arXiv subjects

Lars Hernquist

Publications and source records attributed to Lars Hernquist.

At least 55 records · Page 3Linked to original sources

arXiv2025

AGN feedback in merging galaxies with a SMUGGLE multiphase ISM

We study fast nuclear winds driven by Active Galactic Nucleus (AGN) feedback in merging galaxies using high-resolution hydrodynamics simulations. We use Stars and MUltiphase Gas in GaLaxiEs (SMUGGLE) to explicitly model the multiphase interstellar medium (ISM) and employ sub-grid dynamical friction for massive black holes (BHs). Furthermore, we use a super-Lagrangian refinement scheme to resolve AGN feedback coupling to the ISM at $\sim10-100\,$pc scales. By comparison between merging and isolated galaxies, with and without AGN feedback, we identify trends in the complex interplay between dynamics, BH fueling and feedback, and star formation and feedback. We consider three galaxy types: Milky Way analogs, Sbc-type galaxies, and Small Magellanic Cloud (SMC) analogs. The synergy between AGN feedback and merger dynamics is strongest in the Milky Way-like mergers, where the AGN winds are energetically dominant and entrain more gas when the initially thin disks become thick and amorphous during the merger. In contrast, the merger of thicker, vigorously star-forming Sbc galaxies is not strongly impacted by AGN feedback until star formation declines in the post-merger phase. Finally, while the sub-grid dynamical friction prescription effectively retains BHs in galactic nuclei during more massive mergers, the clumpy multiphase ISM induces significant wandering of low-mass BHs $\mathrm{(<10^5M_\odot)}$ in the shallow potentials of the SMC-like galaxies. These low-mass BHs wander at distances $\gtrsim 2$ kpc from the galactic center, yielding negligible BH accretion and feedback. This has implications for LISA event rates and present a further challenge to understanding the rapid growth of $z\sim7-10$ quasars discovered by JWST.

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arXiv2025

Synthetic JWST galaxy images in the TNG50 simulation - I. Model validation and comparison to observations

We use the TNG50 cosmological simulation and three-dimensional radiative transfer post-processing to generate dust-aware synthetic observations of galaxies at $ 3 \leq z \leq 6 $ and $ \log_{10}(M_\ast/\mathrm{M}_\odot) \geq 8.5 $, tailored to match the depth and resolution of current deep JWST surveys (NGDEEP and JADES). We analyse the performance of spectral energy distribution (SED) fitting on the simulated sample, focusing on the recovery of photometric redshift and stellar mass. At $ z \leq 5 $, we find that 90 per cent of redshifts are recovered within $ \pm0.2 $, but performance declines at $ z = 6 $. Stellar masses are generally well-recovered within a factor of 2, but are systematically underestimated regardless of redshift, a trend that is more pronounced at the high-mass end $ ( \log_{10}(M_\ast/\mathrm{M}_\odot) \geq 10 ) $. In addition, we study the observer-frame colours of galaxies in this redshift range as well as the SED-inferred $UVJ$ diagram. We find that TNG50 galaxies broadly follow the tendencies marked by observations, but tend to be slightly redder at lower masses and bluer at higher masses, regardless of redshift. Finally, using a colour-based definition of quiescence, we determine the fraction of quiescent galaxies as a function of stellar mass at $ 3 \leq z \leq 6 $, which we find to be broadly consistent with observations.

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arXiv2025

Measuring the splashback feature: Dependence on halo properties and history

In this study, we define the novel splashback depth $\mathcal{D}$ and width $\mathcal{W}$ to examine how the splashback features of dark matter haloes are affected by the physical properties of haloes themselves. We use the largest simulation run in the hydrodynamic MillenniumTNG project. By stacking haloes in bins of halo mass, redshift, mass-dependent properties such as peak height and concentration, and halo formation history, we measure the shape of the logarithmic slope of the density profile of dark matter haloes. Our results show that the splashback depth has a strong dependence on the halo mass which follows a power law $\mathcal{D}\propto\left(\log_{10}M\right)^{2.8}$. Properties with strong correlation with halo mass demonstrate similar dependence. The splashback width has the strongest dependence on halo peak height and follows a power law $\mathcal{W}\proptoν^{-0.87}$. We provide the fitting functions of the splashback depth and width in terms of halo mass, redshift, peak height, concentrations and halo formation time. The depth and width are therefore considered to be a long term memory tracker of haloes since they depend more on accumulative physical properties, e.g., halo mass, peak height and halo formation time. They are shaped primarily by the halo's assembly history, which exerts a stronger influence on the inner density profile than short-term dynamical processes. In contrast, the splashback features have little dependence on the short term factors such as halo mass accretion rate and most recent major merger time. The splashback depth and width can therefore be used to complement information gained from quantities like the point of steepest slope or truncation radius to characterise the halo's history and inner structure.

