arXiv subjects

Oscar Agertz

Publications and source records attributed to Oscar Agertz.

At least 19 recordsLinked to original sources

arXiv2026

VINTERGATAN-GM: long-lived satellite planes induced by a massive GSE-like merger

Satellite galaxies in the Local Group tend to be distributed in thin, planar configurations, with many sharing coherent orbital motion. Galaxy formation simulations in $Λ$CDM have historically struggled to produce similar structures, leading to the so-called "planes of satellites problem". In this work, we investigate whether the emergence of such structures is connected to the mass of a major merger at $z\sim2$, analogous to the Gaia-Sausage-Enceladus (GSE) event in the Milky Way. We use the VINTERGATAN-GM suite of high-resolution zoom-in simulations, comprising five realizations of the same Milky Way-mass halo generated through targeted genetic modifications of a GSE progenitor. The GSE-like merger mass ratio is systematically varied from 1:10 to 1:2.1, while keeping the final dynamical mass and large-scale environment fixed. We find a clear and consistent trend: more massive GSE-like mergers lead to satellite populations that are both more planar and more kinematically coherent. In particular, simulations with merger mass ratios larger than 1:6 develop Kinematic Persistent Planes (KPPs), in which at least 40% of satellites co-orbit around a common axis over extended periods, comparable to those observed in the Milky Way. These structures arise when sufficiently massive mergers, accreted along the direction of maximum compression of the Lagrangian volume, produce flattened host halos with anisotropic velocity dispersions aligned with the merger direction. The merger aligns the host halo's minor axis with the direction of flattening of the surrounding cosmic web, and planes of satellites then emerge through two complementary processes: (i) satellites preferentially infall along the host's equatorial plane, and (ii) anisotropic dynamical friction in the non-spherical halo gradually reshapes their orbits toward this plane, generating coherent and long-lived planar configurations.

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arXiv2026

Turbulent gas-rich discs at high redshift: Chaotic orbits and rapid bulge formation at high gas fraction

Using the \nexus\ N-body/hydrodynamic framework, we show that stellar bars that emerge in high-redshift disc galaxies are different in their orbital characteristics compared to their low-redshift counterparts. We identify bar-supporting orbits and, among them, find the fraction of chaotic orbits compared to regular orbits ($f_{\rm chaos}$). This fraction rises above 60\% for models with $f_{\rm gas} > 60 \%$; for $f_{\rm gas} \lesssim 60\%$, we observe chaotic fractions close to zero. The emerging bars seen in the $f_{\rm gas}= 80\%$ and $100\%$ simulations collapse in $\lesssim$1 Gyr to form central bulges supported by rotation, and this collapse follows soon after the chaotic fraction starts to dominate ($f_{\rm chaos} \gtrsim 50\%)$. Moreover, the gravitational potential in those simulations steepens rapidly over that period, and by integrating the chaotic orbits, we find that orbits that deviate from the evolving mean field are more likely to be chaotic. Hence, we argue that the irregularity associated with the rapidly evolving, central potentials drives higher chaotic fractions in these gas-rich bars. As a result, the emergent bars collapse through `violent relaxation' to form oblate rotating bulges. Supporting evidence for this picture comes from the fact that particles of different mass share identical radial profiles, a natural consequence of the rapid relaxation process.

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arXiv2026

The nature of the Norma arm as a bar-driven star-forming shock front

The Milky Way is commonly depicted as having four major spiral arms, including the Norma-Outer arm, and a central stellar bar whose properties remain debated. Masers associated with young high-mass stars have been used to trace a segment of the "Norma arm" extending from one end of the Galaxy's central stellar bar. Recent work suggests that these masers may reside along a spiral arm or occupy the dominant x1 stellar orbits that give the bar its shape. We now use a new generation of N-body, hydrodynamic simulations to argue that these million-year old masers instead trace the Galactic bar's Quadrant I shock front or, equivalently, its dust lane. This implies that the bar dust-lane is star-forming, shedding new light on our understanding of star formation processes in the Milky Way. By associating the masers with the simulated shock front, we determine the bar's parameters to be: a ~ 4.3 +/- 0.2 kpc, b ~ 2.3 +/- 0.2 kpc, phi = 20 (+5/-2) deg, Omega_p <~ 40 km s^-1 kpc^-1, representing the bar's semi-major and semi-minor half-lengths, inclination angle and pattern speed, respectively. This raises questions about the Milky Way's spiral structure and suggests that the Galactic bar is long, slow and star-forming.

