arXiv subjects

Boyuan Liu

Publications and source records attributed to Boyuan Liu.

At least 19 recordsLinked to original sources

arXiv2026

RIGEL: Ultra-faint dwarf galaxy diversity shaped by inhomogeneous cosmic reionization

Ultra-faint dwarf galaxies (UFDs) are among the smallest and oldest galaxies in the Universe and are widely regarded as relics of cosmic reionization. To investigate how reionization quenches star formation and shapes the diversity of UFDs, we present a suite of eight cosmological zoom-in simulations of isolated UFDs with present-day halo masses of $\sim10^9\,{\rm M}_\odot$. The simulations are performed with the radiation-magnetohydrodynamic galaxy formation framework Realistic ISM modeling in Galaxy Evolution and Lifecycles (RIGEL), coupled to realistic large-scale radiation fields extracted from the THESAN reionization simulation. Despite residing in similar $z=0$ halos, the simulated galaxies span nearly two orders of magnitude in stellar mass and broadly reproduce the observed luminosities, sizes, metallicities, and stellar kinematics of Local Group UFDs. We find that reionization quenches star formation through a two-stage process. The arrival of the ionization front rapidly photoionizes the diffuse circumgalactic and intergalactic gas, suppressing further gas accretion onto the galaxy. Star formation nevertheless continues for several hundred Myr using the surviving self-shielded gas reservoir and ceases only after this gas is consumed or dispersed. Within 500 Myr after reionization, less than 40% of the initial gas mass remains in the halo, with photoevaporation constituting the dominant gas-loss channel. We further show that the halo mass at the time of reionization is a key parameter governing the subsequent evolution of UFDs. Galaxies residing in more massive halos at reionization retain gas for longer periods and undergo more extended chemical enrichment. Consequently, the halo mass at reionization strongly correlates with the final stellar mass, stellar age spread, and chemical evolution of the galaxy.

astro-ph.GA↗

arXiv2026

Rapid Growth of Intermediate-Mass Black Holes through Disk-induced Stellar Disruptions

Dense nuclear star clusters provide unique environments for studying the dynamical interactions between stars and massive black holes. When an accretion disk is present, dissipative star--disk interactions can capture surrounding stars, drive their inward migration, and ultimately lead to disk-induced tidal disruption events\,(dTDEs). The long-term feeding rate from this process, however, cannot be inferred from single-orbit migration estimates alone, as it depends on the coupled evolution of disk capture, collisional relaxation, stellar depletion and replenishment, and physical mergers within the star cluster. In this work, we use high-performance direct $N$-body simulations combined with analytic prescriptions for star--disk interactions to follow this coupled evolution for intermediate-mass black holes\,(IMBHs) with accretion disks embedded in dense stellar clusters. The simulations track the formation of the stellar cusp, the capture of stars by repeated disk crossings, their subsequent orbital damping and migration, and their eventual consumption by the central IMBH. We find that dTDEs can sustain stellar mass supply rates of $\sim10^{-3}\,M_\odot \,\mathrm{yr}^{-1}$, which exceeds the Eddington-limited gas accretion rate for IMBHs with $M_\bullet<10^5\,M_\odot$. These results identify dTDEs as an efficient stellar feeding channel for IMBHs in gas-rich dense stellar systems. As one possible application, this mechanism may help transform $\sim10^3\,M_\odot$ IMBHs into more massive black-hole seeds, provided that compact stellar clusters and accretion disks persist for $>30$ Myr.

astro-ph.GA↗

arXiv2026

Linking high-mass X-ray binaries to binary compact object mergers in Milky Way-like galaxies