astro-ph.CO↗

arXiv2025

A smooth filament origin for distant prolate galaxies seen by JWST and HST

The initial gravitational collapse of Dark Matter and gas forms a universal filamentary network where the first galaxies form, with shapes and sizes that depend on the choice of Dark Matter. Claims from deep space imaging surveys that elongated galaxies predominate at $z > 3$ are examined here by comparison with detailed hydrodynamical simulations of Cold Dark Matter (CDM), Warm Dark Matter (WDM), and Wave/Fuzzy Dark Matter, $ψ$DM. For CDM and WDM we have sufficient volume, $10^{3}\,\mathrm{Mpc/h}^{3}$, to generate galaxies with stellar masses $> 10^{9}\,M_{\odot}$ at $z > 2$, allowing comparison with the CEERS and CANDELS surveys. We find the observed tendency towards elongated, prolate-shaped young galaxies is well matched by WDM, from material accreted along smooth filaments during the first $\simeq 500\,\mathrm{Myr}$, with little dependence on stellar mass. This contrasts with CDM, where the stellar morphology is mainly spheroidal, formed from merging of fragmented filaments. For CDM, several subhalos are predicted to be visible, whereas for WDM and $ψ$DM, early merging is rare. Our findings show how the shapes and sizes of early galaxies are sensitive to the smoothness of the underlying filament network, providing a new constraint on the nature of dark matter.

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arXiv2025

Galaxy formation with wave/fuzzy dark matter: The core-halo structure and the solitonic imprint

Dark matter-dominated cores have long been claimed for the well-studied local group dwarf galaxies. More recently, extended stellar halos have been uncovered around several of these dwarfs through deeper imaging and spectroscopy. Such core-halo structures are not a feature of conventional cold dark matter (CDM). In contrast, smooth and prominent dark matter cores are predicted for wave/fuzzy dark matter ($ψ$DM). The question arises as to what extent the visible stellar profiles should reflect this dark matter core structure. Here we compare cosmological hydrodynamical simulations of CDM, ``WDM'' (model used as a proxy for $ψ$DM) \& $ψ$DM, aiming to predict the stellar profiles for these three DM scenarios. We show that cores surrounded by extended halos are distinguishable for $ψ$DM, where the stellar density is enhanced in the core due to the presence of the relatively dense soliton. Our analysis demonstrates that, in our simulations, a distinctive core-halo structure does not appear in the case of CDM in the DM, gas, or stars. Whereas we do find a core-halo transition for DM, gas, and stars for $ψ$DM, and the scale of this transition is in line with the predicted core radius set by the soliton scale anticipated for the adopted boson mass of 2.5$\times10^{-22}$eV. The presence of a core-halo structure in the stellar profile for Galaxy 1 for $ψ$DM is visible for the most massive and the first galaxy to form in the simulation. Clearly, further simulations are needed to establish how strict this possible relationship is between the DM and stellar core-halo profile as a potential observational discriminator. Furthermore, we observe the anticipated asymmetry for $ψ$DM due to the soliton's motion (jumping and random walk), a distinctive characteristic not found in the symmetric distributions of stars in the warm and CDM models.