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arXiv2026

Simulations agree: the Milky Way's satellites are disrupting

Recent analyses of simulated Milky Way-mass galaxies have highlighted the prevalence of satellites undergoing tidal disruption. However, these have focused on single hydrodynamical codes with a number of limitations, making it unclear how generic this result is. We compare satellite disruption in four models: two cosmological hydrodynamical simulations (Auriga and FIRE), cosmological $N$-body simulations with stellar particle tagging (Aquarius), and a semi-analytic model with varying host disk properties (SatGen). For all systems, we compute the fraction of stellar mass retained by the progenitor ($f_\text{bound}$) at the present day. We find a remarkable agreement between FIRE, Aquarius, and SatGen; all predict that $\sim$20 to 30 per cent of classical-mass satellites have $f_\text{bound} < 0.9$ ($\sim$5 to 10 per cent have $f_\text{bound} < 0.5$). Satellites in Auriga experience substantially more mass loss than other simulations, which we attribute to their massive central galaxies and large satellite sizes prior to infall. In the ultra-faint regime captured by Aquarius and SatGen, disruption rates are comparable to or exceeding those for classical satellites. The ubiquity of mass loss across models presents both an exciting opportunity (a plethora of stellar streams awaiting discovery) and a nuisance (for theorists using satellite counts to constrain dark matter).

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arXiv2026

EDGE-INFERNO: The chemical signatures of assembly history in reionization fossils

In galaxies, stellar populations represent a superposition of both in situ and accreted components, tracing internal chemical evolution as well as the histories of merged satellites. We investigate how assembly history shapes the chemical properties of present-day stellar populations in low-mass dwarf galaxies. We analyze a suite of EDGE-INFERNO star-by-star cosmological zoom-in simulations of galaxies with present-day halo mass $\sim1.4\times10^9\,{\rm M}_{\odot}$. We systematically vary galaxy growth while preserving the present-day halo mass, engineering our simulations to span a continuum of assembly histories. Early-forming systems form most of their stars in a single progenitor, with centrally concentrated star formation and a continuous enrichment history. In contrast, our late-forming systems assemble by accretion of multiple progenitors with their own stellar populations, resulting in fragmented star formation across several halos with independent chemical evolution tracks. These regimes produce qualitatively different chemical signatures: single-progenitor systems exhibit connected, well-defined trends in abundance space, while multi-progenitor systems show multiple, distinct stellar populations with limited overlap. We find that metallicity distribution functions provide initial constraints on assembly history for samples of at least $50$-$100$ member stars. Multi-element abundance analysis significantly improves the recovery of distinct components, although robust reconstruction still requires larger samples. Fully separating in situ and accreted populations likely requires kinematic information. Accreted stars can contribute substantially even at high metallicities in multi-progenitor systems, while external enrichment can cause accreted material to follow similar chemical trends as in situ stars, particularly in the single-progenitor regime.

