Weekly issue

Week 13, 2026

Mar 23–29, 2026

Week 13, 2026 includes 6 curated papers, centered on LRD, high-z, spectroscopy.

2603.26872v1

A cosmological framework for stellar collisions at high redshift in proto-globular clusters, nuclear star clusters, and Little Red Dots

Claire E. Williams, Smadar Naoz, Sanaea C. Rose, Blakesley Burkhart, Naoki Yoshida, Avi Chen, Kyle Kremer, William Lake, Federico Marinacci, Shyam H. Menon, Mark Vogelsberger

Theme match 5/5

Digest

Bottom-up, radially resolved analytic framework for stellar dynamics in compact high-redshift systems (proto-globulars, nuclear clusters, and LRD-like nuclei), initialized from high-resolution cosmological simulations and validated against Monte Carlo predictions. Across wide cluster/BH parameter space, main-sequence stellar collisions are ubiquitous and naturally drive runaway growth into very massive stars. In systems with central black holes, inner radii are collision-destructive, rapidly converting stellar mass into dense gas that can feed the BH and yield LRD-like compact environments. The framework ties cluster structure to observable outcomes, outlining a cosmological route from dense clusters to early massive seeds.

Open paper page arXiv abstract arXiv PDF Highlighted PDF

Key figures to inspect

Figure 1 — Use the schematic to locate where constructive vs destructive collisions operate for clusters with and without a central BH, and to trace how merger products migrate inward to assemble a VMS versus how inner destructive zones around BHs build a dense gas reservoir.
Figure 2 — Read the flowchart to understand the model’s decision branches and which outcomes are realized for the explored parameters (grey branches absent; yellow branches only with inner destructive collisions), clarifying when systems evolve toward VMS growth versus BH-fed gas buildup.
Figure 3 — Examine the radial collision counts and timescale crossings (t_coll vs t_MS, t_merge, t_relax) with/without a BH to identify the radii enabling runaway sequences and how a central BH steepens inner destructive-collision rates within ~pc scales.
Figure 4 — Compare predicted maximum VMS mass versus half-mass density to prior fits/simulations, then inspect the implied BH–stellar mass relation (assuming full VMS→BH conversion) against dwarf AGN, JWST high‑z AGN, the Milky Way, and IMBH candidates to gauge seeding efficiency in dense clusters.

Tags

2603.24700v1

A PANORAMIC of UV-optical morphologies of "Little Red Dots": Two groups of LRDs distinguished by UV half-light radius

Aidan P. Cloonan, Katherine E. Whitaker, Sinclaire M. Manning, Christina C. Williams, Jenny E. Greene, Pascal A. Oesch, Andrea Weibel, Gabriel Brammer, Anna de Graaff, Raphael E. Hviding, Pratika Dayal, Christian Kragh Jespersen, Zhiyuan Ji, Ivo Labbe, Mengyuan Xiao, Yunchong Zhang

Theme match 4/5

Digest

Using PANORAMIC pure‑parallel NIRCam imaging, the authors assemble 181 photometric LRDs and map morphology from rest‑UV to optical with single‑Sérsic and joint PSF+Sérsic fits. They find a sharp transition at the Balmer break: rest‑optical light is highly compact/unresolved (R50,opt ≲ 100 pc) while rest‑UV is typically more extended; stacking confirms red‑band profiles track the PSF whereas blue‑band profiles are diffuse. Splitting at the break reveals two groups—objects unresolved there remain compact across the spectrum, while those resolved show extended UV emission (R50,UV > 200 pc)—a dichotomy also seen in a spectroscopic subsample. The patterns favor UV emission from starlight plus a dense, dust‑poor H envelope around an AGN and point to a spread in seed black‑hole masses, with heavier seeds yielding smaller UV half‑light radii.

Open paper page arXiv abstract arXiv PDF Highlighted PDF

Key figures to inspect

Figure 1: Inspect the color–magnitude and color–compactness cuts (plus brown‑dwarf pruning) to see how the 181 LRDs were isolated and how the ‘red1/red2’ color tracks bias the sample toward Balmer‑break systems at higher redshift.
Figure 2: Check the median SED’s ‘v‑shape’ and the location of the Balmer break across F115W–F444W; the small bumps attributed to H lines illustrate how line contamination ties into photometric‑z and the UV/optical split used for morphology.
Figure 3: Size vs. rest‑wavelength shows the collapse of R50 at the Balmer break—UV sizes scatter to extended values while optical sizes hug the resolution limit; the side histograms visualize the two groups separated by UV half‑light radius.
Figure 4: Stacked surface‑brightness profiles contrast blue (diffuse, shallower than PSF) and red (compact, PSF‑like) bands; use this to validate that the compactness in the optical is population‑wide, not fit‑by‑fit noise.

