arXiv subjects

Desika Narayanan

Publications and source records attributed to Desika Narayanan.

At least 19 recordsLinked to original sources

arXiv2026

Simulation-Informed Dust Masses in Typical $6<z<14$ JWST Galaxies

Direct detections of dust in typical $z>8$ galaxies remain untenable, despite the presence of dust suggested by rest-frame UV/optical spectro-photometry. Informed by cosmological zoom-in simulations where dust particles evolve on-the-fly, we present a novel method for inferring dust masses in high-redshift galaxies using stellar half-light radii and UV/optical attenuation. Our methods reproduce observed trends at lower redshift, ultimately diverging from Milky Way-based calibration factors by up to $\sim1.5$ dex. We apply this method to observed JWST galaxies from $6<z<14$, and find dust masses in the range $M_{\rm dust}\sim10^3-10^5\,M_\odot$ that are fully consistent with upper limits from ALMA, and suggest an early dust enrichment period by supernovae followed by efficient grain destruction in the hot gas. Future observations may use our tabulated calibration to infer dust masses over a wide range of stellar masses and redshifts.

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arXiv2026

The Lifecycle and Emission Properties of PAHs in Cosmological Hydrodynamic Galaxy Formation Simulations

We present the first cosmological model for the lifecycle and luminous properties of PAHs in galaxies as they evolve from z=6-->0. We model 40 zoom-in galaxies, coupled with an on-the-fly model for the evolution of dust grains in the ISM. We approximate PAHs as ultrasmall (a < 13 Angstrom) carbonaceous dust grains, and couple this model with single-photon excitation calculations to compute the emergent mid-infrared spectra. (1) If we assume that dust is large upon formation, then PAHs are naturally able to form in situ in the ISM via grain-grain shattering. Interstellar collision velocities increase in low density, diffuse gas in our model; as galaxies evolve, the increase in fractional mass of diffuse gas drives an increase in grain-grain collision velocities and a corresponding rise in the PAH mass fraction (qPAH) from ~5 x 10^{-4} at z~4 to ~10^{-2} at z~0. (2) Increased PAH production in the diffuse ISM results in an inverse relationship between qPAH and the molecular gas fraction. (3) The PAH light-to-mass ratio scales linearly with the radiation field intensity (LPAH/MPAH ~ G_0) but anti-correlates with qPAH, because high-Sigma_SFR galaxies have a denser ISM that suppresses shattering. This means the physical qPAH and observed LPAH/LFIR do not evolve in lockstep. (4) The PAH-metallicity relationship (PZR) arises naturally in this framework: galaxies enrich and grow their diffuse ISM fraction simultaneously, linking rising metallicity to rising qPAH. Our models represent the first to reproduce the PZR observed across z=0-2. (5) The LPAH-SFR and LPAH-M_mol relations emerge from two effects: more massive galaxies have larger PAH reservoirs, and higher-SFR galaxies excite their PAHs more efficiently per unit mass. Taken together, these results suggest that grain-grain shattering in the diffuse ISM is the main driver behind the evolution of cosmic PAH abundances.

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arXiv2026

The Impact of an AGN on PAH Spectral Features in Galaxy Evolution Simulations: Continuum Dilution and Excitation of PAHs

Galaxies hosting an active galactic nucleus (AGN) show diminished power and non-standard spectral feature ratios from polycyclic aromatic hydrocarbons (PAHs) compared with normal star-forming galaxies. Proposed mechanisms for this difference include a combination of AGN-driven PAH grain processing, differential heating from the surrounding radiation field, and continuum dilution. These potentially competing factors are difficult to disentangle observationally. In this paper, we investigate the impact of the AGN radiation field on the excitation properties and potential dilution of the mid-IR emission features via constrained numerical experiments of idealized galaxies in simulations that include a self-consistent model for the evolution of dust. We find that, for the integrated spectrum of our simulated Milky Way analog and independent of grain-processing physics, the apparent PAH emission is progressively diminished as AGN luminosity is increased. For spectra within the central half kiloparsec, PAH features are entirely obfuscated when an AGN of any sampled luminosity is present. We additionally find that the UV radiation from an AGN is capable of impacting PAH band ratios, though note that we do not yet include a model for potential photodestruction of PAHs. Our model suggests that the inferred lack of PAHs via featureless mid-IR spectra from galaxies hosting AGN may simply owe to the direct continuum from the AGN overpowering mid-IR features from PAHs in the ISM of the host galaxy. Future work will investigate the relative role of AGN continuum dilution and PAH heating vs photodestruction/ionization of PAHs.

