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Physics Astrophysics

astro-ph Watch Astrophysics

Wednesday 11 March

Astrophysics of Galaxies

The Kinematically Hot, Extremely Metal-Poor C-19 Stellar Stream in DESI DR2

Nasser Mohammed, Joseph Y. Tang, Ting S. Li et al. (+47 more)

Stellar streams are the result of a host galaxy's gravitational potential tidally disrupting satellite dwarf galaxies and globular clusters (GCs), causing them to grow leading and trailing tidal tails. The C-19 stellar stream is an extremely metal-poor stellar population, showing chemical abundance patterns characteristic of a globular cluster. However, its large velocity dispersion is difficult to reconcile with a conventional, purely baryonic, disrupting-GC progenitor. Current techniques for stream characterization are primarily applied to Gaia DR3, relying heavily on proper motion measurements. Using the Dark Energy Spectroscopic Instrument (DESI), which provides radial velocities and metallicites for over 10 million stars reaching significantly fainter magnitudes than comparable surveys, we employ a mixture model approach to jointly characterize stream populations in proper motions, radial velocities, and metallicities against a Milky Way halo background. By applying this framework to the C-19 stellar stream, we identify a total of 47 spectroscopically confirmed member stars, of which 41 are newly identified and only 6 were previously reported in the literature. In this work, we measure a velocity dispersion of $7.8^{+1.5}_{-1.3}$ km s$^{-1}$ and a mean metallicity of [Fe/H] = $-3.36^{+0.12}_{-0.10}$. We further identify a novel 'spur' feature within the stream. We conclude that our measurements are in line with previous works identifying C-19 as a 'hot', metal-poor stream. In forthcoming work, we will apply this approach to many more streams in the DESI footprint, enabling population-level comparisons with predictions from simulations.
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All authors Nasser Mohammed, Joseph Y. Tang, Ting S. Li, Sergey E. Koposov, Raymond G. Carlberg, Emma Jarvis, Andrew P. Li, Nathan Sandford, Gustavo E. Medina, Wenting Wang, Monica Valluri, Alexander H. Riley, Leandro Beraldo e Silva, Joan Najita, Mika Lambert, Songting Li, J. Aguilar, S. Ahlen, D. Bianchi, D. Brooks, T. Claybaugh, A. P. Cooper, A. de la Macorra, J. E. Forero-Romero, E. Gaztañaga, S. Gontcho A Gontcho, G. Gutierrez, R. Joyce, S. Juneau, R. Kehoe, T. Kisner, A. Kremin, M. Landriau, L. Le Guillou, M. Manera, A. Meisner, R. Miquel, S. Nadathur, W. J. Percival, F. Prada, I. Pérez-Ràfols, G. Rossi, E. Sanchez, D. Schlegel, J. Silber, D. Sprayberry, G. Tarlé, B. A. Weaver, R. Zhou, H. Zou

High Energy Astrophysical Phenomena

Long GRB 250916A: an Off-axis Powerlaw Jet with Thermal Cocoon

Utkarsh Pathak, Sameer K. Patil, Hitesh Tanenia et al. (+22 more)

Some gamma-ray bursts (GRBs) exhibit precursor emission episodes preceding the main emission, with a quiescent period in between. The properties of the precursor emission and the duration of the quiescent interval are related to the central engine activity and jet formation processes, thus providing insights into the physics of GRBs. We present a comprehensive analysis of the prompt emission and multi-wavelength afterglow of GRB 250916A. Using detailed afterglow modeling, we find that the broadband data are best described by a powerlaw structured jet with a relatively narrow core ($\theta_c \approx 0.8^\circ$), viewed moderately off-axis at a viewing angle $\theta_v \approx 2.7^\circ$. The isotropic-equivalent kinetic energy of the jet ($E_{k,iso} \approx 2.4 \times 10^{54}$ erg) is on the higher side for typical GRBs. The precursor emission is well described by a blackbody spectrum with a temperature of kT $\approx$ 13.2 keV and is separated from the main emission by a long quiescent interval of 150 s. Put together, our results indicate that the precursor is likely to be a shock breakout from a cocoon formed by the interaction of the relativistic jet with the progenitor star. The resulting cocoon pressure and shock collimation naturally lead to the launch of a narrowly collimated jet, consistent with the jet geometry inferred from afterglow observations. The long quiescent interval may imply the central engine turn-off in addition to the effect of the off-axis geometry.
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All authors Utkarsh Pathak, Sameer K. Patil, Hitesh Tanenia, Tanishk Mohan, Xander J. Hall, Yogesh Wagh, Viswajeet Swain, Aditya Pawan Saikia, Varun Bhalerao, Tomas Ahumada, G. C. Anupama, Sudhanshu Barway, Malte Busmann, Michael W. Coughlin, Matthew J. Graham, Daniel Gruen, Assaf Horesh, Mansi M. Kasliwal, Russ R. Laher, Frank J. Masci, Antonella Palmese, Josiah Purdum, Argyro Sasli, Roger Smith, Xiaoxiong Zuo

Astrophysics of Galaxies

Large-scale environments of star-forming active galactic nuclei: How black hole mass, accretion rate, and luminosity connect to dark matter halos

G. Mountrichas, F. J. Carrera, F. Shankar et al. (+1 more)

Understanding the relative roles of large-scale environment and internal host-galaxy processes in shaping AGN activity is essential for constraining models of black-hole growth and galaxy evolution. We investigate how the environment of X-ray selected active galactic nuclei (AGN) relates to black-hole growth and accretion properties, and whether these introduce an environmental dependence beyond that expected from the host galaxy itself. Combining the XXL and Stripe 82X surveys, we construct samples of 427 broad-line AGN at $0.5<z<1.2$ and more than $20,000$ galaxies, with host-galaxy properties derived consistently using the same spectral energy distribution fitting methodology. Dark matter halo (DMH) masses are inferred from AGN–galaxy cross-correlation functions, while a multivariate nearest-neighbour matching algorithm is used to isolate trends with black-hole mass ($M_{\mathrm{BH}}$), Eddington ratio ($\lambda_{\mathrm{Edd}}$), and X-ray luminosity ($L_{\mathrm{X}}$) under controlled host-galaxy conditions. Within the uncertainties of the present dataset, X-ray AGN typically reside in halos of $\log(M_{\mathrm{DMH}}/h^{-1}M_\odot)\simeq13$, with no significant variation as a function of $M_{\mathrm{BH}}$, $\lambda_{\mathrm{Edd}}$, or $L_{\mathrm{X}}$. These results suggest that neither long-term black-hole growth nor short-term accretion variability is strongly linked to large-scale environment, and instead support a scenario in which AGN properties are regulated primarily by internal host-galaxy processes, while large-scale structure sets the broader boundary conditions for gas supply and duty cycle.
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All authors G. Mountrichas, F. J. Carrera, F. Shankar, A. Georgakakis