High-mass X-ray binaries (HMXBs) constitute a potential intermediate phase connecting massive stellar binaries to the formation of coalescing binary compact objects (BCOs). Yet, the specific circumstances that allow HMXBs to later develop into merging BCOs are still under debate. In this study, we focus on wind-fed HMXBs and investigate this link by generating synthetic catalogs with the population-synthesis code SEVN and assigning them to a sample of 66 Milky Way-like galaxies. Using stellar particles drawn from the TNG50 cosmological simulation, we assign HMXBs according to particle mass, age, and metallicity, and follow their subsequent evolution to BCOs. In this way, our approach provides a realistic framework for modelling HMXBs in the Milky Way by explicitly accounting for the varied metallicities and spatial distributions of binary systems. Our method recovers the spatial distribution inferred from the observed properties of Galactic HMXBs: their positions follow the spiral arms and agree with the measured radial profile. The age distribution aligns well with observations and shows that BH-HMXBs tend to be younger than NS-HMXBs. We find that the fraction of HMXBs that evolve into merging BCOs within a Hubble time for BH-HMXBs is ~0.2-3.2% (or ~0.3-5.4% when restricting to 10^35 <= L_X <= 10^40 erg s^-1), while for NS-HMXBs is ~3.6-23.4% (or ~3.5-26.0% with the same luminosity cut). We show that common envelope (CE) episodes and natal-kick magnitudes are the primary processes determining the number of merging systems. Successful BCO mergers are typically produced by systems that experienced significant orbital hardening (early CE), whereas stable mass transfer often yields wide, non-merging binaries unless extreme eccentricities are induced by kicks. Metallicity plays a secondary but important role by modulating compact-remnant masses and wind-driven orbital widening.

astro-ph.HE↗

arXiv2026

The Road to Normalcy: Environment-Driven Evolutionary Pathways for Primordial Black Holes

We investigate how cosmological environment regulates the evolution of primordial black hole (PBH) seeds in the early universe using a suite of hydrodynamical simulations. Unlike traditional seeding channels, PBHs provide early BH seeds without the need of special conditions for gas collapse/fragmentation that favor overdense regions, thus covering diverse large-scale environments. We find that PBHs follow distinct evolutionary pathways depending on the gas supply and halo assembly history. In underdense regions, limited inflow suppresses both accretion and star formation, producing faint, metal-poor systems, while in overdense environments, sustained gas inflow drives rapid BH growth and the formation of compact, centrally concentrated stellar components. These differences lead to large variations in BH-to-stellar mass ratio, metallicity, and morphology, collectively bracketing a range of possible evolutionary pathways for PBH-seeded systems. We show that compact, BH-dominated sources resembling recently observed Little Red Dots naturally arise as one phase within these evolutionary pathways before evolving into more extended galaxy--AGN systems. Our results suggest that both the initial seed properties and cosmological environment jointly shape early BH--galaxy co-evolution, while subsequent environmental regulation can erase the memory of the initial seeding channel.

astro-ph.GA↗

arXiv2026

Pebbles to Gems: Intermediate-mass black holes in the first star clusters

The rapid assembly of supermassive black holes (SMBHs) observed at $z\gtrsim7$ requires efficient seeding mechanisms in the early Universe. Population III (Pop. III) star clusters have recently emerged as a promising pathway that may bridge the gap between traditional light- and heavy-seed scenarios by producing intermediate-mass black holes (IMBHs) with masses up to $\sim10^4\,\rm M_{\odot}$. We investigate the properties and number densities of IMBHs forming in Pop. III star clusters with masses $M_{\rm cl}\sim10^3-4\times10^5\,\rm M_{\odot}$, and hosted in isolated dark matter minihalos, using a suite of direct $N$-body simulations. We adopt cosmologically motivated initial conditions and explore different stellar evolution prescriptions, binary orbital parameter distributions, and cluster dynamical configurations. By $z\sim19$, the IMBH mass function consistently peaks at $m_{\rm IMBH}\sim200\,\rm M_{\odot}$, with number densities of $n_{\rm IMBH}\sim0.2-5\,\rm cMpc^{-3}$. In sufficiently dense and massive clusters, IMBHs with masses $>10^3\,\rm M_{\odot}$ can already form by $z\sim19$, reaching number densities of $n_{\rm IMBH}\sim10^{-4}-10^{-2}\,\rm cMpc^{-3}$. The most massive IMBHs in our models reach $\sim6200\,\rm M_{\odot}$ through the collapse of very massive stars assembled by repeated stellar collisions, a process enhanced in fractal clusters. Lower-mass IMBHs form instead predominantly through single and binary stellar evolution and binary stellar mergers. We find that models combining large stellar radii and tight binaries produce the highest IMBH abundances relative to isolated Pop. III evolution. Owing to the high retention fraction of IMBHs ($\gtrsim88\%$), massive dense Pop. III star clusters can act as efficient incubators of both light and heavy SMBH seeds, even if only a fraction of Pop. III stars formed in such environments.

