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Andreas Faisst

Publications and source records attributed to Andreas Faisst.

At least 19 recordsLinked to original sources
arXiv2026

The Roman eXtreme Deep Field (RXDF)

The Roman eXtreme Deep Field (RXDF) program is one of the five General Astrophysics Survey (GAS) programs approved for observing time with the Nancy Grace Roman Space Telescope in Cycles 1 and 2. It has been allocated 386.41 hours to carry out an imaging survey to AB = 30 mag (5-sigma) over ~140x larger area than the Hubble eXtreme Deep Field (HXDF) full-depth area (ACS+WFC3/IR). The RXDF will cover the full Roman wavelength range with 7 bands, reaching AB = 30 mag in RZYJH, 29 mag in F, and 28 mag in K, over a full-depth area of 678.75 arcmin^2 embedded in a total area of 1,243 arcmin^2, and far exceeding the depths of the Roman Core Community Surveys (CCS). The RXDF is within the Euclid Ultra Deep Field (EUDF) near the North Ecliptic Pole (NEP), a strategic long-term field for generational space facilities, with a wealth of multi-wavelength data including extensive coverage from the James Webb Space Telescope (JWST) NEXUS Treasury program. The observations will cover 3 epochs at a 1-year cadence, each epoch divided into 3 sub-epochs ~10 days apart, enabling time-domain studies on time baselines from ~10 days to over ~2 years. The RXDF is uniquely positioned to address critical questions in reionization, large scale structure (LSS), growth of supermassive black holes (SMBHs), little red dots (LRDs), and high-z supernovae (SNe); the volumes probed by HST+JWST are too small at these extreme depths, and even the deepest CCS tiers are too shallow. In addition to our key objectives, a wealth of additional science will be enabled by engaging the community with our rapidly released datasets, revolutionizing a wide range of science for a lasting legacy. This short document, which is converted from the approved RXDF proposal, aims to provide the community with a summary of the program.

astro-ph.GA↗
arXiv2026

Tracing Lyman alpha escape in the CRISTAL-02 galaxy at z~5.3

We investigate the mechanisms regulating Lyman-alpha (Ly$\alpha$) escape in the star-forming galaxy CRISTAL-02 at z~5.3. The galaxy has clumpy morphology, suggestive that it may be interacting with other system(s). Two clumps (A and B, hereafter) are suggested as a site for intense star-formation or potential AGN candidates. We investigate how the local gas, dust, and feedback shape the escape of Ly$\alpha$ photons around these clumps. Using VLT/MUSE and JWST/NIRSpec IFU observations, complemented by NIRCam UV imaging, we constructed spatially matched emission-line maps. We derived flux, line-ratio, and extinction maps, together with spatially resolved Ly$\alpha$ escape fractions and ionizing photon production efficiencies. We find that Ly$\alpha$ is significantly more extended than H$\alpha$ and UV, reaching ~33 kpc and preferentially extending along the cold molecular gas outflow traced by [C II] emission. Clumps A and B show contrasting Ly$\alpha$ properties: Clump A has lower dust attenuation and enhanced Ly$\alpha$/H$\alpha$ ratios and escape fraction, whereas Clump B is brighter in H$\alpha$ and UV but has suppressed Ly-$\alpha$ despite a higher ionizing photon production efficiency. These results indicate that Ly$\alpha$ escape is strongly influenced by the local H I geometry, dust, and outflows and cannot be explained by ionizing photon production alone. The observed Ly$\alpha$ morphology and zELDA radiative transfer modeling favor an outflow-driven escape scenario, while the available data cannot uniquely distinguish between AGN and star formation-driven feedback.

astro-ph.GA↗
arXiv2026

ALMA observations of pre-JWST z ~ 10 galaxy candidates: A CO(J = 9-8) line from a ULIRG at z = 2.54 and revisit of the photometric redshifts with JWST photometry

