@article{21384, abstract = {Cell migration in vivo is often guided by chemical signaling, i.e., chemotaxis. For immune cells performing chemotaxis in the organism, this process is influenced by the complex geometry of the tissue environment. In this study, we use a theoretical model of branched cell migration on a network to explore the cellular response to chemical gradients. The model predicts the response of a branched cell to a chemical gradient: how the cell reorients its internal polarity and how it navigates through a complex environment up a chemical gradient. We then compare the model’s predictions with experimental observations of neutrophils migrating to the site of a laser-inflicted wound in a zebrafish larva fin, and neutrophils migrating in vitro inside a regular lattice of pillars. We find that the model captures the details of the subcellular response to the chemokine gradient, as well as qualitative characteristics of the large-scale migration, suggesting that the neutrophils behave as fast cells, which explains the functionality of these immune cells.}, author = {Liu, Jiayi and Ron, Jonathan E. and Rinaldi, Giulia and Williantarra, Ivanna and Georgantzoglou, Antonios and de Vries, Ingrid and Sixt, Michael K and Sarris, Milka and Gov, Nir S.}, issn = {1553-7358}, journal = {PLOS Computational Biology}, number = {2}, publisher = {Public Library of Science}, title = {{Modelling chemotaxis of branched cells in complex environments provides insights into immune cell navigation}}, doi = {10.1371/journal.pcbi.1013934}, volume = {22}, year = {2026}, } @article{21385, abstract = {We prove that the average size of a mixed character sum (math. formular) (for a suitable smooth function w) is on the order of √x for all irrational real θ satisfying a weak Diophantine condition, where χ is drawn from the family of Dirichlet characters modulo a large prime r and where x 6 r. In contrast, it was proved by Harper that the average size is o(√x) for rational θ. Certain quadratic Diophantine equations play a key role in the present paper. }, author = {Wang, Victor and Xu, Max}, issn = {1473-7124}, journal = {Proceedings of the Royal Society of Edinburgh: Section A Mathematics}, pages = {1--15}, publisher = {Cambridge University Press}, title = {{Average sizes of mixed character sums}}, doi = {10.1017/prm.2026.10123}, year = {2026}, } @phdthesis{21393, abstract = {This thesis documents a voyage towards truth and beauty via formal verification of theorems. To this end, we develop libraries in Lean 4 that present definitions and results from diverse areas of MathematiCS (i.e., Mathematics and Computer Science). The aim is to create code that is understandable, believable, useful, and elegant. The code should stand for itself as much as possible without a need for documentation; however, this text redundantly documents our code artifacts and provides additional context that isn’t present in the code. This thesis is written for readers who know Lean 4 but are not familiar with any of the topics presented. We manifest truth and beauty in three formalized areas of MathematiCS. We formalize general grammars in Lean 4 and use grammars to show closure of the class of type-0 languages under four operations; union, reversal, concatenation, and the Kleene star. Our second stop is the theory of optimization. Farkas established that a system of linear inequalities has a solution if and only if we cannot obtain a contradiction by taking a linear combination of the inequalities. We state and formally prove several Farkas-like theorems over linearly ordered fields in Lean 4. Furthermore, we extend duality theory to the case when some coefficients are allowed to take “infinite values”. Additionally, we develop the basics of the theory of optimization in terms of the framework called General-Valued Constraint Satisfaction Problems, and we prove that, if a Rational-Valued Constraint Satisfaction Problem template has symmetric fractional polymorphisms of all arities, then its basic LP relaxation is tight. Our third stop is matroid theory. Seymour’s decomposition theorem is a hallmark result in matroid theory, presenting a structural characterization of the class of regular matroids. We aim to formally verify Seymour’s theorem in Lean 4. First, we build a library for working with totally unimodular matrices. We define binary matroids and their standard representations, and we prove that they form a matroid in the sense how Mathlib defines matroids. We define regular matroids