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Abstract The unprecedented infrared spectroscopic capabilities of JWST have provided high-quality interstellar medium metallicity measurements and enabled characterization of the gas-phase mass–metallicity relation (MZR) for galaxies atz≳ 5 for the first time. We analyze the gas-phase MZR and its evolution in a high-redshift suite of FIRE-2 cosmological zoom-in simulations atz= 5–12 and for stellar massesM_*∼ 10⁶–10¹⁰M_⊙. These simulations implement a multichannel stellar feedback model and produce broadly realistic galaxy properties, including when evolved toz= 0. The simulations predict very weak redshift evolution of the MZR over the redshift range studied, with the normalization of the MZR increasing by less than 0.01 dex as redshift decreases fromz= 12 toz= 5. The median MZR in the simulations is well approximated as a constant power-law relation across this redshift range given by $\log \left(Z/Z_{\odot }\right)=0.37\log \left(M_{*}/{\mathrm{M}}_{\odot }\right)-4.3$. We find good agreement between our best-fit model and recent observations made by JWST at high redshift. The weak evolution of the MZR atz> 5 contrasts with the evolution atz≲ 3, where increasing normalization of the MZR with decreasing redshift is observed and predicted by most models. The FIRE-2 simulations predict increasing scatter in the gas-phase MZR with decreasing stellar mass, in qualitative agreement with some observations. 

ABSTRACT Our ability to trace the star-forming molecular gas is important to our understanding of the Universe. We can trace this gas using CO emission, converting the observed CO intensity into the H$$_2$$ gas mass of the region using the CO-to-H$$_2$$ conversion factor ($$X_{\rm{{\small CO}}}$$). In this paper, we use simulations to study the conversion factor and the molecular gas within galaxies. We analysed a suite of simulations of isolated disc galaxies, ranging from dwarfs to Milky Way-mass galaxies, that were run using the fire-2 subgrid models coupled to the chimes non-equilibrium chemistry solver. We use the non-equilibrium abundances from the simulations, and we also compare to results using abundances assuming equilibrium, which we calculate from the simulation in post-processing. Our non-equilibrium simulations are able to reproduce the relation between CO and H$$_2$$ column densities, and the relation between $$X_{\rm{{\small CO}}}$$ and metallicity, seen within observations of the Milky Way. We also compare to the xCOLD GASS survey, and find agreement with their data to our predicted CO luminosities at fixed star formation rate. We also find the multivariate function used by xCOLD GASS overpredicts the H$$_2$$ mass for our simulations, motivating us to suggest an alternative multivariate function of our fitting, though we caution that this fitting is uncertain due to the limited range of galaxy conditions covered by our simulations. We also find that the non-equilibrium chemistry has little effect on the conversion factor (<5 per cent) for our high-mass galaxies, though still affects the H$$_2$$ mass and $$L_{\rm{{\small CO}}}$$ by $$\approx$$25  per cent. 

ABSTRACT Recent observations with JWST have uncovered unexpectedly high cosmic star formation activity in the early Universe, mere hundreds of millions of years after the big bang. These observations are often understood to reflect an evolutionary shift in star formation efficiency (SFE) caused by changing galactic conditions during these early epochs. We present FIREbox$$^{\it HR}$$, a high-resolution, cosmological hydrodynamical simulation from the Feedback in Realistic Environments (FIRE) project, which offers insights into the SFE of galaxies during the first billion years of cosmic time. FIREbox$$^{\it HR}$$ re-simulates the cosmic volume ($L=22.1$ cMpc) of the original FIREbox run with eight times higher mass resolution ($$m_{\rm b}\sim {}7800\, M_\odot$$), but with identical physics, down to $$z\sim {}6$$. FIREbox$$^{\it HR}$$ predicts ultraviolet (UV) luminosity functions in good agreement with available observational data. The simulation also successfully reproduces the observed cosmic UV luminosity density at $$z\sim {}6{\!-\!}14$$, demonstrating that relatively high star formation activity in the early Universe is a natural outcome of the baryonic processes encoded in the FIRE-2 model. According to FIREbox$$^{\it HR}$$, the SFE–halo mass relation for intermediate mass haloes ($$M_{\rm halo}\sim {}10^9{\!-\!}10^{11}\, {\rm M}_\odot$$) does not significantly evolve with redshift and is only weakly mass-dependent. These properties of the SFE–halo mass relation lead to a larger contribution from lower mass haloes at higher z, driving the gradual evolution of the observed cosmic UV luminosity density. A theoretical model based on the SFE–halo mass relation inferred from FIREbox$$^{\it HR}$$ allows us to explore implications for galaxy evolution. Future observations of UV faint galaxies at $$z\gt 12$$ will provide an opportunity to further test these predictions and deepen our understanding of star formation during Cosmic Dawn. 

