, as well as larger simulation volumes.

We present the first results from AMBRA simulation ( A STRID with M BH seeding from BRA HMA) evolved to z = 8. AMBRA combines the large cosmological volume and statistical power of ASTRID with the physically motivated gas-based black hole (BH) seeding models from BRAHMA . Motivated by James Webb Space Telescope’s (JWST) discoveries of massive BHs at z ≳ 9, AMBRA adopts a lenient heavy-seed prescription from the BRAHMA suite, allowing for the formation of 4 × 10 ^4−5 M _⊙ seeds in halos with star-forming, metal-poor gas. The seeding model is motivated by scenarios in which heavy seeds form through stellar collisions in star clusters or from the rapid growth of Population III remnants. The improved seeding model enables AMBRA to form BH seeds much earlier and more efficiently compared to ASTRID . This significantly enhances early BH growth, producing a z = 8 BH number density more than an order of magnitude higher than that in ASTRID over the mass range 10 ^5−7 M _⊙ . BHs reaching masses consistent with GN-z11 and CEERS-1019 typically originate in highly compact density peaks and undergo multiple early mergers. In these systems, ∼50% of BH masses by z = 11 is from BH mergers, after which gas accretion becomes the dominant growth channel. Without this early merger-driven assembly, ASTRID cannot reproduce the high-mass BH detected by JWST. Our results indicate that abundant early seed formation combined with frequent mergers can explain several JWST massive BH candidates without requiring sustained super-Eddington accretion. As a testable prediction, AMBRA yields ≈4 Laser Interferometer Space Antenna (LISA) detectable BH merger events per year at z ≥ 8, which is three orders of magnitude higher than that in ASTRID .

AMBRA combines the large cosmological volume and statistical power of ASTRID with the physically motivated gas-based black hole seeding models from BRAHMA. Motivated by JWST's discoveries of massive black holes (BHs) at z≳ 9, AMBRA adopts a lenient heavy-seed prescription from the BRAHMA suite, allowing for the formation of 4× 10^4-5 M_⊙ seeds in halos with star-forming, metal-poor gas. The seeding model is motivated by scenarios in which heavy seeds form through stellar collisions in star clusters or from the rapid growth of Population III remnants. The improved seeding model enables AMBRA to form BH seeds much earlier and more efficiently compared to ASTRID. This significantly enhances early BH growth, producing a z=8 BH number density more than an order of magnitude higher than that in ASTRID over the mass range 10^5-7 M_⊙. BHs reaching masses consistent with GN-z11 and CEERS-1019 typically originate in highly compact density peaks and undergo multiple early mergers. In these systems, ∼50% of BH masses by z=11 is from BH mergers, after which gas accretion becomes the dominant growth channel. Without this early merger-driven assembly, ASTRID cannot reproduce the high-mass BH detected by JWST. Our results indicate that abundant early seed formation combined with frequent mergers can explain several JWST massive BH candidates without requiring sustained super-Eddington accretion. As a testable prediction, AMBRA yields ≈4 LISA detectable BH merger events per year at z≥8, which is three orders of magnitude higher than that in ASTRID.

Recent multiwavelength observations of M87* revealed a high-energy γ -ray flare without a millimeter counterpart. We present a theoretical study of potential millimeter flares in M87*, using general relativistic magnetohydrodynamical simulations with varying black hole (BH) spins and magnetic field configurations. The emergence of a millimeter flare is strongly influenced by both spin and magnetic structure, with limited sensitivity to the electron distribution. We model the intensity light curve with a damped random walk and compare the characteristic timescale ( τ ) with recent Submillimeter Array observations, finding that simulated τ exceeds observed values by over an order of magnitude. For BH spin a = +0.5, we identify a distinct flare followed by an order-of-magnitude flux drop. All Stokes parameters vary near the flare, including a sign reversal in the electric vector position angle. Most β _m modes remain stable, but the EB -correlation phase is sensitive to both the flare peak and decay. We examine polarimetric signatures in photon subrings, focusing on modes n _s = (0, 1). The n _s = 0 signal closely matches the full image, while n _s = 1 exhibits distinct behavior, highlighting the potential of space very long baseline interferometry to isolate subring features. Magnetic reconnection near the disk plane triggers a flux eruption that injects magnetized plasma into the jet funnel, enhancing outflow speeds and temporarily disrupting jet collimation before gradual recollimation. These results suggest transient variability near flares encodes key information about BH spin and magnetic structure, offering new probes of active galactic nuclei.

