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ABSTRACT We update the dust model present within the simba galaxy simulations with a self-consistent framework for the co-evolution of dust and molecular hydrogen populations in the interstellar medium, and use this to explore $$z \ge 6$$ galaxy evolution. In addition to tracking the evolution of dust and molecular hydrogen abundances, our model fully integrates these species into the simba simulation, explicitly modelling their impact on physical processes such as star formation and cooling through the inclusion of a novel two-phase sub-grid model for interstellar gas. Running two cosmological simulations down to $$z \sim 6$$ we find that our simba-EoR model displays a generally tighter concordance with observational data than fiducial simba. Additionally we observe that our simba-EoR models increase star formation activity at early epochs, producing larger dust-to-gas ratios consequently. Finally, we discover a significant population of hot dust at $$\sim 100$$ K, aligning with contemporaneous observations of high-redshift dusty galaxies, alongside the large $$\sim 20$$ K population typically identified. 

Abstract We presentslick(the Scalable Line Intensity Computation Kit), a software package that calculates realistic CO, [Ci], and [Cii] luminosities for clouds and galaxies formed in hydrodynamic simulations. Built on the radiative transfer codedespotic,slickcomputes the thermal, radiative, and statistical equilibrium in concentric zones of model clouds, based on their physical properties and individual environments. We validate our results by applyingslickto the high-resolution run of theSimbasimulations, testing the derived luminosities against empirical and theoretical/analytic relations. To simulate the line emission from a universe of emitting clouds, we have incorporated random forest machine learning (ML) methods into our approach, allowing us to predict cosmologically evolving properties of CO, [Ci], and [Cii] emission from galaxies such as luminosity functions. We tested this model in 100,000 gas particles, and 2500 galaxies, reaching an average accuracy of ∼99.8% for all lines. Finally, we present the first model light cones created with realistic and ML-predicted CO, [Ci], and [Cii] luminosities in cosmological hydrodynamical simulations, fromz= 0 toz= 10. 

Abstract We explore the cosmic evolution of the fraction of dust-obscured star formation predicted by thesimbacosmological hydrodynamic simulations featuring an on-the-fly model for dust formation, evolution, and destruction. We find that up toz= 3, our results are broadly consistent with previous observational results of little to no evolution in obscured star formation. However, atz> 3 we find strong evolution at fixed galaxy stellar mass toward greater amounts of obscured star formation, in tension with high-redshift observations. We explain the trend of increasing obscuration at higher redshifts by evolving star-dust geometry, as the dust-to-stellar mass ratios remain relatively constant across cosmic time. We additionally see that at a fixed redshift, more massive galaxies have a higher fraction of their star formation obscured, which is explained by increased dust-to-stellar mass ratios at higher stellar masses. Finally, we estimate the contribution of dust-obscured star formation to the total star formation rate budget and find that the dust-obscured star formation history peaks aroundz∼ 2−3, and becomes subdominant atz≳ 5. The dominance of obscured star formation at redshiftsz≲ 4 is consistent with our results for the evolution of the obscured star formation fraction at fixed stellar mass to higher values at higher redshift because there exist fewer massive, heavily obscured galaxies at high redshift. 

Abstract In the current era of high-zgalaxy discovery with JWST and the Atacama Large Millimeter/submillimeter Array, our ability to study the stellar populations and interstellar medium conditions in a diverse range of galaxies at Cosmic Dawn has rapidly improved. At the same time, the need to understand the current limitations in modeling galaxy formation processes and physical properties in order to interpret these observations is critical. Here, we study the challenges in modeling galaxy dust temperatures, both in the context of forward modeling galaxy spectral properties from a hydrodynamical simulation and via backwards modeling galaxy physical properties from mock observations of far-infrared dust emission. Using thesimbamodel for galaxy formation combined withpowderdayradiative transfer, we can accurately predict the evolution of dust at high redshift, though several aspects of the model are essentially free parameters (dust composition, subresolution dust in star-forming regions) that dull the predictive power of the model dust temperature distributions. We also highlight the uncertainties in the backwards modeling methods, where we find the commonly used models and assumptions to fit far-infrared spectral energy distributions and infer dust temperatures (e.g., single temperature, optically thin modified blackbody) largely fail to capture the complexity of high-zdusty galaxies. We caution that conclusions inferred from both simulations—limited by resolution and post-processing techniques—and observations—limited by sparse data and simplistic model parameterizations—are susceptible to unique and nuanced uncertainties that can limit the usefulness of current high-zdust measurements. 