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arXiv2025

MISTRAL: a model for AGN winds from radiatively efficient accretion in cosmological simulations

Feedback from active galactic nuclei (AGN) is crucial for regulating galaxy evolution. Motivated by observations of broad absorption line winds from rapidly accreting supermassive black holes (SMBHs), we introduce the Mistral AGN feedback model, implemented in the Arepo code. Mistral comes in two versions: continuous radial (Mistral-continuous) and stochastic bipolar momentum deposition (Mistral-stochastic). Using the framework of the IllustrisTNG simulations, we explore the effect of Mistral on BH and galaxy properties, through an idealized Milky Way-mass galaxy and cosmological zoom simulations run down to $z=2$. Unlike standard thermal AGN feedback prescriptions, Mistral generates galaxy-scale winds that mimic outflows driven by BH accretion. Mistral-continuous produces short-lived galactic fountains, and is inefficient at regulating the growth of massive galaxies at $z=2$. In contrast, Mistral-stochastic efficiently suppresses star formation in massive galaxies, reproduces the empirical stellar-to-halo mass relation, and yields a consistent trend of BH-stellar mass evolution. By supporting large-scale outflows while simultaneously preventing gas inflows, Mistral-stochastic additionally regulates the cold and hot gas fractions at both galaxy and halo scales. Mistral-stochastic therefore works self-consistently across the halo mass range explored $\left(10^{12}-3\times10^{13}\,\rm M_\odot\right)$, without adopting a SMBH-mass dependent AGN feedback scheme such as the one used in IllustrisTNG. Our model is a promising tool for predicting the impact of AGN winds on galaxy evolution, and interpreting the growing population of high-redshift galaxies and quasars observed by JWST. This work is part of the "Learning the Universe" collaboration, which aims to infer the physical processes governing the evolution of the Universe.

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arXiv2025

Little Red Dots Are Nurseries of Massive Black Holes

The James Webb Space Telescope (JWST) has revealed a previously unknown population of compact, red galaxies at $z \sim 5$, known as "Little Red Dots" (LRDs). With effective radii of $\sim 100$ pc and stellar masses of $10^9-10^{11} \, M_\odot$, a purely stellar interpretation implies extreme central densities, $ρ_\star\sim10^4-10^5 \, M_\odot \, \mathrm{pc}^{-3}$ and in some cases up to $\sim 10^9 \, M_\odot \, \mathrm{pc}^{-3}$, far exceeding those of globular clusters. At such densities, the dynamical friction time for $10 \, M_\odot$ stars in the central $0.1$ pc is $< 0.1$ Myr, driving rapid mass segregation. We investigate the dynamical consequences of such an environment using: (i) a Fokker-Planck analysis of long-term core evolution, (ii) an analytical model for the collisional growth of a very massive star (VMS), and (iii) direct $N$-body simulations. All approaches show that runaway collisions produce a VMS with mass $9\times10^3 < M_{\rm VMS} \, [M_\odot] < 5\times10^4$ within $<1$ Myr. Once the supply of massive stars is depleted, the VMS contracts on a $\sim 8000$ yr Kelvin-Helmholtz timescale and undergoes a general relativistic collapse, leaving a massive black hole of mass $M_\bullet \sim 10^4 \, M_\odot$. We conclude that LRDs are natural nurseries for the formation of heavy black hole seeds via stellar-dynamical processes. This pathway produces seed number densities that far exceed those expected from direct collapse models, and, owing to the dense residual stellar core, can sustain high rates of tidal disruption events.

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arXiv2025

Introducing the THESAN-ZOOM project: radiation-hydrodynamic simulations of high-redshift galaxies with a multi-phase interstellar medium

We introduce the THESAN-ZOOM project, a comprehensive suite of high-resolution zoom-in simulations of $14$ high-redshift ($z>3$) galaxies selected from the THESAN simulation volume. This sample encompasses a diverse range of halo masses, with $M_\mathrm{halo} \approx 10^8 - 10^{13}~\mathrm{M}_\odot$ at $z=3$. At the highest-resolution, the simulations achieve a baryonic mass of $142~\mathrm{M}_\odot$ and a gravitational softening length of $17~\mathrm{cpc}$. We employ a state-of-the-art multi-phase interstellar medium (ISM) model that self-consistently includes stellar feedback, radiation fields, dust physics, and low-temperature cooling through a non-equilibrium thermochemical network. Our unique framework incorporates the impact of patchy reionization by adopting the large-scale radiation field topology from the parent THESAN simulation box rather than assuming a spatially uniform UV background. In total, THESAN-ZOOM comprises $60$ simulations, including both fiducial runs and complementary variations designed to investigate the impact of numerical and physical parameters on galaxy properties. The fiducial simulation set reproduces a wealth of high-redshift observational data such as the stellar-to-halo-mass relation, the star-forming main sequence, the Kennicutt-Schmidt relation, and the mass-metallicity relation. While our simulations slightly overestimate the abundance of low-mass and low-luminosity galaxies they agree well with observed stellar and UV luminosity functions at the higher mass end. Moreover, the star-formation rate density closely matches the observational estimates from $z=3-14$. These results indicate that the simulations effectively reproduce many of the essential characteristics of high-redshift galaxies, providing a realistic framework to interpret the exciting new observations from JWST.