to be matroids for which there exists a full representation rational matrix that is totally unimodular, and we prove that all regular matroids are binary. We define 1-sum, 2-sum, and 3 sum of binary matroids as specific ways to compose their standard representation matrices. We prove that the 1-sum, the 2-sum, and the 3-sum of regular matroids are a regular matroid, which concludes the composition direction of the Seymour’s theorem. The (more difficult) decomposition direction remains unproved. In the pursuit of truth, we focus on identifying the trusted code in each project and presenting it faithfully. We emphasize the readability and believability of definitions rather than choosing definitions that are easier to work with. In search for beauty, we focus on the philosophical framework of Roger Scruton, who emphasizes that beauty is not a mere decoration but, most importantly, beauty is the means for shaping our place in the world and a source of redemption, where it can be viewed as a substitute for religion.}, author = {Dvorak, Martin}, isbn = {978-3-99078-074-9}, issn = {2663-337X}, pages = {160}, publisher = {Institute of Science and Technology Austria}, title = {{Pursuit of truth and beauty in Lean 4 : Formally verified theory of grammars, optimization, matroids}}, doi = {10.15479/AT-ISTA-21393}, year = {2026}, } @unpublished{21400, abstract = {This document is a blueprint for the formalization in Lean of the structural theory of regular matroids underlying Seymour's decomposition theorem. We present a modular account of regularity via totally unimodular representations, show that regularity is preserved under 1-, 2-, and 3-sums, and establish regularity for several special classes of matroids, including graphic, cographic, and the matroid R10. The blueprint records the logical structure of the proof, the precise dependencies between results, and their correspondence with Lean declarations. It is intended both as a guide for the ongoing formalization effort and as a human-readable reference for the organization of the proof.}, author = {Sergeev, Ivan and Dvorak, Martin and Rampell, Cameron and Sandey, Mark and Monticone, Pietro}, booktitle = {arXiv}, pages = {18}, title = {{A blueprint for the formalization of Seymour's matroid decomposition theorem}}, doi = {10.48550/arXiv.2601.01255}, year = {2026}, } @phdthesis{21401, abstract = {Runtime verification offers scalable solutions to improve the safety and reliability of systems. However, systems that require verification or monitoring by a third party to ensure compliance with a specification might contain sensitive information, causing privacy concerns when usual runtime verification approaches are used. Privacy is compromised if protected information about the system, or sensitive data that is processed by the system, is revealed. In addition, revealing the specification being monitored may undermine the essence of third-party verification. In this thesis, we propose a protocol for privacy-preserving runtime verification of systems against formal sequential specifications. We develop the protocol in two steps. In the first step, the monitor verifies whether the system satisfies the specification without learning anything else, though both parties are aware of the specification. In the second step, we extend the protocol to ensure that the system remains oblivious to the monitored specification, while the monitor learns only whether the system satisfies the specification and nothing more. Our protocol adapts and improves existing techniques used in cryptography, and more specifically, multi-party computation. The sequential specification defines the observation step of the monitor, whose granularity depends on the situation (e.g., banks may be monitored on a daily basis). Our protocol exchanges a single message per observation step, after an initialization phase. This design minimizes communication overhead, enabling relatively lightweight privacy-preserving monitoring. We implement our approach for monitoring specifications described by register automata and evaluate it experimentally. }, author = {Karimi, Mahyar}, issn = {2791-4585}, keywords = {Privacy-preserving verification, Runtime verification, Monitoring, Reactive functionalities, Cryptographic protocols}, pages = {60}, publisher = {Institute of Science and Technology Austria}, title = {{Privacy-preserving runtime verification}}, doi = {10.15479/AT-ISTA-21401}, year = {2026}, } @article{21406, abstract = {This preliminary study