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arXiv2026

Cosmic evolution of the [CII]-to-dense neutral gas relation

The [CII] 158 $μ$m line is widely used to trace star formation and the gas contents of high-redshift galaxies. However, it remains unclear under which physical conditions it reliably traces the molecular reservoir, and whether a unique conversion factor $α_{\rm [CII]}$ can be applied across cosmic time. We investigated the evolution of the relation between the [CII] luminosity and dense neutral gas mass from $z\simeq10$ to $z\simeq0.2$ using the Vintergatan simulation, a high-resolution cosmological zoom-in of a Milky Way-like galaxy. We post-processed the snapshots with the SKIRT radiative transfer code to generate synthetic [CII] data cubes. We measured global and spatially resolved (100 pc) relations between [CII] luminosity ($L_{\rm [CII]}$), star formation rate (SFR), and dense neutral gas mass ($M_{\rm DN}$). We followed the redshift evolution of the [CII]-to-molecular gas conversion factor $α_{\rm [CII]}$, and link these trends to the evolution of the interstellar medium (ISM) phases. The global $L_{\rm [CII]}$-$M_{\rm DN}$ and $L_{\rm [CII]}$-SFR relations evolve from a steep, [CII]-deficient regime at very low metallicity to an almost linear behaviour, similar to calibrations at $z\approx2$, once the ISM reaches $Z \gtrsim 0.05$-$0.1\,Z_\odot$ at $z\lesssim5$. Over this evolution, $α_{\rm [CII]}$ spans nearly three orders of magnitude, from $\gtrsim 10^4$ down to $\approx10 \,\rm{M_\odot\,L_\odot^{-1}}$, even though the [CII] emission remains spatially correlated with the dense neutral gas. A unique, redshift-independent $α_{\rm [CII]}$ therefore cannot recover dense neutral gas masses across the regimes we explored. [CII] remains a viable tracer of dense neutral gas at very high redshifts, but only when used with conversion factors that explicitly account for metallicity, ISM phase mix, and merger events.

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arXiv2026

Early onset of the hot circumgalactic medium around Milky Way galaxies

Hot coronae are key regulators in Milky Way (MW)-type galaxy formation and are thought to emerge once a galaxy's dark matter halo crosses a critical mass threshold, $M_{\mathrm{vir}}\sim10^{12} {\mathrm{M}}_\odot$, so that accretion shocks can heat infalling gas to virial temperatures, $T_{\mathrm{vir}}$. Here we investigate the roles of stellar feedback and accretion shocks in establishing the circumgalactic medium (CGM) around MW-mass galaxies using the VINTERGATAN cosmological simulation suite. We find that stellar feedback drives the early formation of a hot CGM, with $T_{\mathrm{CGM}}\sim T_{\mathrm{vir}}$. By contrast, in feedback-free simulations, accretion shocks alone do not produce a CGM with comparably high average temperatures at early epochs ($z>2$), independent of gas metallicity. We show that CGM virialisation (defined when halo-gas cooling time exceeds the free-fall time) begins with the emergence of a hot corona ($T>10^5$ K) that can appear as early as $z\approx3$ at virial radius, $R_{\mathrm{vir}}$, when feedback is included. This is much earlier than predicted by accretion-shock theory or feedback-free models ($z\lesssim1$). While accretion shocks alone produce an inside-out virialisation pattern, feedback can alter this behaviour, driving a rapid transition ($\lesssim0.5$ Gyr) that is either outside-in or nearly simultaneous throughout the halo. Finally, these early-forming coronae are largely stable from formation through to the present day, but can appear to evolve simply because the definition of $R_{\mathrm{vir}}$ grows with time. This 'CGM pseudo-evolution' is analogous to the pseudo-evolution of dark matter haloes and should be taken into account when interpreting the evolution of hot coronae.

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arXiv2026

MEGATRON: Reproducing the Diversity of High-Redshift Galaxy Spectra with Cosmological Radiation Hydrodynamics Simulations

