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Physics Astrophysics

astro-ph Watch Astrophysics

Tuesday 31 March

Astrophysics of Galaxies

Distribution function-based modelling of discrete kinematic datasets, in application to the Milky Way nuclear star cluster

Eugene Vasiliev, Anja Feldmeier-Krause and Mattia C. Sormani

We present a method for constructing dynamical models of stellar systems described by distribution functions and constrained by discrete-kinematic data. We implement various improvements compared to earlier applications of this approach, demonstrating with several examples that it can deliver meaningful constraints on the mass distribution even in situations when the density profile of tracers and the selection function of the kinematic catalogue are unknown. We then apply this method to the Milky Way nuclear star cluster, using kinematic data (line-of-sight velocities and proper motions) for a few thousand stars within 10 pc from the central black hole, accounting for the contributions of the nuclear stellar disc and the Galactic bar. We measure the mass of the black hole to be 4x10^6 Msun with a 10% uncertainty, which agrees with the more precise value obtained by the GRAVITY instrument. The inferred stellar mass profile depends on the choice of kinematic data, but the total mass within 10 pc is well constrained in all models to be (2.0-2.3)x10^7 Msun. We make our models publicly available as part of the Agama software framework for galactic dynamics.
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Astrophysics of Galaxies

Constraining the Galactic bar and spiral pattern speeds with the Hyades tidal stream

Zi-yi Zhou, Long Wang, Tereza Jerabkova et al. (+1 more)

We present a suite of direct $N$-body simulations of the Hyades open cluster and its tidal stream in a Milky Way potential that includes a rotating bar and spiral arms. Using the high-resolution code PETAR and an AGAMA-based multi-component Galactic model, we vary the bar and spiral pattern speeds ($\Omega_b$, $\Omega_s$) on a discrete grid and quantify the resulting changes in stream orientation, length, and internal density structure. We compare the simulations to Gaia EDR3 using the convergent point (CP) and compact convergent point (CCP) methods, followed by an adaptive three-dimensional nearest-neighbor matching in Cartesian space $(x,y,z)$. The Gaia candidate members exhibit a pronounced longitudinal density peak at $Y_{\mathrm{rot}} \approx 0.1\,\mathrm{kpc}$ in a stream-aligned coordinate system. Models with $\Omega_s = 22.5\,\mathrm{km\,s^{-1}\,kpc^{-1}}$ and $\Omega_b \simeq 40$–$45\,\mathrm{km\,s^{-1}\,kpc^{-1}}$ best reproduce this feature, while faster-bar models fail to match the observed density structure. These models are consistent with recent constraints favoring a relatively slow Galactic bar, and they illustrate how nearby open-cluster streams can provide an independent, local constraint on non-axisymmetric Galactic dynamics.
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All authors Zi-yi Zhou, Long Wang, Tereza Jerabkova, Zhenghao He

Instrumentation and Methods for Astrophysics

An innovative alternative to traditional funding streams for extragalactic astronomy

Stephen M. Wilkins, Jack Turner, Connor Sant Fournier et al. (+2 more)

With traditional sources of funding for astronomical research under increasing pressure, it is timely to explore innovative alternative mechanisms. We therefore introduce GalaxyCoin, a novel cryptocurrency whose issuance, validation, and economic evolution are anchored to real astrophysical objects - galaxies. GalaxyCoin links digital scarcity to observational astronomy by using galaxy catalogues to parametrise token generation, distribution, and long-term supply growth, providing a transparent, immutable, and independently verifiable foundation for the currency. We present the conceptual design of GalaxyCoin, highlight its potential advantages over conventional cryptocurrencies, and examine its broader implications for sustainability, trust, and public engagement at the intersection of astronomy, data-driven science, and blockchain technology. A central feature of GalaxyCoin is that it directly incentivises the discovery and spectroscopic confirmation of galaxies, aligning financial reward with the production of high-quality astronomical data. In terms of monetary design, its supply elasticity lies between that of fiat currencies and fixed-supply cryptocurrencies, making it distinctive in both economic structure and scientific purpose.
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All authors Stephen M. Wilkins, Jack Turner, Connor Sant Fournier, Behnood Bandi, Aswin Vijayan

Instrumentation and Methods for Astrophysics · Cross-listed from gr-qc

Mexican Burrowing Toads as gravitational wave detectors

Frederic V. Hessman and Christian Jooss

It is generally assumed that gravitational waves are extremely difficult to detect. However, we show that the call of the Mexican Burrowing Toad has an amazing resemblance to cosmic gravitational wave signals due to the merging of neutron stars and/or black holes. It is known that toads exhibit magnetoreception - the ability to detect magnetic fields - and that magnetic fields thus subtly affect ion channel activities in toad neurons. We speculate that gravitational strains produce phonons and magnons in a ferromagnetic substance embedded in the nervous system of the toads and that these coherent signals are exponentially amplified by a Raman laser mechanism to the point where they can be detected. The fine tuning necessary for this mechanism to work would help to explain why this species of toad show this remarkable ability and others do not. We analyze the sound of a pond full of Mexican Burrowing Toads in the hopes of detecting slight phase shifts in their calls due to a gravitational wave event. No effect was found and the the LIGO/VIRGO consortia have not reported an event during the recording, illustrating the power of this approach. We suggest the massive use of these toads would be an inexpensive way to support the operation of optical interferometric gravitational wave detector facilities.
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Astrophysics of Galaxies

PAC in DESI. II. Galaxy-halo connection into the $10^{6}{\rm M}_{\odot}$ frontier

Kun Xu, Carlos S. Frenk, Y. P. Jing et al. (+37 more)