investigates the trace-element composition of ostracod shells (Ostracoda: Crustacea) as biogenic calcium carbonates in their role as environmental sentinels of pollution. Using high-resolution in-situ analysis, we compared two contrasting coastal systems: the highly urbanized seascape of metropolitan megacity Hong Kong (HKSAR) and the agriculturally dominated waters of rural retreat Jeju Island, Republic of Korea (ROK). The goal was to assess whether anthropogenic stress gradients affect trace element-to‑calcium ratios (E/Ca) in the carapaces of shallow-marine Neonesidea Maddocks, 1969 species. Hereby, the focus is laid on potential differences in the effects of extreme urbanization and extreme agriculturalization. We analyzed 12 trace elements commonly incorporated into ostracod shells using Inductively Coupled Plasma–Mass Spectrometry (ICP-MS). Only Mn/Ca, Mg/Ca, and Ni/Ca ratios showed strong correlations with specific seawater physicochemical parameters. Notably, Mn/Ca differed significantly between the two sites, seemingly driven mainly by variations in nitrite nitrogen levels. This suggests that Mn incorporation is sensitive to pollution source, urban versus agricultural, though species-specific uptake effects cannot be excluded. No significant differences in elemental uptake were found between adult and A-1 juvenile stages of Neonesidea mutsuensis Ishizaki, 1961 or Neonesidea elegans (Brady, 1969), supporting the use of both age groups in environmental reconstructions and increasing potential sample yields. While remaining empirical and exploratory, our tentative findings suggest that ostracod geochemistry holds promise for marine pollution monitoring and cautiously supports the application of ostracod Mn/Ca ratios to reconstruct anthropogenic, particularly nitrogen-related, impacts in nearshore environments using sediment core records.}, author = {Jöst, Anna B. and Rodriguez Moreno, Maximiliano J and Kim, Taihun and Baker, David M. and Yasuhara, Moriaki and Not, Christelle A. and Karanovic, Ivana}, issn = {1879-3363}, journal = {Marine Pollution Bulletin}, number = {6}, publisher = {Elsevier}, title = {{Ostracod shell chemistry as proxy for coastal marine conditions of a highly urbanized megacity (Hong Kong SAR) and an agro-centric oceanic province (Jeju Island, Republic of Korea) – a preliminary comparative analysis}}, doi = {10.1016/j.marpolbul.2026.119493}, volume = {227}, year = {2026}, } @article{21407, abstract = {This note proves that only a linear number of holes in a Cech complex of n points in R^d can persist over an interval of constant length. Specifically, for any fixed dimension p < d and fixed ε > 0, the number of p-dimensional holes in the ˇ Cech complex at radius 1 that persist to radius 1+ε is bounded above by a constant times n,where n is the number of points. The proof uses a packing argument supported by relating theCˇ ech complexes with corresponding snap complexes over the cells in a partition of space. The argument is self-contained and elementary, relying on geometric and combinatorial constructions rather than on the existing theory of sparse approximations or interleavings. The bound also applies to Alpha complexes and Vietoris–Rips complexes. While our result can be inferred from prior work on sparse filtrations, to our knowledge, no explicit statement or direct proof of this bound appears in the literature.}, author = {Edelsbrunner, Herbert and Kahle, Matthew and Kanazawa, Shu}, issn = {2367-1734}, journal = {Journal of Applied and Computational Topology}, publisher = {Springer Nature}, title = {{Maximum persistent Betti numbers of Čech complexes}}, doi = {10.1007/s41468-026-00233-3}, volume = {10}, year = {2026}, } @article{21408, abstract = {Rational design strategies for self-assembly require a detailed understanding of both the equilibrium state and the assembly kinetics. While the former is starting to be well understood, the latter remains a major theoretical challenge, especially in programmable systems and the so-called semi-addressable regime, where binding is often nondeterministic and the formation of off-target structures negatively influences the assembly. Here, we show that it is possible to simultaneously sculpt the assembly outcome and the assembly kinetics through the underexplored design space of binding energies and particle concentrations. By formulating the assembly process as a complex reaction network, we calculate and optimize the tradeoff between assembly speed and quality and show that