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arXiv2026

Simulations of Dust Content and Attenuation in $z=5-15$ Galaxies

Correctly modeling the dust attenuation law in galaxies is essential to properly inferring their growth and physical properties. With the rapid discovery of $z>6$ galaxies by JWST and lack of constraints on the properties of dust at these redshifts, we must turn to theoretical models to predict the properties of dust at these redshifts. In this paper, we develop a theoretical foundation for the nature of high-redshift attenuation laws. We analyze $107$ cosmological zoom-in simulations of galaxies run with the SMUGGLE galaxy formation model. SMUGGLE includes an explicitly modeled multiphase interstellar medium and a state-of-the-art dust model with a self-consistently evolving dust grain size distribution. We forward model these simulations with the radiative transfer package POWDERDAY to produce synthetic spectra and attenuation curves for a set of $z=5-15$, $M_*\sim10^{6.5}-10^{10}\; \mathrm{M}_{\odot}$ galaxies. We find evidence for flatter attenuation curves than the commonly used locally-calibrated literature attenuation laws assumed when modeling high-redshift galaxies. The vast majority of our attenuation curves also lack the 2175 Angstrom absorption feature. Our attenuation laws do not evolve strongly over $z=15-5$, which we attribute to relatively constant fractions of small grains up to $z=4$ in our simulations. We present a set of simulation-informed priors on dust attenuation law parameterization that could be implemented in SED fitting codes. There is an anti-correlation between galaxy mass and attenuation curve slope and positive correlation between galaxy mass and $A_V$, which we attribute to dust growth becoming efficient in our massive galaxies with $\log{M_*/\mathrm{M}_{\odot}}\gtrsim9$ and with ISM conditions above a critical metallicity threshold.

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arXiv2026

SQuIGGLE: Radio AGN Found Predominantly in Young, Gas-rich Post-starburst Galaxies at z ~ 0.7

The processes that rapidly quench massive galaxies and maintain their quiescence remain poorly understood. Substantial molecular gas reservoirs can survive this shutdown in post-starburst galaxies, raising the question of what prevents the remaining gas from efficiently forming stars. We present deep, subarcsecond VLA 6 GHz observations of 52 massive SQuIGGLE post-starburst galaxies at $z\sim0.7$ to test the prevalence and mechanical impact of radio activity during the gas-rich post-burst phase. We detect 6 GHz continuum emission in 18/52 galaxies ($\approx$35\%), spanning $\log(L_{1.4\,{\rm GHz}}/{\rm W\,Hz^{-1}})\simeq23.0$--$25.0$ and generally exceeding the contributions expected from ongoing star formation and the fading starburst, favoring an AGN origin. Ancillary data at other wavelengths provide little evidence for a population of luminous, radiatively efficient AGN, suggesting that these sources are instead dominated by comparatively weak, mechanically powered radio AGN. Radio activity is strongly concentrated within the first $\sim$200 Myr post-burst and is predominantly associated with galaxies retaining substantial gas reservoirs. Above a common radio luminosity threshold, the radio detection fraction is enhanced by a factor of $\approx$2.5 relative to the radio AGN candidate fraction in mass- and redshift-matched quiescent LEGA-C galaxies, primarily due to lower power sources. For efficient jet--ISM coupling, the inferred mechanical power could supply the approximate energy required to unbind the molecular gas on timescales comparable to its observed decline. Together, these results identify a population of radiatively inefficient AGN hosted predominantly by young, gas-rich post-starburst galaxies at $z\sim0.7$ and suggest that they may have a role in maintaining inefficient star formation.

