arXiv subjects

Anton M. Koekemoer

Publications and source records attributed to Anton M. Koekemoer.

At least 55 records · Page 3Linked to original sources

arXiv2026

A massive barred spiral galaxy at z = 5.102 discovered by JWST

We report M1149-BSG-z5, a barred spiral galaxy at $z = 5.102$, identified in the parallel field of MACS J1149+2223 with JWST and HST. M1149-BSG-z5 is the highest redshift barred galaxy candidate to date. Both isophote ellipse fitting and structural modeling support a stellar bar of length $a_\mathrm{bar} \approx 4.5$ kpc, and extended spiral arms peaking at $r \approx 5.5$ kpc. M1149-BSG-z5 is a massive main sequence star-forming galaxy, with a stellar mass of $10^{10.45}\rm M_\odot$ and a star-formation rate of $144\,\rm M_\odot/yr$. A concentrated bulge is embedded in an extended disk with a global Sérsic index $n = 2.37$. With an effective radius of $R_{e} = 2.61\rm \ kpc$, M1149-BSG-z5 is larger than typical galaxies at $z \sim 5$ and comparable to barred galaxies at $2 < z < 4$. M1149-BSG-z5 also hosts a broad-line AGN, with a relatively low black-hole-to-stellar mass ratio of $\rm M_{\rm BH}/M_\ast\sim10^{-3}$. Its metal-enriched emission-line properties indicate that it is already chemically evolved. These properties imply M1149-BSG-z5 as an early-assembled and structurally evolved galaxy. We also find that M1149-BSG-z5 resides in an overdense region with a nearby companion galaxy, suggesting an interaction-driven bar formation mechanism. Its concentrated light, early assembly and main-sequence star formation also suggest baryon-dominated, gas-rich conditions, where gravitational instability can further accelerate the bar formation.

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arXiv2026

MEGA and SMILES Find Fewer Dusty Galaxies than Expected at Cosmic Noon

We present infrared (IR) luminsosity functions (LFs) and resulting star formation rate densities using the JWST Mid-infrared Instrument (MIRI) observations from the MIRI EGS Galaxy and AGN (MEGA) survey and Systematic MIRI Legacy Extragalactic Survey (SMILES). JWST allows us to perform a robust analysis on the faint end of the IR LF beyond the local universe. We directly measure the 7.7$μ$m polycyclic aromatic hydrocarbon (PAH) feature using either F1000W, F1500W, or F2100W photometry. This results in a sample of 634 galaxies across the two surveys covering an area of 105 arcmin$^2$ ($\sim$70 in the EGS and $\sim35$ in the GOODS-S/HUDF fields) and spanning $0.2<z<2$. We convert the 7.7$μ$m PAH luminosity to total IR luminosity, resulting in LFs that are two orders of magnitude fainter than previous studies. In contrast to previous extrapolations based on shallower observations, we find a strong flattening in the faint end of the LF with an average slope of $α\sim0.147$. This indicates that less luminous galaxies do not have as much dust obscured star formation as predicted. We measure the star formation rate density (SFRD) by integrating our new IR LFs and find a slightly lower SFRD in all redshift bins than previous studies made with ALMA, Herschel, and Spitzer. We also measure the contribution to the SFRD as a function of luminosity and confirm that LIRGs and ULIRGs remain the dominant contributors to the dust-obscured star formation at $z\sim1-2$.

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arXiv2026

Exploring the Relationship Between Bars, Star Formation Activity, and Host Galaxy Properties from $\mathbf{z \sim 0}$ to $\mathbf{z \sim 2}$

We present the most comprehensive study to date of the relationship between bars, star formation, and galaxy properties from $z \sim$ 0 to $z \sim$ 2. We use a mass-complete sample of 1,171 galaxies from the JWST CEERS survey with $M_\star > 10^{10} M_\odot$ and repeat the analysis using COSMOS-Web data. Our results are: 1) At high redshift ($z \sim$ $1-2$) barred galaxies tend to have high sSFRs and low Sérsic indices ($n \leq 2$), while at low redshifts barred galaxies emerge with both low sSFR and higher $n$, suggestive of quiescent galaxies with bulges. 2) The fractional contribution of barred quiescent galaxies to the bar fraction rises steeply from $z \sim$ 2 to $z \sim$ 0, while that of barred actively star-forming galaxies falls. 3) The fraction of quiescent galaxies that are barred rises steeply over the last 10 Gyr. 4) Our empirical results show good agreement with the TNG50-1 simulations for bars with $a_{\mathrm{bar}}$ $>$ 1.5 kpc. Our results allow for the possibility that bar-driven secular evolution may lead to quiescence and/or that bars are more likely to persist and grow in gas-poor, quiescent galaxies. The steep rise in the quiescent bar fraction over 10 Gyr may represent an evolutionary sequence whereby gas-rich disks at high redshift first develop short, dynamically young bars and over time, repeated bar-driven gas inflows lead to central starbursts and declining gas fractions that strengthen the bar as the galaxy transitions toward quiescence.

