arXiv subjects

Stephane Charlot

Publications and source records attributed to Stephane Charlot.

At least 19 recordsLinked to original sources

arXiv2026

CLASSY. XV. Kinematics and Spatial Distributions of Outflows in Local Highly Star-Forming Galaxies

Star-forming galaxies drive massive outflows that play an important role in galaxy evolution by regulating feedback and influencing the dynamics of surrounding media. Measuring galactic outflow rates is essential for quantifying feedback efficiency and the amount of mass, momentum, and energy deposited into the circumgalactic medium. In this paper, we examine 17 galactic outflows from the CLASSY survey with radiative transfer modeling of UV absorption lines presented in M. Huberty et al. (2024), to study their spatial distributions and kinematic properties. We study the SiII, SiIII, and SiIV ionization states that trace the cool and warm phases of the outflows and find that SiII traced winds generally behave differently than the warmer SiIII and SiIV traced winds. We derive the mass, momentum, and energy loading factors, which we find scale inversely proportional to stellar mass. We find that our measurements of the mass and momentum loading factors are in agreement with the hydrodynamic FIRE-2 simulations. We model the velocity profiles of the winds, with profiles reaching a maximum velocity of 620 km/s on average, in agreement with hydrodynamic simulations from CGOLS. We also investigate the relationship between outflow properties and the age of the stellar population from SED fitting. We find that outflows associated with young star forming regions are more likely to have a column density dominated by cooler gas and have mass outflow rates which decrease with radius.

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arXiv2026

Little Red and Blue Dots: AGN-excited narrow lines, Lyman-$\alpha$ emission, and resemblance to standard quasars

We present an analysis of a sample of 36 Little Red and Blue Dots (LRDs and LBDs) at $2.26<z<7.89$, identified by JWST in the GOODS fields. While both categories are selected to have broad Balmer lines, both of them are extremely X-ray weak. Both classes share the same location on various diagnostic diagrams, consistent with AGN excitation (with some deviations which can be ascribed to low metallicity), although their weak HeII emission suggests a generally softer ionizing spectrum than ordinary AGN. LRDs display Ly$\alpha$ emission stronger than normal star-forming galaxies, and with a broad component consistent with the broad component of H$\alpha$. Overall, these findings indicate that LRDs and LBDs are both powered by growing black holes and their ionizing radiation escapes to ionize the surrounding interstellar medium (ISM). The broad Balmer lines ($H\alpha_b$ and $H\beta_b$) have different apparent properties: LBDs have EW(H$\alpha _b$) and $H\alpha_b/H\beta_b$ broadly consistent with normal AGN, while LRDs have higher values of both quantities, although still in the tail of the quasars distribution. LRD models in which a gas envelope completely encases the black hole, are inconsistent with these results -- these scenarios need modification to include clumpiness, or a (classical) equatorial geometry, letting ionizing photons reach the ISM. The different broad Balmer properties imply that LBDs cannot simply be LRDs with more galaxy contribution. Scenarios in which LRDs are simply dust-obscured LBDs seem broadly consistent with the observations. Finally, these results indicate that LRDs' bolometric luminosities estimated assuming isotropic emission and complete covering by the absorber are inadequate. The few X-ray-detected LRDs suggest no deviation from the standard AGN bolometric corrections, once absorption is accounted for.

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arXiv2026

A Rapid Evolution in the Observed Mbh/M* Relation at z > 3 Revealed via Spectro-photometric SED-Modeling