ABSTRACT Observations indicate dust populations vary between galaxies and within them, suggesting a complex life cycle and evolutionary history. Here we investigate the evolution of galactic dust populations across cosmic time using a suite of cosmological zoom-in simulations from the Feedback in Realistic Environments project, spanning $$M_{\rm vir}=10^{9-12}{M}_{\odot };\, M_{*}=10^{6-11}\, {M}_{\odot }$$. Our simulations incorporate a dust evolution model that accounts for the dominant sources of dust production, growth, and destruction and follows the evolution of specific dust species. All galactic dust populations in our suite exhibit similar evolutionary histories, with gas–dust accretion being the dominant producer of dust mass for all but the most metal-poor galaxies. Similar to previous works, we find the onset of efficient gas–dust accretion occurs above a ‘critical’ metallicity threshold (Zcrit). Due to this threshold, our simulations reproduce observed trends between galactic D/Z and metallicity and element depletion trends in the interstellar medium. However, we find Zcrit varies between dust species due to differences in key element abundances, dust physical properties, and life cycle processes resulting in $$Z_{\rm crit}\sim 0.05{\rm Z}_{\odot },\, 0.2{\rm Z}_{\odot },\, 0.5{\rm Z}_{\odot }$$ for metallic iron, silicates, and carbonaceous dust, respectively. These variations could explain the lack of small carbonaceous grains observed in the Magellanic Clouds. We also find a delay between the onset of gas–dust accretion and when a dust population reaches equilibrium, which we call the equilibrium time-scale (τequil). The relation between τequil and the metal enrichment time-scale of a galaxy, determined by its recent evolutionary history, can contribute to the scatter in the observed relation between galactic D/Z and metallicity. 

ABSTRACT Both observations and simulations have shown strong evidence for highly time-variable star formation in low-mass and/or high-redshift galaxies, which has important observational implications because high-redshift galaxy samples are rest-ultraviolet (rest-UV) selected and therefore particularly sensitive to the recent star formation. Using a suite of cosmological ‘zoom-in’ simulations at z > 5 from the Feedback in Realistic Environments project, we examine the implications of bursty star formation histories for observations of high-redshift galaxies with JWST. We characterize how the galaxy observability depends on the star formation history. We also investigate selection effects due to bursty star formation on the physical properties measured, such as the gas fraction, specific star formation rate, and metallicity. We find the observability to be highly time-dependent for galaxies near the survey’s limiting flux due to the star formation rate variability: as the star formation rate fluctuates, the same galaxy oscillates in and out of the observable sample. The observable fraction $$f_\mathrm{obs} = 50~{{\ \rm per\ cent}}$$ at z ∼ 7 and M⋆ ∼ 108.5–$$10^{9}\, {\rm M}_{\odot }$$ for a JWST/NIRCam survey reaching a limiting magnitude of $$m^\mathrm{lim}_\mathrm{AB} \sim 29{\!-\!}30$$, representative of surveys such as JADES and CEERS. JWST-detectable galaxies near the survey limit tend to have properties characteristic of galaxies in the bursty phase: on average, they show approximately 2.5 times higher cold, dense gas fractions and 20 times higher specific star formation rates at a given stellar mass than galaxies below the rest-UV detection threshold. Our study represents a first step in quantifying selection effects and the associated biases due to bursty star formation in studying high-redshift galaxy properties. 

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 $$10^{12}-10^{15} {\rm M_\odot }$$. 

ABSTRACT ‘Runaway stars’ might play a role in driving galactic outflows and enriching the circumgalactic medium with metals. To study this effect, we carry out high-resolution dwarf galaxy simulations that include velocity ‘kicks’ to massive stars above eigth solar masses. We consider two scenarios, one that adopts a power law velocity distribution for kick velocities, resulting in more stars with high-velocity kicks, and a more moderate scenario with a Maxwellian velocity distribution. We explicitly resolve the multiphase interstellar medium (ISM) and include non-equilibrium cooling and chemistry. We sample individual massive stars from an IMF and follow their radiation input and SN feedback (core-collapse) channel at the end of their lifetime. In the simulations with runaway stars, we add additional (natal) velocity kicks that mimic two- and three-body interactions that cannot be fully resolved in our simulations. We find that including runaway or ‘walkaway’ star scenarios impacts mass, metal, momentum, and energy outflows as well as the corresponding loading factors. The effect on the mass loading factor is small, but we find an increase in the metal loading by a factor of 1.5 to 2. The momentum loading increases by a factor of 1.5–2. The energy loading increases by roughly a factor of 5 when runaway stars are included. Additionally, we find that the overall level of star formation is increased in the models that include runaway stars. We conclude that the inclusion of runaway stars could have an impact on the global star formation and subsequent outflow properties of dwarf galaxies. 

