arXiv subjects

Oscar Agertz

Publications and source records attributed to Oscar Agertz.

At least 19 recordsLinked to original sources

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.

astro-ph.GA↗

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

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

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

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.

astro-ph.GA↗

arXiv2026

Enhanced rates of stellar radial migration in gas-rich discs at high redshift

Radial migration and dynamical heating redistribute stars within galactic discs and thereby modify the chemo-kinematic structure of their host galaxies. Usually, these secular processes are studied in N-body and hydrodynamical simulations of Milky Way analogues with stellar-dominated discs. In contrast, discs at high redshift are gas-rich, which may qualitatively change how secular evolution proceeds. We use the Nexus framework to construct and evolve a suite of isolated galaxies with fixed halo and disc mass but varying initial disc gas fraction, from 0% to 100%. We show that in gas-rich models, the root-mean-square change in stellar angular momentum is up to a factor of two larger than in gas-poor analogues and is accompanied by stronger radial and vertical heating, leading to enhanced radial mixing. We further dissect the role of gas in specific migration channels. For bar-driven migration, corotation resonance dragging dominates in gas-poor discs, whereas in gas-rich discs, stars more readily reach and accumulate at the outer Lindblad resonance, which acts as a barrier. The high radial mixing efficiency in gas-rich phases can flatten the stellar metallicity gradient relative to that of the initial gaseous disc within only a few orbital timescales. Together, these results imply that radial mixing in early, gas-rich discs is substantially more vigorous than in late-time, gas-poor discs, naturally producing distinct evolutionary tracks for chemically bimodal discs such as that of the Milky Way.

astro-ph.GA↗

arXiv2026

The AGORA High-resolution Galaxy Simulations Comparison Project. XI: Solving the Non-Spherical Morphology and Evolution of Dark Matter Halos with Haskap Pie

We introduce a halo solving and tracking procedure that intrinsically treats dark matter halos as non-spherical objects by leveraging the bound particle searching techniques used in Haskap Pie. The AGORA Collaboration's hydrodynamic simulation CosmoRun}project provides a useful laboratory to explore trends in dark matter halo morphology that are revealed by our new procedure in the context of any dispersions or similarities between the codes. We find that several morphological and shape measures were very responsive to high mass ratio mergers. The greatest difference in these measures between the simulation codes were related to timing discrepancies and the dynamical state of the halos prior to the mergers. Most other quantities were similar across codes, including several secular and redshift-dependent trends in various dynamical quantities that showed a departure from Virial Theorem (e.g., overdensity and halo mass). We find that halo spin and the ratio between the semi-major and the semi-minor axis peaked at 4>z>2 before declining at low redshift. Also, halo overdensity is both mass-dependent and redshift-dependent, diverging for low mass halos at low redshift. Our method contributes a new perspective on these trends that have not been fully replicated in other works due to our emphasis on fundamentally non-spherical halos and measures of morphology that correspondingly do not assume spherical symmetry.

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arXiv2026

Cosmic evolution of the [CII]-to-molecular 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 investigate the evolution of the relation between the [CII] luminosity and molecular 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-process the snapshots with the Skirt radiative transfer code to generate synthetic [CII] data cubes. We measure global and spatially resolved (100 pc) relations between [CII] luminosity ($L_{\rm [CII]}$), star formation rate (SFR), and molecular gas mass ($M_{\rm mol}$). We follow 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 $L_{\rm [CII]}$-$M_{\rm mol}$ 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 molecular gas. A unique, redshift-independent $α_{\rm [CII]}$ therefore cannot recover molecular gas masses across the regimes we explore. [CII] remains a viable tracer of molecular gas at very high redshifts, but only when used with conversion factors that explicitly account for metallicity, ISM phase mix, and merger events.

astro-ph.GA↗

arXiv2026

The AGORA High-resolution Galaxy Simulations Comparison Project. X: Formation and Evolution of Galaxies at the High-redshift Frontier