Understanding dwarf galaxy formation is crucial for testing dark matter models and reionization physics. However, constructing stellar-mass complete spectroscopic samples at low masses is increasingly difficult, and the potential existence of a local void complicates studies in an average environment. The Photometric object Around Cosmic webs (PAC) method, which combines deep photometric and spectroscopic data to measure the excess surface density $\bar{n}_2w_{{\rm{p}}}(r_{\rm{p}})$ of photometric objects around spectroscopic tracers, offers a promising path forward. We model 349 $\bar{n}_2w_{{\rm{p}}}(r_{\rm{p}})$ measurements from DESI Y1 BGS and DECaLS, reaching $M_*=10^{6.4}\,{\rm M}_{\odot}$, using a stellar mass-halo mass relation (SHMR)-based subhalo abundance matching framework applied to two high-resolution $N$-body simulations from the Jiutian suite. The resulting SHMR is constrained down to $M_{\rm h}\simeq10^{8.0}\,h^{-1}{\rm M}_{\odot}$, revealing a clear upturn at $\sim10^{10.0}\,h^{-1}{\rm M}_{\odot}$ toward lower masses, indicating rising star-formation efficiency (SFE) in small haloes. This feature persists under extensions of the model that allow mass-dependent scatter, reionization-induced suppression of the halo occupation fraction, galaxy assembly bias, and alternative cosmologies. Combining with the results from Paper I, we find that central red galaxies dominate the low-mass regime. Our results motivate a hypothesis in which SFE is significantly higher than previously thought prior to reionization, enabling relatively massive galaxies to form in small haloes. These systems are subsequently quenched by the UV background, producing the central red dwarf galaxies observed. Finally, we obtain $3\sigma$ and $5\sigma$ upper mass bounds of $10^{8.80}\,h^{-1}{\rm M}_{\odot}$ and $10^{10.24}\,h^{-1}{\rm M}_{\odot}$ on the smallest haloes required to exist.
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All authors Kun Xu, Carlos S. Frenk, Y. P. Jing, Shaun Cole, Sownak Bose, J. Aguilar, S. Ahlen, D. Bianchi, D. Brooks, F. J. Castander, T. Claybaugh, A. de la Macorra, P. Doel, J. E. Forero-Romero, E. Gaztañaga, S. Gontcho A Gontcho, G. Gutierrez, C. Hahn, R. Joyce, S. Juneau, R. Kehoe, T. Kisner, M. Landriau, L. Le Guillou, M. Manera, R. Miquel, J. Moustakas, S. Nadathur, W. J. Percival, F. Prada, I. Pérez-Ràfols, G. Rossi, L. Samushia, E. Sanchez, D. Schlegel, J. H. Silber, D. Sprayberry, G. Tarlé, B. A. Weaver, H. Zou

Astrophysics of Galaxies

Optical spectroscopy of high redshift BL Lac objects

Renato Falomo, Aldo Treves, Simona Paiano et al. (+1 more)

BL Lac objects (BLL) are defined by the presence of very weak (typically $<$ 5 Å) or even absent spectral lines. This makes determining their distance particularly challenging, especially at high redshift, where the sources are fainter and the host galaxy contribution in the optical band becomes negligible. Yet measuring their distance is crucial for deriving and modelling their luminosity, notably in the gamma-ray band, where BLLs dominate the extragalactic sky. In this work, we re-examine the reported high-redshift (z $>$ 0.6) BLL, many of which are commonly cited in the literature despite appearing questionable. We present new spectra for 52 objects obtained with the 10.4 m GTC. For 16 of them we propose a new redshift, or provide a spectroscopic lower limit, while for 14 sources we confirm previously published values. In 22 cases the spectra remain featureless, even with high S/N observations. These objects are likely to lie at 0.3 $<$ z $<$ 1.4 : the lack of host-galaxy features sets a lower limit to their distance, while the absence of intervening absorption systems argues against substantially higher redshifts. We compare our findings with the previous robustly established cases of BLLs at z $>$ 0.6 that meet our selection criteria.
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All authors Renato Falomo, Aldo Treves, Simona Paiano, Riccardo Scarpa

Earth and Planetary Astrophysics

Cow-culation: Reentry Impact Risk to Livestock in the Satellite Megaconstellation Era

Samantha M. Lawler, Michele T. Bannister and Laura E. Revell

The commercial space industry is launching more satellites into Low Earth Orbit every year. Aotearoa New Zealand (NZ) has a thriving dairy and cattle industry. Unfortunately, these industries could come into (high speed) cow-llision, as the rapid launch rate and short operational lifetimes of satellites in megaconstellations like Starlink result in a high reentry rate at NZ's latitudes. This could intersect with NZ's famously large population of livestock. We predict this will be an udder disaster for any cows that are hit, as they are squishy and moo-ve much more slowly than space debris. Using a global bovine density dataset, previously published satellite casualty probability code, and a complete lack of funding to do this calculation carefully enough for submission to a peer-reviewed journal, we calculate a $\simeq 0.3-1% chance of a cow-sualty in NZ from reentering Starlink Gen2 debris over the next 5 years.
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Cosmology and Nongalactic Astrophysics