Spectroscopic observations from JWST have uncovered a plethora of active galactic nuclei (AGN) at z > 4 with black hole (BH) mass (Mbh) to stellar mass (M*) ratios significantly above the local relation when using standard virial mass scaling relations. However, M* estimates of AGN may be inaccurate due to limitations in spectral energy distribution (SED) fitting codes, exemplified by a lack of physically-motivated AGN line emission models. Here, we fit NIRSpec/PRISM spectra of 39 galaxies at z ~ 3.5-7 selected as broad-line AGN from the CEERS and RUBIES surveys. Applying kinematic decompositions from NIRSpec/G395M spectra, we fit their continuum and narrow-component line fluxes using the BEAGLE-AGN SED fitting tool. While limitations of BEAGLE-AGN make it difficult to model little red dots (LRDs), we find that M* estimates of non-LRDs are, surprisingly, only modestly impacted by the inclusion or not of AGN narrow-line region (NLR) and continuum emission model components. We further find that non-LRD AGN at z < 3.5 are consistent with the local Mbh/M* relation while those at z > 4.5 display elevated ratios. While we cannot rule out observational biases or systematic uncertainties as partial causes, this transition over just ~500 Myr is driven entirely by changes in M* rather than an evolving Mbh distribution. These findings are consistent with models in which rapid BH growth results in elevated Mbh/M* ratios at early times, with a swift late-time assembly of host galaxies returning sources to the local relation at z < 4.

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arXiv2026

Origins of Extreme Emission-Line Ratios in z > 3 Galaxies: Insights from the Lumen Model

Optical emission-line ratios in star-forming galaxies at $z \sim 3$-8, such as [OIII]/H$\beta$ and [OIII]/[OII], are strongly offset from those at $z \sim 0$-2, pointing to more extreme ionization and ISM conditions in the early Universe. To constrain the physical origin of these offsets, we developed Lumen, a framework for modelling nebular emission from spatially distributed HII regions in cosmological simulations. We apply Lumen to IllustrisTNG50, validate its predictions at low redshift, and test a suite of proposed mechanisms for producing extreme line ratios at $z = 3$-8. We focus on the [NII]/H$\alpha$ versus [OIII]/H$\beta$ (N2-BPT) diagram, the [SII]/H$\alpha$ versus [OIII]/H$\beta$ (S2-VO87) diagram, and the [OIII]/[OII] versus ([OII]+[OIII])/H$\beta$ (O32-R23) diagram. We find that $\alpha$-enhancement alone cannot explain the bulk of observations. Moderate offsets emerge from the combined effects of $\alpha$-enhancement, a higher IMF upper-mass cutoff, and AGN contributions. The most extreme [OIII]/H$\beta$ and [OIII]/[OII] values require high ionization parameters powered by massive star clusters of $\gtrsim 10^5$-$10^6\,\mathrm{M}_\odot$, consistent with recent JWST observations. Reproducing the highest [NII]/H$\alpha$ ratios additionally requires enhanced nitrogen abundances. Although gas densities of $n \sim 10^4\,\mathrm{cm}^{-3}$ can boost several diagnostic ratios, they suppress [SII]/H$\alpha$ and are therefore in tension with current observations. Overall, models combining harder ionizing spectra, elevated ionization parameters from massive star clusters, and enhanced nitrogen abundances reproduce the observed high-$z$ galaxy population across the N2-BPT, S2-VO87, and O32-R23 diagrams. This successful model also motivates new demarcation lines for star-forming galaxies in the N2-BPT and S2-VO87 diagrams.

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arXiv2026

Metal Mayhem at $\rm z \sim 7-10$: Diversity and Evolution of Gas-Phase Metallicity Gradients