Abstract Recent discoveries of a significant population of bright galaxies at cosmic dawn $\left(z\gtrsim 10\right)$have enabled critical tests of cosmological galaxy formation models. In particular, the bright end of the galaxys’ UV luminosity functions (UVLFs) appear higher than predicted by many models. Using approximately 25,000 galaxy snapshots at 8 ≤z≤ 12 in a suite of FIRE-2 cosmological “zoom-in” simulations from the Feedback in Realistic Environments (FIRE) project, we show that the observed abundance of UV-bright galaxies at cosmic dawn is reproduced in these simulations with a multichannel implementation of standard stellar feedback processes, without any fine-tuning. Notably, we find no need to invoke previously suggested modifications, such as a nonstandard cosmology, a top-heavy stellar initial mass function, or a strongly enhanced star formation efficiency. We contrast the UVLFs predicted by bursty star formation in these original simulations to those derived from star formation histories (SFHs) smoothed over prescribed timescales (e.g., 100 Myr). The comparison demonstrates that the strongly time-variable SFHs predicted by the FIRE simulations play a key role in correctly reproducing the observed, bright-end UVLFs at cosmic dawn: the bursty SFHs induce order-or-magnitude changes in the abundance of UV-bright (M_UV≲ −20) galaxies atz≳ 10. The predicted bright-end UVLFs are consistent with both the spectroscopically confirmed population and the photometrically selected candidates. We also find good agreement between the predicted and observationally inferred integrated UV luminosity densities, which evolve more weakly with redshift in FIRE than suggested by some other models. 

ABSTRACT Forward-modeling observables from galaxy simulations enables direct comparisons between theory and observations. To generate synthetic spectral energy distributions (SEDs) that include dust absorption, re-emission, and scattering, Monte Carlo radiative transfer is often used in post-processing on a galaxy-by-galaxy basis. However, this is computationally expensive, especially if one wants to make predictions for suites of many cosmological simulations. To alleviate this computational burden, we have developed a radiative transfer emulator using an artificial neural network (ANN), ANNgelina, that can reliably predict SEDs of simulated galaxies using a small number of integrated properties of the simulated galaxies: star formation rate, stellar and dust masses, and mass-weighted metallicities of all star particles and of only star particles with age <10 Myr. Here, we present the methodology and quantify the accuracy of the predictions. We train the ANN on SEDs computed for galaxies from the IllustrisTNG project’s TNG50 cosmological magnetohydrodynamical simulation. ANNgelina is able to predict the SEDs of TNG50 galaxies in the ultraviolet (UV) to millimetre regime with a typical median absolute error of ∼7 per cent. The prediction error is the greatest in the UV, possibly due to the viewing-angle dependence being greatest in this wavelength regime. Our results demonstrate that our ANN-based emulator is a promising computationally inexpensive alternative for forward-modeling galaxy SEDs from cosmological simulations. 

ABSTRACT Recent observations indicate that galactic outflows are ubiquitous in high-redshift (high-z) galaxies, including normal star-forming galaxies, quasar hosts, and dusty star-forming galaxies (DSFGs). However, the impact of outflows on the evolution of their hosts is still an open question. Here, we analyse the star-formation histories and galactic outflow properties of galaxies in massive haloes ($$10^{12}\, {\rm M}_{\odot }\ \lt\ M_{\rm vir}\ \lt\ 5\times 10^{12}\, {\rm M}_{\odot }$$) at z ≳ 5.5 in three zoom-in cosmological simulations from the MassiveFIRE suite, as part of the Feedback In Realistic Environments (FIRE) project. The simulations were run with the FIRE-2 model, which does not include feedback from active galactic nuclei. The simulated galaxies resemble z > 4 DSFGs, with star-formation rates of $$\sim\!{1000}\ {\rm M}_{\odot }\, \rm yr^{-1}$$ and molecular gas masses of Mmol ∼ 1010 M⊙. However, the simulated galaxies are characterized by higher circular velocities than those observed in high-z DSFGs. The mass loading factors from stellar feedback are of the order of ∼0.1, implying that stellar feedback is inefficient in driving galactic outflows and gas is consumed by star formation on much shorter time-scales than it is expelled from the interstellar medium. We also find that stellar feedback is highly inefficient in self-regulating star formation in this regime, with an average integrated star formation efficiency (SFE) per dynamical time of 30 per cent. Finally, compared with FIRE-2 galaxies hosted in similarly massive haloes at lower redshift, we find lower mass loading factors and higher SFEs in the high-z sample. We argue that both effects originate from the higher total and gas surface densities that characterize high-z massive systems.