High Energy Astrophysical Phenomena

Gravitational Wave Measurement of the $M_\mathrm{BH}$-$M_\mathrm{bulge}$ Intrinsic Scatter at High Redshift

Cayenne Matt, Kayhan Gültekin, Gabriella Agazie et al. (+109 more)

The observed GWB spectrum is higher in amplitude than model predictions by a factor of 2-3. Using a semi-analytic model, we evaluate the effect of a high-scatter supermassive black hole (SMBH) scaling relation ($M_\mathrm{BH}$-$M_\mathrm{bulge}$) on models of the nanohertz gravitational wave background (GWB). By implementing an intrinsic scatter of the $M_\mathrm{BH}$-$M_\mathrm{bulge}$ relation, which is larger at higher redshift, but matches local observations, we find that the amplitude of GWB models increases to be consistent with the low-frequency end of the GWB spectrum. This amplitude increase is not uniform across frequencies, a strongly evolving scatter preferentially increases the number density of the most massive SMBHs which, in the GWB spectrum, minimizes the strength of the low-frequency turnover. Our models with positively evolving intrinsic scatter can reproduce the electromagnetically observed overmassive SMBHs at $4 < z < 6$ without changing the $M_\mathrm{BH}$-$M_\mathrm{bulge}$ normalization though we find that including moderate normalization evolution marginally improves fits to the GWB data. We conclude that the $M_\mathrm{BH}$-$M_\mathrm{bulge}$ relation which best describes the available GWB and electromagnetic data sets has intrinsic scatter that evolves as $\varepsilon(z) = \varepsilon_0 + (0.56 \pm 0.4) \log_{10}(1 + z)$ and normalization that evolves as $\alpha(z) = \alpha_0 (1 + z)^{0.84 \pm 0.35}$. The results of this work imply that the $M_\mathrm{BH}$-$M_\mathrm{bulge}$ relation we see today is not universal throughout cosmic time and that a diversity of seeding models and growth mechanisms may be at play in the early stages of SMBH-galaxy evolution.
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All authors Cayenne Matt, Kayhan Gültekin, Gabriella Agazie, Nikita Agarwal, Akash Anumarlapudi, Anne M. Archibald, Zaven Arzoumanian, Jeremy G. Baier, Paul T. Baker, Bence Bécsy, Laura Blecha, Adam Brazier, Paul R. Brook, Sarah Burke-Spolaor, Rand Burnette, Robin Case, J. Andrew Casey-Clyde, Maria Charisi, Shami Chatterjee, Tyler Cohen, James M. Cordes, Neil J. Cornish, Fronefield Crawford, H. Thankful Cromartie, Kathryn Crowter, Megan E. DeCesar, Paul B. Demorest, Heling Deng, Lankeswar Dey, Timothy Dolch, Graham M. Doskoch, Elizabeth C. Ferrara, William Fiore, Emmanuel Fonseca, Gabriel E. Freedman, Emiko C. Gardiner, Nate Garver-Daniels, Peter A. Gentile, Kyle A. Gersbach, Joseph Glaser, Deborah C. Good, C. J. Harris, Jeffrey S. Hazboun, Ross J. Jennings, Aaron D. Johnson, Megan L. Jones, David L. Kaplan, Anala Kavumkandathil Sreekumar, Luke Zoltan Kelley, Matthew Kerr, Joey S. Key, Nima Laal, Michael T. Lam, William G. Lamb, Bjorn Larsen, T. Joseph W. Lazio, Natalia Lewandowska, Tingting Liu, Duncan R. Lorimer, Jing Luo, Ryan S. Lynch, Chung-Pei Ma, Dustin R. Madison, Ashley Martsen, Alexander McEwen, James W. McKee, Maura A. McLaughlin, Natasha McMann, Bradley W. Meyers, Patrick M. Meyers, Chiara M. F. Mingarelli, Andrea Mitridate, Cherry Ng, David J. Nice, Shania Nichols, Stella Koch Ocker, Ken D. Olum, Timothy T. Pennucci, Benetge B. P. Perera, Polina Petrov, Nihan S. Pol, Henri A. Radovan, Scott M. Ransom, Paul S. Ray, Joseph D. Romano, Jessie C. Runnoe, Alexander Saffer, Shashwat C. Sardesai, Ann Schmiedekamp, Carl Schmiedekamp, Kai Schmitz, Brent J. Shapiro-Albert, Xavier Siemens, Joseph Simon, Sophia V. Sosa Fiscella, Ingrid H. Stairs, Daniel R. Stinebring, Kevin Stovall, Abhimanyu Susobhanan, Joseph K. Swiggum, Jacob Taylor, Stephen R. Taylor, Mercedes S. Thompson, Jacob E. Turner, Michele Vallisneri, Rutger van Haasteren, Sarah J. Vigeland, Haley M. Wahl, Kevin P. Wilson, Caitlin A. Witt, David Wright, Olivia Young