We present a JWST/NIRSpec-IFU study of metallicity gradients in seven low-metallicity systems at $z=7.2-9.5$. The main sample spans stellar masses of $\rm \log(M_*/M_{\odot}) \sim 7.8-9.5$, star formation rates (SFRs) of $\rm \log(\text{SFR} / M_{\odot} \text{yr}^{-1}) \sim 0.5-2.5$, and gas-phase metallicities of $4\%-15 \%~Z_\odot$. Within our sample, we also identify three low-metallicity satellite galaxies associated with two of our sources, providing a rare view of early-epoch interactions. The three satellites exhibit even more primordial properties, with metallicity $3\% -4\% ~Z_\odot$ and low star-formation activity ($\rm \log(\text{SFR} / M_{\odot} \text{yr}^{-1}) \sim -0.5$ to $-0.9$). We find that our galaxies, and especially the satellites, are significantly offset from the local Fundamental Metallicity Relation (FMR), with deviations reaching $\Delta \text{FMR} \approx -0.9$ dex. This indicates that these galaxies are likely experiencing strong accretion of pristine gas. Overall, we observe a large scatter in radial metallicity gradients, ranging from positive to negative with an average metallicity gradient of $\rm -0.02 \pm 0.04 \ dex \ kpc^{-1}$. Flat gradients are found in systems with confirmed satellites, suggesting that tidal interactions and mergers drive the radial mixing necessary to homogenise the interstellar medium. The (tentative) presence of an AGN in two of our sources suggests that strong feedback may also be responsible for the observed flat gradients. Conversely, the detection of a positive gradient in one source points toward a direct funnelling of metal-poor gas inflow into the central region of the galaxy. These results show that galaxies in the first billion years grow through diverse, episodic processes, suggesting that early evolution is characterised by structural variety rather than a single, predictable path.

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arXiv2026

Reinterpreting the puzzling properties of z>6 galaxies within a variable IMF framework

Recent results form the James Webb Space Telescope (JWST) report space densities for bright and massive galaxies at z>7 that far exceed expectations of theoretical models of galaxy formation, prompting a revision of our understanding of the physical processes leading to the assembly of the first luminous structures. In this work we present predictions from a realization of the GAlaxy Evolution and Assembly (GAEA) model, which implements a prescription for a variable stellar initial mass function (IMF). This prescription is inspired by high-resolution numerical simulations that account for the role of cosmic rays (CR) as regulators of the star formation rate (SFR) in giant molecular clouds. In our approach, SFR density is assumed to be a proxy for the CR density, providing a link between the IMF shape and the predicted physical conditions of the star forming interstellar medium. Our results show that, in our model framework, assuming such a variable IMF reproduces several properties of the z>6 galaxy population, with no further modification of the feedback model, including their UV luminosity functions up to z~13. In order to compare model predictions with available estimates for the galaxy stellar mass function (GSMF), we reconstruct stellar masses from the model's synthetic photometry assuming a universal IMF, reflecting standard observational practice. Under this approach, we show that the model can reproduce the evolution of the GSMF up to the highest redshifts accessible. Our findings highlight the need to consider a variable IMF shape in the error budget associated with stellar mass estimates. We show that the evolution of both the slope and normalization of the gas-phase mass metallicity relation can be used as powerful discriminant between models of early galaxy formation assuming different IMF evolution.

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arXiv2026

The search for Population III: Confirmation of a HeII emitter with no metal lines at z=10.6

We report the confirmation of a HeII$\lambda$1640 emitter located at 3 pkpc from the galaxy GN-z11, at z=10.6. The detection, based on JWST NIRSpec-IFU high-resolution spectroscopy, confirms a previous claim based on medium-resolution spectroscopy. The HeII$\lambda$1640 identification is further supported by the independent detection of H$\gamma$ obtained by \"Ubler et al. (2026) at the same location. The HeII emission is spectrally resolved in two components separated by 120 km/s. The Equivalent Width of the HeII emission is extremely high ($>$20 A). No metal lines are detected. We argue that Population III stars are the most plausible explanation for the observed He II emission, with no satisfactory alternative from other classes of sources or mechanisms.

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arXiv2026

Consistent Gas-Phase Temperatures and Metallicities from UV and Optical Nebular Emission: A Reliable Foundation from z=0 to Cosmic Dawn