The Universe Favors Primes: A Study in the Primality of Cosmic Structures

Nan Li and Shiyin Shen

The cosmological principle states that the universe is uniform and does not favor any specific position or direction. However, research conducted by \cite{Shen2025} has revealed that the universe demonstrates a notable inclination towards parity-odd states. Furthermore, it remains uncertain whether the universe also favors prime numbers. In this study, we examine the largest available catalogs of galaxy groups to investigate this hypothesis. Specifically, we assess whether the number of galaxies within a galaxy group or cluster is more likely to be a prime number. Our results strongly suggest that the universe does indeed have a preference for prime numbers, with findings exceeding the 4.1 sigma significance threshold. This insight explains why the Primes consistently triumphs over Unicorn. Consequently, it may be necessary to consider revising the cosmological principle in the context of a higher-dimensional feature space. Moreover, our research establishes a connection between the Riemann Zeta function and cosmology pioneeringly, paving the way for the development of Cosmozetaology.
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High Energy Astrophysical Phenomena · Cross-listed from gr-qc

Revisiting the Coprecessing Frame in the Presence of Orbital Eccentricity

Lucy M. Thomas, Katerina Chatziioannou, Sam Johar et al. (+2 more)

Accurate inclusion of both spin precession and orbital eccentricity effects in gravitational waveform models represents a key hurdle in our ability to fully characterize the properties of compact binaries. Virtually all efforts to model precession rely on a coprecessing frame transformation, a time-dependent spatial rotation that tracks the dominant emission direction and simplifies the waveform morphology. We assess the utility of the coprecessing frame transformation to separate out the effect of the precession of the orbital plane from the waveform in the presence of non-negligible orbital eccentricity. We rely on 20 numerical relativity simulations, which include the complete physical effects of spin precession and eccentricity in the strong-field, and compare waveforms in both the inertial and coprecessing frames. Comparing against the eccentric, spin-aligned model SEOBNRv5EHM, we find that while the waveform mismatches decrease in the coprecessing frame, they remain above the level required for accurate waveform modeling, $\sim$ 0.01 or higher for large inclinations. Further improvements, e.g., modeling mode asymmetries as already pursued for quasicircular binaries, will likely prove essential. We also find that by removing the dominant amplitude and phase modulations from the waveform, the coprecessing frame facilitates surrogate modeling, achieving lower errors at a fixed number of basis elements compared to the inertial frame. Our results demonstrate both the utility and the limitations of the coprecessing frame as a cornerstone in waveform modeling for eccentric and precessing binaries.
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All authors Lucy M. Thomas, Katerina Chatziioannou, Sam Johar, Taylor Knapp, Michael Boyle

Astrophysics of Galaxies

Spectroscopic study of the broad component of [O III]{\lambda}5007 profile in type 1 AGNs

Qi Zheng, Yansong Ma, Xueguang Zhang et al. (+1 more)

The spectra of type 1 active galactic nuclei (AGNs) often exhibit broad component in [O III]$\lambda$5007, which are typically blue-shifted and associated with strong outflows. We systematically analyze the [O III] emission-line properties of type 1 AGNs with broad components to investigate how these kinematic features relate to the physical properties of the central engine. From a parent sample of 11,557 QSOs at $z<0.3$ in Data Release 16 of the Sloan Digital Sky Survey, we select 2,290 type 1 AGNs exhibiting broad components in [O III]. Previous studies have reported a strong correlation between the blue emission, defined as the full extent of the broad component on the blue side, and black hole mass when the latter is estimated from the $M_{\rm BH}$–$\sigma_{\ast}$ relation using the line width $\sigma$ of the [O III] core component as a surrogate for $\sigma_{\ast}$. By the same way, the black hole mass also shows a strong correlation with the blue emission parameter in our sample. However, this correlation becomes negligible when virial black hole masses are adopted. Besides, the velocity shifts between the broad and core components of [O III] show a weak correlation with the Eddington ratio. This is consistent with the expectation that higher accretion rates enhance radiative pressure, thereby driving faster or more prominent outflows. In future work, we will compare [O III] broad component properties between typical type 1 AGNs and those with double-peaked [O III] to probe differences in narrow-line region kinematics and the impact of outflows or dual AGNs.
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All authors Qi Zheng, Yansong Ma, Xueguang Zhang, Qirong Yuan

Cosmology and Nongalactic Astrophysics

A Lower Bound on the Number of Fundamental Constants

William Luke Matthewson

We describe here, for the first time, a lower bound on the total number of fundamental constants required for a mathematical description of our physical universe to be complete. The answer is shown to be one. The formal arithmetized meta-mathematical proof of this is left to the reader.
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Solar and Stellar Astrophysics

Spectroscopic orbits for three SB2s and one hierarchical triple using SALT data

Mikhail Kovalev, Alexey Kniazev and Oleg Malkov

We confirmed four spectroscopic binary candidates using new observations obtained with SALT. Three SB2 systems (HD 20784, HD 43519A, HD 62153A) exhibit circular orbits with periods shorter than 10 days, whereas one hierarchical triple system (HD 56024) contains a close binary with an inner eccentric orbit with a period of approximately 14 days, composed of nearly identical stellar components, and a rapidly rotating star on an outer eccentric orbit with a period of approximately 400 days. For two additional SB2 candidates (HD 198174 and HD 208433), our new observations do not allow us to derive reliable orbital solutions.
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Instrumentation and Methods for Astrophysics

Synthesis imaging with a lunar orbit array: II. Impacts of instrument-induced phase errors

Meng Zhou, Furen Deng, Yidong Xu et al. (+2 more)