astro-ph.GA↗

arXiv2026

SeFi-Image: A Text-to-Image Foundation Model with Semantic-First Diffusion

Training image generation foundation models consumes substantial resources. Previous methods have attempted to leverage semantic guidance to accelerate the training process, yet their experiments were only conducted on simple datasets such as ImageNet, at low resolutions, and with small-scale models. In this paper, we propose SeFi-Image, a text-to-image foundation model built upon semantic-first diffusion, a novel latent diffusion modeling paradigm. We instantiate SeFi-Image at three model scales, 1B, 2B, and 5B parameters, enabling systematic study of scaling behavior and flexible deployment under varying compute budgets. Notably, our largest 5B model was trained with merely 125K A800 GPU hours, corresponding to roughly 10-20% of the training compute used by Z-Image. However, it achieves results comparable to or even superior to Qwen-Image and Z-Image. Despite this modest training compute, SeFi-Image achieves strong performance on a wide range of benchmarks, including GenEval, DPG, LongTextBench, OneIG, and CVTG-2K. Moreover, we provide DMD2-distilled few-step turbo variants for each model scale to accommodate diverse hardware constraints and latency requirements. We publicly release our code, weights and hope this work offers the community useful insights into semantic-guided diffusion modeling for T2I generation, while also providing practical and readily deployable model options.

cs.CV↗

arXiv2026

AGN-driven BBH mergers: Black hole populations and hierarchical growth across the AGN parameter space

Active galactic nuclei (AGNs) have been proposed as efficient environments for the formation of binary black holes (BBHs). We present an updated semi-analytical framework for BBH formation and evolution in AGN disks, following the capture, migration, pair-up, gas-driven hardening, binary--single encounters, and merger of stellar-origin black holes. We systematically explore the dependence of the resulting BBH merger population on the main AGN parameters, namely the supermassive black hole mass $M_\bullet$, the Eddington ratio $\lambda_\bullet$, and the disk viscosity parameter $\alpha$, and construct an intrinsic BBH population by weighting the simulations according to observed low-redshift AGN properties. We find that AGN disks can produce repeated mergers and build a high-mass tail extending beyond the pair-instability mass gap and into the intermediate-mass range. Hierarchical growth is more efficient in lower-viscosity disks, with $\alpha=0.01$, while higher-viscosity disks suppress the formation of massive remnants. The merger efficiency generally increases with $\lambda_\bullet$, but its dependence on $M_\bullet$ is non-trivial. The AGN-assisted BBH population is characterized by increasingly unequal mass ratios at high primary mass, a correlation between primary mass and $|\chi_{\rm eff}|$, and an effective-spin distribution that depends strongly on the fraction of binaries born in prograde or retrograde configurations. We find that the AGN channel can reproduce systems broadly consistent with the massive BBH events GW190521 and GW231123. We test several variations of the physical model, including different formalisms for migration torques, gas hardening, and three-body encounters. The general properties of the population are robust across these variations, with the high-mass tail and spin signatures persisting in all cases except when gas hardening is switched off.

astro-ph.GA↗

arXiv2026

The 1st PortraitCraft Challenge: A CVPR 2026 Workshop Competition on Portrait Composition Understanding and Generation

This paper presents an overview of the inaugural PortraitCraft Challenge, held as one of the official competitions at CVPR 2026. The challenge focuses on portrait composition understanding and generation, aiming to advance AI research in portrait aesthetics analysis and controllable image synthesis. Unlike existing datasets and tasks that primarily focus on global aesthetic scoring, PortraitCraft introduces a unified evaluation framework comprising two complementary tracks. Track 1 requires models to perform structured portrait composition understanding, and Track 2 requires models to generate portrait images from structured composition descriptions under explicit compositional constraints. To support the challenge, we constructed and publicly released a large-scale portrait composition dataset consisting of approximately 50,000 curated real portrait images, providing multi-level supervision. This report describes the challenge setup, evaluation protocols, dataset composition, and final results, along with an analysis of the technical characteristics of the submitted solutions. The PortraitCraft Challenge provides a standardized and reproducible platform for research on portrait composition understanding and generation, and is expected to foster further progress in the fields of portrait aesthetics and controllable image generation.