Recent observations from JWST have revealed unexpectedly luminous galaxies, exhibiting stellar masses and luminosities significantly higher than predicted by theoretical models at Cosmic Dawn. In this study, we present a suite of cosmological zoom-in simulations targeting high-redshift ($z \geq 10$) galaxies with dark matter halo masses in the range $10^{10} - 10^{11}\ {\rm M}_{\odot}$ at $z=10$, using state-of-the-art galaxy formation simulation codes (Enzo, Ramses, Changa, Gadget-3, Gadget-4, and Gizmo). This study aims to evaluate the convergence of the participating codes and their reproducibility of high-redshift galaxies with the galaxy formation model calibrated at relatively low redshift, without additional physics for high-redshift environments. The subgrid physics follows the AGORA CosmoRun framework, with adjustments to resolution and initial conditions to emulate similar physical environments in the early universe. The participating codes show consistent results for key galaxy properties (e.g., stellar mass), but also reveal notable differences (e.g., metallicity), indicating that galaxy properties at high redshifts are highly sensitive to the feedback implementation of the simulation. Massive halos (${\rm M}_{\rm halo}\geq5\times10^{10}\,{\rm M}_{\odot}$ at $z=10$) succeed in reproducing observed stellar masses, metallicities, and UV luminosities at $10\leq z\leq12$ without requiring additional subgrid physics, but tend to underpredict those properties at higher redshift. We also find that varying the dust-to-metal ratio modestly affects UV luminosity of simulated galaxies, whereas the absence of dust significantly enhances it. In future work, higher-resolution simulations will be conducted to better understand the formation and evolution of galaxies at Cosmic Dawn.

astro-ph.GA↗

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.

astro-ph.GA↗

arXiv2026

The causal effect of cosmic filaments on dark matter halos

The way in which the large-scale cosmic environment affects galactic properties is not yet understood. Dark matter halos, which embed galaxies, initially evolve following linear theory. Their subsequent evolution is driven by non-linear structure formation in the halo region and in its outer environment. In this work, we present the first study where we explicitly control the linear part of the evolution of the halo, thus revealing the role of non-linear effects on halo formation. We focus specifically on the effect of proximity to a large cosmological filament. We employ the splicing method to keep fixed the initial density, velocity, and potential fields where a halo will form while changing its outer environment, from an isolated state to one where the halo is near a large filament. In the regime of Milky Way-mass halos, we find that mass and virial radius of such halos are not affected by even drastic changes of environment, whereas halo spin and shape orientation with respect to a massive filament is largely impacted, with fluctuations of up to 80 % around the mean value. Our results suggest that halo orientation and shape cannot be predicted accurately from a local analysis in the initial conditions alone. This has direct consequences on the modeling of intrinsic alignment for cosmic shear surveys, like Euclid. Our results highlight that non-linear couplings to the large-scale environment may have an amplitude comparable to linear effects, and should thus be treated explicitly in analytical models of dark matter halo formation.

astro-ph.CO↗

arXiv2026

MEGATRON: The environments of Population III stars at Cosmic Dawn and their connection to present day galaxies

We present results of Pop. III formation in the MEGATRON suite of simulations, which self-consistently follows radiation and non-equilibrium chemistry, and resolves gas at near-pc resolution of a Milky Way-mass halo at Cosmic Dawn. While the very first Pop. III stars form in halos with masses well below the atomic cooling limit, whose cooling is dominated by molecular hydrogen, the majority of Pop. III stars form in more massive systems above the $10^4$~K atomic cooling threshold. The shift in cooling regime of halos hosting new Pop. III stars occurs within $100$ Myr of the first Pop. III star as the Lyman-Werner (LW) background rapidly increases to $10^{-21}\,\rm erg\,s^{-1}\,cm^{-2}\,Hz^{-1}\,sr^{-1}$. We find that the global Pop. III star formation rate stabilizes to a value of $10^{-3}\,\rm M_\odot\,yr^{-1}$ at $z=20$. Among the three processes that quench Pop. III star formation in mini-halos, the LW background, gas starvation, and external chemical enrichment, the LW background is most important. A small fraction of haloes undergo multiple episodes of Pop. III star formation when the earlier forming stars all directly collapse to black holes. If the halos become massive enough, they can form up to $\sim100$ Pop. III stars in a single burst, which may be observable by JWST with moderate gravitational lensing. Pop. III stars form at a wide range of distances from UV-bright galaxies, with only $0.06\%$ of Pop. III stars forming within the virial radius of galaxies with $M_{\rm UV} < -17$. Finally, by tracking Pop. III star remnants down to $z=0$, we find that $75-80\,$% reside in the stellar halo of our simulated Milky Way analogue, while the remainder are gravitationally bound to lower-mass systems, including satellite halos.