A lunar orbit interferometer array suffers from a number of systematics. Beyond systematics induced by the imaging algorithm itself and thermal noise considered in Paper I, phase errors due to instrumental inconsistency between receivers, geometric error in baseline determination, and clock synchronization error between satellites will also affect synthesis imaging with the space array. In this paper, we model different sources of phase errors and quantify their impacts on all-sky and patchy-sky map-making, respectively, for the ultra-long wavelength sky ($f\lesssim30$ MHz), using the Discovering the Sky at the Longest wavelength (DSL) mission (also known as the Hongmeng mission) as an example. We find that in the scheme of all-sky imaging, the angular power spectrum can be suppressed uniformly for various sources of phase errors. To ensure a reconstruction of large-scale structures with $\gtrsim 95\%$ of the angular power spectrum, the phase error should be controlled below $\sim 12^\circ$ on the random instrumental component, or below $\sim 12^\circ$ for constant deviation, or below $1.1$ ns on the temporal component. With multiple baseline measurements, the baseline determination errors below $1$ m can also meet the requirement. In the scheme of patchy-sky imaging, the S/N of point source detections does not change significantly, except with instrumental phase errors or at high frequencies. The impact of geometric phase error is relatively stronger in the patchy-sky imaging with higher resolution because longer baselines are used and fewer times of baseline measurements can be averaged over within an integration time. When scaled with wavelength, these results set the basic reference for instrumental requirements for future space interferometers.
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All authors Meng Zhou, Furen Deng, Yidong Xu, Li Zhou, Xuelei Chen

Astrophysics of Galaxies

The Actinide-Boost Star LAMOST J122216.85-063345.2: A Detailed R-process Abundance Study with Gemini-S/GHOST

Miji Jeong, Young Sun Lee, Vinicius M. Placco et al. (+15 more)

We present a detailed chemical-abundance analysis of an actinide-boost ($\log\epsilon$(Th/Dy) = -0.74) star, LAMOST J122216.85-063345.2 (J1222), a very metal-poor ([Fe/H] = -2.45) halo star with moderate enhancement in rapid neutron-capture ($r$-)process elements ([Eu/Fe] = +0.61). From high-resolution spectra (R $\sim$ 55,000) taken with Gemini-S/GHOST, we determine the abundances for 47 elements, including thorium. The abundance pattern of J1222 is consistent with predicted nucleosynthetic yields from neutron star mergers (NSMs) and black hole-neutron star mergers (BH-NSMs), under specific ejecta conditions. Our kinematic analysis of J1222 indicates that it is a member of the I'itoi substructure. A comparative analysis of J1222 and seven other stars from the literature with similar dynamics to the I'itoi substructure exhibits a broad dispersion in $r$-process enrichment - spanning non-enhancement ([Eu/Fe] $\leq$ +0.3), moderate enhancement (+0.3 $<$ [Eu/Fe] $\leq$ +0.7), strong enhancement ([Eu/Fe] $>$ +0.7), and actinide-boost stars (including one additional actinide-boost candidate newly recognized to be associated with I'itoi) - suggesting a complex enrichment history shaped by multiple $r$-process events and inhomogeneous mixing. After exploring several astrophysical scenarios to explain the observed $r$-process abundances, we find that NSMs and BH-NSMs were likely the main contributors to the enrichment, while magneto-rotational supernovae (MR-SNe) may have played a secondary role in enriching some light $r$-process element-rich stars in the I'itoi substructure.
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All authors Miji Jeong, Young Sun Lee, Vinicius M. Placco, Yutaka Hirai, Timothy C. Beers, Jeong-Eun Heo, Kristin Chiboucas, Emily Deibert, Roberto Gamen, Venu M. Kalari, Eder Martioli, Ruben Diaz, Manuel Gomez-Jimenez, Roque Ruiz-Carmona, Greg Burley, J. Gordon Robertson, Kathleen Labrie, Joanna Thomas-Osip

Astrophysics of Galaxies

Ripples of Stellar Enrichment (RoSE) - simulating element production and mixing in a Milky Way-mass galactic disc star-by-star

Chuhan Zhang, Mark R. Krumholz, Melissa K. Ness et al. (+3 more)

We present the Ripples of Stellar Enrichment (RoSE) simulations, which follow an isolated Milky Way-mass disc galaxy with star-by-star feedback and nucleosynthesis from five physical enrichment channels – Wolf-Rayet stars, type II supernovae, type Ia supernovae, asymptotic giant branch stars, and neutron star mergers. We use these simulations to test how elements' diverse nucleosynthetic origins imprint spatial, temporal, and inter-element abundance correlations in gas and newly formed stars. We find that nucleosynthetic source composition is the primary organising principle of elemental structure: elements sharing a dominant production channel exhibit similar spatial and temporal statistics and their abundances are strongly correlated with one another, while mixed-source pairs are much more weakly correlated. We show that a simple linear regression model based only on how element pairs differ in their nucleosynthetic origin is able to predict, with high fidelity, how strongly their abundances correlate, in both interstellar medium gas and coeval stars. Comparison with Milky Way stellar abundance data shows encouraging qualitative agreement, with differences between simulations and observations comparable to the scatter between independent observational datasets. Together, these results show that the covariance of galactic abundances is largely predictable from the mixture of nucleosynthetic sources supplying each element: same-source elements remain tightly correlated in gas and young stars, whereas differing source mixtures produce systematically weaker correlations.
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All authors Chuhan Zhang, Mark R. Krumholz, Melissa K. Ness, Yuan-sen Ting, Zefeng Li, Zipeng Hu

Earth and Planetary Astrophysics · Cross-listed from physics.pop-ph

Lots of Shade on Satellite Constellations

Michael B. Lund

The high frequency of satellite launches, particularly over the last few years, has been a subject of significant concern, particularly relating to the future of observational astronomy, the stability of low Earth orbits, and environmental impacts. We call attention to the insufficiently-addressed silver lining of this looming satellite cloud. If the high rates of satellites continue as we model, we can expect the solar flux received by the Earth to significantly decrease in the relatively near future. We address how this decrease in flux could provide a solution for another major problem, anthropogenic climate change. This would allow us to solve one problem with another problem as early as late March 2031.
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Astrophysics of Galaxies

