arXiv subjects

Irene Shivaei

Publications and source records attributed to Irene Shivaei.

At least 19 recordsLinked to original sources

arXiv2026

Stellar masses of optically dark galaxies: uncertainty introduced by the attenuation law and star-formation histories

Recent studies using JWST have suggested that some high-redshift galaxies may be ultra-massive, thereby challenging standard models of early galaxy formation. We analyse the stellar masses using different modelling assumptions and in conjunction with new data of three galaxies (S1, S2 and S3), whose photometric and NIRCam/grism redshifts were consistent with $z>5$. These three ``optically dark'' galaxies were reported to host exceptionally high stellar masses ($M_{\star}\gtrsim10^{11}~M_{\odot}$) and star-formation rates ($\mathrm{SFR}\gtrsim600~M_{\odot}~\mathrm{yr}^{-1}$), implying extremely high star-formation efficiencies. Recent NIRSpec/IFU observations for S1 indicate a spectroscopic redshift of $z_{\rm spec}=3.2439\pm0.0002$, revising the previous redshift. Using the Bayesian spectral energy distribution (SED) modelling tool \texttt{Prospector}, we investigate the impact of key model assumptions on stellar mass estimates, such as the choice of star-formation history (SFH) priors (constant versus rising SFH base for the non-parametric SFH prior), the dust attenuation law, and the treatment of emission line fluxes. Our analysis yields revised stellar masses of $\log_{10}(M_{\star}/M_{\odot}) \approx 10.36, 10.95$ and $10.31$ for S1, S2, and S3, respectively. We find that adopting a rising SFH base prior results in lower inferred stellar masses compared to a constant SFH base prior. Additionally, we identify a significant degeneracy between the dust attenuation curve slope, the amount of dust attenuation, and stellar mass. Our results highlight various systematics in SED modelling due to SFH priors and dust attenuation that can influence stellar mass estimates. Nevertheless, even with these revised stellar mass estimates, two of the three galaxies remain among the most massive and actively star-forming systems at their respective redshifts, implying high star-formation efficiencies.

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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

Resolved Dust in $z\approx1$ Galaxies with JWST/MIRI MRS: Survey Description and First View on PAHs, Mid-IR Atomic Emission, and Warm H$_2$

Dust is a key component of galaxies that regulates their thermal balance and, consequently, star formation and the build-up of stellar mass. Polycyclic aromatic hydrocarbons (PAHs) are responsible for reprocessing radiative energy of the galaxies thus tracing dust evolution. Using JWST/MIRI MRS, we present the first resolved view of the PAHs and mid-infrared emission lines in a sample of eight $z\approx1$ galaxies. A key novelty is our ability to directly map PAH emission near the end of cosmic noon at JWST's limits. The sample is selected to be on the star-forming main-sequence with stellar masses $M_{\star}=10^{10.6-11.2}\,M_{\odot}$ and infrared luminosities $L_{\rm IR}=10^{11.5-11.9}\,L_{\odot}$. Two of them contain active galactic nuclei (AGN), and two are interacting systems. We detect and quantify primary PAH emissions from 3.3$\,\rm μm$ to 11.3$\,\rmμm$ throughout the galaxies, alongside atomic fine structure lines (Ar, Ne, and Fe), Br$α$, and H$_{2}$ rotational transitions. Through PAH ratio diagnostics and comparison to theoretical models, we qualitatively probe the physical properties of PAH molecules, i.e., size and charge. The AGN and mergers in our sample exhibit a higher fraction of neutral PAHs, possibly related to high radiation intensity and/or shocks, as suggested by increased atomic and H$_{2}$ line ratios. Leveraging the IFU data, we find that the grain sizes of the centrally located PAHs tend to be larger and less ionized than those in the outskirts of the galaxies. Finally, we compare our results to observations of PAHs in local and similar-redshift galaxies, revealing a potential evolutionary trend when controlling $L_{\rm IR}$.