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arXiv2026

The DAWN JWST Archive: A spectroscopically-informed population study of the physical properties of galaxies at z>2

Charting the evolution of star-formation, stellar mass and dust build-up over cosmic time relies on the accurate characterisation of the stellar population properties of high redshift galaxies. We present a catalogue of spectroscopically derived physical properties for ~12,000 galaxies at z>2, with archival JWST/NIRSpec PRISM spectroscopy and JWST/NIRCam imaging from the DAWN JWST Archive. We present a custom version of the SED fitting code BAGPIPES, used to perform joint spectrophotometric modelling of PRISM spectra and broadband NIRCam photometry. In doing so, we publicly release a science-ready catalogue of the physical properties of galaxies beyond cosmic noon. We report key physical properties, such as stellar masses, star-formation rates (SFR) and timescales, and V-band dust attenuation (Av). We further probe the recent star-formation histories (SFH) of galaxies via their SFRs on 3 Myr, 10 Myr, and 100 Myr timescales, capturing both rising and falling recent SFHs. Through dust-corrected ugi colour-colour diagrams of massive galaxies, we show how quiescent and dusty galaxies populate this colour space at z>2, with galaxies experiencing a recent downturn in star-formation occupying distinct parameter space. We also study the dustiest galaxies in our sample, showing that they are preferentially more massive and strongly star-forming relative to the parent sample, residing on the high-mass end of the star-forming main-sequence. The slope of the attenuation curve has a strong dependence on Av, varying from steep to shallow as the dust attenuation increases and the specific star-formation rate decreases. Our findings show that the unparalleled number statistics of this catalogue of spectroscopically derived stellar population properties uniquely enables the study star-formation, quenching and dust properties of galaxies from cosmic noon to cosmic dawn.

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arXiv2026

Evidence for inefficient dust production in a massive, metal-rich galaxy at $z=7.13$ uncovered by JWST and ALMA

Recent observations have revealed a remarkably rapid buildup of cosmic dust in the interstellar medium (ISM) of high redshift galaxies, with complex dust compositions and large abundances already appearing at redshifts $z>6$. Here we present a comprehensive, joint analysis of observations taken with the {\em James Webb Space Telescope} (\jwst) and the Atacama Large Millimetre/submillimetre Array (ALMA) of the highly magnified ($μ= 9.6$), dusty `normal' galaxy, A1689-zD1 at $z=7.13$. We perform detailed spectro-photometric modeling of the rest-frame UV to far-infrared spectral energy distribution (SED) based on archival photometry of the source and report new rest-frame optical strong-line measurements and metallicity estimates from recent \jwst/NIRSpec IFU data. We find that despite its substantial dust mass, $M_{\rm dust}\sim 1.5\times 10^{7}\,M_\odot$, A1689-zD1 has remarkably low dust-to-gas and dust-to-metal mass ratios, ${\rm DTG} = (5.5^{+3.5}_{-1.2})\times 10^{-4}$ and ${\rm DTM} = (6.2^{+4.2}_{-2.6})\times 10^{-2}$, respectively, due to its high metallicity $12+\log({\rm O/H}) = 8.36\pm 0.10$ and substantial gas mass, $M_{\rm gas} = (2.8^{+0.2}_{-1.7})\times 10^{10}\,M_\odot$ inferred from the [\cii] luminosity and bounded by its dynamics. The DTG and DTM mass ratios are an order of magnitude lower than expected for galaxies in the local universe with similar chemical enrichment. These low relative measurements are also consistent with the deficit observed in the $A_V/N_{\rm HI}$ ratio of A1689-zD1 in the line-of-sight. We find that this deviation in the DTG and DTM mass ratios appears to be ubiquitous in other metal-rich galaxies at similar redshifts, $z\gtrsim 6$. This suggests that the processes that form and destroy dust at later times, or the dust emissivity itself, are likely different for galaxies in the early Universe.

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arXiv2026

The abundance and emission of PAHs from the local to early Universe

We present the first large-volume cosmological simulation of galaxy evolution with a model for the abundance and luminous emission of Polycyclic Aromatic Hydrocarbons (PAHs) across a wide range of cosmic time ($0 < z \lesssim 6$). The simulation is built on a semi-analytic model (SAM) that includes a treatment of dust size distribution, with PAHs being identified with the smallest carbonaceous grains ($< 15$ A) residing in the diffuse ISM of galaxies. PAH emission is computed by post-processing the simulation output with the radiative transfer code GRASIL. In our model, where stars only inject large grains, the PAH abundance emerges from the growth and shattering of dust grains in the ISM, naturally reproducing the observed correlation between the PAH mass fraction ($q_{\rm PAH}$) and both metallicity and specific star formation rate (sSFR). We derive a relation to infer $q_{\rm PAH}$ from $L_{\rm PAH}$, $L_{\rm IR}$, and sSFR, with a relatively small scatter (a factor of $\approx 1.3$). Simulated PAH emission correlates well with key galaxy properties such as SFR and molecular gas content, and reproduces observational constraints. Taken together, these results support the viability of a top-down formation scenario for PAHs. We extend our model predictions to higher redshifts, which can be exploited for future missions such as PRIMA. This work is a significant step toward leveraging PAH observations as a tool for studying galaxy evolution.

