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Abstract The dust grain size distribution (GSD) likely varies significantly across star-forming environments in the Universe, but its impact on star formation remains unclear. This ambiguity arises because the GSD interacts nonlinearly with processes like heating, cooling, radiation, and chemistry, which have competing effects and varying environmental dependencies. Processes such as grain coagulation, expected to be efficient in dense star-forming regions, reduce the abundance of small grains and increase that of larger grains. Motivated by this, we investigate the effects of similar GSD variations on the thermochemistry and evolution of giant molecular clouds (GMCs) using magnetohydrodynamic simulations spanning a range of cloud masses and grain sizes, which explicitly incorporate the dynamics of dust grains within the full-physics framework of the STARFORGE project. We find that grain size variations significantly alter GMC thermochemistry: the leading-order effect is that larger grains, under fixed dust mass, GSD dynamic range, and dust-to-gas ratio, result in lower dust opacities. This reduced opacity permits interstellar radiation field and internal radiation photons to penetrate more deeply. This leads to rapid gas heating and inhibited star formation. Star formation efficiency is highly sensitive to grain size, with an order-of-magnitude reduction when grain size dynamic range increases from 10⁻³–0.1μm to 0.1–10μm. Additionally, warmer gas suppresses low-mass star formation, and decreased opacities result in a greater proportion of gas in diffuse ionized structures. 

Abstract Stars form within dense cores composed of both gas and dust within molecular clouds. However, despite the crucial role that dust plays in the star formation process, its dynamics is frequently overlooked, with the common assumption being a constant, spatially uniform dust-to-gas ratio and grain size spectrum. In this study, we introduce a set of radiation-dust-magnetohydrodynamic simulations of star-forming molecular clouds from the STARFORGE project. These simulations expand upon the earlier radiation MHD models, which included cooling, individual star formation, and feedback. Notably, they explicitly address the dynamics of dust grains, considering radiation, drag, and Lorentz forces acting on a diverse size spectrum of live dust grains. We find that once stars exceed a certain mass threshold (∼2M_⊙), their emitted radiation can evacuate dust grains from their vicinity, giving rise to a dust-suppressed zone of size ∼100 au. This removal of dust, which interacts with gas through cooling, chemistry, drag, and radiative transfer, alters the gas properties in the region. Commencing during the early accretion stages and preceding the main-sequence phase, this process results in a mass-dependent depletion in the accreted dust-to-gas (ADG) mass ratio within both the circumstellar disk and the star. We predict that massive stars (≳10M_⊙) would exhibit ADG ratios that are approximately 1 order of magnitude lower than that of their parent clouds. Consequently, stars, their disks, and circumstellar environments would display notable deviations in the abundances of elements commonly associated with dust grains, such as carbon and oxygen. 

ABSTRACT The cold ($$\sim 10^{4}\, {\rm K}$$) component of the circumgalactic medium (CGM) accounts for a significant fraction of all galactic baryons. However, using current galaxy-scale simulations to determine the origin and evolution of cold CGM gas poses a significant challenge, since it is computationally infeasible to directly simulate a galactic halo alongside the sub-pc scales that are crucial for understanding the interactions between cold CGM gas and the surrounding ‘hot’ medium. In this work, we introduce a new approach: the Cold Gas Subgrid Model (CGSM), which models unresolved cold gas as a second fluid in addition to the standard ‘normal’ gas fluid. The CGSM tracks the total mass density and bulk momentum of unresolved cold gas, deriving the properties of its unresolved cloudlets from the resolved gas phase. The interactions between the subgrid cold fluid and the resolved fluid are modelled by prescriptions from high-resolution simulations of ‘cloud crushing’ and thermal instability. Through a series of idealized tests, we demonstrate the CGSM’s ability to overcome the resolution limitations of traditional hydrodynamics simulations, successfully capturing the correct cold gas mass, its spatial distribution, and the time-scales for cloud destruction and growth. We discuss the implications of using this model in cosmological simulations to more accurately represent the microphysics that govern the galactic baryon cycle. 