We investigate how pulsar timing array (PTA) measurements of the nanohertz gravitational-wave background (GWB) can constrain models for the growth history of supermassive black holes (SMBHs) and how active galactic nucleus (AGN) and stellar feedback models can affect GWB predictions. Feedback regulates SMBH growth, altering the black hole mass function (BHMF). Using BHMFs drawn from multiple cosmological simulation suites, including IllustrisTNG, MillenniumTNG, Simba, and CAMELS, and combining these with a quasar-based SMBH binary population framework, we predict the resulting GWB amplitude under a range of different stellar and AGN feedback prescriptions. We find that the choice of both stellar and AGN feedback models alters the high-mass end of the BHMF and changes the predicted GWB amplitude by up to a factor of 2 for the fiducial simulations and a factor of 10 for extreme feedback variations in CAMELS. Models with inefficient or absent AGN feedback produce abundant SMBHs and yield GWB amplitudes consistent with PTA data, yet fail in producing realistic galaxies. Fiducial models of AGN and stellar feedback suppress SMBH growth too much and underpredict the expected signal, an effect that could possibly be mitigated by more realistic black hole seeding and growth prescriptions. The mismatch between the GWB amplitudes predicted by cosmological simulations and that inferred by PTA measurements suggests that SMBH growth is more efficient or occurs earlier than captured by current models. This demonstrates that PTA measurements provide a powerful new probe for not only the SMBH population but also feedback physics.

The dark matter flux in a direct detection experiment depends on its local speed distribution. This distribution has been inferred from simulations of Milky Way-like galaxies, but such models serve only as proxies, given that no simulation directly captures the detailed evolution of our own Galaxy. This motivates alternative approaches that obtain this distribution directly from observations. In this work, we utilize 98 Milky Way analogues from the TNG50 simulation to develop and validate a procedure for inferring the dark matter speed distribution using the kinematics of nearby stars. We find that the dark matter that originated from old mergers, plus that from recent nonluminous accretions, is well described by a Maxwell-Boltzmann speed distribution centered at the local standard-of-rest velocity. Meanwhile, recently accreted dark matter from massive mergers has speeds that can be traced from the associated stellar debris of these events. The stellar populations systematically underestimate the velocity dispersion of their dark matter counterparts, but a simple kinematic boost brings the two into good alignment. Using the TNG50 host galaxies, we demonstrate that combining these two contributions provides an accurate reconstruction of the local dark matter speeds. As an application of the procedure to our own Galaxy, we utilize stellar kinematic data from Gaia to quantify how the dark matter remnants from the Milky Way's last major merger impact its speed distribution in the solar neighborhood.

Empirical models often rely on key relations from the galaxy–halo connection to construct mock galaxy catalogues. These relations typically describe central galaxies more accurately than satellite galaxies, which are generally less massive and orbit within larger haloes. Satellite galaxies are affected by a variety of physical processes that pose significant challenges for modelling. In this work, we use , a state-of-the-art cosmological hydrodynamic simulation, to study the evolution of the baryonic component of satellites. Using the merger trees from this simulation, we follow the evolution of all z=0 satellite galaxies, tracking their stellar mass, gas mass, and r- and U-band magnitudes. We characterise this evolution using proxies including the fraction of subhalo mass and maximum circular velocity remaining relative to infall, the pericentric distance, and the time since infall. All of these quantities are commonly available in gravity-only simulations and can therefore be used to model these trends in simpler galaxy population models. We find that the gas mass, which is well described by the remaining subhalo mass fraction, declines much more rapidly than the other components, with satellites losing ∼ 80% of their gas by the time the subhalo has lost half of its total mass. By contrast, the evolution of stellar mass and magnitudes is overall slower and is better described by the reduction of the host subhalo v_ max. We then examine the evolution of satellite mass profiles. We find that, although stripping is strongest in the outer regions, the intermediate and inner parts of satellites experience mass loss at early times. The results of this work can be used by empirical models and galaxy formation models built on gravity-only simulations to improve their descriptions of satellite galaxies.