Astrophysics of Galaxies

Isophote shape analysis and the unfortunate subtlety of dwarf galaxy structure

A. E. Watkins, I. Lazar, T. Sedgwick et al. (+5 more)

Dwarf galaxies ($M_{*}/M_{\odot} \lesssim 10^{9.5}$), being sensitive to key evolutionary drivers like baryonic feedback and tidal perturbation, are crucial for understanding galaxy evolution as a whole. Their abundance and faintness, however, ensures that most will be studied primarily via broadband imaging for the foreseeable future. It is thus crucial to identify the most informative broadband-derivable quantities in the dwarf regime. As studies of widely used morphological parameters like concentration, asymmetry, and smoothness suggest these lack discriminatory power among dwarfs, we assess alternatives derived from isophotes: position angle twists, ellipticity, deviations from pure ellipses, and residuals to single-Sérsic profile fits. Using these parameters, we compare dwarf populations with massive galaxies of the same morphological class, and among themselves by morphological class. Only dwarf spirals may differ from their massive counterparts, being structurally simpler; dwarf and massive early type galaxy (ETG) isophotal similarity suggests all dwarf ETGs may be triaxial. Among only dwarfs, morphological classes are indistinguishable in this parameter space. A principal component analysis (PCA) using all available morphological, isophotal, and physical parameters expands on this: no PC explains more than $\sim$26% of the population variance, and no clear multimodality appears in any pairwise PC projection. We find similarly moderate spectral clustering, with a silhouette score of only 0.35. Given this self-similarity, parsing dwarf galaxy evolution from photometric parameters alone will likely require detailed statistical analysis of large dwarf populations in a high-dimensional parameter space, a task suitable for up-coming large-scale surveys like the Legacy Survey of Space and Time.
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All authors A. E. Watkins, I. Lazar, T. Sedgwick, G. Martin, S. Kaviraj, D. Kakkad, C. Collins, B. Bichang'a

Cosmology and Nongalactic Astrophysics

FlowSN: Neural Simulation-Based Inference under Realistic Selection Effects applied to Supernova Cosmology

Benjamin M. Boyd, Kaisey S. Mandel, Matthew Grayling et al. (+10 more)

We present FlowSN, a statistical framework using simulation-based inference (SBI) with normalising flows to account for selection effects in observational astronomy. Failure to account for selection effects can lead to biased inference on global parameters. An example is Malmquist bias, where detection limits result in a sample skewed towards brighter objects. In Type Ia supernova (SN Ia) cosmology, these selection effects can systematically shift the inferred posterior distributions of cosmological parameters, necessitating the development of robust statistical frameworks to account for the biases. SBI enables us to implicitly learn probability distributions that are analytically intractable to calculate. In this work, we introduce a novel approach that employs a normalising flow to learn the non-analytic selected SN likelihood for a given survey from forward simulations, independent of the assumed cosmological model. The resulting likelihood approximation is incorporated into a hierarchical Bayesian framework and posterior sampling is performed using Hamiltonian Monte Carlo to obtain constraints on cosmological parameters conditioned on the observed data. The modular learnt likelihood approximation can be reused without retraining to evaluate different cosmological models, providing a key advantage over other SBI approaches. We demonstrate the performance of this methodology by training and testing the SBI technique using realistic LSST-like SNANA simulations for the first time. Our FlowSN approach yields accurate posterior estimates on cosmological parameters, including the dark energy equation of state $w_0$, that are an order of magnitude less biased than those obtained with conventional techniques and also exhibit improved frequentist calibration.
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All authors Benjamin M. Boyd, Kaisey S. Mandel, Matthew Grayling, Ayan Mitra, Richard Kessler, Maximilian Autenrieth, Aaron Do, Madeleine Ginolin, Lisa Kelsey, Gautham Narayan, Matthew O'Callaghan, Nikhil Sarin, Stephen Thorp

Astrophysics of Galaxies

The TRGB-SBF Project. IV. A Color Calibration of the TRGB in the JWST F090W+F150W Filters

Maksim I. Chazov, Dmitry I. Makarov, R. Brent Tully et al. (+7 more)

Observations with JWST in the F090W band provide a powerful tool for determining galaxy distances based on tip of the red giant branch (TRGB) measurements. It is a great convenience that the TRGB lies at an almost constant absolute magnitude level at low metallicities. However, the TRGB becomes fainter at high metallicities in the F090W filter. Details of this break in slope are critical for precision applications in the acquisition of distances. With an absolute scaling set by the maser distance to NGC 4258 (but excluding the uncertainty in that distance), the value $M^\mathrm{TRGB}_\mathrm{F090W} = -4.40 \pm 0.03$ mag (traditional Vega) is found for $(\mathrm{F090W}-\mathrm{F150W})_0<1.65$ mag. The theoretical RGB isochrone that reaches the color 1.65 at the RGB tip corresponds to metallicity $[M/H] = -0.57$ for a 10 Gyr population. The calibration is used to derive distances for 16 galaxies relative to the megamaser host NGC 4258. Revised distances are on average slightly closer than literature values derived from the same data.
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All authors Maksim I. Chazov, Dmitry I. Makarov, R. Brent Tully, Gagandeep S. Anand, Lidia N. Makarova, Yotam Cohen, John P. Blakeslee, Michele Cantiello, Joseph B. Jensen, Gabriella Raimondo

Astrophysics of Galaxies

GSE vs. LMC: reshaping of radially biased stellar haloes by satellites

Adam M. Dillamore, Jason L. Sanders and Richard A.N. Brooks

Perturbations from the Large Magellanic Cloud (LMC) of the Milky Way's stellar and dark matter haloes are well-established. However, studies have generally not considered the high radial anisotropy of the Milky Way's inner halo caused by the accreted debris of Gaia Sausage-Enceladus (GSE). We run a series of test particle simulations of stellar haloes being perturbed by the LMC, with different halo velocity anisotropies $\beta\in[0.5,0.9]$. The LMC causes these initially axisymmetric haloes to become approximately triaxial. Their major axes are aligned with its orbital plane and tilted by up to $\sim14^\circ$ with respect to a fixed Galactic disc. These effects become much more dramatic as $\beta$ increases, causing the halo to fractionate spatially according to anisotropy. This confirms the expectations of an analytical model, which predicts that orbits with eccentricities $e\gtrsim0.95$ should azimuthally align with the tidal field of the LMC. The reshaping of the $\beta=0.9$ halo creates strong overdensities of $\sim40\%$ at heliocentric distances as close as 15 kpc. These coincide with the well-known Virgo Overdensity (VOD) and Hercules-Aquila Cloud (HAC), which have previously been associated with the GSE. We propose that the HAC and VOD were created by the dynamical alignment of highly eccentric orbits by the LMC, and are not necessarily relics of the GSE merger geometry. We conclude that previous works have significantly underestimated perturbations from the LMC in the inner stellar halo by not considering sufficiently high velocity anisotropy. This effect should be corrected for when constructing equilibrium models of the GSE debris.
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Astrophysics of Galaxies

