arXiv subjects

Daniel Schaerer

Publications and source records attributed to Daniel Schaerer.

At least 37 records · Page 2Linked to original sources

arXiv2026

The AURORA Survey: High-Redshift Empirical Metallicity Calibrations from Electron Temperature Measurements at z=2-10

We present detections of auroral emission lines of [OIII], [OII], [SIII], and [SII] in deep JWST/NIRSpec spectroscopy for 41 star-forming galaxies at $z=1.4-7.2$ from the AURORA survey. We combine these new observations with 98 star-forming galaxies at $z=1.3-10.6$ with detected auroral lines drawn from the literature to form a sample of 139 high-redshift galaxies with robust electron temperature and direct-method oxygen abundance determinations. This sample notably covers a wider dynamic range in metallicity than previous work, spanning $0.02-0.9$~Z$_\odot$. We calibrate empirical relations between 19 emission-line ratios and oxygen abundance, providing a robust tool set to infer accurate gas-phase metallicities of high-redshift galaxies when auroral lines are not detected. While calibrations based on lines of $α$ elements (O, Ne, S, Ar) appear reliable, we find significant scatter in calibrations involving lines of N driven by a high dispersion in N/O at fixed O/H, suggesting that N-based line ratios are less reliable tracers of the oxygen abundance at high redshift. These new high-redshift calibrations are notably offset from those based on typical $z\sim0$ galaxy and HII region samples, and are better matched by samples of extreme local galaxies that are analogs of high-redshift sources. The new metallicity calibrations presented in this work pave the way for robust studies of galaxy chemical evolution in the early Universe, leading to a better understanding of baryon cycling and galaxy formation from Cosmic Noon through the Epoch of Reionization.

astro-ph.GA↗

arXiv2026

Non-LTE atmosphere models of very luminous sources and their applicability to Little Red Dots, quasi-stars, and similar objects

We investigate whether atmosphere models traditionally used for massive stars with strong winds can produce synthetic spectra morphologically similar to those of Little Red Dots (LRDs). We compute atmosphere models and synthetic spectra with the code CMFGEN. The models assume a thermalized radiation field at the inner boundary, parameterized by a temperature varying between 5000 and 12000~K. We adopt a typical luminosity of 1e10 Lsun. The models are spherical, assume an expanding atmosphere, and are computed under non-LTE conditions and for several metallicities. The spectral energy distribution (SED) is different from a blackbody, with a blue optical spectrum. Broad hydrogen emission lines are produced, their wings being formed by electron scattering. The SED near the Balmer and Paschen limit is rather continuous. A Balmer break is predicted for the coolest temperature models provided the wind density is reduced. The SED and Balmer decrement of most LRDs is reproduced by the models, provided they are dust-attenuated with Av~1.9-2.7. Assuming the absorbed luminosity is re-radiated in the infrared, the energy output at these wavelengths is consistent with observational constraints. The models predict FeII, oxygen and calcium lines. OI lines at 8446 A and 1.129 um are produced mostly by Lybeta fluorescence. The strength of emission lines from metals depends on input temperature, metallicity, and details of the radiative transfer models. CMFGEN atmosphere models predict a large number of spectral properties observed in many LRDs. They struggle to simultaneously produce a genuine Balmer break and strong emission lines. Whether they are more relevant or not to explain LRDs' spectra compared to alternative models is unclear, leaving open the question of the physical conditions in LRDs.

astro-ph.GA↗

arXiv2026

A Fleeting GLIMPSE of N/O Enrichment at Cosmic Dawn: Evidence for Wolf Rayet N Stars in a z = 6.1 Galaxy