We present Atacama Large Millimetre/submillimetre Array (ALMA) observations targeting the [OIII]$88\,\mu$m line for six $z\sim10$ galaxy candidates selected with the Hubble Space Telescope and the Spitzer Space Telescope. We detect a line ($4.5\sigma$) and dust continuum emission ($30\sigma$) in UDS_18697, while detecting neither robust line nor continuum emission in the remaining five objects. The detected line in UDS_18697 is identified as CO($J=9-8$), because follow-up James Webb Space Telescope (JWST) NIRSpec observations have confirmed the redshift as $z=2.54$. UDS_18697 is classified as an ultra luminous infrared galaxy (ULIRG) with far-infrared (FIR) luminosity of $L_\mathrm{FIR}\approx1.1\times10^{12}\,L_\odot$, assuming a dust temperature of $T_\mathrm{d}\approx42\,$K, estimated using a physically-motivated method. We find that UDS_18697 follows the $L_\mathrm{FIR}-L'_\mathrm{CO}$ relation for local and $z>2$ galaxies, albeit being slightly brighter in CO($J=9-8$). Also, based on the follow-up NIRSpec observations and spectral energy distribution fitting using JWST/NIRCam photometry, we found that most of our targets are suggested to be low-$z$ interlopers. Motivated by these redshift misclassifications, we investigate colour--colour selection criteria for high-$z$ galaxies using JWST spectroscopic survey catalogues. We find that elevating a colour threshold tracing the Lyman break is crucial for constructing a robust high-$z$ sample, particularly for wide field surveys such as Euclid Deep Fields and Roman High-Latitude Wide-Area Survey.

astro-ph.GA↗
arXiv2026

Breathing Fire: Hot Dust in the Big Three Dragons at z = 7.15

We present new Atacama Large Millimeter/submillimeter Array (ALMA) Band 9 ($\lambda_\mathrm{obs} = 0.45\,\mathrm{mm}$) and 4 ($\lambda_\mathrm{obs} = 2.2\,\mathrm{mm}$) observations towards the Big Three Dragons, a pair of merging Lyman-break galaxies (LBGs) at $z=7.15$. The system was previously detected in ALMA Bands 6, 7 and 8 ($\lambda_\mathrm{obs} = 0.73 - 1.32\,\mathrm{mm}$), which, combined with our new observations, allows us to robustly constrain its dust temperature and obscured SFR. The unresolved Band 4 observations yield a $3.5\sigma$ detection, and the $0.4''$ Band 9 observations detect the Eastern and Western LBGs at $3.7$ and $3.5\sigma$, respectively. Through modified blackbody fitting, we infer a global dust temperature of $T_d = 78_{-23}^{+35}\,\mathrm{K}$ for the system, which implies a high IR luminosity of $\log(L_\mathrm{IR}/L_\odot) = 12.32_{-0.41}^{+0.43}$. This makes the Big Three Dragons one of the most IR-luminous systems known at $z>7$, with a total $\mathrm{SFR}_\mathrm{UV+IR} = 267_{-153}^{+418}\,M_\odot\,\mathrm{yr}^{-1}$ that is almost completely obscured ($f_\mathrm{obs} = 0.94_{-0.09}^{+0.04}$). Using resolved ALMA observations in Bands 6, 8 and 9, we confirm both LBGs have hot dust temperatures ($T_d \approx 67 - 84\,\mathrm{K}$) and correspondingly high obscured fractions ($f_\mathrm{obs} \approx 0.88 -0.95$). We find the Western LBG to fall $\sim1\,\mathrm{dex}$ above the canonical IRX-$\beta_\mathrm{UV}$ relation, suggesting patchy dust obscuration. The compact Eastern component, on the other hand, is consistent with a Calzetti- or SMC-like dust screen within the uncertainties. Together with the similarly hot dust temperature recently reported for the $z=8.31$ galaxy MACS0416-Y1, our results suggest a non-negligible fraction of star formation at the bright end of the UV luminosity function is highly dust-obscured, even at $z\gtrsim7$.

