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  1. 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.

     
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  2. Abstract

    Recent discoveries of a significant population of bright galaxies at cosmic dawnz10have 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 (MUV≲ −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.

     
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  3. 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.

     
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  4. 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.

     
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  5. Abstract

    Observations of local star-forming galaxies (SFGs) show a tight correlation between their singly ionized carbon line luminosity ($L_{\rm [C\, \small {II}]}$) and star formation rate (SFR), suggesting that $L_{\rm [C\, \small {II}]}$ may be a useful SFR tracer for galaxies. Some other galaxy populations, however, are found to have lower $L_{\rm [C\, \small {II}]}{}/{}\rm SFR$ than local SFGs, including the infrared-luminous, starburst galaxies at low and high redshifts as well as some moderately star-forming galaxies at the epoch of re-ionization (EoR). The origins of this ‘$\rm [C\, \small {II}]$ deficit’ is unclear. In this work, we study the $L_{\rm [C\, \small {II}]}$-SFR relation of galaxies using a sample of z = 0 − 8 galaxies with M* ≈ 107 − 5 × 1011 M⊙ extracted from cosmological volume and zoom-in simulations from the Feedback in Realistic Environments (fire) project. We find a simple analytic expression for $L_{\rm [C\, \small {II}]}$/SFR of galaxies in terms of the following parameters: mass fraction of $\rm [C\, \small {II}]$-emitting gas ($f_{\rm [C\, \small {II}]}$), gas metallicity (Zgas), gas density (ngas) and gas depletion time ($t_{\rm dep}{}={}M_{\rm gas}{}/{}\rm SFR$). We find two distinct physical regimes: $\rm H_2$-rich galaxies where tdep is the main driver of the $\rm [C\, \small {II}]$ deficit and $\rm H_2$-poor galaxies where Zgas is the main driver. The observed $\rm [C\, \small {II}]$ deficit of IR-luminous galaxies and early EoR galaxies, corresponding to the two different regimes, is due to short gas depletion time and low gas metallicity, respectively. Our result indicates that the $\rm [C\, \small {II}]$ deficit is a common phenomenon of galaxies, and caution needs to be taken when applying a constant $L_{\rm [C\, \small {II}]}$-to-SFR conversion factor derived from local SFGs to estimate cosmic SFR density at high redshifts and interpret data from upcoming $\rm [C\, \small {II}]$ line intensity mapping experiments.

     
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  6. Abstract

    We employ the Feedback In Realistic Environments (FIRE-2) physics model to study how the properties of giant molecular clouds (GMCs) evolve during galaxy mergers. We conduct a pixel-by-pixel analysis of molecular gas properties in both the simulated control galaxies and galaxy major mergers. The simulated GMC pixels in the control galaxies follow a similar trend in a diagram of velocity dispersion (σv) versus gas surface density (Σmol) to the one observed in local spiral galaxies in the Physics at High Angular resolution in Nearby GalaxieS (PHANGS) survey. For GMC pixels in simulated mergers, we see a significant increase of a factor of 5–10 in both Σmolandσv, which puts these pixels above the trend of PHANGS galaxies in theσvversus Σmoldiagram. This deviation may indicate that GMCs in the simulated mergers are much less gravitationally bound compared with simulated control galaxies with virial parameters (αvir) reaching 10–100. Furthermore, we find that the increase inαvirhappens at the same time as the increase in global star formation rate, which suggests that stellar feedback is responsible for dispersing the gas. We also find that the gas depletion time is significantly lower for high-αvirGMCs during a starburst event. This is in contrast to the simple physical picture that low-αvirGMCs are easier to collapse and form stars on shorter depletion times. This might suggest that some other physical mechanisms besides self-gravity are helping the GMCs in starbursting mergers collapse and form stars.

     
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  7. ABSTRACT

    The early growth of black holes (BHs) in high-redshift galaxies is likely feedback regulated. While radiative feedback has been extensively studied, the role of mechanical feedback has received less scrutiny to date. Here, we use high-resolution parsec-scale hydrodynamical simulations to study jet propagation and its effect on 100 M⊙ BH accretion in the dense, low-metallicity gas expected in early protogalaxies. As the jet propagates, it shocks the surrounding gas forming a jet cocoon. The cocoon consists of a rapidly cooling cold phase at the interface with the background gas and an overpressured subsonic phase of reverse shock-heated gas filling the interior. We vary the background gas density and temperature, BH feedback efficiency, and the jet model. We found that the width of the jet cocoon roughly follows a scaling derived by assuming momentum conservation in the jet-propagation direction and energy conservation in the lateral directions. Depending on the assumed gas and jet properties, the cocoon either stays elongated to large radii or isotropizes before reaching the Bondi radius, forming a nearly spherical bubble. Lower jet velocities and higher background gas densities result in self-regulation to higher momentum fluxes and elongated cocoons. In all cases, the outward cocoon momentum flux balances the inward inflowing gas momentum flux near the Bondi radius, which ultimately regulates BH accretion. The time-averaged accretion rate always remains below the Bondi rate, and exceeds the Eddington rate only if the ambient medium is dense and cold, and/or the jet is weak (low velocity and mass loading).