Extreme emission-line galaxies (EELGs) probe chemical enrichment in low-mass, bursty systems where star formation, feedback, and gas accretion are poorly constrained. Using DESI DR1, we select 23 nearby EELGs with detections of 19 ionic species (S/N ≥ 4), stellar masses M_* ≥ 10^7 M_⊙, and extreme Hα and [O III] 5007 equivalent widths (EW ≥ 500 Angstrom). We infer non-parametric star-formation histories and fit a Bayesian single-zone chemical-evolution model to O, N, Ne, S, and Ar, allowing time-dependent star-formation efficiency, outflow mass loading, and evolving inflow metallicity. We find short depletion timescales and large mass-loading factors, indicating rapid gas cycling in a burst-driven, non-equilibrium regime, with depletion times below Kennicutt-Schmidt expectations. Star-formation efficiency and outflows are well constrained, while inflow metallicity is weaker due to degeneracies with metal production. Abundance ratios isolate physical drivers: star-formation efficiency sets evolutionary tracks, outflows regulate metal retention and X/O normalization, and inflow metallicity sets baseline enrichment. N/O strongly constrains burst timing and gas flows, Ne/O remains nearly invariant, and S/O and Ar/O show intermediate sensitivity. These results demonstrate that multi-element abundances provide a direct probe of baryon-cycle processes in extreme low-mass starbursts.

The Tully-Fisher (TF) and Fundamental Plane (FP) relations are widely used to infer extragalactic distances and peculiar velocities, enabling measurements of large-scale velocity statistics and cosmological parameters. Using the Millennium-TNG hydrodynamical simulation, we assess the accuracy of these methods in the presence of realistic galaxy formation physics. We find that, while the 2-point statistics of velocities are reliably inferred on scales larger than ∼10, significant systematic deviations arise on smaller scales. These deviations originate from spatially correlated residuals in the TF and FP relations, driven by correlations between galaxy structural properties, star-formation history, and the local environment. As a result, TF- and FP-inferred velocity fields exhibit spurious correlations with the galaxy density field that cannot be explained by random scatter alone. We show that extending the TF and FP relations to include additional galaxy properties – such as star formation rate, gas mass, and stellar mass – mitigate these environmental correlations, particularly for late-type galaxies. Our results demonstrate that galaxy formation physics induces significant systematics in peculiar velocity measurements on non-linear scales, and that neglecting these effects may bias cosmological analyses.

The distribution of gas-phase metals within galaxies encodes the impact of stellar feedback on galactic evolution. At high-redshift, when galaxies are rapidly assembling, feedback-driven outflows and turbulence can strongly reshape radial metallicity gradients. In this work, we use the FIRE-2, SPICE, Thesan and Thesan Zoom cosmological simulations -- spanning a range of stellar feedback from bursty (time-variable) to smooth (steady) -- to investigate how these feedback modes shape gas-phase metallicity gradients at $3 10^{9}~{\rm M_\odot}$. These results demonstrate that bursty stellar feedback provides sufficient turbulence to prevent strong negative gradients from forming, while smooth stellar feedback does not generically allow for efficient radial redistribution of metals thereby keeping gradients steep. Finally, we compare with recent observations, finding that the majority -- but, notably, not all -- of the observed gradients may favor a bursty stellar feedback scenario. In all, these results highlight the utility of high-resolution observations of gas-phase metallicity at high-redshift as a key discriminator of these qualitatively different feedback types.

Patchy reionization couples the ionized-bubble morphology to the underlying density field, making the CMB Thomson optical depth sensitive to both the global ionization history and anisotropic fluctuations on the sky. Using the large-volume radiation-hydrodynamical Lumina simulation, we compute τ_ CMB in two ways: (i) from global volume- and mass-weighted ionization histories, and (ii) from explicit line-of-sight integrations through on-the-fly light cones. We find that the sightline-averaged optical depth in the light cone, ⟨ τ_ LOS⟩ = 0.0550, exceeds the value inferred from a global volume-weighted history, τ_ CMB,V = 0.0515, by ≈ 7%. This enhancement is largely captured by the global mass-weighted prediction, τ_ CMB,m = 0.0544, indicating that precision comparisons to CMB optical-depth constraints should use mass-weighted electron fractions or explicit light-cone integration rather than volume-weighted ionized fractions alone. The excess optical depth accumulates primarily near z_ LOS = 8.0^+1.9_-1.3, where the combination of high physical density and strong ionization-field patchiness is greatest. The resulting τ_ LOS field is non-Gaussian and exhibits ≳ 5% sightline-to-sightline scatter, with fluctuations tracing rare early-ionized overdensities and large-scale structure. Coarse-graining experiments show that smoothing the ionization field on ≳ 3 cMpc scales suppresses the density-ionization correlation and biases τ_ CMB low relative to the resolved calculation. Finally, angular power spectra and real-space correlation functions decomposed into HII, HeII, and HeIII auto- and cross-contributions reveal scale-dependent departures from simple hydrogen-helium co-tracing and evolving characteristic scales with redshift.