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2603.22277v1

Little Red and Blue Dots: simply stratified Broad Line Regions

J. Scholtz, F. D'Eugenio, R. Maiolino, M. Brazzini, H. Übler, X. Ji, M. Perna, F. Sun, G. Brocchi, S. Carniani, G. Cresci, L. R. Ivey, I. Juodžbalis, A. Marconi, G. Mazzolari, G. Risaliti, B. Trefoloni

Theme match 4/5

Digest

Using high-S/N, R≈1000–2700 Hα spectra of 32 type‑1 AGN spanning Little Red Dots, Little Blue Dots, and X‑ray sources, the authors compare broad-line profile families. Single Gaussians are ruled out, while exponential shapes are not uniquely preferred; Lorentzian or multi‑Gaussian fits match or outperform them for most objects, with no statistical preference in ~60% of cases and a tendency for LRDs to favor Lorentzians and LBDs to show exponentials. They demonstrate that exponential wings arise naturally from a stratified, virial BLR and that stacking multiple broad lines (across objects or internal components) generically produces an exponential profile. This removes the need for a dominant electron‑scattering cocoon and implies standard virial black‑hole mass estimates remain broadly valid, though scattering may contribute at some level.

Open paper page arXiv abstract arXiv PDF Highlighted PDF

Key figures to inspect

Exemplar Hα profile fits for a representative LRD and LBD, showing exponential vs Lorentzian vs multi‑Gaussian models with residuals—use this to see where each model wins (core vs wings) and why single Gaussians fail.
Bar or fractional breakdown of best‑fit profile types across the 32 objects, split by subtype (LRD, LBD, X‑ray)—inspect the higher exponential fraction in LBDs and the Lorentzian tendency in LRDs, and note the ~60% with no unique statistical preference.
Stacking experiment figure—compare individual-object Hα profiles to the stacked result to see how an exponential emerges even when no single source is exponential, illustrating why stacking can bias interpretation.
BLR stratification toy model or simulation—radial/velocity stratification of virial clouds and the resulting synthetic line profile; check that exponential wings appear without invoking electron scattering.
Impact on MBH—comparison of FWHM/line‑widths and inferred black‑hole masses under different profile assumptions vs an electron‑scattering interpretation, showing minimal change to virial MBH estimates.

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2603.21976v1

Observations of Early Black Holes Before and After JWST

Eduardo Banados

Theme match 4/5

Digest

Lecture notes from the 2025 Saas-Fee Course synthesize how our view of supermassive black holes in the first Gyr evolved from ground-based quasar finds to JWST-era infrared constraints. The chapter foregrounds a practical AGN–quasar luminosity taxonomy using M1450 and Lbol, stressing AGN as a time-variable phase and noting that JWST has uncovered AGN at redshifts beyond the most distant known quasars. It then surveys how JWST is reshaping the field—enabling black hole mass estimates, host-galaxy characterization, and environmental mapping—while connecting new results to the pre-JWST baseline. Scope note: the material reflects the state of the field as of January 2025 with targeted updates through early 2026.

Open paper page arXiv abstract arXiv PDF Highlighted PDF

Key figures to inspect

Fig. 2 — Practical luminosity boundaries: read off the M1450 and Lbol demarcations (with the Runnoe+2012 conversion) to see where objects transition from galaxies → AGN → faint/bright quasars and where nomenclature becomes ambiguous.
Fig. 1 — ChatGPT AGN–quasar comparison: use this pedagogical table to pinpoint why luminosity is the real separator and why the pre-JWST “distance” heuristic has been superseded by newer AGN at higher redshift.
Fig. 14 — Reionization-era quasar census (pre-JWST baseline): inspect the redshift–luminosity coverage and selection footprint that anchor the historical census used throughout the notes.
Fig. 33 — Updated quasar census: compare directly to Fig. 14 to visualize the JWST-era additions and how the redshift frontier and number counts have shifted; dataset link provided in comments.