The Evolving Faber-Jackson Relation: A Unifying Framework for Galaxy Ages and the Baryonic Tully-Fisher Connection

Stuart Marongwe and Stuart Kauffman

The baryonic Tully-Fisher relation (BTFR) and Faber-Jackson relation (FJR) represent fundamental scaling laws linking the baryonic mass of galaxies to their kinematics, yet their physical origin and apparent offsets between different galaxy populations have remained enigmatic. Here we present a unified theoretical framework demonstrating that both relations emerge from a common acceleration scale of order $10^{-10}m/s^2$ and evolve with cosmic time through a common exponential kernel. We derive the evolving FJR directly from the evolving BTFR within the Nexus Paradigm of quantum gravity, showing that the normalization scales as $M_b \propto e^{-4\int H(t)\,dt}\sigma^4 $, where $\sigma$ is the velocity dispersion and $ H(t)$ is the time varying Hubble parameter. Using this framework on a sample of 39 galaxies ranging from ultra-faint dwarfs to massive cluster ellipticals, we show that the observed offset between galaxy populations arises naturally from differences in their formation epochs. Ultrafaint dwarf galaxies yield ages of $ 12\pm0.8$ Gyr, in excellent agreement with independent Hubble Space Telescope stellar population ages showing synchronization within $\sim 1$ Gyr. Later-type dwarfs show systematically younger ages of $3.5-6.0$ Gyr. The dynamical age reported by the evolving FJR measures the time since a galaxy last achieved virial equilibrium. Dark matter-deficient galaxies exhibit ancient stellar populations but very young dynamical ages, consistent with a recent violent collision. Independent validation using metallicity-based stellar population ages reveals a Pearson correlation coefficient of $r=0.961$ with our dynamically derived ages for undisturbed systems, providing strong empirical support for the framework. The evolving FJR links pressure-supported systems at every mass scale, making galaxy scaling relations accurate markers of both formation and disturbance events.
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Earth and Planetary Astrophysics

Nonthermal Velocity Dispersion in the Outer Disk of HL Tau

Jinshi Sai, Shigehisa Takakuwa, Hsi-Wei Yen et al. (+2 more)

Turbulence in protoplanetary disks plays a crucial role in the evolution of disk structures and the planet formation process therein. However, the strength of the turbulence remains unclear in young, embedded disks surrounded by infalling envelopes. In this paper, we present the first direct measurement of the nonthermal velocity dispersion within the embedded disk around HL Tau, which possesses a dusty disk with multiple rings and gap structures but is still associated with infalling gas flows from an envelope. Using ALMA archival data of the $\mathrm{H_2CO}$ emission, we measured the local line width through a parametric model fitting that accounts for the contribution of Keplerian shear motion. After subtracting the thermal component, the nonthermal velocity dispersion is $\sim\!\!0.15~\mathrm{km~s^{-1}}$ on average over radii of $80$-$180~\mathrm{au}$, and it slightly increases with radius. The estimated nonthermal motions correspond to a turbulent mach number of $\mathcal{M}\!\!\sim\!\!0.4$ or a viscous $\alpha$ value of $\alpha \!\!\sim\!\!0.16$, assuming that it is entirely caused by turbulence and $\alpha \!\!\sim \!\! \mathcal{M}^2$. Our analysis also suggests that the $\mathrm{H_2CO}$ emission traces near the disk midplane ($z\lesssim 0.1 R$). Turbulence driven by the gravitational instability or infall from the envelope most naturally explains the large nonthermal motions, considering the large disk mass and associated infalling streamers. The strong turbulence measured in the outer disk, in contrast to the vertically settled inner dusty disk, suggests a pronounced radial variation in the turbulence strength and/or an anisotropic nature of the turbulence within the disk.
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All authors Jinshi Sai, Shigehisa Takakuwa, Hsi-Wei Yen, Yusuke Tsukamoto, Yuya Fukuhara

Astrophysics of Galaxies

Schrödinger's Seed: Purr-fect Initialization for an Impurr-fect Universe

Mi chen and Renhao Ye

Context. Random seed selection in deep learning is often arbitrary – conventionally fixed to values such as 42, a number with no known feline endorsement. Aims. We propose that cats, as liminal beings with a historically ambiguous relationship to quantum mechanics, are better suited to this task than random integers. Methods. We construct a cat-driven seed generator inspired by the first Friedmann equation, and test it by mapping 21 domestic cats' physical properties – mass, coat pattern, eye colour, and name entropy – via a Monte “Catlo” sampling procedure. Results. Cat-driven seeds achieve a mean accuracy of 92.58%, outperforming the baseline seed of 42 by $\sim$2.5%. Cats from astrophysicist households perform marginally better, suggesting cosmic insight may be contagious. Conclusions. The Universe responds better to cats than to arbitrary integers. Whether cats are aware of this remains unknown.
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Monday 30 March

Astrophysics of Galaxies

Gas-rich ultra-diffuse galaxies: alleviating the MOND tension with hyperconical modified gravity