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arXiv2026

SQuIGG$\vec{L}$E: Spatially resolved NIRSpec Pa$α$ reveals that the most CO-luminous, $z\sim0.7$ post-starburst galaxies exhibit falling---but still active---star formation behind optically thick dust

One of the fundamental uncertainties in the study of galaxies at the interface between star formation and quiescence--post-starburst galaxies--is their instantaneous star formation rate. While these galaxies appear quiescent in the rest UV-optical, many of the youngest systems are highly CO luminous, implying dense molecular gas, and potentially star formation. However, their instantaneous star formation rates have proved elusive, as measurements with rest-optical emission lines or UV-to-IR SED fitting yield order-of-magnitude uncertainty. Here, we present JWST/NIRSpec G395M/170LP integral field spectroscopy of the three most CO-luminous massive galaxies in the SQuIGG$\vec{L}$E survey, spatially resolving Pa$α$/Br$γ$/Br$β$ to map the dust-corrected star formation rate. We find that these galaxies host star formation rates of $27-80$ $M_\odot \ \mathrm{yr^{-1}}$, though $\sim30-40\%$ of the emission may arise from non-star-forming ionization (AGN or shocks), implied by high [FeII]/Pa$α$ in central spaxels. We also find effective $A_{\mathrm{Pa}α}\sim1$ and individual $\sim700$ pc spaxels with $A_{\mathrm{Paα}}\sim2$, implying significant dust obscuration of nebular regions that contribute little to the rest-optical lines. These galaxies are not consistent with total quiescence; the measured star formation rates place these galaxies on or above the main sequence. However, their star formation rates \textit{are} significantly lower ($\sim0.5-1$ dex) than the peak star formation rates inferred from spectrophotometric fitting. We propose that the CO luminous post-starburst phase represents the last gasp of star formation in galaxies on the rapid quenching pathway, and that dust-obscured star formation, outflows, and (speculatively) an uncertain CO-to-H2 conversion reconcile their gas exhaustion with observed timescales of fading.

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arXiv2026

ALMA Chemical Evolution (ACE) survey: an overview -- extending dust and gas inference to low metallicities at cosmic noon

The baryon cycle governs the exchange of gas, metals, and dust between galaxies and their environments, but simultaneous constraints on these constituents remain scarce beyond z~1. The ALMA Chemical Evolution (ACE) survey is a Cycle 11 ALMA Large Program designed to address this by studying the molecular gas, dust, and metal content of sub-solar metallicity at cosmic noon. ACE consists of CO(3-2) and dust continuum observations of 25 galaxies at z=2.0-2.5 with robust gas-phase metallicity measurements spanning ~35% to 83% of solar metallicity. The survey approximately doubles the number of unlensed main-sequence galaxies at z>1 with CO, dust-continuum and metallicity measurements and extends such studies to almost an order of magnitude lower stellar masses and metallicities than previous surveys. The ACE observations yield 17 CO detections and 17 Band 7 continuum (rest-frame ~270um) detections. CO detectability correlates most strongly with metallicity and stellar mass, while dust continuum detectability is more closely linked to star formation rate (SFR) and infrared luminosity. Using the ACE measurements, we derive a new empirical scaling relation linking CO(3-2) luminosity to stellar mass, SFR, and metallicity, providing a practical benchmark for estimating molecular gas content in low-mass, low-metallicity galaxies. The survey reveals several particularly intriguing systems, including some of the lowest-metallicity CO and dust detections currently known at z>1, galaxies with extreme gas and dust fractions, and systems exhibiting offsets between stellar, dust, and molecular gas emission. ACE provides the first comprehensive view of the interplay between molecular gas, dust, metals, and star formation in sub-solar metallicity galaxies at cosmic noon, enabling direct tests of models for baryon cycling, chemical enrichment, and dust evolution during the peak epoch of galaxy assembly.