cs.CV↗

arXiv2026

Population synthesis of Be X-ray binaries in the Small Magellanic Cloud: Angular momentum recycling and stable mass transfer

Be X-ray binaries (BeXRBs) are key laboratories to constrain binary interaction processes such as mass transfer, angular-momentum transport, and natal kicks. The Small Magellanic Cloud (SMC), hosting a nearly complete and well-characterized BeXRB population, offers a unique opportunity to test these physical processes at low metallicity. We aim to identify the combination of binary-evolution parameters that simultaneously reproduces the observed number and the joint distribution of orbital period and optical magnitude of SMC BeXRBs. We performed an extensive grid analysis of binary population-synthesis models exploring different mass transfer efficiencies, angular-momentum transport prescriptions and Roche-lobe overflow stability criteria. We also considered the impact of natal kicks, and that of the propeller effect of rotating magnetic fields of neutron stars. Synthetic populations obtained with the binary population synthesis code $\texttt{sevn}$ are statistically compared to observations using likelihood-based methods applied to the orbital period and $V$-band magnitude distributions, together with requirements on the total number of systems. We find that models in which mass transfer via Roche-lobe overflow is assumed to be always stable and angular momentum is recycled back into the orbit through tides when the accretor approaches critical rotation provide the best match to observations. Our best-fitting models favor low natal kicks ($\lesssim 100\ \rm km\ s^{-1}$), a moderate mass transfer efficiency ($f_{\rm MT} \simeq 0.6$), a minimum Be threshold spin close to critical rotation, and a strong suppression of accretion onto neutron stars due to the propeller effect. Specifically, the observable population is highly sensitive to the treatment of the propeller effect, which regulates the X-ray luminosity of wide, low-accretion-rate systems.

astro-ph.SR↗

arXiv2026

Towards A Universal Analytical Model of Population III Star Formation: A Bridge Between Cosmological Scales and Protostars

We construct an analytical model of Population III star formation that connects the cosmological radiation background to sub-AU protostellar disk fragmentation, a dynamic range inaccessible to any single simulation. Our approach is based on combining separate models of the disparate relevant scales: from the cosmological environment to the host-halo scale, from the halo scale to the scale of the star-forming cloud, and from the cloud scale to the fragmenting, accreting protostellar disk. Individually and collectively, the models agree well with the predictions of state of the art simulations, while remaining computationally inexpensive and physically transparent. As an example of the applicability of the model, we study the effects of varying the Lyman-Werner flux on the Pop. III star formation efficiency. We show that depending on the halo properties and the strength of the dissociating radiation field, the halo-scale Pop. III star formation efficiency varies by more than two orders of magnitude from $\varepsilon_{\rm SFE,H} \approx 10^{-3}$ to $\varepsilon_{\rm SFE, H} \approx 0.5$. The abrupt transitions between hydrogen-deuteride cooling (in low virial temperature mini-halos subjected to low radiation backgrounds), molecular hydrogen cooling (at intermediate temperatures and radiation intensities), and atomic cooling (in higher temperature halos exposed to strong radiation fields) produces sharp features in the halo-scale star formation efficiency as a function of the halo properties. Meanwhile, at the scale of individual star-forming clouds, the star formation efficiency is $\varepsilon_{\rm SFE,c} \gtrsim 0.2$. That is, pristine gas in a halo is converted into unstable clouds at a wide range of efficiencies, and these unstable clouds are efficiently converted into Pop. III stars.