Solar twins in Gaia DR3 GSP-Spec II. Age distribution and its implications for the Sun's migration

Takuji Tsujimoto, Daisuke Taniguchi, Alejandra Recio-Blanco et al. (+2 more)

Solar twins are among the most powerful tracers of Galactic disk evolution owing to their unique property of sharing nearly solar metallicities ([Fe/H] ~ 0) while spanning a wide range of ages. To grasp solar twins as relics of Galaxy evolution, individual twins must be tagged with ages. A sufficiently large and well-characterized stellar sample then allows us to construct an age distribution that encodes the star formation history beyond our local region, modulated by the efficiency of radial migration of stars. Based on our catalog of 6,594 high-quality local (<~ 300 pc) solar twins from the Gaia Data Release 3 spectroscopic (GSP-Spec) catalog, we derived their age distribution after carefully deconvolving the selection function. We find two distinct features: a narrow peak around ~ 2 Gyr and a broad bump extending over ~ 4–6 Gyr. First, we argue that the former corresponds to a relatively recent burst of star formation that occurred in the disk, including at least a local region within a few kiloparsecs of the Sun, which is in good agreement with previous results deduced from independent works. On the other hand, the older bump, closely associated with the Sun's birth epoch, is intriguing since this finding challenges the predicted presence of a corotation barrier built by the Galactic bar, which is thought to prevent stars born inside R_GC ~ 6 kpc from reaching the solar neighborhood. We propose that the large number of local twins with ages between 4 and 6 Gyr provides compelling evidence that the Sun's long-distance (>= 3 kpc) migration is shared by many inner disk stars. This, in turn, suggests a possible link with the epoch of bar formation, which may have triggered enhanced star formation in the inner disk and induced efficient radial migration.
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All authors Takuji Tsujimoto, Daisuke Taniguchi, Alejandra Recio-Blanco, Pedro A. Palicio, Patrick de Laverny

Cosmology and Nongalactic Astrophysics

Kinematic cosmic dipole from a large sample of strong lenses

Martin Millon, Charles Dalang, Thomas Collett et al. (+1 more)

Measurements of the kinematic cosmic dipole continue to show an intriguing tension between the value inferred from the CMB and that obtained from high-redshift source number counts. While the measured dipole direction appears consistent, the amplitude, set by the observer's peculiar velocity $v_{o}$, remains in significant disagreement. In this paper, we propose using strong gravitational lenses with well-measured Einstein radii to estimate the kinematic cosmic dipole, through the relativistic aberration of the Einstein angle induced by the observer's motion. We show that this effect could be detected solely from measurements of the Einstein radius in wide, high-resolution imaging surveys such as Euclid. However, the precision achievable using Einstein-radius measurements alone, without redshift or lens-galaxy mass information, appears insufficient to discriminate between the CMB value of $v_{o}$ and that derived from source number counts at high statistical significance. Nevertheless, we demonstrate that including a large sample of lenses with available kinematic information, either via the Fundamental Plane relation or, ideally, through spectroscopic velocity-dispersion measurements, drastically reduces the noise and substantially improves the constraining power of this method. We show that, for a realistic sample of strong lenses detected by Euclid and complemented with spectroscopic velocity dispersion measurements from 4MOST or DESI, it is possible to discriminate between the CMB- and source-number-counts-inferred values at the $\sim 5\sigma$ level using a new, fully independent method. We further demonstrate that this technique is only weakly sensitive to strong-lensing selection effects, with selection biases and threshold effects estimated to be well below the 1% level.
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All authors Martin Millon, Charles Dalang, Thomas Collett, Camille Bonvin

Solar and Stellar Astrophysics

AREPAS: A Resource for Exploring Protostellar Accretion Systems - Data Release I

Marbely Micolta, Thanawuth Thanathibodee, Katya Gozman et al. (+1 more)

We present AREPAS, a visualization tool for the exploration of the first data release of the open library of magnetospheric accretion models for T Tauri Stars. The dataset covers the typical observed range of spectral types, mass accretion rates and inclinations of typical protoplanetary disks. This data release includes the emission lines: H$\alpha$, H$\beta$, H$\gamma$, Pa$\beta$, Pa$\gamma$, Pa$\delta$, Br$\gamma$, Ca II K, Ca II 8498 Å, Ca II 8542 Å. AREPAS allows for parameter exploration and the comparison of models to user-input observations.
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All authors Marbely Micolta, Thanawuth Thanathibodee, Katya Gozman, Nuria Calvet

High Energy Astrophysical Phenomena · Cross-listed from hep-ph

Intrinsic Nonlocality of Spin- and Polarization-Resolved Probabilities in Strong-Field Quantum Electrodynamics

Samuele Montefiori, Antonino Di Piazza, Tobias Podszus et al. (+2 more)

Spin and polarization are central to precision tests of fundamental physics and for interpreting radiation from astrophysical sources and ultraintense laser-matter experiments. Here, focusing on the fundamental process of nonlinear Compton scattering, we demonstrate that a key assumption underlying current strong-field quantum electrodynamics (SFQED) models, i.e., that emission can be treated as an instantaneous random event sampled from a local differential rate, is inconsistent once emission angles, electron spin, and/or photon polarization are resolved. Namely, even in strictly constant and uniform fields, the resulting fully differential distribution is sign-indefinite, yielding negative inferred probabilities. The physical reason is that the photon emission probability builds up over a finite length of the electron trajectory, the formation region, during which the electron direction changes by roughly the same small angle that defines the radiation cone. Therefore, we put forward a new method where we integrate over this formation region analytically to obtain a physically consistent electron spin and photon polarization model. Simulations of a GeV-class electron-laser collision accessible at current petawatt facilities and of emission in a pulsar-like magnetic field are shown to reveal spin and polarization patterns that differ even qualitatively from state-of-the-art local models. In particular, our new model predicts substantial angle-dependent circular photon polarization where the well-known collinear-emission approach yields none, and a pronounced helicity bias in the recoiling electrons absent from current predictions. These findings have direct implications for upcoming strong-field QED experiments and for interpreting polarized radiation from extreme astrophysical environments.
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All authors Samuele Montefiori, Antonino Di Piazza, Tobias Podszus, Christoph H. Keitel, Matteo Tamburini