The rest-frame UV spectra of star-forming galaxies are increasingly important as they become one of the primary windows to probe the physical properties of cosmic dawn (z>8) galaxies with the James Webb Space Telescope. However, the systematic discrepancies between UV and optical gas-phase metallicity measurements remain poorly understood in the local universe, partly due to challenges in achieving precise comparisons between UV and optical spectra for the same objects. In this work, we introduce a novel method that leverages the HeII 1640 and HeII 4686 nebular emission lines to achieve accurate aperture and reddening corrections between UV and optical spectra. Here we apply this method to three nearby Blue Compact Dwarf (BCD) galaxies. Our results demonstrate that this approach enables precise measurements, with electron temperatures ($T_e$) derived from UV and optical spectra exhibiting closer agreement compared to previous studies, and O/H abundance agreeing within 0.1 dex. However, two BCDs appear to have lower UV-based electron temperatures $T_{e~1666} < T_{e~4363}$, in contrast to expectations from the temperature fluctuation model. We consider a variety of possible explanations for these unphysical temperatures - differential dust attenuation, aperture differences, and spatial extent of emission lines - but no suitable cause is identified. These findings suggest a complex gaseous environment associated with star formation, and underscore the need for additional observations to further investigate the nature of HeII nebular emission and address the systematic issues between UV and optical nebular properties. Nonetheless, the close empirical agreement of these results indicates that UV- and optical-based nebular temperature and abundance measurements can be reliably compared within 0.1 dex, providing a solid foundation for evolutionary studies from the local Universe to cosmic dawn.

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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 $\mu$m, with MIRI photometry extending the spectral energy distribution to 4 $\mu$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 ($\lambda_{Edd}=0.6\pm0.2$). We find a significant stellar contribution in the far-UV, reaching $\sim80$% in the bluest systems. Possible Wolf-Rayet features (HeII$\lambda$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

JWST Advanced Deep Extragalactic Survey (JADES) Data Release 5: Photometrically Selected Galaxy Candidates at z > 8

We present a sample of 2081 sources selected at photometric redshift $z_{\mathrm{phot}} > 8$ across the JADES DR5 data release in GOODS-S and GOODS-N over a total area of 469 square arcmin. These sources range from $M_{\mathrm{UV}} = -22$ to $M_{\mathrm{UV}} = -16$, with 19 objects at $z_{\mathrm{phot}} > 14$. We estimate the UV slopes for the full sample from fits to the photometry and find evidence for a steepening of the relationship between the UV continuum slope and $M_{\mathrm{UV}}$ to higher redshifts, a result that differs from prior analyses of brighter samples in the literature. We provide evidence that over one quarter of our sources have evidence for being morphologically extended, with many galaxies showing multiple bright knots or clumps even out to $z \sim 13 - 14$, an indication of how galaxies at Cosmic Dawn are growing and evolving. We discuss JADES-GN+189.15982+62.28899, a GOODS-N F200W dropout galaxy at $z_{\mathrm{phot}} \sim 15 - 18$ which has been observed spectroscopically with JWST/NIRSpec in prism mode, resulting in a very low signal-to-noise spectrum that is consistent with the photometry and rules out a number of low-redshift solutions for the source. Finally, we use a subsample of 123 objects in our sample with spectroscopic redshifts to explore the usage of alternate fitting templates and a prescription for Ly-$\alpha$ damping wing absorption, finding that both produce significant improvements to the estimated photometric redshifts.

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arXiv2025

Ionizing Photon Production Efficiencies and Chemical Abundances at Cosmic Dawn Revealed by Ultra-Deep Rest-Frame Optical Spectroscopy of JADES-GS-z14-0