We present the MEGATRON suite of cosmological radiation hydrodynamics simulations following the formation of Milky Way-mass galaxies from the earliest cosmic epochs when Population III stars form to Cosmic Noon. The suite represents the first set of cosmological simulations that couples a vast non-equilibrium thermochemistry network of primordial species, metals, and molecules to multifrequency, on-the-fly radiation transport, allowing us to directly predict the spectral properties of early galaxies. By initializing the simulations at zero metallicity, resolving haloes well below the atomic cooling threshold, reaching parsec-scale resolution, and modeling a Milky Way-mass environment, we aim to address four key science themes: 1) Star formation at cosmic dawn, 2) Galaxy formation and the interstellar medium in the epoch of reionization, 3) The circumgalactic medium towards cosmic noon, and 4) Reionization in a local volume environment and near-field cosmology. In this introductory work, we present an overview of the physical characteristics of high-redshift MEGATRON galaxies and their environment at $z>8$. We present a library of $>175,000$ simulated galaxy spectra and demonstrate how much of the diversity of galaxy spectra seen by JWST is naturally reproduced in the context of a $Λ$CDM cosmology. Caveats are discussed, such as the lack of AGN in our simulations and the limitations of our adopted stellar population and chemical yield models. This project represents a step towards making more direct comparisons between simulations and observations and is particularly applicable for optimizing methods to infer galaxy properties from existing high-redshift JWST spectra and imaging data.

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arXiv2026

MEGATRON: the impact of non-equilibrium effects and local radiation fields on the circumgalactic medium at cosmic noon

We present three cosmological radiation-hydrodynamic zoom simulations of the progenitor of a Milky Way-mass galaxy from the MEGATRON suite. The simulations combine on-the-fly radiative transfer with a detailed non-equilibrium thermochemical network (81 ions and molecules), resolving the cold and warm gas in the circumgalactic medium (CGM) on spatial scales down to 20 pc and on average 200 pc at cosmic noon. Comparing our full non-equilibrium calculation with local radiation to traditional post-processed photoionization equilibrium (PIE) models assuming a uniform UV background (UVB), we find that non-equilibrium physics and local radiation fields fundamentally impact the thermochemistry of the CGM. Recombination lags and local radiation anisotropy shift ions away from their PIE+UVB values and modify covering fractions (for example, HI damped Ly$α$ absorbers differ by up to 40%). In addition, a resolution study with cooling-length refinement allows us to double the resolution in the cold and warm CGM gas, reaching 120 pc on average. When refining on cooling length, the mass of the lightest cold clumps decreases tenfold to $\approx 10^4\,M_\odot$, their boundary layers develop sharper ion stratification, and the warm gas is better resolved, boosting the abundance of warm gas tracers such as CIV and OIII. Together, these results demonstrate that non-equilibrium thermochemistry coupled to radiative transfer, combined with physically motivated resolution criteria, is essential to predict circumgalactic absorption and emission signatures and to guide the design of targeted observations with existing and upcoming facilities.

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arXiv2026

Nexus-CDM: Isolated Galaxy Simulations with Cosmologically Evolving Dark-Matter Halos I. Method and Validation

Galaxy evolution can be modelled in two complementary ways. Standalone simulations place a single galaxy in isolation, evolving it under controlled initial conditions without the surrounding cosmic web. This approach offers high resolution and computational efficiency, making it well suited to disentangle specific physical processes (e.g. feedback and disc instabilities) and testing them cleanly, though at the cost of ignoring environmental context, gas accretion from the broader large-scale structure, and the hierarchical assembly that feeds real galaxies over cosmic time. Cosmological simulations, by contrast, form galaxies self-consistently within an expanding universe, capturing large-scale structure formation, mergers, gas inflows, and environmental effects such as tidal stripping and ram-pressure. This realism comes at a steep computational cost, requiring coarser resolution or simplified sub-grid physics for star formation and feedback. Here, we explore a new paradigm that combines the relative merits of both methods -- what we call the Nexus-CDM framework. This approach naturally incorporates galaxy mass growth, which is absent from existing standalone simulations, reinforcing the limitations of static halo models used in galaxy formation and evolution research. The viability of the framework is demonstrated in an idealised setup, where it successfully reproduces the formation and subsequent evolution of a realistic stellar disc. We present some early results, in particular, discs form readily in a gravitational potential with a shallow central gradient, contrary to recent claims.