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arXiv2026

Cloud-scale gas properties, depletion times, and star formation efficiency per free-fall time in PHANGS--ALMA

We compare measurements of star formation efficiency to cloud-scale gas properties across PHANGS-ALMA. Dividing 67 galaxies into 1.5 kpc scale regions, we calculate the molecular gas depletion time, tau_dep= Sigma_mol/Sigma_SFR, and the star formation efficiency per free-fall time, eff=tau_ff/tau_dep, for each region. Then we test how tau_dep and eff vary as functions of the regional mass-weighted mean molecular gas properties on cloud scales (60-150pc): gas surface density, , velocity dispersion, , virial parameter, , and gravitational free-fall time, . and tau_dep correlate positively, consistent with the expectation that gas density plays a key role in setting the rate of star formation. Our fiducial measurements suggest tau_dep \propto ^0.5 and eff \approx 0.39%, though the exact numbers depend on the adopted fitting methods. We also observe anti-correlations between tau_dep and and between tau_dep^mol and . All three correlations may reflect the same underlying link between density and star formation efficiency combined with systematic variations in the degree to which self-gravity binds molecular gas in galaxies. We highlight the tau_dep- relation because of the lower degree of correlation between the axes. Contrary to theoretical expectations, we observe an anti-correlation between tau_dep^mol and and no significant correlation between eff and . Our results depend sensitively on the adopted CO-to-H2 conversion factor, with corrections for excitation and emissivity effects in inner galaxies playing an important role. We emphasize that our simple methodology and clean selection allow easy comparison to numerical simulations and highlight this as a logical next direction.

astro-ph.GA↗

arXiv2026

The Impact of Star Formation and Feedback Recipes on the Stellar Mass and Interstellar Medium of High-Redshift Galaxies

We introduce MEGATRON, a new galaxy formation model for cosmological radiation hydrodynamics simulations of high-redshift galaxies. The model accounts for the non-equilibrium chemistry and heating/cooling processes of $\geq 80$ atoms, ions, and molecules, coupled to on-the-fly radiation transfer. We apply the model in a cosmological setting to the formation of a $10^9\ {\rm M_{\odot}}$ halo at $z=6$, and run 25 realizations at pc-scale resolution, varying numerous parameters associated with our state-of-the-art star formation, stellar feedback, and chemical enrichment models. We show that the overall budget of feedback energy is the key parameter that controls star formation regulation at high redshift, with other numerical parameters (e.g. supernova clustering, star formation conditions) having a more limited impact. As a similar feedback model has been shown to produce realistic $z=0$ galaxies, our work demonstrates that calibration at $z=0$ does not guarantee strong regulation of star formation at high-redshift. Interestingly, we find that subgrid model variations that have little impact on the final $z=6$ stellar mass can lead to substantial changes on the observable properties of high-redshift galaxies. For example, different star formation models based on, e.g. density thresholds or turbulence inspired criteria, lead to fundamentally distinct nebular emission line ratios across the interstellar medium (ISM). These results highlight the ISM as an important resource for constraining models of star formation, feedback, and galaxy formation in the JWST era, where emission line measurements for $>1,000$ high-redshift galaxies are now available.

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