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arXiv2026

ALMA Chemical Evolution (ACE) Survey: Molecular gas properties of low-mass, low-metallicity galaxies at cosmic noon

Molecular gas plays a central role in regulating star formation and galaxy evolution, yet observational constraints at cosmic noon remain biased toward massive, metal-rich systems. We present ALMA Band 3 observations of the CO J=3-2 transition in 26 unlensed star-forming galaxies from the ALMA Chemical Evolution (ACE) Large Program at z~2-2.5, probing stellar masses of $10^{9} < M_\star < 10^{10.5}\,\mathrm{M}_{\odot}$ and sub-solar metallicities (8.2 < 12 + log(O/H) < 8.6). We derived molecular gas masses using a metallicity-dependent CO-to-H$_2$ conversion factor and alongside analysed stacking measurements and a homogenized literature compilation spanning both local and high-redshift galaxies. We find that the ACE galaxies extend established molecular-gas scaling relations to an order of magnitude lower stellar masses than previously explored at cosmic noon. The molecular gas mass ($M_{\rm mol}$) correlates tightly with star formation rate (SFR), while molecular gas fractions show a strong dependence on specific star formation rate (sSFR) and offset from the star-forming main sequence. In contrast, molecular gas fractions show only weak trends with stellar mass and no significant dependence on metallicity. Molecular gas depletion times are ~1 Gyr and vary little with stellar mass or metallicity, and weakly with sSFR and offset from the star-forming main sequence. Together this further reinforces that the availability of molecular gas is the primary driver of the SFR in galaxies, with changes in star-formation efficiency playing a secondary role. Leveraging the expanded parameter space probed by the ACE and literature samples, we derive a new empirical prescription for predicting $M_{\rm mol}$ as a function of SFR and sSFR. The persistence of the observed scaling relations suggests a largely universal framework governing the molecular gas-star formation cycle across cosmic time.

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arXiv2026

JWST Whirlpool Galaxy Treasury: Mid-Infrared Emission in M51 and its Relation to Gas Column and Star Formation

Using JWST/MIRI imaging of M51 in eight broadband filters, we investigate correlations of mid-infrared emission from polycyclic aromatic hydrocarbons (PAHs) and dust continuum with molecular, atomic, and ionized gas traced by CO(1-0), HI, and Pa-alpha, respectively. In molecular gas-dominated regions, PAH-dominated filters (F560W, F770W, F1130W, F1280W) exhibit near-linear correlations with CO(1-0) at 40 pc scale, indicating that PAHs are well-mixed with gas and experience relatively constant radiation field intensities. The F1500W, F1800W, and F2100W dust continuum-dominated filters show shallower slopes with CO(1-0), reflecting contributions from star-forming regions with high radiation field intensities. This is reinforced by the near-linear scaling between F2100W and Pa-alpha. PAH-dominated bands do not show this linear trend with Pa-alpha, likely due to their destruction in ionized regions. F1000W behaves similarly to PAH bands in its correlations with CO(1-0) and Pa-alpha. Modeling mid-infrared emission with an empirical decomposition into gas- and star-formation-associated components shows that PAH-dominated filters receive comparable contributions from both, while the relative contribution associated with the Pa-alpha template increases toward longer wavelengths, reaching $\sim$75% in F2100W. These results demonstrate that mid-infrared simultaneously traces the gas column and star formation, but with a systematic wavelength-dependent shift in what drives the correlations: PAHs being more gas-tracing and dust-continuum reflecting star formation. Lastly, considering both HI and H$_2$ at 440 pc resolution, we find a tight, linear relation between $Σ_{HI+H_2}$ and PAH-dominated filters. Although most of our coverage is in H$_2$-dominated regions, we note similar observations with HI, suggesting that PAHs are also well-mixed with atomic gas.