parameter optimization can speed up assembly by many orders of magnitude without lowering the yield of the target structure. Although the exact speedup varies from design to design, we find the largest speedups for nondeterministic systems where unoptimized assembly is the slowest, sometimes even making them assemble faster than optimized, fully addressable designs. Therefore, these results not only solve a key challenge in semi-addressable self-assembly but further emphasize the utility of semi-addressability, where designs have the potential to be faster as well as cheaper (fewer particle species) and better (higher yield). More broadly, our results highlight the importance of parameter optimization in programmable self-assembly and provide practical tools for simultaneous optimization of kinetics and yield in a wide range of systems.}, author = {Hübl, Maximilian and Goodrich, Carl Peter}, issn = {1089-7690}, journal = {Journal of Chemical Physics}, number = {8}, publisher = {AIP Publishing}, title = {{Simultaneous optimization of assembly time and yield in programmable self-assembly}}, doi = {10.1063/5.0304731}, volume = {164}, year = {2026}, } @article{21409, abstract = {Meiotic drivers are selfish genetic elements that gain transmission advantages by distorting equal, Mendelian segregation. For decades, biologists have considered meiotic drivers as interesting, albeit esoteric, case studies. It is now clear, however, that meiotic drive is more common and phylogenetically widespread than previously supposed. Indeed, intensive study of a few well-known cases has begun to reveal the evolutionary genomic consequences of meiotic drive. We argue here that many features of genome evolution, content, and organization that are seemingly inexplicable by organismal adaptation or nearly neutral processes are instead best accounted for by recurrent histories of meiotic drive. We review how meiotic drive can affect the evolution of sequences, gene copy numbers, genes with functions in meiosis and gametogenesis, signatures of “selection,” chromosome rearrangements, and karyotype evolution. We also explore the interactions of meiotic drive elements with other classes of selfish genetic elements, including satellite DNAs, transposable elements, and with the endogenous host genes involved in drive suppression. Finally, we argue that some aspects of drive-mediated genome evolution are now sufficiently well established that we might reverse the direction of discovery—rather than ask how drive affects genome evolution, we can use genome data to discover new putative drive elements.}, author = {Presgraves, Daven C. and Dawe, R. Kelly and Dyer, Kelly A. and Fishman, Lila and Bhide, Soumitra A. and Bradshaw, Sasha L. and Brady, Meghan J. and Burga, Alejandro and Courret, Cécile and Fagen, Brandon L. and Machado Ferretti, Ana Beatriz Stein and Kelemen, Réka K and Kitano, Jun and Liu, Yiran and Martí, Emiliano and Erlenbach, Theresa and Reinhardt, Josephine A. and Ross, Laura and Runge, Jan Niklas and Swanepoel, Callie M. and Vicoso, Beatriz and Vogan, Aaron A. and Lindholm, Anna K. and Larracuente, Amanda M. and Unckless, Robert L.}, issn = {1537-1719}, journal = {Molecular Biology and Evolution}, number = {2}, publisher = {Oxford University Press}, title = {{The evolutionary genomics of meiotic drive}}, doi = {10.1093/molbev/msag020}, volume = {43}, year = {2026}, } @inproceedings{21410, abstract = {Given a finite set of red and blue points in R^d, the MST-ratio is defined as the total length of the Euclidean minimum spanning trees of the red points and the blue points, divided by the length of the Euclidean minimum spanning tree of their union. The MST-ratio has recently gained attention due to its direct interpretation in topological models for studying point sets with applications in spatial biology. The maximum MST-ratio of a point set is the maximum MST-ratio over all proper colorings of its points by red and blue. We prove that finding the maximum MST-ratio of a given point set is NP-hard when the dimension is part of the input. Moreover, we present a quadratic-time 3-approximation algorithm for this problem. As part of the proof, we show that in any metric space, the maximum MST-ratio is smaller than 3. Furthermore, we study the average MST-ratio over all colorings of a set of n points. We show that this average is always at least n-2/n-1, and for n random points uniformly distributed in a d-dimensional unit cube, the average tends to (math formular) in expectation as n approaches infinity.