astro-ph.GA↗

arXiv2026

Impact of Stochastic Pop~III X-ray Binaries on the Cosmological 21-cm Signal

High-mass X-ray binaries are one of the primary drivers of the 21-cm signal from Cosmic Dawn and Reionization, playing a leading role in the thermal history of the intergalactic medium. In traditional semi-numerical simulations, a deterministic scaling relation between the total X-ray luminosity of high-mass X-ray binaries, $L_{\rm X}$, and star formation rate (SFR) is usually adopted. However, this assumption is inaccurate for high-redshift low-SFR regions hosting few sources. The spatial variation in the number and luminosity of these sources is expected to enhance fluctuations in the Cosmic Dawn 21-cm signal. Here we quantify this effect by introducing a stochastic $L_{\rm X}$ model sampled from a power-law X-ray luminosity function. Implementing this in 21cmSPACE, a large-scale simulation framework of Cosmic Dawn and Reionization, we find that the stochasticity leads to enhanced fluctuations in X-ray heating rate fields, and affects the 21-cm power spectrum on small scales ($k>0.3~ \mathrm{cMpc^{-1}}$). The impact of stochasticity on the global 21-cm signal and on the large-scale power spectrum is found to be negligible. Our results suggest these effects will remain undetected by the upcoming Square Kilometer Array. However, large-scale lunar-based experiments may be sensitive to the signatures of stochastic X-ray heating at $z\sim 25$. Quantifying these corrections is a vital step toward robust 21-cm modeling and ensuring that future precision data interpretation is free from astrophysical biases.

astro-ph.CO↗

arXiv2026

The Cliff: A Metal-Poor Little Red Dot Hosting an Overmassive Black Hole at $z = 3.55$

JWST has revealed a large population of massive black holes (BHs) in the early Universe with unusual properties which mark them as distinct from low-redshift active galactic nuclei. Such findings have prompted the development of new models of BH formation and growth, and of their co-evolution with host galaxies. Linking the gas-phase metallicity of BH environments to seed masses is key to understanding which evolutionary pathways could explain the population of JWST-discovered BHs. We present new high-resolution JWST NIRSpec/IFU observations covering the rest-frame optical emission lines of a Little Red Dot (LRD) at $z=3.55$, known as The Cliff, from the `Red Unknowns: Bright Infrared Extragalactic Survey' (RUBIES). We find evidence for low metallicity ($Z=0.017\pm0.004 \ Z_\odot$) based on the low narrow-line [OIII]$\lambda5007$/H$\beta$ ratio, supported by the non-detection of low-ionisation emission lines such as [OII]$\lambda\lambda3727,3729$ and [NII]$\lambda\lambda6548,6583$. We find that the observed properties of The Cliff, including its overmassive BH, can be reproduced by some simulations of black hole growth and evolution down to $z\sim3.5$. However, these simulation runs require high seed masses ($10^4 - 10^5\ M_\odot$) and appear as rarely in the simulation volume as in the RUBIES survey volume over redshifts $3<z<4$, highlighting the unusual nature of The Cliff. Future simulations and numerical models will help to uncover how such a metal poor system managed to develop a massive black hole and persist to such low redshift.

astro-ph.GA↗

arXiv2026

Direct N-body simulations of rotating and extremely massive Population III star clusters

Aims. We present eight direct N-body simulations with NBODY6++GPU of extremely massive, initially rotating Population III star clusters with 1.01 x 10^5 stars. Methods. Our models include primordial binaries, a continuous initial mass function, differential rotation, tidal mass loss, updated fitting formulae for extremely massive metal-poor Population III stars, and general-relativistic merger recoil kicks. We assess their impact on cluster dynamics. Results. All runs form black holes below, within, and above the pair-instability gap, with multi-generation growth. Faster-rotating clusters core-collapse earlier; post-collapse clusters host a rotating, axisymmetric subsystem of intermediate-mass black holes (IMBHs) at the centre and an expanding halo of lower-mass objects. Pair-instability supernovae and compact-object formation at ~2-3 Myr sharply reduce total mass and a large fraction of the cluster's angular momentum. All Population III clusters in our simulations have the gravothermal-gravogyro catastrophe phase. Conclusions. We confirm two of the hypothesized formation channels of galactic nucleus seed black holes: gravitational runaway mergers of black holes and of Population III stars, which core-collapse into IMBHs thereafter. A higher initial star cluster bulk rotation correlates with earlier core collapse and, in the event counts reported here, with more coalescences and collisions, as well as lower retained (compact) binary abundances. Initial bulk rotation is a primary control parameter of cluster evolution: faster rotation accelerates early angular-momentum transport, gravothermal collapse, mass segregation, and amplifies post-collapse expansion, which also favours the formation of a compact central IMBH subsystem.

astro-ph.GA↗

arXiv2026

GLIMPSE-DDT spectroscopic properties of faint-end galaxies at $z\sim6$: Towards first metal enrichment, dust production, and ionizing photon production