Instrumentation and Methods for Astrophysics

Masses of Potentially Habitable Planets Characterized by the Habitable Worlds Observatory

Kaz Gary, B. Scott Gaudi, Eduardo Bendek et al. (+5 more)

Constraints on the masses of exoplanets directly imaged and characterized by the Habitable Worlds Observatory (HWO) are crucial for categorizing these planets and interpreting their spectra. In particular, achieving a mass measurement with a precision of approximately 10% or better may be necessary to identify the dominant gaseous species in the atmospheres of Earth-like planets. This is essential for assessing their habitability and interpreting potential biosignatures (arXiv:2502.01513). Space-based astrometry will be required to measure the masses of planets in face-on systems, or planets orbiting hot and rapidly rotating or highly active stars. Astrometric uncertainties are dominated by the number and magnitude of background reference stars needed to precisely measure the astrometric wobble of the target star induced by the planet. To that end, we propose a program to measure the masses of Earth analogs orbiting HWO target stars with ultra-high-precision astrometry obtained with the HWO high-resolution instrument. We assess the photon-noise error budget for these observations. We find that, for a field of view spanning a few square arcminutes, the astrometric uncertainty due to the number and brightness of reference stars dominates the photon-noise error budget, particularly for targets near the Galactic poles. We explored the impact of filter choice and location in the sky on the photon-noise astrometric uncertainties by simulating the magnitude distribution of reference stars across different filters at a range of galactic longitudes and latitudes. We find that a ~ 200-day survey in the Gaia G band consisting of 100 epochs per target star distributed over the 5-year prime mission with a 6m aperture HWO equipped with a 6' x 6' field-of-view would be required to achieve the photon-noise sensitivity to measure the masses of the ~ 40 Earth-mass habitable-zone planets to ~10%.
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All authors Kaz Gary, B. Scott Gaudi, Eduardo Bendek, Tyler Robinson, Renyu Hu, Breann Sitarski, Aki Roberge, Eric Mamajek

Earth and Planetary Astrophysics

Martian concretion sizes predicted from two independently constrained inputs: atmospheric dust grain size and obliquity-forced wetting duration

Samuel Cody

Diagenetic concretions have been identified at multiple widely separated sites on Mars, including Meridiani Planum (Opportunity), Gale crater (Curiosity), and Jezero crater (Perseverance). Solid concretions at all sites fall within the millimetre size range (typically 1-6 mm diameter), despite differing cement mineralogies. The one substantial outlier – centimetre-to-decimetre-scale hollow concretions on Bradbury Rise – formed in coarser basaltic sandstone via a distinct mechanism. I propose that this size convergence reflects a common physical control: the globally uniform fraction of ultra-fine (~3 um), amorphous, equant atmospheric dust incorporated into sediments at all sites. I derive the diagenetic timescale from Mars' ~120 kyr obliquity cycle, which drives periodic subsurface wetting: each high-obliquity pulse (~10^4-10^5 yr) sets the available growth time. Using a diffusion-reaction model with nucleation competition, I show that the low effective diffusivity imposed by the fine dust matrix limits concretion growth to the observed millimetre scale, independent of local fluid chemistry. Formation efficiency in dust-rich sediment exceeds 90%, making concretion formation essentially inevitable wherever liquid water contacts the dust. This mechanism depends on the non-phyllosilicate, equant-grain mineralogy of Martian dust, which maintains connected pore networks unlike terrestrial clays. Growth is self-limiting: the first wetting pulse exhausts reactive phases in the depletion halo, so successive obliquity cycles produce new concretions in fresh sediment rather than enlarging existing ones. Each concretion records a single wetting episode. The narrow size distributions at all sites suggest that Martian concretion populations may constitute a sedimentary archive of the planet's obliquity history.
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Instrumentation and Methods for Astrophysics

The ocean worlds science case for the Pollux spectropolarimeter

Vincent Hue, Bilal Benmahi, Mathieu Barthelemy et al. (+9 more)

Pollux is a candidate European instrument contribution to the Habitable Worlds Observatory (HWO), designed to advance our understanding of the formation and evolution of cosmic structures in the universe, and specifically search signs of life on extrasolar planets. This high-resolution spectrograph (R\,$>$\,40,000) with polarimetric capabilities offers nearly continuous and simultaneous coverage from the FUV ($\sim$100\,nm) to the NIR ($\sim$1.9\,$\micron$), making it a versatile tool for a wide range of scientific investigations from solar system studies to cosmology. Several Solar System ocean worlds have been the focal point of the scientific community to understand the conditions of their internal saline oceans, as well as the possible emergence of life beyond Earth. The ocean world science case will leverage Pollux's UV spectropolarimetric capabilities to investigate surface reflectance and composition, characterize airglow emissions in the environments of giant-planet moons, as well as constrain the microphysical properties of atmospheric aerosols.
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All authors Vincent Hue, Bilal Benmahi, Mathieu Barthelemy, Tracy M. Becker, Jean-Claude Bouret, Richard J. Cartwright, Jean-Yves Chaufray, Luca Fossati, Gabriel Giono, Olivier Poch, Ujjwal Raut, Lorenz Roth

Instrumentation and Methods for Astrophysics

SpectralUnmix: A Torch-Based Regularized Non-negative Matrix Factorization

Rafael S. de Souza, Paula Coelho, Niranjana P et al. (+2 more)

We present SpectralUnmix, an R package for regularized non-negative matrix factorization (NMF), implemented in torch with optional GPU acceleration. The package estimates low-rank non-negative representations through proximal-gradient updates and allows smoothness regularization along the spectral axis. As a compact demonstration, we apply the method to a subset of stellar spectra and compare the recovered NMF components with principal-component directions and representative stellar spectra. The package is released under the MIT license at \href{https://rafaelsdesouza.github.io/SpectralUnmix/}{this repository}.
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All authors Rafael S. de Souza, Paula Coelho, Niranjana P, Ana L. Chies-Santos, Rogério Riffel

Instrumentation and Methods for Astrophysics

Searching for Life-As-We-Don't-Know-It: Mission-relevant Application of Assembly Theory for Exoplanet Life Detection

Sara Walker, Estelle Janin, Evgenya Shkolnik et al. (+2 more)