We present the discovery of extreme nitrogen enrichment by Wolf Rayet nitrogen stars (WN) in the metal-poor (~10% Z_odot), lensed, compact (R_eff ~ 20 pc) galaxy RXCJ2248 at z=6.1, revealed by unprecedentedly deep JWST/NIRSpec medium-resolution spectroscopy from the GLIMPSE-D Survey. The exquisite S/N reveals multiple high-ionization nebular lines and broad Balmer and [OIII] components (FWHM~700-3000 km/s). We detect broadened HeII 1640 and 4687 (FWHM~530 km/s) and strong NIII] 4642 emission consistent with a population of WN stars, making RXCJ2248 the most distant galaxy with confirmed WR features to date. We measure the multi-phase nebular density across five ions, the direct-method metallicity (12+log(O/H)= 7.749+/-0.023), and a non-uniform elemental enrichment pattern of extreme N/O enhancement (log(N/O)=-0.390+/-0.035 from N^+, N^+2, and N^+3) and suppressed C/O relative to empirical C/N trends. We show that this abundance pattern can be explained by enrichment from a dual-burst with a low WC/WN ratio, as expected at low metallicities. Crucially, these signatures can only arise during a brief, rare evolutionary window shortly after a burst (~3-6 Myr), when WN stars dominate chemical feedback but before dilution by later yields (e.g., supernovae). The observed frequency of strong N emitters at high-z implies a ~50 Myr burst duty cycle, suggesting that N/O outliers may represent a brief but ubiquitous phase in the evolution of highly star-forming early galaxies. The detection in RXCJ2248, therefore, provides the first direct evidence of WN-driven chemical enrichment in the early Universe and a novel timing argument for the bursty star formation cycles that shaped galaxies at cosmic dawn.

astro-ph.GA↗

arXiv2026

A Glimpse of the Low-Mass End of the Direct Mass-Metallicity Relation at $z\sim6-8$

The competition between metal synthesis and feedback from massive stars establishes the mass-metallicity relation (MZR) at low-redshifts. Examining this relation at higher redshifts, particularly at the low-mass end $\lesssim10^{8}\,{\rm M_\odot}$, is essential for understanding chemical enrichment and stellar feedback. In this study, we utilize the deep ($\sim30\,$hrs) JWST/NIRSpec G395M GLIMPSE-D survey of the lensed field Abell S1063, to explore the low-mass end of the MZR at high redshift ($z\sim6-8$). We identify eight [OIII]$λ$4364 emitters, enabling the most reliable "direct" metallicity measurements in galaxies down to stellar masses of $\sim10^{6-8}\,{\rm M_\odot}$. By combining our sample and galaxies with [OIII]$λ$4364 detections from the literature, we calculate direct metallicities for 21 galaxies. We compare our direct metallicities to those derived from strong-line diagnostics, and find them to be consistent with previous calibrations. We fit the MZR at $10^{6.7-9}\,M_{\odot}$ with $\sim0.3-0.5$ dex lower metallicity than local galaxies at similar stellar mass. We find the slope to be $0.25\pm0.10$, comparable to the local MZR; and the MZR exhibits a scatter of $\sim0.2\,$dex, which is larger than the local MZR, The lower metallicities may reflect denser, more gas-rich early environments, with continuous inflow of metal-poor gas diluting the ISM metallicity. In addition, we show that in extremely high electron densities ($n_e \gtrsim 10^5\,{\rm cm^{-3}}$), metallicities can be significantly underestimated ($\sim0.5$ dex), if lower $n_e$ are assumed for galaxies with high $n_e$. In a nutshell, these observations provide the first glimpse of the low-mass MZR at $z\sim6-8$ using direct metallicity measurements. More deep spectroscopic observations in lensed fields will be critical to robustly characterize the MZR and chemical evolution in the early universe.

astro-ph.GA↗

arXiv2026

The Pristine HeII Emitter near GN-z11: Constraining the Mass Distribution of the First Stars

The properties of the first metal-free stars remain largely unknown, and so far, the only data-driven constraints on their mass distribution (IMF) come from near-field cosmology. Here, we interpret new observations of the C1 and C2 components of Hebe, the HeII emitter near the galaxy GN-z11. Using a locally calibrated model, we robustly confirm the pristine (PopIII) nature of both components, showing that the measured upper limits on metal lines can only be reproduced by galaxies with $>50\%$ of their stellar mass in PopIII stars. We find that C1 is consistent with a purely PopIII system and adopt a simple parametric approach to infer the implications for the PopIII IMF and stellar mass. The observed $\rm HeII/H_γ$ ratio excludes steep IMFs, favoring top-heavy distributions, especially for young stellar ages ($\leq 1$ Myr). Combined with the HeII luminosity, this implies a total PopIII stellar mass of $2 \cdot 10^4 < M_\star/M_\odot < 6 \cdot 10^5$. While degeneracies between IMF, stellar mass, and age remain, adopting the lower stellar masses predicted by simulations ($M_\star < 10^5\,M_\odot$) strengthens the preference for top-heavy IMFs. Combining these results with near-field constraints, which instead exclude the flattest IMFs, we define a data-driven range of viable PopIII IMFs, linking characteristic mass and slope. This work demonstrates that direct observations of high-$z$ PopIII systems can place independent constraints on the IMF of the first stars, opening a new window on their formation and properties.