Abstract Simulations and observations suggest that galaxy interactions may enhance the star formation rate (SFR) in merging galaxies. One proposed mechanism is the torque exerted on the gas and stars in the larger galaxy by the smaller galaxy. We analyze the interaction torques and star formation activity on six galaxies from the FIRE-2 simulation suite with masses comparable to the Milky Way galaxy at redshiftz= 0. We trace the halos fromz= 3.6 toz= 0, calculating the torque exerted by the nearby galaxies on the gas in the central galaxy. We calculate the correlation between the torque and the SFR across the simulations for various mass ratios. For near-equal-stellar-mass-ratio interactions in the galaxy sample, occurring betweenz= 1.2−3.6, there is a positive and statistically significant correlation between the torque from nearby galaxies on the gas of the central galaxies and the SFR. For all other samples, no statistically significant correlation is found between the torque and the SFR. Our analysis shows that some, but not all, major interactions cause starbursts in the simulated Milky Way-mass galaxies, and that most starbursts are not caused by galaxy interactions. The transition from “bursty” at high redshift (z≳ 1) to “steady” star formation state at later times is independent of the interaction history of the galaxies, and most of the interactions do not leave significant imprints on the overall trend of the star formation history of the galaxies. 

ABSTRACT We present an investigation of clustered stellar feedback in the form of superbubbles identified within 11 galaxies from the FIRE-2 (Feedback in Realistic Environments) cosmological zoom-in simulation suite, at both cosmic noon (1 < z < 3) and in the local universe. We study the spatially resolved multiphase outflows that these supernovae drive, comparing our findings with recent theory and observations. These simulations consist of five Large Magellanic Cloud–mass galaxies and six Milky Way-mass progenitors (with a minimum baryonic particle mass of $$m_{\rm b.min} = 7100\,{\rm M}_{\odot }$$). For all galaxies, we calculate the local and galaxy-averaged mass and energy-loading factors from the identified outflows. We also characterize the multiphase morphology and properties of the identified superbubbles, including the ‘shell’ of cool ($$T\lt 10^5$$ K) gas and break out of energetic hot ($$T\gt 10^5$$ K) gas when the shell bursts. We find that these simulations, regardless of redshift, have mass-loading factors and momentum fluxes in the cool gas that largely agree with recent observations. Lastly, we also investigate how methodological choices in measuring outflows can affect loading factors for galactic winds. 

Abstract The origins and mergers of supermassive black holes (SMBHs) remain a mystery. We describe a scenario from a novel multiphysics simulation featuring rapid (≲1 Myr) hyper-Eddington gas capture by a ∼1000M_⊙“seed” black hole (BH) up to supermassive (≳10⁶M_⊙) masses in a massive, dense molecular cloud complex typical of high-redshift starbursts. Due to the high cloud density, stellar feedback is inefficient, and most of the gas turns into stars in star clusters that rapidly merge hierarchically, creating deep potential wells. Relatively low-mass BH seeds at random positions can be “captured” by merging subclusters and migrate to the center in ∼1 freefall time (vastly faster than dynamical friction). This also efficiently produces a paired BH binary with ∼0.1 pc separation. The centrally concentrated stellar density profile (akin to a “protobulge”) allows the cluster as a whole to capture and retain gas and build up a large (parsec-scale) circumbinary accretion disk with gas coherently funneled to the central BH (even when the BH radius of influence is small). The disk is “hypermagnetized” and “flux-frozen”: dominated by a toroidal magnetic field with plasmaβ∼ 10⁻³, with the fields amplified by flux-freezing. This drives hyper-Eddington inflow rates ≳1M_⊙yr⁻¹, which also drive the two BHs to nearly equal masses. The late-stage system appears remarkably similar to recently observed high-redshift “little red dots.” This scenario can provide an explanation for rapid SMBH formation, growth, and mergers in high-redshift galaxies. 

Abstract Cosmic rays (CRs) are the primary driver of ionization in star-forming molecular clouds (MCs). Despite their potential impacts on gas dynamics and chemistry, no simulations of star cluster formation following the creation of individual stars have included explicit cosmic-ray transport (CRT) to date. We conduct the first numerical simulations following the collapse of a 2000M_⊙MC and the subsequent star formation including CRT using the STAR FORmation in Gaseous Environments framework implemented in the GIZMO code. We show that when CRT is streaming-dominated, the CR energy in the cloud is strongly attenuated due to energy losses from the streaming instability. Consequently, in a Milky Way–like environment the median CR ionization rate in the cloud is low (ζ≲ 2 × 10⁻¹⁹s⁻¹) during the main star-forming epoch of the calculation and the impact of CRs on the star formation in the cloud is limited. However, in high-CR environments, the CR distribution in the cloud is elevated (ζ≲ 6 × 10⁻¹⁸), and the relatively higher CR pressure outside the cloud causes slightly earlier cloud collapse and increases the star formation efficiency by 50% to ∼13%. The initial mass function is similar in all cases except with possible variations in a high-CR environment. Further studies are needed to explain the range of ionization rates observed in MCs and explore star formation in extreme CR environments. 