This white paper introduces a framework for applying Assembly Theory (AT) to planetary atmospheres as a biosignature framework suitable for the Habitable Worlds Observatory (HWO). AT quantifies the minimum combinatorial complexity required to co-construct an observed ensemble of molecular species, providing a measure of how much selection and evolution is encoded in a planetary atmosphere's chemical space, without assuming any specific biochemistry, kinetics nor metabolism. We outline some forthcoming results applying this framework and how it can be extended to population-level exoplanet studies, validated against existing spectroscopic data, and used to directly inform HWO instrumental requirements. Rather than imposing a binary alive/dead classification, AT-based atmospheric analysis would provide a continuous measure of planetary complexity, opening a path toward detecting life-as-we-don't-know-it.
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All authors Sara Walker, Estelle Janin, Evgenya Shkolnik, Louie Slocombe, Leroy Cronin

High Energy Astrophysical Phenomena

XMM-Newton Observation and Optical Monitoring of the Candidate Redback Millisecond Pulsar 1FGL J0523.5$-$2529

J. P. Halpern and S. Bogdanov

1FGL J0523.5$-$2529 is a Fermi selected redback millisecond pulsar candidate that exhibited luminous optical and X-ray flares in 2020-2021. We obtained a simultaneous X-ray and $U$-band observation with XMM-Newton in 2025, the first to cover the 16.5 hr orbit of 1FGL J0523.5$-$2529. The X-ray luminosity was in an intermediate state with a power-law photon spectral index of $\Gamma=1.53\pm0.02$. Apparent fluctuations were superposed on a broad, single-peaked modulation, the latter characteristic of intrabinary shock models in which the shock front is wrapped around the pulsar. The $U$-band light curve was dominated by ellipsoidal modulation of the nearly Roche lobe filling companion star, similar to that seen in ground-based optical photometry. We also used this effect in 10 years of ATLAS monitoring to improve the precision of the orbital period to 0.6881366(19) days. Considering that searches for radio pulsations from 1FGL J0523.5$-$2529 at all orbital phases have been unsuccessful, it may be that the shocked wind usually surrounds the pulsar.
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Cosmology and Nongalactic Astrophysics

Blind mitigation of foreground-induced biases on primordial $B$ modes for ground-based CMB experiments

Aliza Mustafa, Alessandro Carones, Nicoletta Krachmalnicoff et al. (+2 more)

Observations of the Cosmic Microwave Background (CMB) B-mode polarisation provide a unique probe of inflationary physics. Extracting a reliable constraint on the tensor-to-scalar ratio $r$ nonetheless demands stringent suppression of diffuse Galactic foregrounds, whose residuals can bias the inferred signal. This work introduces and evaluates two extensions of the Needlet Internal Linear Combination (NILC) framework aimed at reducing foreground-induced biases on $r$. The first extension implements the deprojection of selected foreground moments directly within the component-separation step. The second performs a likelihood-level marginalisation over residual foreground power using a data-driven template. Using Simons Observatory Small Aperture Telescope (SO-SAT) - like simulations, we show that both methods effectively control residual contamination, yielding unbiased estimates of $r$ and a consistent reconstruction of the lensing B-mode amplitude. These results indicate that enhanced foreground-mitigation strategies will be useful for next-generation CMB polarisation analyses seeking a robust detection of primordial B-modes.
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All authors Aliza Mustafa, Alessandro Carones, Nicoletta Krachmalnicoff, Marina Migliaccio, Carlo Baccigalupi

Earth and Planetary Astrophysics

Oxygenated False Positive Biosignatures in Mars-like Exoplanet Atmospheres

Margaret Turcotte Seavey, Shawn Domagal-Goldman, Amber Young et al. (+3 more)

Oxygen is a well-studied biosignature. Studying potential abiotic pathways for O2 build-up in exoplanet atmospheres is essential for evaluating whether the detection of O2 would constitute a biosignature detection on other worlds. Previous modeling efforts in the literature demonstrated that detectable abiotic O2 and O3 can be produced through CO2 photolysis for rocky planets around M dwarf stars. Building on modeling approaches from previous studies, we use photochemical simulations to reassess the conditions under which O2 and O3 may accumulate through similar photochemical mechanisms. Using a Mars-like atmospheric composition and planetary parameters, we vary the hydrogen mole fraction to assess how changes in HOx chemistry can affect the resulting accumulation of abiotic O2 and O3. Across the range of hydrogen mole fractions explored, we obtain a maximum O2 abundance of ~2.7% for H = 0.0065 ppm, about an order of magnitude lower than reported in the literature. This reduction is consistent with the elevated water vapor abundance adopted in our simulations, which enhances HOx-driven recycling of CO and O and thereby suppresses the accumulation of O2 and O3. Our improved understanding of how this cycle results in atmospheric false positive biosignatures in crucial towards developing future exoplanet characterization strategies.
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All authors Margaret Turcotte Seavey, Shawn Domagal-Goldman, Amber Young, Jaime Crouse, Jacob Lustig-Yaeger, Giada Arney

Solar and Stellar Astrophysics

No Gravitational Anomaly in Wide Binaries from Forward Modeling of 3D Orbits

Serat M. Saad and Yuan-Sen Ting

Recent work by Chae et al. (2026) reported a gravitational anomaly in 36 wide-binary pairs, finding a gravity boost factor of $\gamma \equiv G_{\rm eff}/G_{\rm N} \approx 1.60_{-0.14}^{+0.17}$ at low accelerations, consistent with predictions from Modified Newtonian Dynamics (MOND). We reanalyze the same dataset using a hierarchical Bayesian model that infers a global $\gamma$ across all systems while fitting three-dimensional orbital elements. Our model yields $\gamma = 1.00^{+0.24}_{-0.19}$, consistent with Newtonian gravity ($\gamma = 1$). To identify the source of the discrepancy, we perform a test using an approach similar to Chae et al. (2026), replacing the semi-major axis with a geometric de-projection of the observed projected separation. This test yields $\gamma = 1.56^{+0.21}_{-0.18}$, closely matching the result of Chae et al. (2026). This shows that if the three-dimensional orbital separation is forward modeled using an independent semi-major axis parameter and then projected into the space of the observables, the relative velocities of wide binaries are consistent with Newtonian gravity.
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Solar and Stellar Astrophysics