Robert Monjo

Gas-rich ultra-diffuse galaxies (UDGs) are an unusually sharp test for gravity models tied to the baryonic Tully–Fisher relation because several systems appear to rotate too slowly for their baryonic masses. This study revisits the six isolated gas-rich UDGs analysed by Mancera Piña et al. with the current outer-radius prescription of hyperconical modified gravity (HMG), using the published baryonic masses and circular velocities at the outer radii. The scan over the neighbourhood-scale parameter drives the model towards the asymptotic branch of HMG. For that limit, the HMG velocities are still systematically high for four of the six galaxies. Relative to the observed values, the fixed asymptotic branch gives $\chi^2\simeq18.1$ for six objects, whereas Newtonian baryons alone give $\chi^2\simeq9.7$, but MOND interpolation is much worse ($\chi^2\simeq 615.7$). Using combined uncertainties, the per-galaxy HMG tension ranges from $0.2\sigma$ to $2.1\sigma$, very similar to the $0.1\sigma$ to $1.7\sigma$ found for Newtonian baryons, and much smaller than the $3.7\sigma$ to $5.9\sigma$ obtained for MOND. We conclude that the present outer-radius HMG implementation alleviates the difficulties of MOND, but is still not sufficient to account for the published central values of the UDG sample. Gas-rich UDGs therefore provide a useful discriminant between MOND and HMG.
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Solar and Stellar Astrophysics

Data-driven discovery of dynamo cycle equations

Anna Guseva, Calum Skene and Steve Tobias

Many low-mass stars like the Sun host periodic, oscillatory magnetic fields that lead to variable levels of stellar activity, driving space weather that affects the habitability and detection of exoplanets. Owing to the intrinsic difficulty in modeling stellar magnetohydrodynamics across scales, realistic numerical simulations of this process are very challenging, and developing reduced-order models is of interest. In this work, we develop a framework to recover such models directly from numerical data by using a combination of Dynamic Mode Decomposition (DMD) to identify coherent magnetic structures, and the Sparse Identification of Nonlinear Dynamics (SINDy) framework to model their dynamics. We compare these models to those obtained using the classic mathematical method of weakly nonlinear (WNL) analysis. This approach is implemented on a one-dimensional mean-field dynamo model that parameterizes the main components of a convective dynamo in a low-mass star – helical convection and differential rotation. We recover oscillatory dynamo models as a function of the dynamo strength parameter $D\sim \alpha \Omega'$, magnetic dissipation parameter $\kappa$, and a comprehensive dynamo model that predicts the magnetic state for a combination of these two parameters. Our results suggest that equations discovered with SINDy are more robust than equations from WNL analysis, and can predict the saturation amplitude of magnetic fields in parameter regimes far from the onset of dynamo action characterized by stiff nonlinearities. This includes unstable, and typically unknown, subcritical branches. Further to this, SINDy is able to find equations in parameter regimes where the nonlinearity is not analytic and WNL analysis cannot be applied. These properties of data-driven SINDy models suggest them as a viable alternative for modeling of stellar dynamo cycles directly from the data.
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High Energy Astrophysical Phenomena · Cross-listed from gr-qc

No hair but plenty of feathers: are birds black holes?

Andrew Laeuger and Taylor Knapp

The imitative verb "chirp" is thought to originate from 16th-century Middle English. Meanwhile, this same word has been used to describe the gravitational waves (GWs) emitted from the merger of compact objects, such as black holes and neutron stars, since at least the 1990s. Motivated purely by this linguistic overlap, we study whether the chirps of birds can be modeled by compact binary waveforms. In particular, we consider a test case of the Northern cardinal (Cardinalis cardinalis), finding that its time-reversed chirp can be approximately modeled by that of a high mass ratio, precessing black hole binary, with a number of indications towards extreme matter effects or beyond the Standard Model physics. Importantly, this waveform correspondence is not so straightforward for all bird species, as some chirp morphologies are far more akin to glitches seen in GW observatories. With these comparisons made, we propose an alternative solution to the longstanding philosophical conundrum: rather than the chicken or the egg, perhaps it was the Big Bang which truly came first.
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Astrophysics of Galaxies

Pervasive Cavity-Ring Structure for Star Formation in Dwarf Irregular Galaxies

Bruce G. Elmegreen and Deidre A. Hunter

Unsharp-mask images of HI emission from 36 dwarf irregular (dIrr) galaxies illustrate star formation in dispersed clouds and on the rims of large cavities. The cavities can extend for a radial scalelength and typically have circular or slightly sheared forms. The average surface density of cloud peaks is ~20 Msun/pc2, and, combined with their average FUV star formation rate, suggests a gas consumption time of ~3.2 Gyr. Vertical hydrostatic equilibrium calculations for 24 of these dIrrs give a typical scale height of ~400 pc, which combines with the gas and star formation surface densities to suggest an efficiency per free fall time of ~1%. These values are comparable to those in the molecular clouds of spiral galaxies, suggesting the primary difference between clouds is the presence of CO at higher metallicity in the spirals. U-B color images of the dIrrs suggest that cavity ages range between 10^7 and 10^8 years, with the longer times explaining the common lack of bright OB associations in their centers and their low expansion speeds. Most are circular because the shear time exceeds 100 Myr, although some of the HI has spiral structure. These observations suggest that star formation in dIrrs proceeds slowly in a sequential fashion in dispersed clouds and on the periphery of giant cavities that move and expand during the ~50 Myr supernova era of the previous generation. In contrast, spiral galaxies have shear times 10 times shorter and more important stellar dynamics that compresses the gas into filaments.
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High Energy Astrophysical Phenomena

AGN Disks as Supernova Mufflers I: 3D Local Hydrodynamic Models

Harrison E. Cook, Wladimir Lyra, Mordecai-Mark Mac Low et al. (+2 more)