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arXiv2026

The driving mode of turbulence in disc galaxy simulations with adaptive mesh refinement

Turbulence is a key ingredient in controlling the structure of the interstellar medium (ISM) and star formation, yet we still lack a detailed understanding of its drivers in galaxies. Previous idealised simulations of turbulence employ a stochastic forcing field to drive turbulence. The geometry of this forcing field - whether it is predominantly solenoidal or compressive - is a key parameter governing how turbulence shapes the ISM density distribution and regulates the star formation rate. The turbulence driving parameter ($b$) quantifies the relative contribution of compressive versus solenoidal driving.. Therefore, accurate knowledge of the driving parameter is essential for understanding and predicting star formation, and for sub-grid modelling of ISM physics and the star formation rate. In this work, we introduce an algorithm to measure the turbulence driving parameter in adaptive mesh refinement (AMR) simulations of galaxies. We focus our analysis on a synthetic Large Magellanic Cloud (LMC), present-day analogue with a total mass $M \approx 10^{11}\,\mathrm{M}_\odot$. We find that turbulence is driven primarily solenoidally ($b<0.4$) within the inner $\sim4\,\mathrm{kpc}$ of the galaxy, and becomes increasingly compressive with $b>0.4$ towards the outskirts, $R\gtrsim4.5\,\mathrm{kpc}$. The volume-weighted median across the disc, $b\simeq0.4$, is consistent with the natural mixture of driving modes. We further find that $b$ is weakly correlated with the strength of shear, in that solenoidal driving tends to be associated with regions of higher shear, as expected for the more central parts of galaxies. These trends persist over $\sim\!2\,\mathrm{Gyr}$ of the galaxy's evolution.

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arXiv2026

Turbulent gas-rich discs at high redshift: the origin of early massive stellar bars

Recent observations combining the power of ALMA and JWST have revealed large ($3-7$ kpc), massive ($3-10\times10^{10}\,\mathrm{M}_\odot$) stellar bars at $z=4-5$ when the Universe was only 1.2-1.6 Gyr old. At this early epoch, the host galaxy was baryon-dominated (typically 75\% gas, 25\% stars) within the observed extent of the disc ($8-15$ kpc). Using NEXUS $N$-body/hydrodynamic simulations, we show that such bars can form promptly (400$-$800 Myr), provided the disc mass fraction is high ($f_{\rm disc}\gtrsim 70\%$) and the bar is gas-dominated at the time of its formation, consistent with the observations. In this limit, gas-free bars are unstable to vertical bending modes, but a dominant gas component suppresses this instability. Unlike massive bars in the local Universe, these early bars were sites of vigorous star formation, as we show. Remarkably, for gas-rich models with $f_{\rm gas}\lesssim60\%$, the bars develop X-shaped boxy bulges; at higher gas fractions ($f_{\rm gas}> 60\%$), diffusion suppresses resonant orbit trapping and the emerging bar collapses within 1 Gyr to form a classical bulge. The bar formation time, length, mass, and $m=2$ Fourier amplitude are all inversely related to $f_{\rm gas}$. We present a simple analytic model for how stochastic forcing shifts the bar onset time, defined as the time at which the growing bar amplitude reaches a specified threshold.

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arXiv2026

MEGATRON: how the first stars can create an iron metallicity plateau in the smallest dwarf galaxies

We study the stellar mass-iron metallicity relation of dwarf galaxies in the new high-resolution MEGATRON cosmological radiation-hydrodynamics simulations. These simulations model galaxy formation up to $z\approx8$ in a region that will collapse into a Milky-Way-like galaxy at $z=0$, while self-consistently tracking Population III and II (Pop.~III, Pop.~II) star formation, feedback and chemical enrichment. MEGATRON dwarf galaxies are in excellent agreement with the observed stellar mass-metallicity relation at $z=0$, including an over-abundance of dwarfs along a flat plateau in metallicity ($\langle [\rm{Fe}/\rm{H}] \rangle \approx -2.5$) at low stellar masses ($M_{\star} \leq 10^5 \, \rm{M}_{\odot}$). We tie this feature to the chemical enrichment of dwarf galaxies by Pop.~III pair-instability supernova (PISN) explosions. The strong Lyman-Werner background (LW) from the protogalaxy ensures that PISNe occur in haloes massive enough ($\approx 10^7\, \rm{M}_{\odot}$) to retain their ejecta. We also predict a tail of $\approx 20\%$ of iron-deficient ($\langle [\rm{Fe}/\rm{H}] \rangle \leq - 3$) dwarf galaxies. We show that both plateau and tail (i) are robust to large variations in Pop.~II feedback assumptions, and (ii) survive in bound satellites surrounding the central galaxy at $z=0$.