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arXiv2025

Heavy seeds and the first black holes: Insights from the BRAHMA simulations

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

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arXiv2025

The Cosmic Rush Hour: Rapid Formation of Bright, Massive, Disky, Star-Forming Galaxies as Signatures of Early-Universe Physics

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

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arXiv2025

Star Formation Rates, Metallicities, and Stellar Masses on kpc-scales in TNG50

Integral field units (IFU) have extended our knowledge of galactic properties to kpc (or, sometimes, even smaller) patches of galaxies. These scales are where the physics driving galaxy evolution (feedback, chemical enrichment, etc.) take place. Quantifying the spatially-resolved properties of galaxies, both observationally and theoretically, is therefore critical to our understanding of galaxy evolution. To this end, we investigate spatially-resolved scaling relations within galaxies of $M_\star>10^{9.0}$ at $z=0$ in IllustrisTNG. We examine both the resolved star-forming main sequence (rSFMS) and the resolved mass-metallicity relation (rMZR) using $1~{\rm kpc}\times1~{\rm kpc}$ maps. We find that the rSFMS in IllustrisTNG is well-described by a power-law, but is significantly shallower than the observed rSFMS. However, the disagreement between the rSFMS of IllustrisTNG and observations is likely driven by an overestimation of AGN feedback in IllustrisTNG for the higher mass hosts. Conversely, the rMZR for IllustrisTNG has very good agreement with observations. Furthermore, we argue that the rSFMS is an indirect result of the Schmidt-Kennicutt (SK) law and local gas relation, which are both independent of host galaxy properties. Finally, we expand upon a localized leaky-box model to study the evolution of idealized spaxels and find that it provides a good description of these resolved relations. The degree of agreement, however, between idealized spaxels and simulated spaxels depends on the `net' outflow rate for the spaxel, and the IllustrisTNG scaling relations indicate a preference for a low net outflow rate.

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arXiv2025

How does feedback affect the star formation histories of galaxies?

Star formation in galaxies is regulated by the interplay of a range of processes that shape the multiphase gas in the interstellar and circumgalactic media. Using the CAMELS suite of cosmological simulations, we study the effects of varying feedback and cosmology on the average star formation histories (SFHs) of galaxies at $z\sim0$ across the IllustrisTNG, SIMBA and ASTRID galaxy formation models. We find that galaxy SFHs in all three models are sensitive to changes in stellar feedback, which affects the efficiency of baryon cycling and the rates at which central black holes grow, while effects of varying AGN feedback depend on model-dependent implementations of black hole seeding, accretion and feedback. We also find strong interaction terms that couple stellar and AGN feedback, usually by regulating the amount of gas available for the central black hole to accrete. Using a double power-law to describe the average SFHs, we derive a general set of equations relating the shape of the SFHs to physical quantities like baryon fraction and black hole mass across all three models. We find that a single set of equations (albeit with different coefficients) can describe the SFHs across all three CAMELS models, with cosmology dominating the SFH at early times, followed by halo accretion, and feedback and baryon cycling at late times. Galaxy SFHs provide a novel, complementary probe to constrain cosmology and feedback, and can connect the observational constraints from current and upcoming galaxy surveys with the physical mechanisms responsible for regulating galaxy growth and quenching.

astro-ph.GA↗

arXiv2025

Accretion onto supermassive and intermediate mass black holes in cosmological simulations