astro-ph.GA↗

arXiv2026

A GLIMPSE into the very faint-end of the H$β$+[OIII]$λλ$4960,5008 luminosity function at z=7-9 behind Abell S1063

We use the ultra-deep GLIMPSE JWST/NIRCam survey to constrain the faint-end of the H$β$+[OIII]$λλ$4960,5008 luminosity function (LF) down to $10^{39}$ erg/s at z=7-9 behind the lensed Hubble Frontier Field Abell S1063. We perform SED fitting on a Lyman-Break Galaxy sample, measuring combined H$β$+[OIII] fluxes to construct the emission-line LF. The resulting LF ($α$=-1.55 to -1.78) is flatter than the UV LF ($α<-2$), indicating a lower number density of low H$β$+[OIII] emitters at fixed MUV. We explore three explanations: (i) bursty star formation histories reducing the H$β$+[OIII]-to-UV ratio, (ii) metallicity effects on [OIII]/H$β$, or (iii) a faint-end turnover in the UV LF. Assuming an evolving [OIII]/H$β$ ratio, we derive a flatter [OIII]$λ$5008 LF ($α$=-1.45 to -1.66) and a steeper H$β$ LF ($α$=-1.68 to -1.95). The combination of decreasing metallicity and bursty star formation can reconcile the UV and H$β$+[OIII] LF differences. Converting the LF to the ionising photon production rate, we find that galaxies with H$α$ flux $>10^{39}$ erg/s (SFR(H$α$)$>5\times10^{-3} M_\odot$/yr) contribute 31%-90% and 46%-156% of the ionising photon budget at 7<z<8 and 8<z<9, respectively (for $f_{esc}=0.14$). The LF shape suggests faint galaxies contribute minimally to the ionising photon production rate. Our cosmic star formation rate density (CSFRD) estimates align with previous work, but GLIMPSE's sensitivity to low SFRs confirms that very faint galaxies are minor contributors to both the ionising photon production rate and the CSFRD. Our results suggest that GLIMPSE has detected the bulk of the total H$β$+[OIII] emission from star-forming galaxies, with fainter sources playing a limited role in cosmic reionisation.

astro-ph.GA↗

arXiv2026

Radio Spectral Energy Distribution of Low-$z$ Metal Poor Extreme Starburst Galaxies: Novel insights on the escape of ionizing photons

Recent optical surveys have identified a rare population of low-$z$ ($z \sim 0.01 - 0.06$) extreme star-forming galaxies (xSFGs) characterized by very low metallicity, strong emission lines, extremely high specific star-formation rate, low stellar mass, and strong Ly~$α$ emission. Their global properties resemble recently discovered $z > 6$ reionization-era star-forming galaxies. We present new multi-frequency radio continuum (RC) observations of $8$ xSFGs using the upgraded Giant Metrewave Radio Telescope (uGMRT) at $1.25$ GHz, the Karl G. Jansky Very Large Array (VLA) at $1.5, 3.0, 6.0, 10.0$ and $15.0$ GHz, along with archival LOw Frequency ARray (LOFAR) data at $150$ MHz for several sources. These data allow construction of the radio spectral energy distribution (radio-SED) from $\sim 1$ GHz (down to $150$ MHz for some sources) to $15$ GHz, spanning nearly two orders of magnitude in frequency. We find that xSFGs exhibit a flat spectral index between $6$ and $15$ GHz, while a subset shows spectral turnovers at $0.3 - 3$ GHz. Our Bayesian radio-SED modeling indicates that these features are consistent with a high thermal fraction combined with free-free absorption, requiring high emission measures in some systems. By comparing modeled thermal radio emission with observed H$β$ line flux density, we find evidence for dust in several xSFGs. Finally, we confirm a previously reported correlation between Lyman continuum escape fraction, ionization state, and radio spectral index, particularly among strong leakers.