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arXiv2026

NOEMA probes the [CII] and dust content in a 2175Å UV Bump Galaxy at $z=7.1$

The detection of the $2175$Å UV bump at $z>6$ challenges existing models of dust formation, suggesting rapid formation of small carbonaceous dust grains within the first billion years of cosmic time. We present the results of the first direct attempt at linking far-infrared (FIR) observations to the UV bump within the Epoch of Reionisation (EoR), through NOEMA observations of GNWY-7379420231 at $z=7.108$, a galaxy exhibiting the strongest known UV bump feature at $z>4$. We detect the [CII] 158$μ$m emission line at 5.1$σ$ at $z_\mathrm{[CII]} = 7.1078 \pm 0.0005$, in excellent agreement with the redshift derived from the [OIII] $λ5007$Å line. The {\cii} luminosity implies $\mathrm{SFR}_\mathrm{[CII]}=16.2^{+5.8}_{-5.5}M_\odot \mathrm{yr}^{-1}$, consistent with short timescale SFR tracers such as dust corrected $\mathrm{SFR}_\mathrm{Hα}=18.0\pm3.9 M_\odot \mathrm{yr}^{-1}$ and $\mathrm{SFR}_\mathrm{10~Myr}=20.5^{+3.8}_{-5.0}M_\odot \mathrm{yr}^{-1}$ from SED fitting. The dust continuum is not detected suggesting an obscured SFR of $\mathrm{SFR}_\mathrm{IR} < 32 ~M_\odot \mathrm{yr}^{-1}$ and a dust mass of $M_\mathrm{d} < 5.3\times10^6 ~M_\odot$ $(M_\mathrm{d}/M_\star<2\%)$. Finally, the [CII]-derived dynamical mass of $\log_{10}(M_\mathrm{dyn}/M_\odot)=8.95^{+0.51}_{-0.66}$ and stellar mass of $\log _{10}\left(\mathrm{M}_{\star} / \mathrm{M}_{\odot}\right) = 8.39_{-0.09}^{+0.13}$ suggest a gas-rich moderately massive galaxy. Taken together these results rule out GNWY-7379420231 being a heavily dust obscured or massive galaxy, but rather a `normal' EoR galaxy with a recent upturn in star-formation. Our results suggest efficient shattering of larger dust grains in the diffuse, turbulent ISM and/or fortunate line of sight alignment are needed to explain the UV bump properties of GNWY-7379420231.

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arXiv2026

Little Red Dots: One Photometric Tag Concealing Diverse Spectroscopic Flavors of Massive Star Formation and Black Hole Activity

We compile JWST/NIRSpec prism and MIRI data for 249 Little Red Dots (LRDs) at 2.3<z<9.3, forming a representative spectroscopic subset of NIRCam-selected LRDs. We derive a median stacked spectrum covering rest-frame 0.09-1.2 $μ$m, with MIRI photometry extending the spectral energy distribution to 4 $μ$m. Four additional stacks for subsamples defined by optical-to-UV luminosity ratios show that LRDs form a heterogeneous population spanning diverse continuum slopes and line properties. Assuming LRDs host super-massive black holes (BHs) surrounded by dense gas clouds, and stars accompany this core, we infer masses of $M_{BH}\sim10^{6.0-6.5}$ M$_\odot$ and $M_\bigstar\sim10^{8.3}$ M$_\odot$, corresponding to BH-to-stellar mass ratios of 1-2%. The stacks show ubiquitous UV and optical FeII emission, indicating a direct view of the broad-line region and high (but sub-Eddington) accretion ($λ_{Edd}=0.6\pm0.2$). We find a significant stellar contribution in the far-UV, reaching $\sim80$% in the bluest systems. Possible Wolf-Rayet features (HeII$λ$4687, nitrogen lines) are identified, tracing a young (3-7 Myr) compact starburst event. We also detect strong Balmer breaks and atypical Balmer, Paschen, [OIII], and optical and near-infrared HeI line ratios, and an absorption at $\sim4550$ Angstrom (probably linked to FeII), all consistent with radiative-transfer effects in high-density gas with warm temperatures (4000-7000 K). We find a diversity of LRD flavors modulated by the luminosity ratio between between a short ($\lesssim20$ Myr) and intense phase of BH activity, the most extreme stage lasting $\sim3-7$ Myr, characterized by near-Eddington-limit radiation, and a nuclear and compact starburst dominated by massive stars (even super-massive, $\mathrm{M}_\mathrm{SMS}\sim10^{5}$ M$_\odot$), all embedded in dense gas with modest dust content producing a variety of optical depths.

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arXiv2026

Can Little Red Dots Contribute To The Early Universe Cosmic Dust Budget?