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arXiv2026

Magnetic destabilisation in disc galaxies: Filament/feather formation

Gas gravitational instability plays a crucial role in secular galactic evolution, but the role played by magnetic fields in mediating this instability - despite their dynamical relevance and ubiquity - is still not understood. To investigate this question we conduct a parameter study using numerical simulations of 3D isolated disc galaxies that are initialized in equilibrium, but have a range of initial magnetisation, quantified by $β\in \{0.1, 0.5, 1, 10, 100, \infty\}$, where $β$ is the ratio of thermal to magnetic pressure. We analyse how magnetic field strength influences the formation of dense filaments and feathers driven by gravitational instability. The simulations show that filament growth rate and spacing are significantly altered by dynamically strong fields ($β\lesssim 10$), and that these effects depend on the value of $β$ and strength of shear in the disc. Magnetic fields either stabilise or destabilise, with destabilisation dominating in regions with low $β$, and low shear. This makes the destabilisation particularly important in dwarf galaxies with low-shear rotation curves. Filament spacings are similarly affected differently in different galactic regions, depending upon the local field strength and shear. Our results are in good agreement with predictions from the magneto-Jeans mechanism.

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arXiv2026

The AGORA High-resolution Galaxy Simulations Comparison Project. IX - Part 1: Effects of a Major Galaxy Merger on Star Formation of a Milky Way-mass Galaxy Progenitor

Given their highly nonlinear dynamics and sensitivity to initial conditions, galaxy mergers are a compelling area to conduct a simulation code comparison. We perform a comparative study of a major galaxy merger at $z \approx 4.5$ in cosmological zoom-in hydrodynamic simulations of a Milky Way-mass galaxy progenitor. The comparison employs the AGORA CosmoRun suite of nine well-calibrated, state-of-the-art numerical codes, each adopting a different stellar feedback scheme. We find that the evolution of the star formation rate (SFR) during the interaction is strongly shaped by the stellar feedback type. Using kinetic feedback in the feedback model drives a pronounced merger-induced starburst that starts to subside before coalescence; using thermal feedback without kinetic feedback yields prolonged SFR growth even after coalescence; and using delayed cooling or radiation pressure results in highly fluctuating SFR. Tracking gas particles in particle-based codes reveals that kinetic feedback facilitates gas inflow from the secondary galaxy onto the primary galaxy between the first periapsis and apoapsis, thus producing an earlier and more prominent starburst. In contrast, thermal feedback, augmented by superbubble or delayed-cooling feedback, suppresses gas cooling, creates a more extended gas distribution, and hinders strong starbursts during the merger. We also observe an inverse correlation between burst fraction and pre-merger gas fraction that is independent of feedback models. Overall, these results highlight the sensitivity of simulated galaxy mergers' star formation response to stellar feedback prescriptions. This study indicates that galaxy mergers may serve as a good testbed for stellar feedback processes in cosmological simulations.

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arXiv2026

The AGORA High-resolution Galaxy Simulations Comparison Project. IX - Part 2: Effects of a Major Galaxy Merger on the Stellar Morphology of a Milky Way-mass Galaxy Progenitor