astro-ph.GA↗
arXiv2026

SPHEREx 0.75 to 5 $\mu$m Spectra for a Sequence of Nearby Brown Dwarfs

The SPHEREx all-sky survey has now measured the R$\sim$40-100 infrared spectra of thousands of nearby brown dwarfs in the chemically rich 0.75-5 $\mu$m range. The survey's wide spectral coverage and high S/N permits flux measurements that capture several broadband molecular absorption features, and upwards of 80$\%$ of the total bolometric luminosity of most brown dwarfs. Atmospheric models are known to yield systematic disagreements in the inferred temperatures and radii of brown dwarfs, necessitating benchmarking against observations. In this work, we present SPHEREx spectra across a broad sequence of 37 nearby field brown dwarfs, ranging from L0 to Y4 ($\sim$2500-250 K) and compare them to theoretical expectations. We additionally compile spectra for separate low-gravity and low-metallicity objects, and show how they trend with constant spectral type. We fit the measured spectra to the well-known forward model grids Sonora Diamondback, Elf Owl, BT-Settl, ATMO2020 and ATMO2020++ and compare their goodness-of-fit as a function of wavelength, spectral type, and treatment of clouds and chemistry. We find that the models continue to struggle to simultaneously fit the J/H/K peaks and the 4 $\mu$m opacity window, especially in L/T transition objects. The largest deviations appear around the chemistry-sensitive CO$_2$ and CO features. Despite these offsets, the models broadly capture their trends across the L/T transition, with the observed sample of field dwarfs strongly preferring the weak vertical mixing ($k_\mathrm{zz}$ = 10$^4$ cm$^2$s$^{-1}$) Elf Owl models over strong mixing. The spectra shown here along with future SPHEREx data will help guide improvements to models.

astro-ph.SR↗
arXiv2026

Observations of Atmospheric Helium and Oxygen with SPHEREx

We present measurements of near-infrared (NIR) terrestrial airglow produced by helium and oxygen in the exosphere as observed by SPHEREx. Using eight months of survey data obtained from a 680 km low-Earth orbit, emission from HeI $\lambda$10830, OI $\lambda$8446, and OI $\lambda$11287 is mapped with both global spatial and multi-season temporal coverage. These measurements are obtained along upward looking lines of sight as part of the astrophysical survey, in contrast to conventional nadir-viewing Earth remote sensing, which probes the behavior of low-density material in the thermo- and exosphere. We describe an analytical framework to extract atmospheric emission lines in the presence of astrophysical backgrounds including stars, resolved galaxies, and the diffuse Zodiacal light. The resulting global measurements reveal temporal variability over the survey period and systematic dependencies on geographic location. We interpret these variations in the context of the variable Solar illumination and seasonal effects. SPHEREx, an astrophysical space observatory, is demonstrated to be a promising new platform for monitoring NIR airglow and investigating its coupling to Solar activity and global geophysical processes.

physics.ao-ph↗
arXiv2026

The SPHEREx Ices Investigation: An Overview

SPHEREx is a NASA mission designed to perform an all-sky spectroscopic survey in the 0.75 - 5 $\mu$m wavelength range. Its primary science objectives are to investigate: (1) inflationary cosmology, (2) the history of galaxy formation, and (3) the abundance of molecular ices - critical for prebiotic chemistry - found on the surfaces of interstellar dust grains within planet-forming regions. This paper focuses on the third theme, the SPHEREx Ices investigation, for which SPHEREx is conducting a spectroscopic survey of nearly ten million preselected sources throughout the Milky Way and Magellanic Clouds to characterize their ice absorption features. By selecting targets based on infrared color, spatial isolation, and brightness, the Ices Investigation secures high-signal-to-noise spectra across a broad range of astrophysical environments that are relatively free of spectral contamination. Rather than attempting to decompose each spectrum into its individual ice components, the Ices Investigation prioritizes accurate measurements of the integrated optical depths of key molecular ice absorption features. This approach enables statistically powerful correlation studies between ice abundances and environmental parameters - including extinction, temperature, gas composition, radiation field strength, cosmic ray flux, and star formation activity. The data pipeline developed for this purpose incorporates machine learning for continuum estimation, drawing on both SPHEREx and ancillary datasets. Ultimately, the expansive spectral archive produced by SPHEREx, combined with targeted follow-up from facilities like JWST, will transform our understanding of Galactic ice formation, evolution, abundance and their inheritance into planetary systems and prebiotic inventories.

astro-ph.GA↗
arXiv2026

Mature but Still Growing: JWST Detection of the Earliest Intracluster Light at z ~ 2