     
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  8. ABSTRACT

    Negative feedback from accreting supermassive black holes is considered crucial in suppressing star formation and quenching massive galaxies. However, several models and observations suggest that black hole feedback may have a positive effect, triggering star formation by compressing interstellar medium gas to higher densities. We investigate the dual role of black hole feedback using cosmological hydrodynamic simulations from the Feedback In Realistic Environment (FIRE) project, incorporating a novel implementation of hyper-refined accretion-disc winds. Focusing on a massive, star-forming galaxy at z ∼ 2 ($M_{\rm halo} \sim 10^{12.5}\, {\rm M}_{\odot }$), we demonstrate that strong quasar winds with a kinetic power of ∼1046 erg s−1, persisting for over 20 Myr, drive the formation of a central gas cavity and significantly reduce the surface density of star formation across the galaxy’s disc. The suppression of star formation primarily occurs by limiting the availability of gas for star formation rather than by evacuating the pre-existing star-forming gas reservoir (preventive feedback dominates over ejective feedback). Despite the overall negative impact of quasar winds, we identify several potential indicators of local positive feedback, including (1) the spatial anticorrelation between wind-dominated regions and star-forming clumps, (2) higher local star formation efficiency in compressed gas at the edge of the cavity, and (3) increased contribution of outflowing material to local star formation. Moreover, stars formed under the influence of quasar winds tend to be located at larger radial distances. Our findings suggest that both positive and negative AGN feedback can coexist within galaxies, although the local positive triggering of star formation has a minor influence on global galaxy growth.

     
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  9. ABSTRACT

    We examine the azimuthal variations in gas-phase metallicity profiles in simulated Milky Way-mass disc galaxies from the Feedback in Realistic Environments (FIRE-2) cosmological zoom-in simulation suite, which includes a sub-grid turbulent metal mixing model. We produce spatially resolved maps of the discs at z ≈ 0 with pixel sizes ranging from 250 to 750 pc, analogous to modern integral field unit galaxy surveys, mapping the gas-phase metallicities in both the cold and dense gas and the ionized gas correlated with H ii regions. We report that the spiral arms alternate in a pattern of metal rich and metal poor relative to the median metallicity of the order of ≲0.1 dex, appearing generally in this sample of flocculent spirals. The pattern persists even in a simulation with different strengths of metal mixing, indicating that the pattern emerges from physics above the sub-grid scale. Local enrichment does not appear to be the dominant source of the azimuthal metallicity variations at z ≈ 0: there is no correlation with local star formation on these spatial scales. Rather, the arms are moving radially inwards and outwards relative to each other, carrying their local metallicity gradients with them radially before mixing into the larger-scale interstellar medium. We propose that the arms act as freeways channeling relatively metal poor gas radially inwards, and relatively enriched gas radially outwards.

     
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  10. ABSTRACT

    Feedback from accreting supermassive black holes (SMBHs) is thought to be a primary driver of quenching in massive galaxies, but how to best implement SMBH physics into galaxy formation simulations remains ambiguous. As part of the Feedback in Realistic Environments (FIRE) project, we explore the effects of different modelling choices for SMBH accretion and feedback in a suite of ∼500 cosmological zoom-in simulations across a wide range of halo mass (1010–1013 M⊙). Within the suite, we vary the numerical schemes for BH accretion and feedback, accretion efficiency, and the strength of mechanical, radiative, and cosmic ray feedback independently. We then compare the outcomes to observed galaxy scaling relations. We find several models satisfying observational constraints for which the energetics in different feedback channels are physically plausible. Interestingly, cosmic rays accelerated by SMBHs play an important role in many plausible models. However, it is non-trivial to reproduce scaling relations across halo mass, and many model variations produce qualitatively incorrect results regardless of parameter choices. The growth of stellar and BH mass are closely related: for example, overmassive BHs tend to overquench galaxies. BH mass is most strongly affected by the choice of accretion efficiency in high-mass haloes, but by feedback efficiency in low-mass haloes. The amount of star formation suppression by SMBH feedback in low-mass haloes is determined primarily by the time-integrated feedback energy. For massive galaxies, the ‘responsiveness’ of a model (how quickly and powerfully the BH responds to gas available for accretion) is an additional important factor for quenching.

     
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