Tags

2603.21714v1

A quasi-star is born: formation and evolution of accreting quasi-stars as a metallicity-independent pathway to Little Red Dots

J. Roman-Garza, D. Schaerer, C. Charbonnel, T. Fragos, E. Cenci, R. Marques-Chaves, P. Oesch, M. Xiao

Theme match 4/5

Digest

Using MESA, the authors follow rapidly accreting proto-stars (Ṁ=0.01–1 M_sun/yr; Z=0–0.01) into supermassive stars and then quasi-stars supported by a central black hole to explain the rest-frame optical output of Little Red Dots. For Ṁ≥0.1 M_sun/yr the envelopes stay near the Hayashi regime (Teff≈4–9 kK) until general-relativistic instability at M*≈3.5×10^4–6.6×10^4 M_sun (L≈10^9 L_sun), after which quasi-stars live 10^7–10^8 yr with evolution essentially independent of metallicity; surface CNO abundances should vary with brief high-N phases. Matching model luminosities to LRDs at z<4.5 (L_bol≈10^9.5–10^11.5 L_sun) implies quasi-star masses ≈10^{4.5}–10^{6.5} M_sun and progenitor accretion rates ≳0.1 M_sun/yr. This provides a metallicity-independent pathway to LRD optical emission and long duty cycles for early black-hole growth.

Open paper page arXiv abstract arXiv PDF Highlighted PDF

Key figures to inspect

Fig. 1 Kippenhahn diagram: read off when the general-relativistic instability triggers BH formation, how the convective envelope and nitrogen mass fraction evolve, and how BH and envelope masses grow for accreting vs non‑accreting quasi-stars—including the crash/end points.
Fig. 2 HR diagram: compare color-coded accretion-rate tracks against the de Graaff et al. (2025) LRD locus; verify the Teff≈4–9 kK Hayashi-like regime and the extrapolated luminosity maxima relative to observed LRD luminosities.
Accreting vs non-accreting QS comparison (dashed/orange vs solid tracks across Figs. 1–2): assess how continued external accretion shifts final QS/BH masses and where models terminate.
Appendix D summary table (with Appendix B extrapolations): extract lifetimes (10^7–10^8 yr) and final BH seed masses as functions of Ṁ and the BH accretion-efficiency parameter to gauge duty cycle and seed growth.

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2603.24893v1

Revisiting the Claim for a Direct-Collapse Black Hole in UHZ1 at $z=10.05$

Fan Zou, Elena Gallo, Zihao Zuo, Edmund Hodges-Kluck, Dieu D. Nguyen, Guido Roberts-Borsani, Piero Madau, Fabio Pacucci, Anil C. Seth, Tommaso Treu

Theme match 3/5

Digest

Reanalyzing the full 2.2 Ms Chandra/ACIS data set for the lensed z=10.054 galaxy UHZ1 and adding new JWST/MIRI imaging, the authors reassess the DCBH claim. The hard 2–7 keV signal reaches only 2.3–2.9σ depending on astrometric registration and does not strengthen with the extra 0.95 Ms, arguing against a persistent source. UHZ1 is undetected in all nine MIRI bands, with SED fits limiting any buried AGN to L_bol < 1.3×10^45 erg s^-1; independent JWST spectroscopy also finds no AGN features. The multiwavelength picture favors a low-mass, metal-poor, star-forming galaxy rather than a luminous obscured DCBH, underscoring the sensitivity of such claims to Chandra astrometry and cluster-background treatment.

Open paper page arXiv abstract arXiv PDF Highlighted PDF

Key figures to inspect

Figure 1: Inspect the Monte Carlo histograms of detection significance versus astrometric registration to see how rarely the Bogdán et al. (2024) 4.2–4.4σ value is recovered and how the preferred 2.3–2.9σ range emerges under plausible alignments.
Figure 2: Check the per-epoch and binned 2–7 keV net count rates; the flat trend and consistency with zero in the recent 0.95 Ms show the excess does not build with exposure, disfavoring a steady hard X-ray source.
Figure 3: Examine the SED with MIRI upper limits and the scaled Torus/Hot DOG templates; the mid-IR calorimetric constraint (set by F1500W/F1800W limits) caps L_bol well below the Compton-thick interpretation, aligning with a star-forming, metal-poor host.