We present a JWST analysis of intracluster light (ICL) in XLSSC 122 at z = 1.98, currently the most distant known strong lensing galaxy cluster with an evolved member population. Using deep JWST imaging complemented by HST data and careful control of systematics, we robustly detect diffuse emission extending to several hundred kpc from the brightest cluster galaxy (BCG) down to about 29 mag arcsec^-2. Multi component PSF convolved Sersic modeling separates the surface brightness profiles into three components: a BCG core, a BCG envelope, and an ICL component, with stable Sersic indices across wavelengths. Nearly flat color profiles indicate minimal radial variation in the stellar populations of the BCG envelope and the ICL. The median ICL fraction measured across seven bands is about 17 percent, demonstrating that the buildup of intracluster stars in massive halos was already well underway by z about 2. The ICL fraction peaks near 5000 Angstrom in the rest frame, resembling the behavior observed in dynamically active clusters. We also detect a southern excess of ICL relative to the best fit Sersic model and quantify it using wavelet based modeling, providing additional support that this system is dynamically active. The BCG + ICL light distribution and strong lensing mass map show strong morphological agreement within about 100 kpc. These findings establish the ICL as an early forming and dynamically informative component of massive halos.

astro-ph.GA↗
arXiv2026

Spectral response of SPHEREx

The Spectro Photometer for the History of the Universe, Epoch of Reionization, and Ices Explorer (SPHEREx) is conducting the first all-sky near infrared spectral survey spanning 0.75 to 5.0um with resolving power R~35 to 130. Linear variable filters mounted in front of six H2RG detectors produce a position dependent spectral response across the focal plane. This paper presents the ground-based spectral calibration of SPHEREx, including the cryogenic apparatus, optical configuration, measurement strategy, analysis pipeline, and resulting calibration products. Monochromatic wavelength scans are used to derive the spectral response function, band center, and resolving power for every pixel. Band centers are measured to better than 1nm for Bands 1 through 4 (0.75 to 3.82um) and better than 10nm for Bands 5 and 6 (3.82 to 5.0um). Out-of-band leakage is negligible for detectors above 1.64um and is present at the percent level below this wavelength. The resolving power is measured to within 5% and agrees with design expectations to within 10%. An on-sky spectrum of the Cat's Eye Nebula (NGC 6543) constructed from repeated observations provides in-flight verification and shows agreement between ground calibrated response and astrophysical emission features. Calibration products, including per-pixel band center and resolving power maps, are released through IPAC to support community use of SPHEREx data. The absolute spectral calibration will continue to improve through in-flight measurements, with further reductions in uncertainty expected for the longest-wavelength bands.

astro-ph.IM↗
arXiv2026

Illuminating the Physics of Cosmic Origin and Evolution: A UK Space Frontiers 2035 White Paper

Understanding the Universe's origins and evolution remains one of the most fundamental challenges in modern cosmology. This white paper explores three key science priorities in this field: unravelling the physics of cosmic inflation, investigating the accelerating expansion of the Universe, and precisely measuring the sum of the neutrino masses. Achieving these goals requires a dedicated survey to map the large-scale structure at high redshift in unprecedented detail. We describe how this can be achieved through a mission concept called SIRMOS, providing a high-throughput, highly multiplexed spectroscopic capability to obtain accurate redshifts for over 100 million galaxies over a wide sky area. Such a survey would leverage the deepest existing wide-area photometric catalogues for targeting, with spectra offering continuous 1.25-2.5~$\mu$m wavelength coverage at moderate resolution, allowing precise redshift measurements in the $1<z<4$ range with minimal bias. We outline the scientific opportunities this presents. Recent years have seen significant advances in instrumentation, including digital micromirror devices, complex telescope mirrors, large detector arrays, and data processing pipelines. While these technologies have been demonstrated in terrestrial applications, such a survey is a unique opportunity to apply these proven capabilities in space to address fundamental questions in cosmology. Participation in such a mission will simultaneously deliver a compelling science case, help align UK Space Agency and STFC strategies, demonstrate the UK's growing capability in end-to-end space missions, and strengthen the national space economy through high-value industrial participation.