astro-ph.GA↗

arXiv2026

The Ly$α$ and Continuum Origins Survey. III. Investigating the Link between Galaxy Morphology, Merger Properties and LyC Escape

Characterizing the mechanisms and galaxy properties conducive to the emission and escape of ionizing (LyC) emission is necessary to accurately model the Epoch of Reionization, and identify the sources that powered it. The Ly$α$ and Continuum Origins Survey (LaCOS) is the first program to obtain uniform, multi-wavelength subkiloparsec imaging for a large sample (42) of galaxies observed in LyC and enable statistically robust studies between LyC and resolved galaxy properties. Here, we characterize the morphology and galaxy merger properties of LaCOS galaxies and investigate their connection with the escape fraction of LyC emission $f_{esc}^{LyC}$. We find strong anticorrelations between $f_{esc}^{LyC}$ and size ($r_{20}$, $r_{50}$, and $r_{80}$) measured in filters containing emission from star-forming regions, and with the asymmetry and clumpiness in F150LP, the bluest filter in our dataset, tracing UV continuum and Ly$α$. We find that $\geq48\%$ of LaCOS galaxies, and $\geq41\%$ of LaCOS LyC-emitters are visually classified as galaxy mergers. Galaxies robustly identified as mergers in LaCOS are at advanced stages of interaction, close to coalescence. The $f_{esc}^{LyC}$ properties of robust mergers and low-probability mergers cannot be differentiated statistically, and we only find significant difference between the two populations in terms of their of their sizes and LyC luminosity: robust mergers having larger values. We conclude that (i) $f_{esc}^{LyC}$ tends to be larger in galaxies with a small number of compact, centrally-located, UV-emitting star-forming regions, (ii) at advanced stages of interaction represent a sizable fraction of LyC-emitting samples at $z\sim0.3$, $z\sim0$, and (iii) mergers can facilitate the escape of LyC photons from galaxies.

astro-ph.GA↗

arXiv2026

A first GLIMPSE into star clusters populations across cosmic time

We present the first sample of 222 high-redshift (z>0.5) star clusters, detected with JWST/NIRCam in 78 magnified galaxies from different galaxy cluster fields. The majority of the systems (~60%) is observed in the very deep NIRCam observations of the cluster AbellS1063 (GLIMPSE program), showing the power that deep observations, combined with lensing, has to reveal these primordial stellar structures. We perform simultaneous size-flux estimates in all available NIRCam filters and spectral energy distribution (SED) fitting analysis to recover star cluster physical properties. All star cluster candidates have very high magnification. Star clusters and clumps show similar ages and redshift distributions, although noticeable differences are seen in their masses, sizes and stellar surface densities inherent to the lack of resolution in the latter group. We reconstruct the formation redshift of star clusters and find that the large majority of the observed star clusters show young ages (<100 Myr) and seems to form at cosmic noon (CN,1 1, based on a subsample of 60 star clusters younger than 100 Myr and with masses above 2e6 Msun. The star cluster mass function is well described by a power-law with slope beta = -1.89 suggesting that a power-law -2 function might already be in place in the distant Universe.

astro-ph.GA↗

arXiv2026

A Cosmic Miracle: A Remarkably Luminous Galaxy at $z_{\rm{spec}}=14.44$ Confirmed with JWST