The origin of dust in the early Universe remains uncertain. We explore whether Little Red Dots (LRDs) could provide a high-redshift dust-production channel. Motivated by an analogy with Type IIn supernovae, we investigate whether LRDs may share the efficient dust-forming conditions. We consider dust formation in outer winds and later in a shielded cold dense shell after the central source fades. Such dust may avoid a global remnant-phase reverse shock, while longer LRD lifetimes may promote grain growth. We model the SEDs of two low-redshift LRD analogs as a thermal pseudo-photosphere plus optically thin dust components, obtaining dust reservoirs of order $10^{2}$-$10^{3}\,M_{\odot}$. Although photometry cannot exclude pre-existing dust, the narrow Balmer components of both analogs are close to the Case B ratio and their narrow-line environments are metal-poor, disfavoring a dominant diffuse host-ISM origin for the inferred reservoir. Formation within the LRD outflow is consistent with our optical-depth, sublimation, and energy-balance checks. In this scenario, the large dust masses do not produce a strong optical attenuation because a clumpy or asymmetric distribution can leave the dominant optical sightlines relatively unobscured. Our population calculation shows that the LRD contribution to early-Universe dust production can range from negligible to dominant. At higher efficiencies, LRDs can approach or exceed the lower CCSN contribution above $z\approx5$, whereas less favorable assumptions yield a minor contribution. We therefore propose that LRDs can act as early-Universe dust factories and, under some conditions, may dominate over CCSNe or provide seed grains for subsequent growth in the early ISM.

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arXiv2026

LEGGOS I: The JWST LEGGOS Survey -- LEnsing and Galaxy Growth: Observing Substructures -- Unpacks the Nature of Clumpy Star Formation and Quenching in Gravitationally Lensed Galaxies beyond Cosmic Noon

We present first results from the JWST LEGGOS Survey (LEnsing and Galaxy Growth: Observing Substructures), aimed at studying the physics of clumpy star formation and quenching in eight lensed galaxies at $z\sim2$--4. LEGGOS combines multiple Cycle 2 JWST GO programs (GO 4125, GO 3843) and Cycle 1 archival data, and utilizes strong gravitational lensing with NIRCam imaging and NIRSpec integral-field spectroscopy. LEGGOS targets UV-bright, highly magnified systems to resolve $\sim$10--200 pc regions in both rest-frame optical continuum and nebular emission. This overview paper describes the survey design, data reduction and calibration strategy, and science-quality data products, and highlights early examples demonstrating how spectroscopy breaks key degeneracies inherent to photometry-only clump studies, including identifying recent quenching in previously-thought UV star forming galaxies. We introduce a uniform analysis framework that jointly models lensing reconstruction, multi-band photometry, and integral field spectroscopy to disentangle multiple stellar populations within individual clumps and their surrounding diffuse regions. Using maps of Balmer recombination lines and key emission line diagnostic ratios, we connect star formation histories, dust attenuation, and nebular conditions on sub-kpc scales -- LEGGOS galaxies range from uniform metallicities across the whole galaxy, to having higher clump metallicities and harder ionization conditions relative to diffuse regions. The full survey dataset, with simultaneous flux and morphology constraints on clumpy source-plane regions, and a flexible spectrophotometric SPS modeling approach, provides a direct bridge between parsec-scale star formation physics and galaxy assembly at and beyond cosmic noon, offering a robust and efficient means of resolving star formation in the first galaxies.

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arXiv2026

PAHSPECS: Spatially Resolved PAH Spectroscopy at cosmic noon with JWST MIRI MRS

We present spatially resolved spectroscopy with JWST/MIRI MRS of a representative sample of normal star-forming galaxies at $z\sim1.1$ as part of the PAHSPECS program. To extract emission from Polycyclic Aromatic Hydrocarbon (PAH) features, we forward model the data cubes with non-parametric spatial distributions, accounting for convolution with the PSF. With this method we are able to recover accurate spatial profiles of the 3.3 $μ$m, 6.2 $μ$m, 7.7 $μ$m, 11.3 $μ$m PAHs and [ArII] (6.98 $μ$m) emission and produce PAH ratio maps at cosmic noon. From the PAH ratio maps we find that PAHs become larger and more neutral with increasing galactocentric radius, which is the opposite of trends in local galaxies, indicating radial ISM gradients in normal star-forming galaxies are different at cosmic noon. Through spatially resolved SED fitting of HST and JWST photometry we measure the UV radiation field hardness through the intrinsic ratio of UV to optical flux and find the 3.3/11.3 PAH ratio to decrease with increasing hardness and the 11.3/7.7 to increase. This may suggest photo-destruction of small/ionized PAHs is driving the observed PAH ratio trends and may explain the overall lower 3.3/11.3 PAH at cosmic noon compared to the local universe. This work demonstrates that PAH properties hold crucial information on the resolved ISM physics of galaxies at cosmic noon.