astro-ph.IM↗
arXiv2026

Reducing the Dimensions of AGN Lightcurve Manifolds

The Active Galactic Nuclei (AGN) glossary is vast and complex. Depending on selection method, observing wavelength, and brightness, AGNs are assigned distinct labels, yet the relationship between different selection methods and the diversity of time-domain behavior within and across classes remains difficult to characterize in a unified framework. Changing-look AGNs (CLAGNs), which transition between classifications over time, further complicate this picture. In this work, we learn a data-driven, low-dimensional representation of multi-wavelength photometric light curves of AGNs, in which the structure of the projected manifold correlates with AGN class and independent spectroscopic properties. Using the NASA Fornax Science Platform, we assemble light curves from ZTF, Pan-STARRS, Gaia, and WISE/NEOWISE for two samples: (1) a heterogeneous set of $\sim$2000 AGNs spanning $z \lesssim 1$, including SDSS quasars, variability-selected sources, and CLAGNs; and (2) a homogeneous sample of $\sim$65000 narrow-line AGNs at $z \approx 0.1$ with well-characterized optical emission-line measurements. Without using class labels during training, the learned manifolds organize variability-selected AGNs into coherent regions of the low-dimensional space, distinguish between turn-on and turn-off CLAGNs, and place tidal disruption events in distinct regions. Manifold coordinates correlate with key spectroscopic and host-galaxy properties, including stellar mass, [OIII] luminosity, and D$_n$(4000), demonstrating that heterogeneous multi-band variability can be combined in a purely data-driven manner to recover correlations with independent physical diagnostics, without requiring explicit physical modeling. These results show that manifold learning offers a practical, assumption-light approach for integrating time-domain surveys and prioritizing spectroscopic follow-up.

astro-ph.GA↗
arXiv2026

A spectroscopically confirmed, strongly lensed, metal-poor Type II supernova at z = 5.13

Observing supernovae (SNe) in the early Universe (z > 3) provides a window into how both galaxies and individual stars have evolved over cosmic time, yet a detailed study of high-redshift stars and SNe has remained difficult due to their extreme distances and cosmological redshifting. To overcome the former, searches for gravitationally lensed sources allow for the discovery of magnified SNe that appear as multiple images - further providing the opportunity for efficient follow-up. Here we present the discovery of "SN Eos": a strongly lensed, multiply-imaged, SN II at a spectroscopic redshift of z = 5.133 +/- 0.001. SN Eos exploded in a Lyman-{\alpha} emitting galaxy when the Universe was only ~1 billion years old, shortly after it reionized and became transparent to ultraviolet radiation. A year prior to our discovery in JWST data, archival HST imaging of SN Eos reveals rest-frame far ultraviolet (~1,300{\AA}) emission, indicative of shock breakout or interaction with circumstellar material in the first few (rest-frame) days after explosion. The JWST spectroscopy of SN Eos, now the farthest spectroscopically confirmed SN ever discovered, shows that SN Eos's progenitor star likely formed in a metal-poor environment (<= 0.1 Z_{\odot}), providing the first direct evidence of massive star formation in the metal-poor, early Universe. SN Eos would not have been detectable without the extreme lensing magnification of the system, highlighting the potential of such discoveries to eventually place constraints on the faint end of the cosmic star-formation rate density in the very early Universe.

astro-ph.HE↗
arXiv2025

A first systematic study of [OIII] 88$\mu$m at $z>8$: two luminous oxygen lines and a powerful ionized outflow in the first 600 million years

We present deep ALMA Band 7 observations of the [OIII] $88\mu$m line and underlying dust continuum emission in four UV-bright, gravitationally lensed (magnification $\mu = 1.4-3.8$), JWST-selected galaxies at $z = 8.5 - 10.3$, with observed magnitudes $-22.5 \lesssim M_\mathrm{UV} \lesssim -20.5$. [OIII] $88\mu$m is confidently detected in UNCOVER-10646 at $z=8.5080 \pm 0.0011$ ($15\sigma$) and DHZ1 at $z=9.3113 \pm 0.0006$ ($6\sigma$), with both being intrinsically luminous systems [$L_\text{[OIII]} = (1.1 - 1.6) \times 10^9\,L_\odot$] that follow the local [OIII]-SFR relation. [OIII] $88\mu$m remains undetected in the two $z>10$ targets, including in the $z=10.07$ X-ray AGN UHZ1, where we obtain a deep limit of $L_\text{[OIII]} < 6 \times 10^7\,L_\odot$. Dust emission is not detected in any individual source nor in a stack ($<3\sigma$). The high S/N [OIII] $88\mu$m detection in UNCOVER-10646 uniquely reveals an additional broad component ($\mathrm{FWHM} = 1366_{-329}^{+473}\,\mathrm{km/s}$; $\Delta\mathrm{BIC}\approx20$) indicative of an ionized outflow. We infer a high outflow rate of $\dot{M}_\mathrm{out} = 128_{-46}^{+80}\,M_\odot\,\mathrm{yr}^{-1}$, corresponding to a mass loading factor $\eta = \dot{M}_\mathrm{out}/\mathrm{SFR} = 2.9_{-1.0}^{+1.8}$ that matches or exceeds theoretical predictions and JWST-based studies of ionized outflows at high redshift. While high-resolution ALMA follow-up is required to confirm and spatially resolve the outflow, this first systematic study at $z>8$ highlights the unique diagnostic power of [OIII] $88\mu$m in characterizing galaxies in the early Universe.