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arXiv2026

Dust in the Average Galaxy: Attenuation, Emission, and Opacity from $0<z<7$

We present constraints on the dust emission and attenuation properties of galaxies across 0 10^{10.5}M_\odot$. We derive empirical relationships for the effective attenuation, dust temperature, fraction of star formation that is unobscured, and dust-to-stellar mass ratio as functions of redshift and stellar mass. We separate the first order effect of star/dust geometry from dust grain properties by combining constraints on the IR SED, UV SED, and dust mass surface density. Importantly, we measure over an order of magnitude decrease in $κ_{UV}/κ_{FIR}$--the ratio of dust mass absorption coefficients in the UV at 1600Å and FIR at 500$μ$m--from z~0 to z~7. A depressed $κ_{UV}/κ_{FIR}$ is consistent with a deficit of small dust grains, possibly attributable to the intense radiation fields of high-$z$ star formation; indeed, we find a redshift-invariant inverse relationship between $κ_{UV}/κ_{FIR}$ and $Σ_{SFR}$. Most evolution in the dust-to-stellar ratio is at $z<1$, the product of mild downward evolution in the dust-to-gas ratio combined with steep evolution in the gas-to-stellar ratio. The significant evolution and dynamic range of $κ_{UV}/κ_{FIR}$ and prevailing disconnect between the UV/optical and FIR regimes emphasize that direct dust constraints are irreplaceable for the majority of star-forming galaxies at z<7, not just the most extreme star-formers.

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arXiv2026

Dust and Grain Size Evolution in Galaxy Simulations: What Matters and What Does Not

We present the first implementation of an evolving dust grain size distribution (GSD) within a semi-analytic cosmological model (SAM) of galaxy evolution. This flexible model self-consistently accounts for stellar dust production, shattering, coagulation, accretion of gas-phase metals, and destruction in supernova-driven shocks and hot gas, successfully reproducing key observational constraints. The purpose of this paper is to present the key physical elements of this novel dust implementation in a SAM and to explore controlled numerical experiments to identify the mechanisms shaping the GSD and extinction law in galaxies. Our results show that the GSD evolves from a large-grain-dominated regime at high redshift to a flatter, MRN-like shape at low redshift. This transition occurs earlier for massive galaxies, at a characteristic metallicity determined by the galaxy depletion time. The resulting extinction curves show an increase of the UV/optical slope and a pronounced $2175$ A bump toward lower redshift, in good agreement with the extinction properties of the MW. Through numerical experiments, we find that once stars provide the initial reservoir of large grains, shattering and ISM accretion are the principal mechanisms driving the growth of small grains. When accretion is included, the model robustly reproduces the observed $z \approx 0$ dust masses, largely independent of the specific assumptions adopted for grain-size physics. The extinction properties of MW-like galaxies are also generally recovered, except in extreme cases, such as when grain velocities in turbulent media are assumed to be independent of grain size.

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arXiv2026

The HII Regions' Molecular Law of Star Formation

We combine imaging data from the HST, JWST, and ground-based millimeter facilities to investigate the correlation between star formation rate (SFR) and molecular gas at the ~100 pc scale of HII regions in three nearby galaxies: NGC628, NGC5194 and NGC5236. The JWST 21 micron maps of the three galaxies offer a unique insight into the dust-absorbed SFR at high resolution. We find that the relation between the surface densities of SFR and molecular gas has a slope of ~1.85, in log-log scale, significantly steeper than previous results for nearby galaxies but closer to the trends found for molecular clouds in the Milky Way. The steep relation also holds on larger, ~500 pc, scales, and results from the high-resolution imaging that cleanly isolates the star-forming region emission from the underlying galaxy's diffuse contribution. The diffuse emission at 21 micron is, in fact, found to correlate with the galaxy's stellar mass. Comparisons with physical models of star formation are inconclusive; they overlap with the locus of the 100 pc data, but have difficulties in reproducing the data scatter. Possible exceptions are models that add a power law tail to the gas density probability distribution, due to the large range of free parameters allowed. We find that local HII regions, high redshift star-forming clumps, and low and high redshift starburst galaxies form a single sequence of star formation over three orders of magnitude in gas surface density.