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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 assume that PAHs are 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

PAHSPECS: Polycyclic aromatic hydrocarbon properties at cosmic noon with JWST/MIRI MRS

Context. Cosmic noon (z ~ 1-3) marks the peak of the cosmic star-formation rate density, when dust-obscured star formation dominated galaxy growth. Mid-infrared spectroscopy probes the interstellar medium through PAH emission, whose band ratios trace PAH charge, size, and local radiation-field conditions. Aims. We characterize the PAH properties of five z ~ 1.1 star-forming galaxies from the PAHSPECS survey and investigate how their PAH luminosities and band ratios relate to global galaxy properties. We compare them with local luminous infrared galaxies (LIRGs) to assess whether PAH emission at cosmic noon differs from that nearby. Methods. We analyze JWST/MIRI MRS observations of five ASPECS galaxies in the HUDF. Integrated spectra are extracted with wavelength-dependent apertures and modeled with CAFE, including ancillary photometry to constrain the dust emission. Stellar masses and SFRs are derived with Prospector. Results. Compared to local LIRGs, most PAHSPECS sources show higher 6.2/7.7 and lower 11.3/7.7 ratios, suggesting an ionized PAH component weighted toward smaller grains. The 3.3/11.3 ratio is less constrained, since the 3.3 micron feature is detected in only two sources. Within the sample, 11.3/7.7 increases with sSFR and star-formation surface density, while 6.2/7.7 decreases with sSFR, consistent with preferential processing of small ionized PAH carriers. ASPECS-15, the AGN-hosting source, has the lowest 6.2/7.7 ratio and highest sSFR, suggesting a reduced contribution from small PAHs, potentially due to AGN activity. The 7.7 micron luminosity follows the local L7.7-SFR relation, supporting its use as a star-formation tracer at z ~ 1. Conclusions. PAH emission at cosmic noon appears shaped by different ISM conditions than in nearby starburst galaxies, likely reflecting more intense radiation fields, while the 7.7 micron feature remains a robust SFR tracer.

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arXiv2026

3D-Herschel: Constraining Dust Emission with Panchromatic Modeling of 3D-HST Galaxies

We present 3D-Herschel, a new publicly released 0.3-350$μ$m photometric catalog that combines deblended Herschel far-infrared (FIR) imaging with the CANDELS/3D-HST legacy fields to probe the dust-obscured universe. Using the 17-parameter Bayesian fitting code Prospector-$β$, we model 41,387 galaxies spanning 0.5 $< z <$ 2.5 to measure stellar and dust properties. Comparing fits with and without FIR constraints, we find that for the 3.2$\%$ of galaxies with $>3 σ$ detections in $\geq2$ Herschel bands, UV-MIR-only models (0.3-24$μ$m) recover robust stellar ages, SFRs, and stellar masses (50-70$\%$ within the median 1$σ$ error). Consequently, the Prospector-$β$ star-forming sequence is unchanged by the inclusion of FIR data (average deviation 0.1$\pm$0.07 dex between UV-MIR and UV-FIR fits at fixed stellar mass), confirming that the offset relative to UV+IR-based estimates reported by Leja et al. 2022 is robust to the lack of direct FIR constraints. However, the use of rigid log-average IR templates with fixed dust emission parameters ($γ$, $U_{\mathrm{min}}$, $q_{\mathrm{PAH}}$) in UV-MIR modeling yields cold dust temperatures ($\sim$7K colder than Herschel-informed fits at all redshifts) and an unevolving MIR-to-IR luminosity ratio, with $\sim$0.2 dex lower IR-to-7.7$μ$m luminosity ratios at the low-mass end of Herschel-detected galaxies (log($M_{\star}$) $\sim$ 9.6 $M_{\odot}$). These results demonstrate that MIR-to-IR conversions depend on stellar mass, cautioning against $L_{\mathrm{IR}}$-independent templates without FIR data. For galaxies with $<10^{11} \ M_{\odot}$ at $z>1.5$, Herschel can at best provide upper limits due to source confusion; next-generation FIR telescopes will be essential for distant galaxies.