From the luminous quasars at z ∼ 6 to the recent z ∼ 9–11 active galactic nuclei (AGN) revealed by JWST, observations of the earliest black hole (BH) populations can provide unique constraints on BH evolution. We use the BRAHMA simulations with constrained initial conditions to investigate BH assembly in extreme overdense regions. The simulations implement heavy ∼10 ^4 –10 ^5 M _⊙ seeds forming in dense, metal-poor gas exposed to sufficient Lyman–Werner flux. With gas accretion modeled via the Bondi–Hoyle formalism and BH dynamics with a subgrid dynamical friction scheme, we isolate the impact of seeding, dynamics, accretion, and feedback on BH evolution. With fiducial stellar and AGN feedback inherited from IllustrisTNG , accretion is suppressed at z ≳ 9, leaving mergers as the dominant growth channel. Gas accretion dominates at z ≲ 9, where permissive models (super-Eddington or low radiative efficiency) build ∼10 ^9 M _⊙ BHs powering quasars by z ∼ 6, while stricter IllustrisTNG -based prescriptions yield much smaller BHs (∼10 ^6 –10 ^8 M _⊙ ). Our seed models strongly affect mergers at z ≳ 9: only the most lenient models (with ∼10 ^5 M _⊙ seeds) produce enough BH mergers to reach ≳10 ^6 M _⊙ by z ∼ 10, consistent with current estimates for GN-z11. Our dynamical friction model gives low merger efficiencies. Therefore, even in such extreme regions, we are unable to produce ≳10 ^7 M _⊙ BHs by z ∼ 9–10, as currently inferred for GHZ9, UHZ1, and CAPERS-LRD-z9. If the BH-to-stellar mass ratios of these sources are indeed so extreme, they would require either very short BH merger timescales or reduced AGN thermal feedback. Weaker stellar feedback boosts both star formation and BH accretion and cannot raise these ratios.

The initial gravitational collapse of dark matter and gas formed a universal filamentary network where the first galaxies formed, with shapes and sizes that depended on the type of dark matter. Claims from deep-space imaging surveys that elongated galaxies predominate at z > 3 are examined here by comparison with detailed hydrodynamical simulations of cold dark matter (CDM), warm dark matter (WDM) and wave/fuzzy dark matter (psi DM). For CDM and WDM, we have sufficient volume, 10(3) Mpc/h(3), to generate galaxies with stellar masses >10(9) M-circle dot at z > 2, which allows a comparison with the CEERS and CANDELS surveys. Here we find that the observed tendency towards elongated, prolate-shaped young galaxies is well matched by WDM based on material accreted along smooth filaments during the first similar to 500 Myr, with little dependence on stellar mass. This contrasts with CDM, where the stellar morphology is mainly spheroidal and formed from the merging of fragmented filaments. For CDM, several subhaloes are predicted to be visible, whereas for WDM and psi DM, early merging is rare. Our findings show how the shapes and sizes of early galaxies are sensitive to the smoothness of the underlying filament network, which provides a new constraint on the nature of dark matter.