Accretion is the dominant contribution to the cosmic massive black hole density in the Universe today. Yet, modelling it in cosmological simulations is challenging due to the dynamic range involved, as well as the theoretical uncertainties of the underlying mechanisms driving accretion from galactic to black hole horizon scales. We present a simple, flexible parametrization for gas inflows onto massive black holes in order to manage this uncertainty in large-volume cosmological simulations. This is done as part of the "Learning the Universe'' collaboration, which aims to jointly infer the initial conditions and physical processes governing the evolution of the Universe using a Bayesian forward-modelling approach. To allow such a forward-modelling, we update the prescription for accretion with a two-parameter free-fall based inflow estimate that allows for a radius-dependent inflow rate and add a simple model for unresolved accretion disks. We use uniform resolution cosmological hydrodynamical simulations and the IllustrisTNG framework to study the massive black hole population and its dependence on the introduced model parameters. Once the parameters of the accretion formula are chosen to result in a roughly similar redshift zero black hole mass density, the differences caused by the details in the accretion formula are moderate in the supermassive black hole regime, indicating that it is difficult to distinguish between accretion mechanisms based on luminous active galactic nuclei powered by supermassive black holes. Applying the same models to intermediate mass black holes at high redshift, however, reveals significantly different accretion rates in high redshift, moderate luminosity active galactic nuclei and different frequencies and mass distributions of intermediate mass black hole mergers for the same black hole formation model.

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arXiv2025

Development of Turbulence in Post-shock Regions

Supersonic isothermal turbulence is ubiquitous in the interstellar medium. This work presents high-resolution AREPO hydrodynamical simulations of isolated shocks moving through supersonic turbulence to study the development and evolution of turbulence in the pre- and post-shock regions. We find that shocks can amplify turbulent energy in the post-shock region while inducing a preferential orientation for the vorticity. This results in the creation of anisotropic turbulence in the post-shock region. Turbulent energy and dissipation are also strongly enhanced near the shock front. By applying typical scalings from the cold neutral medium to simulations, we find that shocks moving into turbulence on the scale of superbubbles can generate compressive flows on the order of $10^{3} M_{\odot}$/Myr. Our results also show good agreement with related studies on turbulent fluctuations generated by shocks in pure fluid mechanics.

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arXiv2025

How Early Could the Milky Way's Disk Form?

We investigate early, $z > 3$, galaxy formation in a cosmological zoom-in simulation of a close, early-forming Milky Way (MW) analog extracted from TNG50 simulation and resimulated with detailed modeling of cold interstellar medium (ISM) formation, coupled with on-the-fly UV radiative transfer, turbulence-regulated star formation, and stellar feedback. In our enhanced-physics simulation, the galaxy develops a bistable ISM structure (warm, with $T \sim 10^4$ K, and cold, with $T < 100$ K) and exhibits significantly more efficient, early, and bursty star formation than in TNG. Notably, the stellar disk of this MW progenitor forms extremely early, around $z\sim6-7$, and exhibits chemo-kinematic properties consistent with the low-metallicity population of the MW stars. The disk forms rapidly, on a timescale of $\sim$0.2 Gyr which is significantly shorter than the timescale implied by the observable chemo-kinematic signatures of disk spinup, $\sim$0.7 Gyr, due to the scatter in the age--metallicity relation. The rotational support of the gas disk and the location of the galaxy on the main sequence are consistent with early disk galaxies observed by JWST and ALMA at $z\sim4-7$, suggesting that some of these galaxies could be progenitors of MW-like systems. Remarkably, the variation of the global star formation rate (SFR) before disk formation is similar to the observed SFR scatter at these early times. Our findings underscore the critical role of modeling a turbulent cold ISM and turbulence-regulated star formation and feedback in driving early SFR variability, while at the same time enabling early disk formation, without destroying it with overly efficient stellar feedback.