Supernova (SN) shocks that originate from stars on orbits embedded in dense active galactic nuclei (AGN) accretion disks evolve differently from those that occur in the interstellar medium. We aim to assess how shocks evolve in this dense stratified medium and understand where SNe are muffled and have their kinetic energy absorbed by an AGN disk versus escaping. We use Sirko & Goodman (SG) and Thompson, Quataert & Murray (TQM) AGN disk models for midplane radial profiles, generated with the pAGN code; we compare the disk pressure to the energy of a standard core-collapse SN ($10^{51}\,{\rm erg}$) to find radii where shock breakout can occur. For verification, we evolve three-dimensional hydrodynamic shearing box simulations of stratified Gaussian disks constructed from the midplane values that are injected with energy and mass from SNe placed at multiple radii and vertical locations, using the Athena code. We find SN shocks in SG disks around black holes with mass $\Mbh=10^6\,\Msun$ become muffled beyond $R\sim10^6\,\Rs$, and that this muffling radius is inversely proportional to supermassive black hole (SMBH) mass with muffling occurring at $R\sim10^2\,\Rs$ for $\Mbh=10^9\,\Msun$. Around TQM disks, the muffling radius occurs at $R\sim10^6\,\Rs$, independent of $\Mbh$. The largest determining factor for muffling a SN shock is the local scale height of the AGN disk. In conclusion, we developed a predictive analytic criterion to identify where AGN disks can muffle SNe shocks depending on their density and vertical scale.
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All authors Harrison E. Cook, Wladimir Lyra, Mordecai-Mark Mac Low, K. E. Saavik Ford, Barry McKernan

High Energy Astrophysical Phenomena

Late-Time HST UV Detections Reveal Eruptive Mass Loss and Circumstellar Interaction in a Quarter of Stripped-Envelope Supernovae

C. Fremling, S. Covarrubias, J. Sollerman et al. (+19 more)

We present HST WFC3/UVIS F275W near-UV imaging of 91stripped-envelope supernovae (SE SNe; Types IIb, Ib, Ic) from Snapshot program SNAP-16657, observed at phases of 270-1845 days (median 952 days) after first optical detection. We detect UV counterparts in 13 SE~SNe, of which 6 are classified as secure and 7 as ambiguous after comparison to nearby Hii regions, interpreting the secure sources as signatures of interaction with circumstellar material (CSM). Independent WISE W1/W2 light curves show $>300$ day mid-IR excesses in two of the secure UV sources, corroborating the interaction interpretation, and reveal two additional IR-only candidates without UV counterparts, indicating dust-obscured interaction episodes missed by the UV survey. A forward-modeling MCMC analysis using a physics-based CSM interaction model with three free parameters, the interaction fraction $f_\mathrm{CSM}$, shell mass $M_\mathrm{CSM}$, and thickness fraction $f_\mathrm{thick}$, yields $f_\mathrm{CSM} = 0.23^{+0.17}_{-0.09}$, $M_\mathrm{CSM} \approx 0.013~M_\odot$, and $f_\mathrm{thick} \approx 0.07$. The inferred thin-shell geometry implies an ejection duration of $\sim$6 yr for an outflow velocity of $300$ km s$^{-1}$, two to three orders of magnitude shorter than the thermal timescale of stable Roche-lobe overflow. This result disfavors steady binary mass transfer as the origin of the detected CSM and instead points to eruptive pre-supernova mass ejection in the final years before core collapse, either from wave-driven outbursts or from mass transfer triggered by late-stage progenitor re-expansion.
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All authors C. Fremling, S. Covarrubias, J. Sollerman, K. De, T. X. Chen, T.-W. Chen, R. Dekany, C. Fransson, A. Gal-Yam, S. L. Groom, W. V. Jacobson-Galán, M. M. Kasliwal, R. Lunnan, E. O. Ofek, D. A. Perley, J. N. Purdum, S. Schulze, Y. Sharma, N. Sravan, A. Wei, Lin Yan, Y. Yao

Instrumentation and Methods for Astrophysics

AI Cosplaying as Astrophysicists: A Controlled Synthetic-Agent Study of AI-Assisted Astrophysical Research Workflows

Chun Huang

Large Language Models (LLMs) are now widely used in astrophysics, but do they actually make our lives easier, or do they merely invent new physics with enough confidence to hide a minus sign? In a specialized field where checking fluent hallucinations is itself labor-intensive, AI assistance can demand as much work as the task it claims to simplify. To evaluate where AI genuinely improves scientific workflows, we bypassed human trials and instead forced AI agents to cosplay as astrophysicists. We simulated 144 synthetic researchers, varying in career stage, AI awareness, and willingness to verify outputs, across 2,592 daily astrophysics research assignments. Comparing solo work against four styles of AI assistance produced 12,960 scored episodes. No assisted policy universally outperformed unassisted work in the primary Qwen production run. Instead, performance depends strongly on the task, the style of AI use, and the identity of the actor. While cautious assistance helps on creative, extractive, and critique-oriented tasks, it can fail catastrophically on derivation-heavy physics. A full actor-swap DeepSeek rerun changes that picture materially: verification-heavy use becomes the strongest assisted policy, two assisted modes enter the higher-utility/lower-risk quadrant, and the derivation-heavy fragility that dominates the Qwen production run largely disappears. In its current form, AI is useful, but only conditionally, its value is uneven, task-specific, and shaped jointly by workflow, usage policy, and which LLM you are using.
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Cosmology and Nongalactic Astrophysics · Cross-listed from hep-ph

Baryogenesis from Exploding Primordial Black Holes

Alexandra P. Klipfel, Miguel Vanvlasselaer, Sokratis Trifinopoulos et al. (+1 more)

Exploding primordial black holes can source baryon asymmetry soon after the electroweak phase transition, as high-energy Hawking radiation drives ultrarelativistic shocks in the surrounding plasma. The shocks and their trailing rarefaction waves delineate two bubble-like walls around a shell of superheated fluid, in which electroweak symmetry is restored. These moving interfaces source chiral charge, which is converted to baryon number. Upon adding a simple CP-violating operator at the TeV scale, this mechanism yields the observed baryon asymmetry with minimal dependence on PBH model parameters.
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All authors Alexandra P. Klipfel, Miguel Vanvlasselaer, Sokratis Trifinopoulos, David I. Kaiser