JWST has discovered an early period of galaxy formation that was more vigorous than expected, which has challenged our understanding of the early Universe. In this work, we present the longest spectroscopic integration ever acquired by JWST/MIRI. This spectrum covers the brightest rest-frame optical nebular emission lines for the luminous galaxy JADES-GS-z14-0 at $z > 14$. Most notably, we detect $[\mathrm{OIII}] \lambda\lambda 4959,5007$ at $\approx 11 \sigma$ and $\mathrm{H}\alpha$ at $\approx 4 \sigma$ with these ultra-deep observations. These lines reveal that JADES-GS-z14-0 has low dust attenuation with a recent star-formation rate of $\mathrm{SFR} \approx 10 \pm 2\ M_{\odot} / \mathrm{yr}$, star-formation rate surface density of $\Sigma_{\mathrm{SFR}} \approx 23 \pm 5\ M_{\odot}/\mathrm{yr}/\mathrm{kpc}^{2}$, and ionizing photon production efficiency of $\xi_{\mathrm{ion}} \approx 10^{25.3 \pm 0.1}\ \mathrm{Hz/erg}$. Using standard strong-line diagnostics, we infer a gas-phase oxygen abundance of $[\mathrm{O/H}] \approx -1.1 \pm 0.4$ ($\approx 10\%\ Z_{\odot}$), carbon-to-oxygen ratio of $[\mathrm{C/O}] \approx -0.4 \pm 0.4$, ionization parameter of $\mathrm{log}_{10}(U) \gtrsim -2.4$, and density of $n_{\mathrm{H}} \approx 720 \pm 210\ \mathrm{cm}^{-3}$. Using detailed photoionization modeling, we instead derive $[\mathrm{O/H}] \approx -0.3_{-0.4}^{+0.4}$ ($\approx 50\%\ Z_{\odot}$) and $\mathrm{log}_{10}(U) \approx -1.5_{-0.4}^{+0.3}$. The inferred properties of JADES-GS-z14-0 are similar to those measured for similarly luminous galaxies at $z > 10$ with previous MIRI/Spectroscopy, such as GHZ2/GLASSz12, GN-z11, and MACS0647-JD1. Existing simulations are unable to reproduce the empirical and inferred properties of JADES-GS-z14-0. This work demonstrates an important step toward understanding the formation of the first stars and heavy elements in the Universe. [Abridged]

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arXiv2025

GA-NIFS: A smouldering disk galaxy undergoing ordered rotation at z=4.26

Rotating galaxies with relaxed gaseous disks have been discovered across cosmic time, from the local Universe to high redshift (z>4). But few such sources have been confirmed at z>4, making them a precious sample to examine what conditions result in such ordered kinematics in an early, more chaotic Universe. One of the best examples of this sample is the galaxy DLA0817g1 (z=4.2603), which shows remarkably clear rotation in ALMA [CII] data. We present recent JWST/NIRSpec IFU data (R~2700) of DLA0817g1, which we combine with archival ALMA [CII] observations to place constraints on its ISM conditions and morpho-kinematics. From a combination of line ratios, we find a high gas-phase metallicity (~0.7 solar), high fraction of obscured star formation, low ionisation (compared to other high-redshift galaxies observed with JWST), and no significant evidence for AGN (based on the WHAN diagnostic). Dynamical modelling with 3DBarolo reveal nearly identical rotation in Halpha and [CII], but with a higher velocity dispersion in the former. Using our metallicity estimate and previous CO and [CII] detections, we derive a new estimate of the molecular gas mass, relieving a previous strain in the mass budget. Altogether, we suggest that this is a 'smouldering' galaxy, where past star formation resulted in significant chemical enrichment (i.e., Zgas and Mdust), but the current activity is low (i.e., lower ionisation parameter and electron temperature). These new observations have opened a window into questions regarding the interplay of gas, metallicity, star formation, and kinematics in a prototypical early disk galaxy.

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arXiv2025

Significant Evidence of an AGN Contribution in GHZ2 at z = 12.34

GHZ2 is among the highest-redshift galaxies discovered to date, exhibiting a spectrum rich with prominent emission lines in the rest-frame ultraviolet (UV) and optical. These features raise critical questions about the mechanism powering this nebular emission, in particular the extremely strong C IV1548 emission (rest-frame EW = 45 Angstrom). Here we aim to quantify the AGN contribution within this system using the BEAGLE-AGN tool to simultaneously fit the spectrum and photometry of GHZ2. We consider a range of models with and without AGN components, allowing us to disentangle the stellar and AGN contribution of GHZ2 for the first time. We conclude that a partial contribution by an AGN is significantly favored based on the Bayes factor comparison to models without an AGN component, measuring an AGN contribution of 54$^{+1}_{-1}$% and 26$^{+4}_{-2}$% for the C IV$\lambda$1548 and C III]$\lambda$1908 emission lines, respectively. We obtain an estimate for the black hole mass using the accretion luminosity ($L_{acc}$) from the best fit BEAGLE-AGN model, computing a value of log$_{10}$(M$_{BH}$/M$_{\odot}$) = 7.20$^{+0.04}_{-0.04}$, for an Eddington ratio of $\eta$ = 0.5 (with a systematic uncertainty of $\sim$1 dex). The inferred black hole mass to stellar mass ratio is 0.05$^{+0.02}_{-0.02}$, consistent with other high redshift AGN systems. If the black hole interpretation is confirmed, GHZ2 would represent the most distant black hole identified to date, making it an ideal laboratory to study AGN growth and their role in shaping high-redshift galactic evolution.