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arXiv2026

Summary of Discussion Sessions from "The Dusty Universe 2025: The Fifth Pandust Conference"

"The Dusty Universe: The Fifth Pandust Conference" took place in Tucson (AZ, USA) from November 10 until 14, 2025. The goal of this meeting was to get the dust community together to review where we are, hear exciting new results, and make plans for the future. The meeting encompassed all aspects of dust investigations including observations, theory, modeling, and laboratory studies. The conference consisted of invited review talks, contributed talks and posters. Science topics included interstellar dust (Milky Way & nearby galaxies), circumstellar dust (including Solar System & exoplanets), dust in Galaxies (including high-z), lifecycle of dust, and future needs (laboratory, theory, & observations), with a particular focus on results from JWST and ALMA, and on nanodust (including PAHs). On November 12th, we organized breakout discussion sessions covering a wide range of interesting dust-related topics. The purpose of this document is to capture the main topics/questions that were discussed, the key conclusions of these discussions, the challenges and possible solutions that were brought up, and the open questions that still remain to be answered. We hope that this document records our findings and challenges for the future generation.

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arXiv2026

JADES Data Release 4 Paper I: Sample Selection, Observing Strategy and Redshifts of the complete spectroscopic sample

This paper accompanies Data Release 4 of the JWST Deep Extragalactic Survey (JADES), which presents the full NIRSpec spectroscopy of the survey. We provide spectra of 5190 targets across GOODS-North and GOODS-South (including the Hubble Ultra Deep Field), observed with the low-dispersion (R $\sim$ 30-300) prism and three medium-resolution (R $\sim$ 1000) gratings spanning 0.8 $< λ<$ 5.5 microns; 2654 were also observed with the higher-resolution (R $\sim$ 2700) G395H grating. The tiered survey design obtained more than 20 hr exposures for $\sim$ 700 galaxies in the Deep and Ultra Deep tiers, and shallower observations ($\sim$ 1-3 hr per setting) of $>$ 4400 galaxies in the Medium tiers. Targets were selected from photometric redshifts or colours, with priority given to rest-UV-selected galaxies at $z > 5.7$ and F444W-selected galaxies at $1.5 < z < 5.7$. We describe the full target selection and present spectroscopic redshifts and success rates. In total we obtain robust redshifts for 3297 galaxies, including 396 at $z > 5.7$ and 2545 at $1.5 < z < 5.7$. To facilitate uniform analyses, we define 'gold' sub-samples based on UV- and F444W-selection. Using the parent samples and redshift success rates, we construct rest-UV luminosity functions at $6 \lesssim z \lesssim 9$ from the Medium- and Deep-JWST tiers. Our number densities agree well with previous determinations from both photometric and spectroscopic samples, with modest interloper fractions confirming the reliability of photometric UV-bright galaxy selections at these redshifts.

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arXiv2026

A first empirical derivation of the average dust attenuation law at 2<z<7

Dust attenuation strongly affects the observed spectral energy distributions of galaxies, introducing significant uncertainties in the derivation of key physical properties such as star formation rates, stellar masses, and metallicities. While attenuation curves have been extensively studied in the local Universe and at intermediate redshift, direct spectroscopic constraints at earlier cosmic epochs have remained limited prior to JWST. We aim to derive the average dust attenuation law of star-forming galaxies over the redshift range 2 9) of 120 galaxies with reliable Balmer decrement (Ha/Hb), we construct stacked spectral energy distributions in bins of Balmer optical depth and derive the selective attenuation curve following the empirical methodology introduced by Calzetti et al. (2000). The wavelength coverage is further extended toward the near-infrared using MIRI photometry. The resulting attenuation curve spans the rest-frame range 0.16-1.14mu and is well described by a smooth function. We derive a normalization factor R_V=3.98, finding that the average attenuation law is consistent with the local starburst relation in both slope and normalization. Compared to several determinations at intermediate redshift, however, our curve appears systematically flatter in the ultraviolet. We find no significant evidence for a 2175A UV bump in the average attenuation curve. Our results provide the first empirical determination of the average dust attenuation law for star-forming galaxies at 2<z<7 based on JWST spectroscopy. Despite the diversity of attenuation properties observed in individual systems, the ensemble-average behavior remains consistent with the local starburst relation.