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arXiv2026

PEARLS: NuSTAR and XMM-Newton Extragalactic Survey of the JWST North Ecliptic Pole Time Domain Field VI: Multiwavelength SED Analysis

We model spectral energy distributions of 261 X-ray sources to $z \sim 5$ in the North Ecliptic Pole Time Domain Field, extending prior XMM-Newton and NuSTAR analyses. Using the star-forming main sequence (SFMS) and black hole accretion rate (BHAR) frameworks, we find that SFRs generally lie below the SFMS while most BHARs exceed the population average, as expected for X-ray-selected samples. There is a strong correlation ($ρ=+0.73$) between SFR relative to the SFMS and specific AGN luminosity, $L_{AGN}/M_*$; galaxies with the highest $L_{AGN}/M_*$ exist at or above the SFMS. X-ray luminosity correlates with SFR ($ρ=+0.80$), revealing a star-forming and X-ray luminous "cold quasar" population consistent with dramatic, short-timescale accretion episodes. Low-mass galaxies show BHARs well above the population averaged value for their mass whereas high-mass galaxies' SMBHs accrete at the population averaged BHAR, suggesting "growth spurt" and "maintenance-mode" accretion, respectively. Traditional AGN classifications (obscured, unobscured, or radio-loud) do not reveal these distinctions, demonstrating the X-ray perspective's unique ability to identify rare AGN phases that are critical for the instantaneous link between galaxies and their SMBHs.

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arXiv2026

COSMOS-Web: A Multi-wavelength Morphological Catalog of ~780,000 Galaxies

We present multi-wavelength morphological measurements for all galaxies in the COSMOS-Web survey, i.e., $\sim$780,000 galaxies contained in the COSMOS2025 catalog. We perform both parametric (e.g., single and double Sérsic modeling) and non-parametric (e.g., Gini-$M_{20}$) morphology analyses in four NIRCam bands, independently. Our parametric fits reveal a strong correlation between galaxy structure and star formation activity up to $z\sim4$, as evidenced by the dependence of the Sérsic index ($n_{\rm sérsic}$) and bulge-to-total ratio ($B/T$) on the position of the star formation rate-stellar mass plane. A tight correlation between $n_{\rm sérsic}$ and $B/T$ is observed. The evolution of $n_{\rm sérsic}$ and $B/T$ depends on stellar mass; for example, the median $n_{\rm sérsic}$ increases from $\sim1$ at $z\sim6$ to $\sim2.5$ at $z\sim2$ for massive galaxies with $M_*>10^{10.5} M_{\odot}$, while lower mass galaxies remain $n_{\rm sérsic}\sim1.2$ at all epochs. The UV $n_{\rm sérsic}$ values are systematically smaller than those in the optical, although both exhibit similar evolutionary trends. From non-parametric analyses, we demonstrate the distribution of galaxies on the Gini-$M_{20}$ and asymmetry-concentration planes, and find that morphological classifications based on non-parametric indicators are consistent with those derived from the Sérsic index. The resulting catalog provides the largest and most detailed set of JWST multi-wavelength morphological measurements to date, serving as a valuable community resource for studies of structural transformation, bulge growth, and galaxy-supermassive black hole coevolution across cosmic time.