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arXiv2025

JADES: The Star Formation and Dust Attenuation Properties of Galaxies at 3<z<7

We present the star formation and dust attenuation properties for a sample of 602 galaxies at redshifts $\rm{3<z<7}$, as part of the JADES survey. Our analysis is based on measurements of the $\rm{H}\alpha/\rm{H}\beta$ Balmer Decrement using medium resolution (R$\sim$1000) spectroscopic observations with the JWST/NIRSpec Micro-Shutter Assembly. Stellar masses and star formation rates (SFRs) are inferred with \texttt{Prospector} using deep multi-band imaging. We utilize the Balmer decrement to measure dust-corrected H$\alpha$-based SFRs, taking into account the subsolar metallicities observed in galaxies at high redshift. We confirm, with our large sample size, that the correlation between the Balmer decrement and stellar mass is already established out to $z\sim7$. We find that the relation between the Balmer decrement and stellar mass does not significantly evolve from the local universe to $z\sim7$. We investigate the UV slope as a function of the Balmer optical depth and find that the best-fit correlation for our high redshift sample is sSFR dependent and significantly different at high redshift when compared to galaxies at $z\approx 0$ and $z \approx 2$. For the highest sSFR galaxies in our sample, there is no significant correlation between the UV slope and Balmer optical depth. This is evidence that the UV slope should be used with great caution to correct for dust in high redshift galaxies.

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arXiv2025

JADES reveals a large population of low mass black holes at high redshift

JWST has revealed a large population of active galactic nuclei (AGN) in the distant universe, which are challenging our understanding of early massive black hole seeding and growth. We expand the exploration of this population to lower luminosities by stacking $\sim 600$ NIRSpec grating spectra from the JWST Advanced Deep Extragalactic Survey (JADES) at $3<z<7$, in bins of redshift, [OIII]5007 luminosity and equivalent width, UV luminosity and stellar mass. In various stacks, we detect a broad component of H$\alpha$ without a counterpart in [OIII], implying that it is not due to outflows but is tracing the Broad Line Region (BLR) of a large population of low-luminosity AGN not detected in individual spectra. We also consider the possible contribution from Supernovae (SNe) and Very Massive Stars and conclude that while this is very unlikely, we cannot exclude some potential contribution by SNe to some of the stacks. The detection, in some stacks, of high [OIII]4363/H$\gamma$, typical of AGN, further confirms that such stacks reveal a large population of AGN. We infer that the stacks probe black holes with masses of a few times $10^6~M_\odot$ accreting at rates $L/L_{Edd}\sim 0.02-0.1$, i.e. a low mass and dormant parameter space poorly explored by previous studies on individual targets. We identify populations of black holes that fall within the scatter of the local $M_{BH}-M_{*}$ scaling relation, indicating that there is a population of high-z BHs that are not overmassive relative to their host galaxies and which have been mostly missed in previous JWST observations. Yet, on average, the stacks are still overmassive relative the local relation, with some of them 1-2 dex above it. We infer that the BH mass function (BHMF) at $3<z<5$ rises steeply at low masses. The BHMF is consistent with models in which BHs evolve through short bursts of super-Eddington accretion.