Galaxy mergers, with their high sensitivity to initial conditions, provide a valuable setting for comparative studies of galaxy simulation codes. Following our first paper focusing on merger-driven star formation, we present a code comparison examining the morphological transformation impact of a major galaxy merger at $z \approx 4.5$ on a Milky Way-mass galaxy progenitor. Our analysis employs nine state-of-the-art codes from the AGORA CosmoRun cosmological zoom-in simulation suite. For this merger, we show that the adopted stellar feedback type influences the galaxy's compaction and stellar disc formation. Codes with purely thermal feedback produce a merger remnant that forms a disc and becomes compact primarily during and after coalescence; codes that include kinetic feedback begin disc formation and compaction around the first periapsis; and codes with strong delayed cooling or superbubble feedback suppress disc formation and produce a more extended remnant. In contrast, the orientation of the remnant disc is code-independent. In all codes, the rotational angular momentum of the remnant disc aligns with the interaction's orbital angular momentum rather than the pre-merger rotational axis, implying that the infalling gas preserves its orbital angular momentum to form a new disc. Comparisons with the Santa Cruz semi-analytic model show reasonable agreement in stellar mass and half-mass radius, yet the model underpredicts (overpredicts) the dark matter fraction and velocity dispersion for codes exhibiting strong compaction (expansion). The systematic dependence of our remnants' morphology on feedback schemes demonstrates that merger remnant morphology may serve as a powerful probe of stellar feedback processes.

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arXiv2026

A steep mass transition for bar-driven ISM structuring revealed by PHANGS-JWST

Galactic bars play a critical role in the secular evolution of their hosts by reorganising the ISM. We use a sample of 57 star-forming disc galaxies observed with JWST at 3 and 7.7 $μ$m to probe how the spatial distribution of PAH emission, as a structural marker of the cold ISM, depends on stellar mass and bar presence. We find evidence for a "watershed" at a stellar mass of $10^{10}$ Msun, marking a fundamental transition in the bar-driven distribution of PAH emission. This confirms trends previously predicted by numerical simulations and observed via ionised gas or UV light. While lower-mass galaxies exhibit a disordered and clumpy distribution of PAH emission regardless of bar presence, higher-mass barred hosts display well-structured dynamical features traced by PAH emission with significant gas reservoirs (e.g., discs and rings) within the central 15% of the bar radius (Rb). Furthermore, we observe a systematic depletion of PAH emission within the [0.2-0.8] Rb range in barred systems with stellar masses above $10^{10}$ Msun. Such central discs, rings, and associated radial dips ("bar deserts") appear to be a mass-dependent phenomenon: ubiquitous in massive galaxies but mostly absent in lower-mass counterparts. In contrast to the structured features in massive hosts, the disorganised ISM in lower-mass galaxies masks commonly observed bar-driven signatures. This suggests that tracer selection and dust obscuration may significantly bias observed bar fractions. Our study underlines two regimes of secular evolution, with different impacts and observability of bar-driven processes: it reaffirms bars as primary drivers of rapid secular evolution in galaxies above $10^{10}$ Msun$, while their impact is significantly reduced or delayed below this threshold. It further underscores the need to account for these processes when modelling galaxy evolution in cosmological simulations.

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arXiv2026

A stellar bar hidden in an extreme gas-rich disk galaxy at z=4.055

The classical picture for the formation of stellar bars -- key dynamical drivers of the evolution of galaxies -- is through secular evolution of instability in gas poor, stellar-dominated disks. The detection with the James Webb Space Telescope (JWST) of stellar bars and spiral arms in galaxies at early cosmic times has thus challenged LambdaCDM-based expectations, which recent studies reconcile by suggesting that these galaxies are baryon-dominated and have already consumed most of their gas. Yet, a paradox arises, as early galaxies are expected to be increasingly rich in gas, which is generally considered to prevent or slow down stellar bar formation. Here, we show the detection of a stellar bar in GN20, a gas-rich star-forming disk galaxy at a redshift of z=4.055, only 1.5 billion years after the Big Bang. Simultaneous observations of the stars, gas, and dust reveal that GN20 is indeed baryon-dominated (over dark matter; 72+/-34%), but the baryonic mass is largely in the form of gas (74+/-25%). This discovery demonstrates that gas-rich disks do support rapid stellar bar formation in the early Universe, motivating a new theoretical perspective on bar formation in gas-rich systems, and providing a potential new mechanism for very early galaxy assembly and quenching.

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