}, author = {Jabal Ameli, Afrouz and Motiei, Faezeh and Saghafian, Morteza}, booktitle = {20th International Conference and Workshops on Algorithms and Computation}, isbn = {9789819571260}, issn = {1611-3349}, location = {Perugia, Italy}, pages = {386--401}, publisher = {Springer Nature}, title = {{On the MST-ratio: Theoretical bounds and complexity of finding the maximum}}, doi = {10.1007/978-981-95-7127-7_26}, volume = {16444}, year = {2026}, } @inproceedings{21411, abstract = {To achieve fast recovery from link failures, most modern communication networks feature fully decentralized fast re-routing mechanisms. These re-routing mechanisms rely on pre-installed static re-routing rules at the nodes (the routers), which depend only on local failure information, namely on the failed links incident to the node. Ideally, a network is perfectly resilient: the re-routing rules ensure that packets are always successfully routed to their destinations as long as the source and the destination are still physically connected in the underlying network after the failures. Unfortunately, there are examples where achieving perfect resilience is not possible. Surprisingly, only very little is known about the algorithmic aspect of when and how perfect resilience can be achieved. We investigate the computational complexity of analyzing such local fast re-routing mechanisms. Our main result is a negative one: we show that even checking whether a given set of static re-routing rules ensures perfect resilience is coNP-complete. Additionally, we investigate other fundamental variations of the problem. In particular, we show that our coNP-completeness proof also applies to scenarios where the re-routing rules have specific patterns (known as skipping in the literature). On the positive side, for scenarios where nodes do not have information about the link from which a packet arrived (the so-called in-port), we present a linear-time algorithm to realize perfect resilience whenever possible (which we show can also be determined in linear time). }, author = {Bentert, Matthias and Ceylan, Esra and Hübner, Valentin and Schmid, Stefan and Srba, Jiří}, booktitle = {29th International Conference on Principles of Distributed Systems}, isbn = {9783959774093}, issn = {1868-8969}, location = {Iaşi, Romania}, publisher = {Schloss Dagstuhl - Leibniz-Zentrum für Informatik}, title = {{Fast re-routing in networks: On the complexity of perfect resilience}}, doi = {10.4230/LIPIcs.OPODIS.2025.31}, volume = {361}, year = {2026}, } @misc{21422, author = {Sunko, Veronika}, publisher = {Institute of Science and Technology Austria}, title = {{Data underpinning "Magneto-optical Kerr effect in an A-type antiferromagnet"}}, doi = {10.15479/AT-ISTA-21422}, year = {2026}, } @phdthesis{21423, author = {Dunajova, Zuzana}, isbn = {978-3-99078-076-3}, issn = {2663-337X}, pages = {110}, publisher = {Institute of Science and Technology Austria}, title = {{Geometry-driven self-organization of migrating cells and chiral filaments}}, doi = {10.15479/AT-ISTA-21423}, year = {2026}, } @article{21436, abstract = {The cobalt-intercalated transition metal dichalcogenide CoxTaS2 hosts a rich landscape of magnetic phases that depend sensitively on x. While the stoichiometric compound with x = 1/3 exhibits a single magnetic transition, samples with x≤0.325 display two transitions with an anomalous Hall effect (AHE) emerging in the lower temperature phase. Here, we resolve the spin structure in each phase by employing a suite of magneto-optical probes that include the discovery of anomalous magneto-birefringence: a spontaneous time-reversal sensitive rotation of the principal optic axes. A symmetry-based analysis identifies the AHE-active phase as an anisotropic (2+1)Q state, in which magnetic modulation at one wavevector (Q) differs in symmetry from that at the remaining two. The (2+1)Q state naturally exhibits scalar spin chirality as a mechanism for the AHE and expands the classification of multi-Q magnetic phases.