Searching for Magnetic White Dwarfs in LAMOST DR10

Si-Cheng Yu, Juan-Juan Ren, Vitaly V. Neustroev et al. (+7 more)

Magnetic white dwarfs (MWDs) are key to understanding the origin and evolution of magnetic fields in compact stars. While large spectroscopic surveys such as SDSS have greatly expanded the known sample, the potential of LAMOST has not yet been fully explored. Our aim is to identify and characterize isolated MWDs in the LAMOST DR10 database. We cross-matched LAMOST DR10 spectra with white dwarf candidates from Gaia EDR3 and with recent SDSS-based catalogs of MWDs. Zeeman splitting in Balmer and helium absorption lines was used as the primary diagnostic to identify magnetic fields and to estimate their strengths. Reference objects from SDSS catalogs were used to test the detectability of MWDs in LAMOST low-resolution spectra. We identified 63 isolated MWDs in LAMOST DR10, of which 32 are new discoveries. Surface magnetic field strengths were measured from Zeeman splitting, covering a range from a few MG up to several tens of MG. For previously known SDSS MWDs, our LAMOST-based field measurements show mostly agreement with published values. This work demonstrates the capability of LAMOST low-resolution spectroscopy to identify and characterize isolated MWDs. The newly discovered objects expand the known population and provide valuable targets for future high-resolution spectroscopic and polarimetric follow-up studies. Our results highlight the potential of combining LAMOST with Gaia and other large surveys to build a more complete census of MWDs.
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All authors Si-Cheng Yu, Juan-Juan Ren, Vitaly V. Neustroev, Thomas Hackman, Hao-Tong Zhang, Yi-Qiao Dong, Zhong-Rui Bai, Hai-Long Yuan, Mengxin Wang, Ming Zhou

High Energy Astrophysical Phenomena

Universal behaviour of $\alpha$-viscosity in black hole accretion discs

Marek A. Abramowicz, Axel Brandenburg, Jiří Horák et al. (+4 more)

The Shakura-Sunyaev $\alpha$-viscosity coefficient, defined as the ratio of total stress to total pressure, $\alpha= \mathbb{T}/p$, began to play an important role in the development of accretion disc theory in the early 1970s. The origin of the turbulence that causes the stress $\mathbb{T}$ was unknown at that time; Shakura and Sunyaev assumed $\alpha=$ const. Today we know that this was not quite realistic - modern general relativistic magneto-hydrodynamic simulations (GRMHD) of black hole accretion discs have revealed that $\alpha$ changes by about an order of magnitude within the disc, being smaller far away from the black hole and larger in the plunging region close in, and it has been found that the behaviour of $\alpha$ reflects some underlying, fundamental properties of the stress $\mathbb{T}$. In particular, it has been argued by several authors, that $\mathbb{T}$ must be zero at the black hole horizon. We note that the stress calculated in three independent GRMHD simulations of accretion discs around non-rotating black holes, made by a variety of authors (including ourselves), each has its prominent maximum close to the location of the circular photon orbit. We propose a formula that accurately describes this 'universal' behaviour of $\alpha$ in terms of the 'gyration radius', a physical characteristic of rotation well known in Newtonian dynamics and in the black hole case uniquely defined by the Kerr space-time geometry. Analytic and semi-analytic models of black hole accretion discs provide an invaluable insight into fundamental physics, and the GRMHD simulations do not aspire to replace them. Rather, simulations could help to improve analytic models by making them more realistic. For example, our $\alpha$-formula, deduced from the GRMHD simulations, may be useful in the construction of improved versions of thin and slim disc models.
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All authors Marek A. Abramowicz, Axel Brandenburg, Jiří Horák, Debora Lančová, John C. Miller, Ewa Szuszkiewicz, Maciek Wielgus

Cosmology and Nongalactic Astrophysics · Cross-listed from gr-qc

Holographic dark energy from a new two-parameter entropic functional

G. G. Luciano and E. N. Saridakis

We formulate an extended holographic dark energy scenario based on a recently proposed two-parameter generalized entropic functional. Unlike constructions that phenomenologically impose modified entropy-area relations at the horizon level, the present framework is rooted in a microscopic entropy functional and the corresponding microstate counting. For bounded systems, the entropy acquires a generalized holographic scaling with two independent area contributions, recovering the Bekenstein-Hawking entropy in the appropriate limits. Implementing this entropy within the holographic principle, we derive a generalized dark energy density containing two distinct holographic sectors, naturally embedding standard holographic dark energy and $\Lambda$CDM as limiting cases. We analyze the cosmological evolution for both Hubble and future event horizon cutoffs and show that the model successfully reproduces the matter-to-dark-energy transition. The two entropic exponents enrich the dynamics, allowing for quintessence-like behavior or phantom regimes, while remaining compatible with the standard thermal history of the Universe.
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Earth and Planetary Astrophysics

Gaia-GIC-1: An Evolving Catastrophic Planetesimal Collision Candidate

Anastasios Tzanidakis and James R. A. Davenport

We report the discovery of the optical dipper and low-luminosity infrared stellar transient Gaia20ehk (hereafter, Gaia-GIC-1), which is currently undergoing high-amplitude variability due to transiting dusty material. In this work, we identify Gaia-GIC-1 as a likely young F-type star based on the spectral energy distribution before the onset of the high-amplitude optical variability. We detect a significant periodic modulation of 380.5 days in Gaia-G band before the onset of the infrared brightening, consistent with a $\sim$1.1 AU orbit assuming circular orbits and a 1.3 M$_{\odot}$ star. The system has remained in an infrared bright state for $>$4 years since the last near-infrared detection, confirmed by recent SPHEREx observations, while continuing to undergo large amplitude irregular optical dimming. We measure the dust temperature from the freshly generated debris to be $\sim$900 Kelvin based on available WISE photometry, and the dust clump size to have a minimum cross-sectional area of 0.13 AU$^{2}$, and the dust mass $4\times 10^{20}$ kg. Currently, optical follow-up spectroscopy has not revealed any prominent features in the system, likely due to its highly variable nature. We hypothesize that Gaia-GIC-1 represents debris recently formed in a planetary collision, which produced a clumpy dust cloud on a bound orbit, producing the observed dimming events. The ongoing collisional activity in this system presents a unique opportunity for understanding terrestrial planet formation.
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Earth and Planetary Astrophysics