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arXiv2026

Spatially resolved gas-phase metallicity at z~2-3 with JWST/NIRISS

Spatially resolved gas-phase metallicity maps are a crucial element in understanding the chemical evolution of galaxies. We present spatially resolved metallicity maps obtained from NIRISS/WFSS observations. This is the first such work presenting multiple individual galaxies. We investigate the source of ionisation, metallicity and its relation to star-formation in a spatially-resolved sense for a sample of eight galaxies -- four from JWST-PASSAGE and four from GLASS-JWST ERS. All but one galaxy are in the redshift range $1.9 \leq z \leq 2$, the outlier being at $z = 3.1$. Our sample covers a range of $8.0 <$ \logM $< 9.5$ in stellar mass, $0.2 <$ $\log{\rm{(SFR}}$/\Msunpyr) $< 1.1$ in star-formation rate (SFR) and $7.8 <$ \logOH $< 9.0$ in global metallicity. As a solution to the challenge of SF-AGN demarcation in absence of resolved \halpha, we present a new SF-demarcation line in the \textit{OHNO} parameter space based on MAPPINGS v5.1 publicly available \hii region model grids. We present the mass-metallicity gradient relation for our sample, which showed no clear trend with stellar mass, perhaps hinting at the fact that the high-$z$ galaxies have not yet started their accretion dominated phase. By interpreting the correlation between spatially resolved metallicity and SFR maps as a proxy for effective timescales of metal-transport in galaxies, we find a weak trend such that this timescale increases with stellar mass, implying a more effective feedback in lower mass galaxies.

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arXiv2026

A stellar dynamical mass measurement of an inactive black hole at redshift 2

Supermassive black holes and their host galaxies grow together over time, producing correlations between the black hole mass and various galaxy properties. Determining the evolution of these correlations requires precise measurements of the masses of distant black holes. We observe the gravitationally lensed quiescent galaxy MRG-M0138, at redshift 1.95, using JWST integral field spectroscopy to spatially resolve the kinematics of stars within the black hole's sphere of influence. By using a foreground lens model and fitting stellar dynamical models, we determine the mass of its inactive black hole, $M_{\bullet}=6.0^{+2.1}_{-1.7}\times10^9$ solar masses. Comparing this measurement to local galaxies, we find that $M_{\bullet}$ is higher than expected given the galaxy's bulge mass, but consistent with the correlation with stellar velocity dispersion.

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arXiv2026

The impact of cosmic filaments on starburst galaxies across cosmic times

Cosmological simulations suggest that various galaxy properties depend on their location within the cosmic web. Yet direct observational evidence of the dependence of star formation activity on distance to filaments remains scarce and is missing at z>1. We investigate how starburst, main-sequence (MS), and quenched galaxies are distributed with respect to cosmic web filaments, and how this distribution evolves with redshift. We first use the SIMBA cosmological simulation to predict the redshift evolution of the mean distance to the closest filament from z=3 to z=0 for different galaxy populations after removing stellar-mass dependencies. We then measure the corresponding signal in the COSMOS field, using COSMOS2020 and COSMOS-Web data, where accurate photometric redshifts enable reconstruction of the projected cosmic web from z=2 to z=0.5, and starbursts are identified through far-infrared spectral energy distribution fitting. In agreement with the results from SIMBA, starburst galaxies are found closer to filaments at z>1 and at larger distances at z<1, MS galaxies occupy intermediate environments with little evolution, and quenched galaxies show progressively shorter distances to filaments toward low redshift, with a crossing between starburst and MS populations around z~1. In COSMOS-Web, the relative evolution in the average distance to filaments between starburst and MS galaxies is detected at a significance level of at least 5σ. We show that a minimal toy model in which the only environmental ingredient is the sSFR-filament distance modulation measured in simulations is sufficient to reproduce the observed differential evolution of the average filament distance between starburst and MS galaxies. These results show that the imprint of large-scale environmental effects on the star formation activity of galaxies, predicted by simulations, is detectable from z=2 down to z=0.5.

astro-ph.CO↗

arXiv2026

OCEANS of Absorption: High-resolution NIRSpec Spectroscopy Reveals Diverse Balmer-line Absorption in Little Red Dots

The ``Little Red Dots' (LRDs) that appeared in JWST deep field images have been the subject of significant study since their discovery. In this work, we present high-resolution follow-up spectroscopy from the OCEANS program of 10 LRDs with Ha coverage at 3<z<7 in the CEERS/EGS field. We find Balmer-line absorption in 4 of these LRDs, a detection rate higher than the fractions reported in lower-resolution NIRSpec surveys. All of the absorbers are presented in high-resolution for the first time here and two have Balmer-line absorption detected for the first time. Of the 10 LRDs, 7 are best fit by Ha profiles with exponential wings. We find that absorbers tend to be blue-shifted with a median velocity offset of (-49 km/s) and absorption equivalent width of 5.3 Angstroms. Trends are explored to compare LRD absorption properties along the sequence of LRDs. We confirm an LRD with statistically significant absorption velocity offsets between Ha and Hb. The diversity of absorption properties can be effectively explained by a model with a radial distribution of partial-covering absorbing gas that is often co-located near the broad-line emission regions, along with a radial gradient of close inflow and distant outflow velocities for the absorbing gas. We present other interesting LRDs, including an outflow-dominated LRD and an LRD with relatively blue UV-to-optical colors but clear Balmer-line absorption. This high occurrence of absorbing hydrogen in LRDs, evident by both the Balmer-line absorption features and Balmer break strengths, implies a near-ubiquitous presence of dense, excited n=2 hydrogen.