}, author = {Kruppe, Jonathon and Rodriguez, Josue and Xu, Catherine and Analytis, James and Orenstein, Joseph and Sunko, Veronika}, issn = {2397-4648}, journal = {npj Quantum Materials}, publisher = {Springer Nature}, title = {{Anisotropic multi-Q order in CoxTaS2}}, doi = {10.1038/s41535-026-00856-w}, year = {2026}, } @unpublished{21438, abstract = {Antiferromagnets (AFMs) hold promise for applications in digital logic. However, switching AFM domains is challenging, as magnetic fields do not couple to the bulk antiferromagnetic order parameter. Here we show that magnetic-field-driven switching of AFM domains can in many cases be enabled by a generic reduction of magnetic exchange at surfaces. We use statistical mechanics and Monte Carlo simulations to demonstrate that an inequivalence in magnetic exchange between top and bottom surface moments, combined with the enhanced magnetic susceptibility of surface spins, can enable deterministic selection of antiferromagnetic domains depending on the magnetic-field ramping direction. We further show that this mechanism provides a natural interpretation for experimental observations of hysteresis in magneto-optical response of the van der Waals AFM $\mathrm{MnBi_2Te_4}$. Our findings highlight the critical role of surface spins in responses of antiferromagnets to magnetic fields. Furthermore, our results suggest that antiferromagnetic domain selection via purely magnetic means may be a more common and experimentally accessible phenomenon than previously assumed.}, author = {Weber, Sophie F. and Sunko, Veronika}, booktitle = {arXiv}, title = {{Deterministic domain selection of antiferromagnets via magnetic fields}}, doi = {10.48550/arXiv.2601.06646}, year = {2026}, } @misc{21439, abstract = {These files contain supplementary movies accompanying the PhD thesis “Geometry-driven self-organization of migrating cells and chiral filaments” by Zuzana Dunajova (2026). The videos provide additional visual material supporting the experiments and results described in the thesis.}, author = {Dunajova, Zuzana}, publisher = {Institute of Science and Technology Austria}, title = {{Supplementary movies to PhD thesis “Geometry-driven self-organization of migrating cells and chiral filaments”}}, doi = {10.15479/AT-ISTA-21439}, year = {2026}, } @misc{21442, author = {Schlögl, Alois}, keywords = {hypocampus, ca3 simulations, modelling}, publisher = {Institute of Science and Technology Austria}, title = {{CA3Simu v1.06 (vargas2026v1)}}, doi = {10.15479/AT-ISTA-21442}, year = {2026}, } @article{21449, abstract = {Three-dimensional (3D) crystals offer a route to scaling up trapped-ion systems for quantum sensing and quantum simulation applications; however, engineering coherent spin-motion couplings and effective spin-spin interactions in large crystals poses technical challenges associated with decoherence and prolonged timescales to generate appreciable entanglement. Here, we explore the possibility of speeding up these interactions in 3D crystals via parametric amplification. For this purpose, we derive a general Hamiltonian for the parametric amplification of spin-motion coupling that is broadly applicable to normal modes with motion transverse to or along the spatial extent of the crystal. Unlike in lower-dimensional crystals, we find that the ability to faithfully (uniformly) amplify the spin-spin interactions in 3D crystals depends on the physical implementation of the spin-motion coupling. We consider the light-shift gate, and the so-called phase-insensitive and phase-sensitive Mølmer-Sørensen (MS) gates, and we find that only the phase-sensitive MS gate can be faithfully amplified in general 3D crystals. We discuss a situation where nonuniform amplification can be advantageous. We also reconsider the effect of counter-rotating terms on parametric amplification and find that they are not as detrimental as previous studies suggest.}, author = {Hawaldar, Samarth and Nikhil, N. and Rey, Ana Maria and Bollinger, John J. and Shankar, Athreya}, issn = {2331-7019}, journal = {Physical Review Applied}, number = {3}, publisher = {American Physical Society}, title = {{Parametric amplification of spin-motion coupling in three-dimensional trapped-ion crystals}}, doi = {10.1103/h1m9-h3yw}, volume = {25}, year = {2026}, } @article{21450, abstract = {Stellar wind mass loss of massive stars is often assumed to depend on their metallicity Z. Therefore, evolutionary models predict that massive stars in lower-Z environments are able to retain more of their hydrogen-rich layers and evolve into brighter cool supergiants (cool SGs; Teff < 7 kK). Surprisingly, in galaxies in the metallicity range 0.2 ≲ Z/Z⊙ ≲ 1.5, previous studies have not found a metallicity dependence on the upper luminosity limit Lmax of cool SGs. Here, we add four galaxies to the sample studied for this purpose with data from the Hubble Space Telescope and the James Webb Space Telescope (JWST). Observations of the extremely metal-poor dwarf galaxy