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arXiv2025

From UV-bright Galaxies to Early Disks: the Importance of Turbulent Star Formation in the Early Universe

Bursty star formation at early times can explain the surprising abundance of early UV-bright galaxies revealed by JWST but can also be a reason for the delayed formation of galactic disks in high-resolution cosmological simulations. We investigate this interplay in a cosmological simulation of an early-forming Milky Way analog with detailed modeling of the cold turbulent interstellar medium (ISM), star formation, and feedback. We find that the modeling of locally variable star formation efficiency (SFE) coupled with the ISM turbulence on the scales of star-forming regions is important for producing both early bursty evolution and early formation and survival of galactic disks. Such a model introduces a qualitatively new channel of the global star formation rate (SFR) burstiness caused by chaotic fluctuations in the average SFE due to changes in the ISM turbulence, which, in our simulation, dominates the short-term SFR variability. The average SFE stays low, close to $\sim 1\%$ per freefall time, and its variation decreases when the gas disk forms, leading to only mild effects of stellar feedback on the early disk, enabling its survival. By rerunning our simulation with fixed SFE values, we explicitly show that low SFEs lead to smoother SFR histories and disk survival, while high SFEs lead to bursty SFRs and hinder disk formation. The model with variable SFE switches between these two regimes at the moment of disk formation. These trends are missing in more commonly used star formation prescriptions with fixed SFE; in particular, the prescriptions tying star formation to molecular gas should be interpreted with caution because the two are decoupled at early times, as we also show in this paper.

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arXiv2025

Metallicity Gradients in Modern Cosmological Simulations I: Tension Between Smooth Stellar Feedback Models and Observations

The metallicity of galaxies, and its variation with galactocentric radius, provides key insights into the formation histories of galaxies and the physical processes driving their evolution. In this work, we analyze the radial metallicity gradients of star forming galaxies in the EAGLE, Illustris, IllustrisTNG, and SIMBA cosmological simulations across a broad mass ($10^{8.0}M_\odot\leq M_\star \lesssim10^{12.0}M_\odot$) and redshift ($0\leq z\leq8$) range. We find that all simulations predict strong negative (i.e., radially decreasing) metallicity gradients at early cosmic times, likely due to their similar treatments of relatively smooth stellar feedback not providing sufficient mixing to quickly flatten gradients. The strongest redshift evolution occurs in galaxies with stellar masses of $10^{10.0}-10^{11.0}M_\odot$, while galaxies with stellar masses $< 10^{10}M_\odot$ and $>10^{11}M_\odot$ exhibit weaker redshift evolution. Our results of negative gradients at high-redshift contrast with the many positive and flat gradients in the $1 6$, the negative gradients observed with JWST and ALMA are flatter than those in simulations, albeit with closer agreement than at lower redshift. Overall, we suggest that these smooth stellar feedback galaxy simulations may not sufficiently mix their metal content radially, and that either stronger stellar feedback or additional subgrid turbulent metal diffusion models may be required to better reproduce observed metallicity gradients.

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arXiv2025

Invisible Accretion: Ionized Envelopes of TNG50 HVCs can Sustain Star Formation

Galactic high-velocity clouds (HVCs) are known to be complex, multiphase systems consisting of neutral and/or ionized gas moving at high velocities relative to the rotation of the disk. In this work, we investigate Milky Way-like galaxies from the TNG50 simulation to characterize the properties, morphology, and accretion rates of the warm and hot ionized material comoving with neutral HVCs visible in HI. We find that the ionized gas forms an envelope around the neutral material, and in most cases (73% of the HVCs) it is prolate in morphology. We also find that the ionized mass is ~6 times greater than the neutral mass, which leads to significantly more accretion possible from the ionized gas ($\dot{M}_\mathrm{ion}$) than the neutral gas ($\dot{M}_\mathrm{neut}$), consistent with estimates made from observations of our own Galaxy. We investigate the accretion rates from both phases of HVCs around 47 Milky Way-like galaxies and find that $\dot{M}_\mathrm{ion}$ scales with $\dot{M}_\mathrm{neut}$, and both scale with the star formation rate of the galaxy. Finally, we find that, on average, $\dot{M}_\mathrm{ion}$ could account for 81% of the galactic star formation rate (assuming the material can sufficiently cool and condense), while $\dot{M}_\mathrm{neut}$ can only balance 11%. Thus, the diffuse, ionized, high-velocity circumgalactic medium plays a defining role in the evolution and growth of galaxies at low redshift.

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