astro-ph.GA↗
arXiv2025

An Active Galaxy Cluster Merger at Cosmic Noon Revealed by JWST Weak Lensing and Multiwavelength Probes

The galaxy cluster XLSSC~122 is a rare system at $z = 1.98$, hosting surprisingly evolved member galaxies when the Universe was only one-third of its present age. Leveraging deep JWST/NIRCam imaging, we perform a weak-lensing analysis and reconstruct the cluster's mass distribution, finding a mass peak that coincides with both the X-ray peak and the position of the brightest cluster galaxy. We obtain a mass estimate of $M_{200\rm c}=1.6 \pm 0.3\times 10^{14}~M_{\odot}$ and a concentration of $c_{200 \rm c}=6.3 \pm 0.3$ implied by the preferred concentration--mass relation, in agreement with recent strong-lensing estimates. The high concentration in particular motivates tests against empirical and simulation-derived concentration--mass relations. Placing our weak-lensing mass map in the context of Chandra X-ray data, MeerKAT radio imaging, ALMA+ACA/ACT Sunyaev-Zel'dovich (SZ) mapping, and new JWST intracluster light measurements, we identify consistent NE--SW elongation across datasets and a pronounced offset along the same axis between the SZ and mass/X-ray peaks, pointing to significant merger activity. XLSSC~122 thus serves as a JWST pilot study for high-$z$ lensing, demonstrating the telescope's unique ability to map cluster mass distributions at $z\sim 2$ and motivating a uniform sample of analogous systems with joint lensing, X-ray, SZ, and radio data to probe cluster assembly at cosmic noon.

astro-ph.GA↗
arXiv2025

Extended Components of Little Red Dots in the Rest-Frame Optical

Recent JWST observations have revealed a population of red, compact, high-redshift objects called Little Red Dots (LRDs), whose host components have remained largely unconstrained, possibly due to their extreme compactness. Current morphological studies suggest the presence of extended emission in LRDs at rest-frame ultraviolet wavelengths. However, in the rest-frame optical regime, investigations have been limited by small sample sizes and insufficient imaging depth, hindering reliable separation between point-like and potential extended components. Here we perform the image stacking analysis of 217 LRDs in four NIRCam bands, a large and homogeneous sample observed with the COSMOS-Web survey. Our results reveal the detection of faint extended emission in the F444W band, with a typical size of ~200 parsecs and magnitude of ~27.7 AB at z~6.5. We perform four-band photometric spectral energy distribution fitting based on galaxy templates and derive an average stellar mass of log(M*/M_sun) = 9.02 +0.20/-0.18. Given this stellar mass, the host galaxy is compact, that is, ~2.5 times smaller than star-forming galaxies of similar mass at comparable redshifts. This work provides direct observational evidence for the existence of LRD host galaxies at rest-frame optical wavelengths and offers new insights into the stellar buildup of these systems within the first billion years after the Big Bang.

astro-ph.GA↗
arXiv2025

The ALPINE-CRISTAL-JWST Survey: Stellar and nebular dust attenuation of main-sequence galaxies at z~4-6