Ultra-faint galaxies at high-$z$ are fundamental elements of the early galaxy assembly, and spectroscopic characterization of this population is essential to understand the earliest galaxy evolution. Leveraging the ultra-deep JWST/NIRCam and NIRSpec observations of a gravitational lensing field of Abell S1063, taken as part of the GLIMPSE survey, we present spectroscopic properties of 16 galaxies fainter than $M_{\rm UV}=-17$ mag, including the metallicity, dust attenuation, and the ionizing photon production efficiency. The emission lines are generally quite strong, roughly half of which cannot be replicated with standard stellar populations and require an extreme ionizing source. We also identify relatively strong [OIII] emission lines from all sample galaxies, which indicates that the low-mass end of the mass-metallicity relation is extended down to $M_\star\sim10^6\ M_\odot$ at $z\sim6$. The strong [OIII] line detection from the lowest-mass galaxy among the sample ($M_\star\sim10^{5.6}\ M_\odot$) stands in contrast to recent reports of extremely metal-poor galaxy candidates at similar mass and redshift, suggesting that there could be two distinct pathways of the earliest metal enrichment as simulations have predicted. Interestingly, we detect both dust attenuation and galactic outflow in one of the sample galaxies with $M_\star=10^{6.6}\ M_\odot$ at $z=5.5$. All the dust, metal, and outflow contents in this galaxy can be consistently explained by supernovae (SNe), indicative of the key roles of SNe in the earliest galaxy assembly such as dust production, metal enrichment, stellar feedback, and baryon cycle.

astro-ph.GA↗

arXiv2025

Primordial Black Holes as Seeds for Extremely Overmassive AGN Observed by JWST

The James Webb Space Telescope (JWST) has recently identified Abell 2744-QSO1 as a compact, metal-poor, black hole (BH) dominated galaxy at $z\simeq 7$. This system exhibits an extreme black-hole-to-stellar mass ratio and unusually low metallicity, posing significant challenges to BH seeding models. Motivated by these discoveries, we perform high-resolution cosmological simulations with a massive primordial black hole (PBH; $M_{\rm BH}=5\times10^7\,M_\odot$) seed, incorporating for the first time a fully coupled treatment of PBH accretion, BH feedback, and Population~III/II star formation and stellar feedback. Although PBHs accelerate structure formation through the seed effect, the associated strong thermal feedback from the accretion delays the onset of star formation to $z\lesssim 10$, producing short, bursty episodes throughout the subsequent evolution. PBH-driven outflows expel enriched gas from the nucleus, while sustained inflows from the intergalactic medium continuously replenish pristine material. This feedback-regulated cycle naturally yields low accretion rates ($\dot{m}_{\rm BH}/\dot{m}_{\rm edd} \sim 1-10\%$), subsolar metallicities ($Z/Z_\odot\lesssim10^{-2}$) and extreme $M_{\rm BH}/M_\star$ ratios during both the initial star-forming phase and the subsequent quenching phases, in excellent agreement with JWST observations. Our results demonstrate that massive PBHs offer a viable pathway for forming the most extreme high-redshift systems, providing a physically motivated explanation for the extraordinary properties of Abell 2744-QSO1, as a sub-class of the broader population of JWST-discovered "little red dots".

astro-ph.GA↗

arXiv2025

RIGEL: Feedback-regulated cloud-scale star formation efficiency in a simulated dwarf galaxy merger