Baryonic physics is anticipated to be a major source of systematic uncertainty in current and future large-scale cosmological surveys. We investigate how baryonic effects on halo density profiles vary with secondary halo properties at fixed halo mass, using the large-volume MillenniumTNG hydrodynamical simulation and its dark matter-only counterpart. We focus on the impact of halo concentration and large-scale environment on the ratio of density profiles of matched halos in the hydrodynamical and dark matter-only simulations. At redshift z = 0.0, we find a strong dependence on halo concentration, especially at lower halo mass (12.5 < log(M_h/h^-1M_⊙) < 13.0), where more concentrated halos exhibit weaker inner enhancement and stronger intermediate-radius suppression at fixed halo mass, with variations reaching ∼ 15% at small scales and decreasing toward larger scales. This trend weakens and reverses at higher halo mass. In contrast, the secondary dependence on large-scale environment is weaker (∼ 2%) and largely scale-independent, with halos in denser regions exhibiting slightly weaker intermediate suppression. By separating internal profile redistribution from total mass suppression, we show that concentration impacts both components, whereas the environmental dependence is primarily associated with an overall mass shift. These secondary dependencies persist at z = 0.5 and correlate with variations in internal baryonic properties. We examine additional halo properties, including halo spin and velocity dispersion, and find significant secondary dependence. Overall, our results highlight the important role of secondary halo properties in modulating baryonic effects on halo density profiles, with potential implications for future modeling efforts.

ABSTRACT The timing of cosmic reionization across Local Group (LG) analogues provides insights into their early histories and surrounding large-scale structure. Using the radiation-hydrodynamic simulation thesan-1 and its dark-matter-only counterpart thesan-dark-1, we track the reionization histories of all haloes, including 224 LG analogues within the proximity of any of the 20 Virgo-like clusters with halo masses above $10^{14}\, \text{M}_{\odot }$ at $z = 0$ and their environments. The statistically controlled samples quantify how the reionization redshift ($z_\text{reion}$) correlates with halo mass, local overdensity, and present-day pair properties. Even at fixed mass, haloes in denser regions ionize earlier, and increasing the overdensity smoothing scale systematically suppresses small-scale structure, including local variations and environmental gradients in $z_\text{reion}$. Virgo-like clusters accelerate reionization in their surroundings out to $\sim$5–10 cMpc, beyond which local overdensity again becomes the dominant factor. Within LG pairs, reionization timing offsets reach up to $\sim$150 Myr and correlate with present-day halo separation, reflecting sensitivity to large-scale structure rather than mass ratio in driving asynchronous reionization. The results support an extreme inside-out picture where clustered sources rapidly ionize their immediate neighbourhoods, while lower density regions self-ionize later and voids wait for external homogenization. These links between environment and reionization timing explain the influence of protoclusters and help interpret fossil records in LG dwarfs around the Milky Way. For Milky Way analogues, we find a reionization redshift as early (late) as $z_{\rm reion} = 12.7^{+2.0}_{-1.7}$ ($8.88^{+0.66}_{-0.70}$) when considered on 125 ckpc (500 ckpc) scales, with LG analogues following an inside-out reionization picture.

Understanding how galaxies and active galactic nuclei (AGN) jointly drive the reionization of the intergalactic medium (IGM) across cosmic time remains a major challenge in cosmology. We present Lumina, a large-volume radiation-hydrodynamic simulation that self-consistently follows the coupled evolution of the intergalactic medium, galaxies, and AGN through HI, HeI, and HeII reionization down to redshift z=3. Lumina evolves a cosmological volume of comoving side length L_box=500 cMpc with 2× 6000^3 resolution elements, corresponding to baryonic and dark-matter mass resolutions of 3.6× 10^6 M_⊙ and 1.9× 10^7 M_⊙, respectively. The simulation uses the moving-mesh code AREPO, combining the IllustrisTNG galaxy-formation model with a GPU-accelerated M1 radiation-transport solver in six frequency bins. The initial conditions employ separate transfer functions for baryons and dark matter and include their relative streaming velocity. Lumina predicts a late, predominantly stellar-driven hydrogen reionization, with the median sub-volume fully ionized by z≈ 5.2 and residual neutral HI patches persisting until z≈ 4.75. HeII reionization is driven self-consistently by AGN and is nearly complete by z=3. The simulation yields a Thomson-scattering optical depth in excellent agreement with Planck, an IGM thermal history and photoionization background broadly consistent with observational constraints, and a clear late-time thermal boost associated with HeII reionization. Its galaxy population remains consistent with the original IllustrisTNG project, while the larger volume improves statistics for rare objects, large-scale environments, and cosmic variance, enabling forward modelling of observables linking HI and HeII topologies to the evolving galaxy and AGN populations.