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arXiv2026

Calibrating Photometric Mid-Infrared Star Formation Rates for JWST

The mid-infrared (IR) spectrum of galaxies has a long history as a valuable proxy for the dust-obscured star formation rate (SFR) in massive galaxies. Now, with JWST, we can explore the mid-IR's full potential as a SFR tracer over four orders of magnitude in total infrared luminosity (9<~log LIR/Lo<~13). First, combining the SMILES and FRESCO surveys, we evaluate MIRI photometry against the Pa-alpha emission line - a gold standard SFR indicator - in Main Sequence (MS) galaxies at cosmic noon. We find the rest-frame 6-8um luminosity has a steeply superlinear relation with SFR(Pa-alpha) below ~8 Mo/yr, in contrast with the unity slope seen in coeval massive galaxies. We derive broken power-law SFR indicators from single-band MIRI photometry plus a representative dust template, with a scatter typical of IR SFRs (~0.2-0.3 dex). Despite the break in the mid-IR behavior and our simplifying assumption of a single dust SED, we next successfully formulate a UV+IR composite relation (scatter ~0.15 dex) under the usual assumption of energy balance. This implies that the rest-frame 6-8um primarily tracks the global dust-obscuration fraction - which decreases rapidly at log M*/Mo<~10 - rather than reflecting a deficit in PAH abundances at low mass. Our results thus support MIRI photometry as a robust SFR proxy at log M*/Mo>~9 up to z~3. Finally, extending to local and z>~1 ultraluminous infrared galaxies not represented in SMILES, we examine when Pa-alpha and the IR reliably track SFR in the bright regime.

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arXiv2026

MARTA: The connection between chemical enrichment, feedback, and dust in a Wolf-Rayet galaxy at z${\sim}$2

We present the analysis of the stellar and interstellar medium (ISM) properties of MARTA-4327, a star-forming galaxy at z=2.224 observed by means of deep JWST/NIRSpec spectroscopy in both medium- and high-resolution gratings as part of the "Measuring Abundances at high Redshift with the Te Approach" (MARTA) programme. We report one of the highest-redshift detections of the Wolf-Rayet (WR) blue and red bumps in a non-lensed system. The broad He ii$λ$4686 feature is consistent with a young (${\sim 5-6}$ Myr) burst dominated by WN stars, although both SSP models and empirical templates struggle to reproduce the nitrogen stellar features at ${\approx}$ 4640 A. Based on the relative strength of the available optical stellar features, we disfavor the presence of very massive stars (VMS) in this system. Elemental abundance ratios such as Ne/O, N/O, and Ar/O align with observations of local star-forming galaxies (including WR galaxies), suggesting that any impact of the WR population on the chemical enrichment of the ISM is strongly localized. However, the gas-phase Fe/O ratio appears enhanced compared to local galaxies of similar metallicity, which we interpret as evidence for reduced Fe depletion onto dust grains, possibly linked to localized destruction in WR-driven wind environments. In addition, we detect a broad and blueshifted (~70 km/s) H$α$ component, revealing the presence of an ionized outflow with a mass loading factor ${η\sim 0.2}$. Finally, we report the robust detection of O I$λ$8446 emission (among the firsts at high redshift), which we interpret as originating from Ly$β$ fluorescence and/or collisional excitation in dense clumps. Overall, MARTA-4327 represents a unique system for studying the role of massive stars in shaping the ISM in galaxies at Cosmic Noon.

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arXiv2026

JWST+ALMA reveal the build up of stellar mass in the cores of dusty star-forming galaxies at Cosmic Noon

Dusty star-forming galaxies have long been suspected to serve as the missing evolutionary bridge between the star-forming and quiescent phases of massive galaxy evolution. With the combined power of JWST and ALMA, it is now possible to use high resolution imaging at rest-frame ultraviolet (UV), optical, near-infrared (NIR), and sub-mm wavelengths to study the multi-wavelength morphologies tracing both the stellar populations and dust during this key phase. We present the joint analysis of JWST/NIRCam imaging in GOODS-S and mm dust emission traced by ALMA for a sample of 33 galaxies at $z=1.5$ to $z=5.5$ selected from the 1.1mm GOODS-ALMA 2.0 survey, and compare the morphologies of this population to mass- and redshift-selected samples of field star-forming and quiescent galaxies. The 1.1mm-selected sample is morphologically distinct from other similarly massive star-forming galaxies; we find a steeper size-wavelength gradient from 1.5-4.4$μ$m, with a more dramatic decrease in size towards longer wavelengths. While the rest-NIR surface brightness profiles of the 1.1mm-selected galaxies are brighter in the inner regions relative to the field star-forming population, they are remarkably similar to the quiescent population. These morphological differences could suggest that dusty star-forming galaxies, unlike more typical star-forming galaxies, have already built up stellar mass in a severely dust-obscured core, leading to extended and clumpy morphologies at rest-UV and rest-optical wavelengths and more compact emission in the rest-NIR that is co-spatial with dust. If the bulge is already established, we speculate that mm-selected galaxies may imminently evolve to join their quiescent descendants.

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