Abstract We present JWST and Atacama Large Millimeter/submillimeter Array (ALMA) imaging for the lensing system SPT0418−47, which includes a strongly lensed, dusty, star-forming galaxy at redshiftz= 4.225 and an associated multiply imaged companion. The JWST NIRCam and MIRI imaging observations presented in this paper were acquired as part of the Early Release Science program Targeting Extremely Magnified Panchromatic Lensed Arcs and Their Extended Star formation (TEMPLATES). This data set provides robust mutiwavelength detections of stellar light in both the main (SPT0418A) and companion (SPT0418B) galaxies, while the ALMA detection of [Cii] emission confirms that SPT0418B lies at the same redshift as SPT0418A. We infer that the projected physical separation of the two galaxies is 4.42 ± 0.05 kpc. We derive total magnifications ofμ= 29 ± 1 andμ= 4.1 ± 0.7 for SPT0418A and SPT0418B, respectively. We use bothprospectorandcigaleto derive stellar masses. We find that SPT0418A has a stellar mass of $M_{*}={3.4}_{-0.6}^{+1.1}\times {10}^{10}{M}_{\odot }$fromprospector orM_*= 1.5 ± 0.3 × 10¹⁰M_⊙fromcigale. The stellar mass ratio of SPT0418A and SPT0418B is roughly between 4 and 7 ( ${4.2}_{-1.6}^{+1.9}$forprospectorand 7.5 ± 3.7 forcigale). We see evidence of extended structure associated with SPT0418A that is suggestive of a tidal feature. These features, along with the close projected proximity, imply that the system is interacting. Interestingly, the star formation rates and stellar masses of both galaxies are consistent with the main sequence of star-forming galaxies at this epoch, indicating that this ongoing interaction has not noticeably elevated the star formation levels. 

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. 

Abstract The SPT 0311–58 system atz= 6.900 is an extremely massive structure within the reionization epoch and offers a chance to understand the formation of galaxies at an extreme peak in the primordial density field. We present 70 mas Atacama Large Millimeter/submillimeter Array observations of the dust continuum and [Cii] 158μm emission in the central pair of galaxies and reach physical resolutions of ∼100–350 pc, among the most detailed views of any reionization-era system to date. The observations resolve the source into at least a dozen kiloparsec-size clumps. The global kinematics and high turbulent velocity dispersion within the galaxies present a striking contrast to recent claims of dynamically cold thin-disk kinematics in some dusty galaxies just 800 Myr later atz∼ 4. We speculate that both gravitational interactions and fragmentation from massive parent disks have likely played a role in the overall dynamics and formation of clumps in the system. Each clump individually is comparable in mass to other 6 <z< 8 galaxies identified in rest-UV/optical deep field surveys, but with star formation rates elevated by a factor of ~3-5. Internally, the clumps themselves bear close resemblance to greatly scaled-up versions of virialized cloud-scale structures identified in low-redshift galaxies. Our observations are qualitatively similar to the chaotic and clumpy assembly within massive halos seen in simulations of high-redshift galaxies. 

Abstract The Baldwin, Philips, & Terlevich diagram of [Oiii]/Hβversus [Nii]/Hα(hereafter N2-BPT) has long been used as a tool for classifying galaxies based on the dominant source of ionizing radiation. Recent observations have demonstrated that galaxies atz∼ 2 reside offset from local galaxies in the N2-BPT space. In this paper, we conduct a series of controlled numerical experiments to understand the potential physical processes driving this offset. We model nebular line emission in a large sample of galaxies, taken from thesimbacosmological hydrodynamic galaxy formation simulation, using thecloudyphotoionization code to compute the nebular line luminosities from Hiiregions. We find that the observed shift toward higher [Oiii]/Hβand [Nii]/Hαvalues at high redshift arises from sample selection: when we consider only the most massive galaxiesM_*∼ 10^10–11M_⊙, the offset naturally appears, due to their high metallicities. We predict that deeper observations that probe lower-mass galaxies will reveal galaxies that lie on a locus comparable toz∼ 0 observations. Even when accounting for samples-selection effects, we find that there is a subtle mismatch between simulations and observations. To resolve this discrepancy, we investigate the impact of varying ionization parameters, Hiiregion densities, gas-phase abundance patterns, and increasing radiation field hardness on N2-BPT diagrams. We find that either decreasing the ionization parameter or increasing the N/O ratio of galaxies at fixed O/H can move galaxies along a self-similar arc in N2-BPT space that is occupied by high-redshift galaxies. 