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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

The ALPINE-CRISTAL-JWST Survey: The Fast Metal Enrichment of Massive Galaxies at z~5

We present the stellar mass-metallicity relation (MZR) and mass-metallicity-star formation relation ("fundamental metallicity relation"; FMR) of 18 massive (log(M/Msun) = 9.5 - 11) main-sequence galaxies at z~5 from the ALPINE-CRISTAL-JWST sample. This sample complements recent studies by JWST at up to two orders of magnitude lower stellar masses. The metallicities are derived using strong optical lines, and verified by temperature-based oxygen abundance measurements for five galaxies for which faint auroral lines are detected. We find that the metal abundance evolves, on average, from 40% to 60% solar between z~5 and cosmic noon (z~2) at the massive end of the MZR, suggesting already significant metal enrichment at early times. The FMR at z=5 exhibits a 5x larger scatter (preferentially to lower metallicities) compared the local FMR relation. This scatter can be explained by a bursty star formation and the direct build-up of metals in early galaxies as well as differences in age and outflow efficiencies. Capitalizing on all available samples, we find that the observed MZR and FMR over three orders of stellar mass is generally in good agreement with results from cosmological simulation, although some underestimate the metal enrichment at low stellar masses. This may be due to too efficient metal-rich outflows. We show that the ALPINE-CRISTAL-JWST galaxies likely joined the current FMR at z~10 and will evolve into massive (log(M/Msun)~11.4) galaxies with super-solar metallicities by z=0.

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arXiv2026

Discovery and Analysis of a Type II Supernova Candidate at z = 3.19 from JWST's COSMOS-Web Survey

The launch of the James Webb Space Telescope (JWST) has enabled the discovery of a small but increasing sample of high-redshift core-collapse supernovae (CC SNe), which provide new tests of massive star evolution in the early Universe. In this study, we report the discovery of SN 2023aeaf in COSMOS-Web survey observations, which at $z = 3.195$ has one of the highest SN spectroscopic redshifts to date. Using two epochs of JWST photometry separated by $\sim$1 month in the rest frame, we photometrically classify SN 2023aeaf by comparing the JWST photometry to spectrophotometric CC SN and Type Ia (SN Ia) models and UV observations of SNe from the Swift telescope, finding that SN 2023aeaf is highly likely to be a Type II SN. A spectrum of the SN$+$host galaxy was also obtained $\sim$30 rest-frame days after discovery but shows no clearly identifiable SN features, with H$α$ emission from the host potentially masking emission from the SN. Although the limited photometric coverage prevents strong constraints on the explosion properties, we find that the data are most consistent with a $\sim$12$M_\odot$ progenitor with $\sim$0.5$M_{\odot}$ of circumstellar material. We next use the host-galaxy spectrum and photometry to model the host spectral energy distribution (SED) using the Prospector Bayesian inference framework. We find that the host is a star-forming galaxy with a sSFR of $ \log_{10}(\rm sSFR/yr^{-1})= -10.17^{+0.13}_{-0.10}$, a stellar mass of $\log(M_\star/M_\odot) = 9.04^{+0.03}_{-0.04}$, and a gas-phase metallicity of $12 +{\rm log_{10}}({\rm O/H}) = 7.82\pm0.02$. SN 2023aeaf joins a growing sample of early Universe CC SNe with high luminosities, dense CSM, and low-metallicity environments.