JWST has revealed a stunning population of bright galaxies at surprisingly early epochs, $z>10$, where few such sources were expected. Here we present the most distant example of this class yet -- MoM-z14, a luminous ($M_{\rm{UV}}=-20.2$) source in the COSMOS field at $z_{\rm{spec}}=14.44^{+0.02}_{-0.02}$ that expands the observational frontier to a mere 280 million years after the Big Bang. The redshift is confirmed with NIRSpec/PRISM spectroscopy through a sharp Lyman-$α$ break and $\sim3σ$ detections of five rest-UV emission lines. The number density of bright $z_{\rm{spec}}\sim14-15$ sources implied by our "Mirage or Miracle" survey spanning $\sim350$ arcmin$^{2}$ is $>100\times$ larger ($182^{+329}_{-105}\times$) than pre-JWST consensus models. The high EWs of UV lines ($\sim15{-}35$ Å) signal a rising star-formation history, with a $\sim10\times$ increase in the last 5 Myr ($\rm{SFR_{\rm{5Myr}}}/\rm{SFR_{\rm{50Myr}}}=9.9^{+3.0}_{-5.8}$). The source is extremely compact (circularized $r_{\rm{e}} = 74^{+15}_{-12}$ pc), and yet elongated ($b/a=0.25^{+0.11}_{-0.06}$), suggesting an AGN is not the dominant source of UV light. The steep UV slope ($β=-2.5^{+0.2}_{-0.2}$) implies negligible dust attenuation and a young stellar population. The absence of a strong damping wing provides tentative evidence that the immediate surroundings of MoM-z14 may be partially ionized at a redshift where virtually every reionization model predicts a $\sim100\%$ neutral fraction. The nitrogen emission and highly super-solar [N/C]$>1$ hint at an abundance pattern similar to local globular clusters that may have once hosted luminous supermassive stars. Since this abundance pattern is also common among the most ancient stars born in the Milky Way, we may be directly witnessing the formation of such stars in dense clusters, connecting galaxy evolution across the entire sweep of cosmic time.

astro-ph.GA↗

arXiv2026

GLIMPSE-DDT spectroscopic properties of faint-end galaxies at $z\sim6$: Towards first metal enrichment, dust production, and ionizing photon production

Ultra-faint galaxies at high-$z$ are fundamental elements of the early galaxy assembly, and spectroscopic characterization of this population is essential to understand the earliest galaxy evolution. Leveraging the ultra-deep JWST/NIRCam and NIRSpec observations of a gravitational lensing field of Abell S1063, taken as part of the GLIMPSE survey, we present spectroscopic properties of 16 galaxies fainter than $M_{\rm UV}=-17$ mag, including the metallicity, dust attenuation, and the ionizing photon production efficiency. The emission lines are generally quite strong, roughly half of which cannot be replicated with standard stellar populations and require an extreme ionizing source. We also identify relatively strong [OIII] emission lines from all sample galaxies, which indicates that the low-mass end of the mass-metallicity relation is extended down to $M_\star\sim10^6\ M_\odot$ at $z\sim6$. The strong [OIII] line detection from the lowest-mass galaxy among the sample ($M_\star\sim10^{5.6}\ M_\odot$) stands in contrast to recent reports of extremely metal-poor galaxy candidates at similar mass and redshift, suggesting that there could be two distinct pathways of the earliest metal enrichment as simulations have predicted. Interestingly, we detect both dust attenuation and galactic outflow in one of the sample galaxies with $M_\star=10^{6.6}\ M_\odot$ at $z=5.5$. All the dust, metal, and outflow contents in this galaxy can be consistently explained by supernovae (SNe), indicative of the key roles of SNe in the earliest galaxy assembly such as dust production, metal enrichment, stellar feedback, and baryon cycle.

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-α 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 (~1,300Å) 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↗

arXiv2026

VENUS: Two Faint Little Red Dots Separated by $\sim70\,\mathrm{pc}$ Hidden in a Single Lensed Galaxy at $z\sim7$

We report the identification of a pair of faint little red dots (LRDs), dubbed Red Eyes, in a strongly-lensed galaxy at $z\sim7$ behind the PLCKG004.5-10.5 cluster, identified from the JWST Treasury program VENUS. Red Eyes are spatially resolved on the image plane with distinct colors, while the critical curve lies far north of Red Eyes, clearly requiring two different LRDs rather than a single LRD. Red Eyes is an extremely close pair of LRDs separated by $\sim70\,\mathrm{pc}$ in the source plane with a magnification of $μ\sim20$, which consistently explains another counter-image detected to the north-west. Red Eyes is hosted in a typical star-forming galaxy with $M_{\mathrm{UV,int}}\sim -19$, but its own UV emission is very faint ($M_{\mathrm{UV,int}} \gtrsim -16$). Moreover, Red Eyes does not reside at the galaxy center but lies at an offset position of approximately one effective radius $R_{\mathrm{e}}$ away from the galaxy center. If observed without lensing, Red Eyes would appear as a typical star-forming galaxy at $z\sim 7$ with $M_{\mathrm{UV}}\sim -19$, showing no apparent LRD signatures in either morphology or SED. These results suggest that multiple off-center LRDs, similar to Red Eyes, may be commonly hidden in a typical high-$z$ star-forming galaxy. In this case, various plausible scenarios may emerge, one of which is that intermediate-mass black holes (IMBHs) with $M_\mathrm{BH}\sim10^{4\text{--}6}\,M_\odot$ may form in star clusters on a stellar disk and contribute to the growth of the central supermassive black hole via mergers, with some IMBHs detectable as luminous LRDs in a sufficiently active and massive phase.