ABSTRACT Fuelling star formation in large, discy galaxies requires a continuous supply of gas accreting into star-forming regions. Previously, we characterized this accretion in four Milky Way mass galaxies ($$M_{\rm halo}\sim 10^{12}{\rm M}_{\odot }$$) in the FIRE-2 cosmological zoom-in simulations. At $$z\sim 0$$, we found that gas within the inner circumgalactic medium (iCGM) approaches the disc with comparable angular momentum (AM) to the disc edge, joining in the outer half of the gaseous disc. Within the disc, gas moves inwards at velocities of $$\sim$$1–5 km s$$^{-1}$$ while fully rotationally supported. In this study, we analyse the torques that drive these flows. In all cases studied, we find that the torques in discs enable gas accreted near the disc edge to transport inwards and fuel star formation in the central few kpc. The primary sources of torque come from gravity, hydrodynamical forces, and the sub-grid $$P \mathrm{ d}V$$ work done by supernova (SN) remnants interacting with gas on $$\lesssim$$10 pc scales. These SNe remnant interactions induce negative torques within the inner disc and positive torques in the outer disc. The gas–gas gravitational, hydro, and ‘feedback’ torques transfer AM outwards to where accreting gas joins the disc, playing an important role in driving inflows and regulating disc structure. Gravitational torques from stars and dark matter provide an AM sink within the innermost regions of the disc and iCGM, respectively. Feedback torques are dominant within the disc, while gravitational and hydrodynamical torques have similar significance depending on the system/region. Torques from viscous shearing, magnetic forces, stellar winds, and radiative transfer are less significant. 

ABSTRACT The observationally inferred size versus stellar–mass relationship (SMR) for low-mass galaxies provides an important test for galaxy formation models. However, the relationship relies on assumptions that relate observed luminosity profiles to underlying stellar mass profiles. Here we use the Feedback in Realistic Environments simulations of low-mass galaxies to explore how the predicted SMR changes depending on whether one uses star-particle counts directly or mock observations. We reproduce the SMR found in The Exploration of Local Volume Satellites survey remarkably well only when we infer stellar masses and sizes using mock observations. However, when we use star particles to directly infer stellar masses and half-mass radii, we find that our galaxies are too large and obey an SMR with too little scatter compared to observations. This discrepancy between the ‘true’ galaxy size and mass and those derived in the mock observation approach is twofold. First, our simulated galaxies have higher and more varied mass-to-light ratios (MLR) at a fixed colour than those commonly adopted, which tends to underestimate their stellar masses compared to their true, simulated values. Second, our galaxies have radially increasing MLR gradients therefore using a single MLR tends to underpredict the mass in the outer regions. Similarly, the true half-mass radius is larger than the half-light radius because the light is more concentrated than the mass. If our simulations are accurate representations of the real Universe, then the relationship between galaxy size and stellar mass is even tighter for low-mass galaxies than is commonly inferred from observed relations. 

ABSTRACT In the absence of supplementary heat, the radiative cooling of halo gas around massive galaxies (Milky Way mass and above) leads to an excess of cold gas or stars beyond observed levels. Active galactic nucleus jet-induced heating is likely essential, but the specific properties of the jets remain unclear. Our previous work concludes from simulations of a halo with $$10^{14} \,\mathrm{ M}_\odot$$ that a successful jet model should have an energy flux comparable to the free-fall energy flux at the cooling radius and should inflate a sufficiently wide cocoon with a long enough cooling time. In this paper, we investigate three jet modes with constant fluxes satisfying the criteria, including high-temperature thermal jets, cosmic ray (CR)-dominant jets, and widely precessing kinetic jets in $$10^{12}-10^{15}\, {\rm M}_{\odot }$$ haloes using high-resolution, non-cosmological magnetohydrodynamic simulations with the FIRE-2 (Feedback In Realistic Environments) stellar feedback model, conduction, and viscosity. We find that scaling the jet energy according to the free-fall energy at the cooling radius can successfully suppress the cooling flows and quench galaxies without violating observational constraints. On the contrary, if we scale the energy flux based on the total cooling rate within the cooling radius, strong interstellar medium cooling dominates this scaling, resulting in a jet flux exceeding what is needed. Among the three jet types, the CR-dominant jet is most effective in suppressing cooling flows across all surveyed halo masses due to enhanced CR pressure support. We confirm that the criteria for a successful jet model work across a wider range, encompassing halo masses of $$10^{12}-10^{15} {\rm M_\odot }$$.