astro-ph.HE↗

arXiv2026

A Post-starburst Galaxy Undergoing Ram-pressure Stripping at Redshift 3.06

Understanding how galaxies ignite and extinguish their star formation remains a cornerstone question in modern astrophysics. Recent JWST surveys have revealed an overabundance of massive quiescent galaxies in the first billion years of the Universe, challenging current models of galaxy evolution. In the nearby Universe, ram pressure stripping (RPS) is a major environmental mechanism capable of rapidly shutting down star formation, yet direct observation remains scarce at redshift $z\gtrsim1$, and its role at $z>2$ is even poorly constrained by simulations. Here, we utilize JWST and ALMA observations to present direct evidence of RPS in the post-starburst galaxy A2744-JF-z3, residing in a galaxy group at redshift 3.06, the earliest such detection to date. Spectroscopic diagnostics and spectral energy distribution modeling reveal the ongoing removal of cold gas and dust, coincident with the abrupt cessation of star formation. Contrary to hydrodynamical simulations that predict a reduced incidence of RPS at high redshift, our results instead imply that RPS can operate at $z>3$, suggesting a highly stochastic and impulsive stripping within a clumpy, filamentary intra-group and circumgalactic medium. These observations extend environmental quenching well into the epoch of galaxy assembly, highlighting RPS as a previously overlooked decisive pathway to rapid quenching in nascent groups and protoclusters in the early Universe.

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arXiv2026

Testing Lens Models of PLCK G165.7+67.0 Using Lensed SN H0pe

Supernova H0pe is a multiply-imaged Type Ia supernova (SN~Ia) and the second lensed SN to yield a measurement of the Hubble constant by the time-delay cosmography method, finding $H_0 = 75.4^{+8.1}_{-5.5} \text{km s}^{-1} \text{Mpc}^{-1}$ (Pascale et. al 2025). We investigate the seven lens modeling approaches used to derive $H_0$, assessing their agreement with $Λ\text{CDM}$ constraints from SN~Ia surveys through a purely observational comparison. \textbf{We test each lens model by combining its predicted magnifications with the observed time delays to reconstruct the intrinsic SN~Ia luminosity and corresponding distance modulus.} While photometrically derived magnifications yield distance moduli in line with $Λ\text{CDM}$ expectations, our comparison reveals that lens model predictions, even the most precise ones, \textbf{consistently overestimate the magnification, with an offset $> 1$~mag}. This known bias, already appreciated by modeling teams, is independently confirmed through our analysis and highlights the value of lensed SNe as a tool to test model accuracy. If unaccounted for, such magnification biases can propagate into uncertainties in derived cosmological parameters, including $H_0$, a critical challenge for precision cosmology using strongly lensed transients.

astro-ph.CO↗

arXiv2026

SDSS-V: Revealing a weak accretion state in X-ray selected red quasars

Red quasars (rQSOs) have been recognized as a short-lived, early stage in the evolutionary cycle of Active Galactic Nuclei (AGN), with fundamental differences in their intrinsic properties compared to blue quasars (bQSOs). In this work, we present the first large X-ray sample of 380 rQSOs, selected from the eROSITA/SDSS-V collaboration, providing uniform X-ray detection with optical spectroscopy accros half the sky, in the German hemisphere of eROSITA. We combine X-ray imaging, optical spectroscopy, and multi-wavelength photometry to fully probe the accretion, absorption and host properties of rQSOs. Independent Component Analysis is used to reconstruct optical spectra in a data-driven and non-parametric approach, while accounting for dust reddening and host contamination. rQSOs are intrinsically X-ray weak compared to bQSOs, with a higher fraction found at low X-ray luminosities (over 50$\%$ of the rQSO sample have $L_X < 10^{43.5} \, \rm erg \, s^{-1}$). We investigate the relative X-ray strength of rQSOs by measuring the spectral slope indicator $α_{OX}$. Despite their suppressed X-ray emission, rQSOs are not optically faint, but show low $α_{OX}$ values, indicating weak X-ray emission relative to their bright optical continua. X-ray spectral measurements reveal large gas column densities relative to optical reddening due to dust, implying that X-ray absorption could arise from dust-free gas close to the supermassive Black Hole (BH) rather than a classical dusty torus, while the dust responsible for optical reddening likely resides on larger host-galaxy scales or is associated with dusty gas carried in disc winds. rQSOs trace a phase of suppressed BH assembly relative to stellar mass growth, suggesting that they represent a distinct evolutionary stage where BH accretion is suppressed while the host galaxy continues to grow.