astro-ph.GA↗

arXiv2025

GLIMPSE-D: An Exotic Balmer-Jump Object at z=6.20? Revisiting Photometric Selection and the Cosmic Abundance of Pop III Galaxies

We present deep JWST/NIRSpec G395M spectroscopy of GLIMPSE-16043, a promising $z\sim6$ Pop III candidate originally identified through NIRCam photometry as having weak [OIII]$λ\lambda4959,5007$ emission. Our follow-up reveals clear [OIII] emission, ruling out a genuine zero-metallicity nature. However, the combination of the measured line fluxes and photometry indicates that its spectral energy distribution requires an extraordinarily strong Balmer jump ($-1.66 \pm 0.47$ mag) and H$α$ equivalent width ($3750\pm1800$ Å), features that cannot be reproduced by current stellar+nebular or pure nebular photoionization models. The only models approaching the observations to almost within $1σ$ involve a hot ($T_{\rm eff}\!\simeq\!10^{4.7}$ K) single blackbody embedded in a low-$T_{\rm e}$ nebular environment, suggestive of scenarios such as a tidal-disruption event or a microquasar with strong disk winds. This cautions that photometric Pop~III selections are vulnerable to contamination when the rest-frame optical continuum is undetected. Motivated by this, we refine the photometric Pop III selection criteria to exclude the locus of extreme Balmer-jump objects. The revised criteria also recover the recently reported spectroscopic candidate AMORE6, demonstrating that the updated selection preserves sensitivity to genuine Pop III-like sources while removing key contaminants. Applying the refined criteria across legacy survey fields and five newly released CANUCS lensing cluster fields, we revisit the Pop III UV luminosity function and estimate the Pop III cosmic star-formation rate density to be $\approx[10^{-6}$--$10^{-4}]$~$M_{\odot}$~yr$^{-1}$~cMpc$^{-3}$ at $z\simeq6$--7, falling in the range of current theoretical predictions.

astro-ph.GA↗

arXiv2025

Lyman-$α$ Visibility During the Epoch of Reionization: Combining JWST FRESCO Grism Data with Keck Archival Spectroscopy

The visibility of Lyman-$α$ emission at $z>7$ provides crucial insights into the reionization process and the role of galaxies in shaping the ionized intergalactic medium. Using JWST FRESCO data, we investigate the environments of Lyman-$α$ emitters (LAEs) in the GOODS-N and GOODS-S fields by identifying [OIII] emitters and analyzing their large-scale distribution. Using the FRESCO redshifts, we recover eight new LAEs from archival Keck/MOSFIRE observations at $z=7.0-7.7$, including a potential AGN candidate at $z \sim 7.2$. Complemented by six literature LAEs, our sample consists of 14 LAEs in total, all of which are [OIII] emitters except for one very faint source not detected by FRESCO. We define seven groups of [OIII] emitters centered around the brightest LAEs and find that these bright LAEs do not reside in more overdense environments than the average galaxy population. The overdensity parameters for LAEs and [OIII] emitters without Lyman-$α$, calculated for sources with $\mathrm{M_{UV}<-19.5}$ to ensure completeness, are similar, indicating that overdensities alone cannot fully explain Lyman-$α$ visibility. While LAEs have slightly higher recent star formation (SFR$_{10}$/SFR$_{50} \approx 1.3\times$) and [OIII] EW ($\approx1.5\times$), they show no significant differences from [OIII] emitters in UV slope ($β$), UV magnitude ($\mathrm{M_{UV}}$), or stellar mass ($\log_{\mathrm{M}_{\star}}$). Our results suggest that other factors may contribute to the observability of Lyman-$α$ emission. Future spectroscopic surveys with broader wavelength coverage and more complete sampling will be crucial for refining our understanding of reionization.

astro-ph.GA↗

arXiv2025

Little red dot variability over a century reveals black hole envelope via a giant Einstein cross