Characterizing dust attenuation is crucial for revealing the intrinsic physical properties of galaxies. We present an analysis of dust attenuation in 18 spectroscopically confirmed star-forming main-sequence galaxies at $z = 4.4-5.7$ observed with JWST/NIRSpec IFU and NIRCam, selected from the ALPINE and CRISTAL ALMA large programs. We fit the emission line fluxes from NIRSpec and the broad-band photometry from NIRCam with Prospector, using both spatially integrated emission and $\sim0.6$ kpc pixel-by-pixel measurements. We derive the stellar-to-nebular dust attenuation ratio ($f=E(B-V)_{\mathrm{star}}/E(B-V)_{\mathrm{neb}}$) from the SED fits and the Balmer decrement with H$\alpha$ and H$\beta$. Although individual galaxies show large scatter, the best-fit value is $f = 0.51^{+0.04}_{-0.03}$, slightly higher than that measured for local starburst galaxies. We find weak correlations of $f$ with galaxy properties, increasing with higher specific star-formation rates, younger stellar ages, and more recent star-formation. For the range of $E(B-V)_{\mathrm{star}} = 0.009-0.15$ mag for in our sample, assuming $f = 1$ (often adopted in high-redshift studies) instead of $f = 0.51$ underestimate line luminosities and ionizing photon production efficiency $\xi_\text{ion}$ by $\sim3-36\%$ and $\sim4-46\%$, respectively. We also find that the total stellar masses estimated from spatially-integrated SED fits with a delayed-$\tau$ star-formation histories are systematically smaller than the sum of pixel-by-pixel SED fits, with a median offset of $\sim 0.26$ dex, likely because the integrated fits are biased toward luminous young stellar populations.

astro-ph.GA↗
arXiv2025

Galaxy Protoclusters as Drivers of Cosmic Reionization: I. Bubble Overlap at Redshift z ~ 7 in LAGER-z7OD1

Since the launch of JWST, the sample size of reionization-era Lyman-alpha-emitters (LAEs) has been steadily growing; yet inferences about the neutral hydrogen fraction in the intergalactic medium exhibit increasing variance at redshift z ~ 7, possibly indicating significant field-to-field fluctuations in the progression of cosmic reionization. In this paper, we present new JWST/NIRSpec and Keck/LRIS spectra of nine LAEs in the redshift z ~ 7 protocluster, LAGER-z7OD1. Measurements of Lyman-alpha-transmission and Lyman-alpha velocity offset along multiple sightlines map the Lyman-alpha damping wing optical depth across the galaxy overdensity. In the standard context of inside-out ionization, we estimate radii of ionized bubbles (R(min) = 0.07 - 0.69 Mpc) based on the distance from each LAE to the first neutral patch along the sightline. The resulting 3D topology reveals three distinct sub-clusters where the ionized bubbles are approaching overlap. Five of the nine LAEs plausibly ionized their bubbles, a few bursts of star formation and a modest escape fraction are sufficient. We demonstrate, however, that the actual ionized volumes are likely larger, at least R(ism) = 0.42 - 1.29 Mpc, based on an empirical model for interstellar attenuation of Lyman-alpha. Modeling galactic attenuation of Lyman-alpha significantly increases the inferred intergalactic transmission (thus enlarging the ionized pathlength). The errorbars on the reddening correction allow fully overlapping bubbles, and our results are consistent with accelerated reionization in the protocluster.

astro-ph.GA↗
arXiv2025

Need for PRIMA to understand the nature and ISM physical conditions of HST-dark galaxies

One of the main open issues in galaxy formation and evolution is the early assembly of the most massive galaxies and their contribution to the stellar mass and star formation rate densities at early epochs. Massive red sources already in place at z > 2 to 3 have been found in deep Spitzer-IRAC and ALMA surveys. They are often called optically and near-IR dark, or HST-dark, being undetected even in the deepest HST frames. The submillimeter (i.e., ALMA) detection of these sources confirms their high-z dusty nature: they are massive (e.g., log(M*/Msun) > 10) and dusty star-forming galaxies with estimated redshifts in the 2.5 to 7 range. They seem to lie mostly below the main sequence (MS) of star-forming galaxies and show gas depletion times <1 Gyr. Imaging with the PRIMA/PRIMAger instrument over the full 25 to 265 micron range will allow us to characterize their still uncovered spectral energy distributions between JWST and ALMA spectral windows, probing their dust content and properties (e.g., temperature, mass), whereas spectroscopic observations with FIRESS will be the key to investigate the nature of their powering source (e.g., AGN or star formation) and to study the physics of their ISM, by detecting and measuring fine structure lines in the mid- and far-IR domain.

astro-ph.GA↗