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arXiv2026

Changing-Look Active Galactic Nuclei in SDSS-V: Host-Galaxy Properties and Black-Hole Scaling Relations

Changing-look active galactic nuclei (CL-AGNs) exhibit dramatic spectral variability on unexpectedly short timescales, challenging standard accretion flow models. Despite growing samples, the physical drivers of this extreme variability, and the potential link to host-galaxy properties, remain unknown. Regardless of the underlying mechanism, the transition between AGN-dominated and host-dominated spectra offers a unique opportunity to study relations between AGNs and their hosts within the same objects. We present intermediate-resolution spectroscopy of 23 CL-AGNs identified by the Sloan Digital Sky Survey V (SDSS-V), obtained with the Very Large Telescope/X-shooter and Gemini-N/GMOS. An analysis of the Mgii emission line observed in the spectra demonstrates that the majority of these sources cannot be driven by variable obscuration. Our CL-AGNs roughly follow the M_BH-sigma_* and M_BH-M_* relations of inactive galaxies, with a median black hole-to-stellar mass ratio of 0.38%. We find no evidence that the stellar population properties of our CL-AGNs, including stellar mass, age, young stellar fraction, and star-formation rate, differ from those of type 2 AGNs in SDSS. These results suggest that CL-AGNs reside in typical AGN host galaxies and that their extreme variability is likely unrelated to host-galaxy environment, supporting the idea that CL-AGNs are not a distinct population, but rather represent a phase of normal AGN activity. This result, in turn, implies that CL-AGNs can serve as useful probes of the AGN-host connection, providing access to both AGN-dominated and host-dominated spectra of the same systems.

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arXiv2026

Massive Galaxies Form Early and Gray: Stellar Assembly and Dust Attenuation at $\mathbf{z>3.5}$ from CAPERS

The stellar mass assembly of massive galaxies in the first few billion years of cosmic history remains a central challenge in galaxy formation. Galaxies with $M_\star \gtrsim 10^{10}M_\odot$ observed at $z \gtrsim 4$ must grow rapidly under conditions of intense gas accretion, feedback, and dust production. Observationally, their star-formation histories (SFHs) have been poorly constrained due to degeneracies inherent to broadband photometry. The advent of JWST enables direct spectroscopic access to detailed continuum shapes and rest-frame optical diagnostics at high redshift, providing a critical opportunity to reconstruct formation timescales of massive early galaxies. Here, we investigate massive galaxies using joint spectro-photometric SED fitting of JWST/NIRSpec prism spectroscopy from the CANDELS-Area Prism Epoch of Reionization Survey (CAPERS). Our sample comprises 148 galaxies selected photometrically with log $(M_\star/M_\odot) > 9.5$ at $z > 3.5$. We find that the most massive galaxies (log $(M_\star/M_\odot) > 10.5$) preferentially exhibit shallow, gray dust attenuation curves, consistent with higher dust optical depths and large grain sizes. We also find significant diversity in the time at which galaxies form 25% of their stellar mass. While formation timescales converge toward later cosmic times, galaxies with lower sSFR ($\lesssim -9$) at the observation epoch formed significantly earlier than systems with higher sSFRs. Across the full mass range, inferred assembly times are systematically earlier than model predictions, suggesting more rapid early growth than currently captured theoretically. These results underscore the importance of spectroscopic constraints and flexible SFH and dust models for reconstructing high-redshift massive galaxy formation histories.

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arXiv2026

The JWST EXCELS survey: Outflows in 1.5 < z < 5 quiescent and recently quenched galaxies are likely relics from episodic AGN activity

We investigate the presence and origin of neutral gas outflows and inflows in 13 post-starburst (PSB) and quiescent galaxies at redshifts 1.8 $\leq$ z $\leq$ 4.6, using JWST NIRSpec spectroscopy from the EXCELS survey. NaD absorption profiles reveal that 3 out of 13 exhibit blueshifted absorption indicative of outflows, and a further 2 objects show signs of inflowing gas. Outflow velocities range from $\approx$ 300 - 1200 kms$^{-1}$, and we find gas flows are detected exclusively in objects that quenched $\lt$ 600 Myr ago. This result holds when we include comparable objects from recent literature. We derive mass outflow rates over two orders of magnitude higher than current levels of star formation in our sample, indicating that the winds are unlikely to be driven by supernovae, and likely play a significant role in keeping the galaxies quenched. The majority of the outflow sample have anomalously high energy and momentum outflow rates compared to those predicted for current levels of star formation or AGN activity. We conclude that we are likely observing fossil outflows driven by previous, more luminous AGN activity which has since faded. We then compare with the EAGLE simulation to explore a potential 'outflow cycle', finding that our observations are consistent with a model in which z $\sim$ 3 quiescent galaxies undergo short $\simeq$ 5 Myr periods of AGN activity strong enough to drive outflows, which occur every $\simeq$ 40 Myr on average. This AGN activity drives observable outflows that persist for up to $\simeq$ 10 Myr after the AGN fades, followed by a $\simeq$ 20 Myr lull, and a subsequent short inflow, which eventually re-ignites AGN activity, and the cycle repeats.

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