Astrophysics of Galaxies

The Evolution of the Spin Alignments of Dark Matter Halos in the Cosmic Web

David Tobar, Rory Smith, Antonio Montero-Dorta et al. (+2 more)

We investigate the evolution of dark matter halo spin alignments with respect to cosmic filaments, exploring how halo mass, proximity to filaments, and major mergers influence their orientation over time. We perform a suite of dark matter-only zoom-in N-body simulations centered on ten filaments extracted from a cosmological box using the 1DREAM structure finder. This approach allows us to resolve low-mass halos within filaments while preserving the large-scale environment. Halos are identified with the Amiga Halo Finder (AHF), and their evolutionary histories are reconstructed to trace the spin, shape, and distance to the filament from redshift $z = 1$ to $z = 0$. We confirm a strong mass-dependent alignment signal: low-mass halos tend to align parallel to the filament, while high-mass halos preferentially exhibit perpendicular orientations, despite limited statistics. Perpendicular alignments become dominant at the highest halo masses in our sample, around $\log_{10}(M_\mathrm{h}/h^{-1}\mathrm{M_\odot}) \sim 12$. We also find that major mergers can induce sharp spin reorientations and temporary transitions toward more prolate halo shapes, particularly in massive halos located near the filament core, suggesting a preferential merger direction within filaments. Overall, halo mass emerges as the primary factor governing spin-filament alignments in our sample. By analyzing the global evolution, we find that the average orientations at z = 0 do not differ significantly from those at $z = 1$, indicating that the present-day spin configuration is largely established at earlier stages of halo evolution. Major mergers, although relatively rare, represent one of the few mechanisms capable of disrupting this initial alignment.
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All authors David Tobar, Rory Smith, Antonio Montero-Dorta, Katarina Kraljic, Pablo López

High Energy Astrophysical Phenomena

Eccentricity constraints disfavor single-single capture in nuclear star clusters as the origin of all LIGO-Virgo-KAGRA binary black holes

Nihar Gupte, M. Coleman Miller, Rhiannon Udall et al. (+12 more)

Multiple formation pathways have been proposed for the origin of binary black holes (BBHs). These include isolated binary evolution and dynamical assembly in dense stellar environments such as nuclear or globular star clusters. Yet, the fraction of BBHs originating from each channel remains uncertain. One way to constrain this fraction is by investigating the orbital eccentricities of the BH coalescences detected by the LIGO-Virgo-KAGRA (LVK) Collaboration. We analyze 84 BBHs from the first part of the fourth LVK observing run (O4a) using a multipolar, eccentric, aligned-spin effective-one-body waveform model. We perform parameter inference with neural posterior estimation and nested sampling. After incorporating astrophysical prior odds and comparing to the quasicircular precessing-spin hypothesis, we find that no candidates reach a high enough significance to claim a confident detection of eccentricity. We use these upper limits to explore a model, in which all O4a BBHs originate from single-single gravitational wave (GW) captures. We perform hierarchical inference on the velocity dispersion of the host environment of the BBHs and find $\sigma$ < 19.7 km/s (95% credible upper bound). This disfavors single-single capture in nuclear star clusters (approximately 20-200 km/s) as the dominant source of all observed BBH mergers. Our analysis also jointly infers the mass, spin and redshift distributions and takes into account selection effects due to using quasi-circular templates for BBH detection. Our results place improved constraints on the number of eccentric BBHs and highlight the importance of eccentricity measurements in disentangling compact-binary formation channels in current and future GW detectors.
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All authors Nihar Gupte, M. Coleman Miller, Rhiannon Udall, Sophie Bini, Alexandre Toubiana, Alessandra Buonanno, Jonathan Gair, Aldo Gamboa, Lorenzo Pompili, Antoni Ramos-Buades, Maximilian Dax, Stephen R. Green, Annalena Kofler, Jakob Macke, Bernhard Schölkopf

Solar and Stellar Astrophysics

Introducing PxP: A Population Synthesis Framework for Predicting YSO Properties

J. Peltonen, E. Rosolowsky, A Ginsburg et al. (+3 more)

The most direct method of measuring the star formation rate is with young stellar objects (YSOs), but this requires high-resolution observations and high-quality models. Using the latest YSO radiation transfer and stellar evolution models, we have developed a population synthesis code that generates model YSO populations that can be observed by JWST. We combine these model populations with principal component analysis (PCA) and maximum likelihood fitting to create a complete framework for predicting the age and mass of YSO populations. We dub this combination of Population synthesis and PCA, PxP, and show that it is effective at predicting mass and age with self-fitting tests. We apply PxP to the Spitzer identified YSOs in N44 and find a mass of (1.1+-0.1)*10^4 M_sun and an age of 0.74^{+0.06}_{-0.03} Myr, consistent with previous work. Next, we identify 112 YSO candidates in the archival JWST observations of NGC 604. Applying PxP to this newly identified population we find a mass of (2.2+-0.2)*10^4 M_sun and an age of 0.62+-0.01 Myr. This first look at this framework demonstrates its effectiveness with a specific set of models and leaves clear opportunities for future exploration. PxP allows us to directly determine the recent (<3~Myr) star formation history, giving an unprecedented look at the effect of the large-scale environment on individual star formation.
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All authors J. Peltonen, E. Rosolowsky, A Ginsburg, R. Indebetouw, T. Richardson, M. Jimena Rodriguez