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arXiv2025

The Stellar Populations and Rest-Frame Colors of Star-Forming Galaxies at $z \approx 8$: Exploring the Impact of Filter Choice and Star Formation History Assumption with JADES

Our understanding of the physical properties of star-forming galaxies during the Epoch of Reionization (EoR, at $z > 6$) suffers from degeneracies among the apparent properties of the stars, the nebular gas, and the dust. These degeneracies are most prominent with photometry, which has insufficient (1) spectral resolution and (2) rest-frame spectral coverage. We explore ways to break these degeneracies with a sample of $N = 22$ high-redshift star-forming galaxies at $7 < z_{\mathrm{phot}} \leq 9$, using some of the deepest existing imaging from JWST/NIRCam and JWST/MIRI with JADES. Key to this study is the imaging from JWST/MIRI at $7.7\ \mu\mathrm{m}$, which provides coverage of the rest-frame $I$-band at the observed redshifts. We infer stellar population properties and rest-frame colors using a variety of filter sets and star formation history assumptions to explore the impact of these choices. Evaluating these quantities both with and without the $7.7\ \mu\mathrm{m}$ data point shows that dense spectral coverage with JWST/NIRCam (eight or more filters, including at least one medium-band) can compensate for lacking the rest-frame $I$-band coverage for the vast majority ($\approx 80\%$) of our sample. Furthermore, these galaxy properties are most consistently determined by assuming the delayed-tau star formation history, which provides the smallest offsets and scatters around these offsets when including JWST/MIRI. Within extragalactic surveys like JADES and CEERS, our findings suggest that robust characterization of the stellar population properties and rest-frame colors for high-redshift star-forming galaxies is possible with JWST/NIRCam alone at $z \approx 8$.

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arXiv2025

A black hole in a near-pristine galaxy 700 million years after the Big Bang

The recent discovery of a large number of massive black holes within the first two billion years after the Big Bang, as well as their peculiar properties, have been largely unexpected based on the extrapolation of the properties of luminous quasars. These findings have prompted the development of several theoretical models for the early formation and growth of black holes, which are, however, difficult to differentiate. We report the metallicity measurement around a gravitationally lensed massive black hole at redshift 7.04 (classified as a Little Red Dot), hosted in a galaxy with very low dynamical mass. The weakness of the [OIII]5007 emission line relative to the narrow H$\beta$ emission indicates extremely low metallicity, about $4\times 10^{-3}$ solar, and even more metal poor in the surrounding few 100 pc. We argue that such properties cannot be uncommon among accreting black holes around this early cosmic epoch. Explaining such a low chemical enrichment in a system that has developed a massive black hole is challenging for most theories. Models assuming heavy black hole seeds (such as Direct Collapse Black Holes) or super-Eddington accretion scenarios struggle to explain the observations, although they can potentially reproduce the observed properties in some cases. Models invoking "primordial black holes" (i.e. putative black holes formed shortly after the Big Bang) may potentially explain the low chemical enrichment associated with this black hole, although this class of models also requires further developments for proper testing.

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arXiv2025

Semi-empirical versus Theoretical Stellar Population Models: a comparison with Star Clusters

Stellar population synthesis (SPS) models are a key tool for deriving the age, metallicity, radial velocity and reddening of star clusters from their integrated spectra. Using a sample of 129 star clusters with high-quality spectra, we analyze the uncertainties associated with selecting an empirical versus a theoretical stellar spectral library in the SPS models. We find that the fits from the different models agree on the goodness of fit metrics and inferred reddening. However, the derived age and metallicity can be affected by the choice of the stellar library, with synthetic libraries tending to give lower age and metallicity, especially for spectra with low SNR. Ages and reddening values from SSP-equivalent fits are consistent with the multi-population fits, however, SSP-equivalent metallicities are affected by the coarse coverage of the SPS grid in [Fe/H]. When comparing the spectral fitting results with the literature, we find that (1) all models underestimate age for old and metal-poor systems; (2) on average, SPS models based on synthetic stellar libraries better match the isochrone ages and metallicities from high-resolution stellar spectroscopy.

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