Abstract We investigate the fine-structure [Cii] line at 158μm as a molecular gas tracer by analyzing the relationship between molecular gas mass (M_mol) and [Cii] line luminosity (L_[CII]) in 11,125z≃ 6 star-forming, main-sequence galaxies from thesimbasimulations, with line emission modeled by the Simulator of Galaxy Millimeter/Submillimeter Emission. Though most (∼50%–100%) of the gas mass in our simulations is ionized, the bulk (>50%) of the [Cii] emission comes from the molecular phase. We find a sublinear (slope 0.78 ± 0.01) $\log L_{\left[\mathrm{C}\mathrm{II}\right]}–\log M_{\mathrm{mol}}$relation, in contrast with the linear relation derived from observational samples of more massive, metal-rich galaxies atz≲ 6. We derive a median [Cii]-to-M_molconversion factor ofα_[CII]≃ 18M_⊙/L_⊙. This is lower than the average value of ≃30M_⊙/L_⊙derived from observations, which we attribute to lower gas-phase metallicities in our simulations. Thus, a lower, luminosity-dependent conversion factor must be applied when inferring molecular gas masses from [Cii] observations of low-mass galaxies. For our simulations, [Cii] is a better tracer of the molecular gas than COJ= 1–0, especially at the lowest metallicities, where much of the gas isCO-dark. We find thatL_[CII]is more tightly correlated withM_molthan with star formation rate (SFR), and both the $\log L_{\left[\mathrm{C}\mathrm{II}\right]}–\log M_{\mathrm{mol}}$and $\log L_{\left[\mathrm{C}\mathrm{II}\right]}–\log \mathrm{SFR}$relations arise from the Kennicutt–Schmidt relation. Our findings suggest thatL_[CII]is a promising tracer of the molecular gas at the earliest cosmic epochs. 

The physical mechanisms that link the termination of star formation in quiescent galaxies and the evolution of their baryonic components, stars, and the interstellar medium (ISM; dust, gas, and metals) are poorly constrained beyond the local Universe. In this work, we characterise the evolution of the dust content in 545 quiescent galaxies observed at 0.1 < z < 0.6 as part of the hCOSMOS spectroscopic redshift survey. This is, to date, the largest sample of quiescent galaxies at intermediate redshifts for which the dust, stellar, and metal abundances are consistently estimated. We analyse how the crucial markers of a galaxy dust life cycle, such as specific dust mass (M_dust/M_⋆), evolve with different physical parameters, namely gas-phase metallicity (Z_gas), time since quenching (t_quench), stellar mass (M_⋆), and stellar population age. We find morphology to be an important factor in the large scatter inM_dust/M_⋆(∼2 orders of magnitude). Quiescent spirals exhibit strong evolutionary trends of specific dust mass withM_⋆, stellar age, and galaxy size, in contrast to the little to no evolution experienced by ellipticals. When transitioning from solar to super-solar metallicities (8.7 ≲ 12 + log(O/H)≲9.1), quiescent spirals undergo a reversal inM_dust/M_⋆, indicative of a change in dust production efficiency. By modelling the star formation histories of our objects, we unveil a broad dynamical range of post-quenching timescales (60 Myr < t_quench < 3.2 Gyr). We show thatM_dust/M_⋆is highest in recently quenched systems (t_quench < 500 Myr), but its further evolution is non-monotonic, as a consequence of different pathways for dust formation, growth, or removal on various timescales. Our data are best described by simulations that include dust growth in the ISM. While this process is prevalent in the majority of galaxies, for ∼15% of objects we find evidence of additional dust content acquired externally, most likely via minor mergers. Altogether, our results strongly suggest that prolonged dust production on a timescale of 0.5 − 1 Gyr since quenching may be common in dusty quiescent galaxies at intermediate redshifts, even if their gas reservoirs are heavily exhausted (i.e. cold gas fraction < 1 − 5%).