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arXiv2026

The Drivers of Cosmic Dust Temperature Evolution

Observations of the rest-frame far-infrared (far-IR) emission of galaxies suggest a mild increase of dust temperature $T_{\rm dust}$ with redshift, although constraining $T_{\rm dust}$ in high-redshift systems remains challenging due to limited sampling of the far-IR spectral energy distribution (SED). We present and discuss the redshift evolution of $T_{\rm dust}$ predicted by a cosmological galaxy evolution simulation with dust treatment, and interpret its dependence on other galaxy physical properties. We use a semi-analytic model of galaxy formation that includes an explicit treatment of dust, post-processed with radiative transfer. Dust temperatures are derived by applying modified blackbody SED fitting to the simulated galaxies, mirroring the methodology adopted in most observational studies. The dust temperature of simulated galaxies increases with redshift, in broad agreement with observational results. A feature-importance analysis reveals that the star formation rate surface density $Σ_{\rm SFR}$ and the dust-to-gas ratio (DTG) are the main drivers of dust temperature, tracing the intensity of the interstellar radiation field and the optical depth of warm molecular clouds, respectively. Galaxies with higher star formation rate surface density and lower DTGs -- common conditions at high$-z$ -- are associated with warmer dust. We provide a simple relation to estimate DTG from $Σ_{\rm SFR}$, $T_{\rm dust}$, and redshift. Variations in dust grain size and chemical composition have a negligible impact on $T_{\rm dust}$. Our results are particularly relevant to the study of dust properties with observations of high-z galaxies, where far-IR dust emission is not fully sampled.

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arXiv2026

JWST spectroscopy of galaxies at $z>10$: Damped Ly$α$ absorbers reveal efficient star formation and hidden redshift biases

Recent observations with JWST have revealed a remarkable population of surprisingly luminous galaxies at redshifts $z>10$. Their abundance exceed predictions from simulations and empirical extrapolations from lower redshifts, suggesting a transition in the physical conditions under which the first stars formed. Here we investigate the physical conditions of a select sample of 25 galaxies with robust redshift measurements at $z_{\rm spec}\geq 10$ observed with JWST/NIRSpec Prism. We characterize their star-formation efficiency, `burstiness', and presence of strong rest-frame UV nebular lines in relation to the density of the local neutral atomic hydrogen (HI) gas reservoirs they are embedded in. We find that the prominence of strong rest-UV lines are correlated with the burstiness of the galaxies, defined as ${\rm SFR_{10\,Myr} / SFR_{100\,Myr}}$. In contrast, there are no strong connections between the HI gas column density derived from the damped Ly$α$ absorption (DLA) and the $M_{\rm UV}$ brightness, ${\rm SFR_{10\,Myr} / SFR_{100\,Myr}}$, and prominence of rest-UV lines. The most bursty galaxies show a large variation in star-formation efficiencies and HI gas surface densities, though typically with very short depletion timescales, $t_{\rm dep} \lesssim 20$\,Myr. This necessites rapid gas depletion times and external replenishment from infalling, pristine gas, powering starburst episodes on equally short timescales. We further quantify the impact of strong DLAs in galaxy spectra on photometric and Ly$α$-break redshift-inferences, finding average redshift biases of $\langle z \rangle =0.39$ and $0.14$, respectively, when not incorporating DLAs on the emergent spectra. We show the effect of this bias on new measurements of the cosmic UV luminosity density, $ρ_{\rm UV}$, derived here at $z>10$, finding that this has a marginal impact on the UV luminosity function.

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arXiv2026

Reimagining SED Fitting with Cosmological Galaxy Simulations and Machine Learning

SED fitting is the most common technique to recover galaxy physical properties from observed photometry. However, SED fitting requires many assumptions that essentially collapse a galaxy from a three-dimensional spatially varying object with complex structure into a scalar point. Moreover, modern inference techniques are computationally intensive, which presents a unique challenge in the era of extremely large datasets. We present \textsc{Phot-Gal}, a new galaxy SED modeling tool that solves the inverse problem of SED fitting by training a machine learning model on photometry generated from 3D radiative transfer of simulated galaxies with a wide range of implemented physics. \textsc{Phot-Gal} is designed to accept an arbitrary amount of input photometry by utilizing a $K$-nearest neighbors imputation strategy. Our fiducial model predicts redshift, stellar mass, dust mass, and star formation rate with uncertainties based on the provided input photometry. We evaluate the performance of \textsc{Phot-Gal} relative to the commonly-used SED fitting tool \textsc{prospector} in successfully recovering each of these properties with several metrics for the inferred values and uncertainties and find that it outperforms the accuracy of standard SED fitting software on the testing set. However, with fewer photometric constraints, \textsc{Phot-Gal} is more likely to have output uncertainties that do not reflect the offset from the ground truth. We dissect the components of \textsc{Phot-Gal} to find reasonable physical justifications for the photometry it relies on most, understand how each step in its workflow contributes to the eventual output posterior, and evaluate its ability to generalize to novel data.

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