Major mergers of galaxies are likely to trigger bursty star formation activities. The accumulation of dense gas and the boost of star formation efficiency (SFE) are considered to be the two main drivers of starbursts. However, it remains unclear how each process operates on the scale of individual star-forming clouds. Here, we present a high-resolution (2 Msun) RHD simulation of a gas-rich dwarf galaxy merger using the RIGEL model to investigate how mergers affect the properties of the structure of dense star-forming gas and the cloud-scale SFE. We tracked the evolution of sub-virial dense clouds in the simulation by mapping them across successive snapshots taken at intervals of 0.2 Myr. We find that the merger triggers a 130 fold increase in the SFR and shortens the galaxy-wide gas-depletion time by two orders of magnitude compared to those in two isolated galaxies. However, the depletion time of individual clouds and their lifetime distribution remained unchanged over the simulation period. The cloud life cycles and cloud-scale SFE are determined by the local stellar feedback rather than such environmental factors as tidal fields regardless of the merger process, and the integrated SFE ($\epsilon_{\rm int}$) of clouds in complex environments remains well-described by an $\epsilon_{\rm int}-\Sigma_{\rm tot}$ relation found in idealized isolated-cloud experiments. During the peak of the starburst, the media SFE was lower by only 0.17-0.33 dex compared to the value when the galaxies were not interacting. The merger boosts the SFR through the accumulation and compression of dense gas fueling star formation. Strong tidal torques assemble $>10^5$ Msun clouds, which seed massive star clusters. The average separation between star-forming clouds decreases during the merger, which in turn decreases the cloud--cluster spatial de-correlation from >1 kpc to 0.1 kpc depicted in tuning fork diagrams.

astro-ph.GA↗

arXiv2025

The Entangling of Supernova Feedback Impacts with Coarsening Simulation Resolution

It is often understood that supernova (SN) feedback in galaxies is responsible for regulating star formation (SF) and generating gaseous outflows. However, a detailed look at their small-scale effects on the interstellar medium (ISM) in simulations shows that these processes proceed in distinct and separate channels. We demonstrate this finding in two independent simulations of isolated dwarf galaxies with very high ($m_{\rm gas}$ $\sim \msun$) numerical resolution, {\small LYRA} and {\small RIGEL}. Focusing on the immediate environment surrounding SNe, our findings suggest that the macroscopic effect of a given SN on the galaxy is best predicted by its local density. Outflows are driven by SNe in diffuse regions expanding to their cooling radii on large ($\sim$ kpc) scales, while dense SF regions are disrupted in a localized ($\sim$ pc) manner. However, these separate feedback channels are only distinguishable at very high resolutions capable of following mass scales $\lesssim 10^2 \,\msun$. When averaging on coarser scales, ISM densities are greatly mis-estimated, and variations between different SF and SNe-affected regions are severely washed out. It therefore cannot be self-consistently determined, from coarse-resolution information \textit{alone}, (1) whether a SN tends to contribute to outflows or direct SF suppression, and (2) the rate of SF in a given region. In particular, commonly used parameters in coarse-resolution (subgrid) models, such as the SN cooling radius and SF density threshold, may require more detailed treatments informed by high-resolution studies.

astro-ph.GA↗

arXiv2025

BlackTHUNDER: Evidence of three massive black holes in a z~5 galaxy

We present observational evidence of three massive, accreting black holes (BHs) in the z=5.0167 galaxy J0148-4214 from JWST/NIRSpec-IFU spectroscopy. The BHs are revealed through broad H$\alpha$ emission (FWHM=430-2920 km/s) without a forbidden-line counterpart in the bright [OIII] doublet. Channel maps of the asymmetric central H$\alpha$ profile isolate two spatially distinct broad-line regions (BLRs), separated by $190\pm40$ pc, while a third BLR is found in the galaxy outskirts with a projected separation of 1.7 kpc. We discuss whether this emission could be due to supernovae, shocks, winds, or massive stars, but find the BLR origin most likely. Using single-epoch virial relations, we estimated BH masses of $\log(M_\bullet/M_\odot)=7.9\pm0.4$ (primary), $5.8\pm0.5$ (secondary), and $6.3\pm0.5$ (third off-nuclear). We argue that the two central BHs will likely rapidly merge, with a simple dynamical friction time estimate of the order of $\lesssim700$ Myr. Assuming that the third off-nuclear BH is also in the process of sinking towards the centre, it will likely lead to a second merger, and we investigated the detection probability of such mergers with LISA. Alternatively, the third BH may be the result of a previous central three-body interaction or a gravitational recoil, where our observations would provide evidence that such BHs may retain their accretion discs and BLRs even in the aftermath of such extreme dynamical interactions. The possible discovery of a BH triplet at high z, together with other recent results on distant BH pairs, indicates that multiple massive BH systems were likely common in the early Universe. Our results highlight the importance of IFU observations for the detection of massive BH multiplets in distant galaxies, the progenitors of massive BH mergers that may be detected with next-generation gravitational wave observatories.

astro-ph.GA↗