I Zw 18 from JWST allow us to extend the studied metallicity range down to Z/Z⊙ ≈ 1/40. For cool SGs in all studied galaxies, including I Zw 18, we find a constant value of Lmax ≈ 105.6 L⊙, similar to literature results for 0.2 ≲ Z/Z⊙ ≲ 1.5. In I Zw 18 and the other studied galaxies, the presence of Wolf-Rayet stars has been previously inferred. Although we cannot rule out that some of them become intermediate-temperature objects, this paints a picture in which evolved stars with L > 105.6 L⊙ burn helium as hot, helium-rich stars down to extremely low metallicity. We argue that metallicity-independent late-phase mass loss would be the most likely mechanism responsible for this. Regardless of the exact stripping mechanism (winds or, for example, binary interaction), for the Early Universe our results imply a limitation on black hole masses and a contribution of stars born with M ≳ 30 M⊙ to its surprisingly strong nitrogen enrichment. We propose a scenario in which single stars at low metallicity emit sufficiently hard ionizing radiation to produce He II and C IV lines. In this scenario, late-phase metallicity-independent mass loss produces hot, helium-rich stars. Due to the well-understood metallicity dependence on the radiation-driven winds of hot stars, a window of opportunity would open below 0.2 Z⊙, where self-stripped helium-rich stars can exist without dense Wolf-Rayet winds that absorb hard ionizing radiation.}, author = {Schootemeijer, Abel and Götberg, Ylva Louise Linsdotter and Langer, Norbert and Bortolini, Giacomo and Hirschauer, Alec S. and Patrick, Lee}, issn = {1432-0746}, journal = {Astronomy & Astrophysics}, publisher = {EDP Sciences}, title = {{A constant upper luminosity limit of cool supergiant stars down to the extremely low metallicity of I Zw 18}}, doi = {10.1051/0004-6361/202557675}, volume = {707}, year = {2026}, } @article{21451, abstract = {The population of the little red dots (LRDs) may represent a key phase of supermassive black hole (SMBH) growth. A cocoon of dense excited gas is emerging as a key component to explain the most striking properties of LRDs, such as strong Balmer breaks and Balmer absorption, as well as the weak IR emission. To dissect the structure of LRDs, we analyzed new deep JWST/NIRSpec PRISM and G395H spectra of FRESCO-GN-9771, one of the most luminous known LRDs at z = 5.5. These spectra reveal a strong Balmer break, broad Balmer lines, and very narrow [O III] emission. We revealed a forest of optical [Fe II] lines, which we argue are emerging from a dense (nH = 109 − 10 cm−3) warm layer with electron temperature Te ≈ 7000 K. The broad wings of Hα and Hβ have an exponential profile due to electron scattering in this same layer. The high Hα : Hβ : Hγ flux ratio of ≈10.4 : 1 : 0.14 is an indicator of collisional excitation and resonant scattering dominating the Balmer line emission. A narrow Hγ component, unseen in the other two Balmer lines due to outshining by the broad components, could trace the ISM of a normal host galaxy with a star formation rate of ∼5 M⊙ yr−1. The warm layer is mostly opaque to Balmer transitions, producing a characteristic P Cygni profile in the line centers suggesting outflowing motions. This same layer is responsible for shaping the Balmer break. The broadband spectrum can be reasonably matched by a simple photoionized slab model that dominates the λ > 1500 Å continuum and a low-mass (∼108 M⊙) galaxy that could explain the narrow [O III], with only a subdominant contribution to the UV continuum. Our findings indicate that Balmer lines are not directly tracing the gas kinematics near the SMBH and that the BH mass scale is likely much lower than virial indicators suggest.}, author = {Torralba Torregrosa, Alberto and Matthee, Jorryt J and Pezzulli, Gabriele and Naidu, Rohan P. and Ishikawa, Yuzo and Brammer, Gabriel B. and Chang, Seok Jun and Chisholm, John and De Graaff, Anna and D’Eugenio, Francesco and Di Cesare, Claudia and Eilers, Anna Christina and Greene, Jenny E. and Gronke, Max and Iani, Edoardo and Kokorev, Vasily and Kotiwale, Gauri and Kramarenko, Ivan and Ma, Yilun and Mascia, Sara and Navarrete, Benjamín and Nelson, Erica and Oesch, Pascal and Simcoe, Robert A. and Wuyts, Stijn}, issn = {1432-0746}, journal = {Astronomy & Astrophysics}, publisher = {EDP Sciences}, title = {{The warm outer layer of a little red dot as the source of [Fe ii] and collisional Balmer lines with scattering wings}}, doi = {10.1051/0004-6361/202557537}, volume = {707}, year = {2026}, }