"Little red dots" (LRDs) represent a new population of astronomical objects uncovered by JWST whose nature remains debated. Although many LRDs are suspected as active galactic nuclei (AGN), they show little variability on days-years timescales. We report the discovery of two gravitationally lensed LRDs at redshift $\sim$4.3 behind the cluster RXCJ2211-0350, one of which (RX1) is quadruply imaged with time delays spanning $\sim$130 years. RX1 exhibits intrinsic color and brightness variations of up to 0.7 magnitude among its images. These changes are consistent with blackbody-temperature variations of a photosphere, indicating long-term variability analogous to Cepheid-like pulsations but in a far more extended ($R \sim 2000$ AU) and massive ($M \gtrsim 10^6 \, M_{\odot}$) systems. These results suggest LRDs as a distinct class of AGN with stellar-like envelopes.

astro-ph.GA↗

arXiv2025

The Deepest GLIMPSE of a Dense Gas Cocoon Enshrouding a Little Red Dot

The detection of strong Balmer breaks and absorption features in Little Red Dots (LRDs) suggests they host AGN embedded within dense gas envelopes, potentially powered by super-Eddington accretion. We present GLIMPSE-17775, a luminous ($L_{\rm bol}\sim10^{45}$ erg s$^{-1}$) LRD at $z=3.501$ behind Abell S1063 ($μ\sim2$), observed with deep JWST/NIRCam and a $\sim$20 hr (80 hr de-lensed) NIRSpec/G395M spectrum. The data reveal 40+ emission and absorption features, including a rich forest of low-ionization FeII lines and numerous broad hydrogen recombination transitions. We use this depth to test the dense-gas interpretation through five independent diagnostics. Nearly all permitted lines show exponential wings with consistent FWHM, the signature of Thomson scattering requiring $n_e\gtrsim10^8$ cm$^{-3}$. Adopting this width yields $M_{\rm BH}\sim10^{6.7}M_\odot$, a factor of ten lower than Gaussian fits, and $λ_{\rm Edd}\sim1.8$. Additional diagnostics support the same picture: a pronounced Balmer break ($f_{ν,4050}/f_{ν,3670}=2.0\pm0.1$), enhanced HeI $\lambda7065$ and $\lambda10830$ with P-Cygni absorption, Bowen-fluorescent OI $\lambda8446$-$\lambda11290$ emission requiring Ly$β$ pumping, and 16 FeII lines matching fluorescence models. These features indicate a dense ($n\sim10^8$ cm$^{-3}$), partially ionized cocoon where scattering and fluorescence dominate line formation, providing strong evidence that at least some LRDs are powered by super-Eddington black-hole growth in the early Universe.

astro-ph.GA↗

arXiv2025

Radio Astronomy in the Era of Vision-Language Models: Prompt Sensitivity and Adaptation

Vision-Language Models (VLMs), such as recent Qwen and Gemini models, are positioned as general-purpose AI systems capable of reasoning across domains. Yet their capabilities in scientific imaging, especially on unfamiliar and potentially previously unseen data distributions, remain poorly understood. In this work, we assess whether generic VLMs, presumed to lack exposure to astronomical corpora, can perform morphology-based classification of radio galaxies using the MiraBest FR-I/FR-II dataset. We explore prompting strategies using natural language and schematic diagrams, and, to the best of our knowledge, we are the first to introduce visual in-context examples within prompts in astronomy. Additionally, we evaluate lightweight supervised adaptation via LoRA fine-tuning. Our findings reveal three trends: (i) even prompt-based approaches can achieve good performance, suggesting that VLMs encode useful priors for unfamiliar scientific domains; (ii) however, outputs are highly unstable, i.e. varying sharply with superficial prompt changes such as layout, ordering, or decoding temperature, even when semantic content is held constant; and (iii) with just 15M trainable parameters and no astronomy-specific pretraining, fine-tuned Qwen-VL achieves near state-of-the-art performance (3% Error rate), rivaling domain-specific models. These results suggest that the apparent "reasoning" of VLMs often reflects prompt sensitivity rather than genuine inference, raising caution for their use in scientific domains. At the same time, with minimal adaptation, generic VLMs can rival specialized models, offering a promising but fragile tool for scientific discovery.

astro-ph.IM↗