TOI-4616 b: a benchmark Earth-sized planet transiting a nearby M4 dwarf

F. Zong Lang, B. O. Demory, Y. Gomez Maqueo Chew et al. (+44 more)

Rocky exoplanets are particularly abundant around M-type stars. Their small radii and low luminosities provide favourable conditions for detecting transiting terrestrial planets and probing their atmospheric properties. We report the discovery and statistical validation of TOI-4616 b, an Earth-sized planet transiting a nearby mid-M dwarf observed by the Transiting Exoplanet Survey Satellite (TESS). We confirm the planetary nature of the signal and determine the system parameters by combining TESS photometry with ground-based multi-band transit observations, high-resolution imaging, and optical and near-infrared spectroscopy. The host star lies at a distance of 28.10 +(-) 0.07 pc and has a radius of 0.1889 +(-)0.0096 solar radii, a mass of 0.1881 +(-) 0.0094 solar masses, and an effective temperature of 3150 +(-) 75 K. TOI-4616 b has a radius of 1.22 Earth radii and an orbital period of 1.55 days. The planet receives an incident flux of approximately 40 times that of Earth, corresponding to an equilibrium temperature of about 525 K. This places TOI-4616 b in a regime intermediate between Earth-sized planets orbiting early M dwarfs and those around ultra-cool hosts. Statistical validation with the TRICERATOPS framework, supported by high-resolution imaging and chromatic transit constraints, yields a false-positive probability of 0.0135, below the recommended validation threshold of 0.015, confirming TOI-4616 b as a validated planet. Owing to its proximity to Earth, well-constrained stellar properties, and extensive multi-band follow-up, TOI-4616 b constitutes a valuable benchmark system for comparative studies of terrestrial planets around mid-M dwarfs and for future atmospheric investigations.
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All authors F. Zong Lang, B. O. Demory, Y. Gomez Maqueo Chew, Y. Schmid, M. Timmermans, F. J. Pozuelos, M. Gillon, Artem Y. Burdanov, Benjamin V. Rackham, Didier Queloz, Keivan G. Stassun, Khalid Barkaoui, Amaury Triaud, Julien de Wit, S. Zuniga-Fernandez, A. J. Burgasser, Elsa Ducrot, Madison G. Scott, D. Sebastian, A. Soubkiou, M. Lendl, I. Plauchu-Frayn, U. Schroffenegger, Erik Meier V., P. Pedersen, A. Khandelwal, Roman Gerasimov, C. Aganze, Chih-Chun Hsu, J. M. Jenkins, Aishwarya R. Iyer, C. Watkins, C. A. Theissen, K. A. Collins, H. P. Osborn, A. Shporer, Claudia Jano Munoz, Toshi Suganuma, Norio Narita, Akihiko Fukui, F. Murgas, J. de Leon, Enric Palle, Yasmin Davis, D. Kitzmann, M. Pichardo Marcano, M. J. Hooton

Solar and Stellar Astrophysics

Catalogue and statistics of greater than 100 MeV solar proton events during solar cycles 23-25 from SOHO-ERNE observations

M. Jarry, C. Palmroos, E. Lavasa et al. (+12 more)

The SPEARHEAD (specification, analysis, and re-calibration of high-energy particle data) project, funded by the European Union Horizon Europe programme, enhances the accuracy and usability of high-energy particle measurements. It investigates particle acceleration, release, and transport during solar eruptions by refining instrument response functions and cross-calibrating datasets. We present a comprehensive catalogue of greater than 100 MeV proton events identified from May 1996 to August 2024 using SOHO-ERNE penetrating-particle rates, together with associated solar phenomena derived from multi-instrument observations. The SEP events were detected through a systematic scan of ERNE-HED counter data and cross-calibrated with SOHO-EPHIN to derive peak fluxes and fluences. Each event was associated with its likely parent eruption using X-ray (XR) (GOES-XRS, RHESSI, SolO-STIX), radio (Wind-WAVES, STEREO-WAVES, ground-based observatories), and gamma-ray (Fermi-LAT) observations, CMEs (SOHO-LASCO), and ground-level enhancements (GLEs) (neutron monitors). Timings and physical properties were systematically compared to investigate the relationships between SEP onset, flare evolution, CME kinematics, and radio signatures. Statistical analyses reveal that most SEP releases closely follow flare and CME onsets, with moderate SEP-XR-CME correlations, and a strong SEP-GLE fluence link. These results indicate that high-energy SEP events are typically associated with strong solar activity signatures, with the observed intensities and timings strongly modulated by magnetic connectivity and coronal conditions. This catalogue provides the most extensive reference to date for high-energy SEP events over solar cycles 23-25, establishing a unified framework for future investigations of extreme particle acceleration.
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All authors M. Jarry, C. Palmroos, E. Lavasa, N. Talebpour Sheshvan, M. Koeberle, B. Heber, A. P. Rouillard, A. Papaioannou, J. Gieseler, C. Ngom, P. Oleynik, E. Riihonen, R. Vainio, G. Vasalos, A. Anastasiadis

High Energy Astrophysical Phenomena

X-ray polarization in the soft state of Cyg X-1

A. Niedzwiecki, M. Szanecki, A. Veledina et al. (+5 more)

We aim to identify the physical mechanism responsible for the observed X-ray emission and polarization of Cyg X-1 in the soft state. We performed a detailed spectral analysis of X-ray data obtained with NICER, NuSTAR, and INTEGRAL during observations simultaneous with IXPE on 2023 June 20, supplemented at higher energies with archival CGRO data. We developed a new model, retBB, which describes thermal disk emission and its returning reflection, and applied it together with accurate models of Comptonization and relativistically broadened reflection. Using the resulting spectral solution, we computed the expected polarization signal and compared it with the IXPE measurements. Our spectropolarimetric modeling shows that the observed polarization is in agreement with being produced by Comptonization in a corona undergoing a semi-relativistic outflow at a velocity of ~0.3c. Our spectral solutions admit either low or high black-hole spin values, depending on the adopted model setup. However, the observed polarization strongly favors a low spin. At high spin, the polarization angle would inevitably rotate significantly across the energy band, which is inconsistent with the observations. Apart from this rotation of the polarization angle, general relativistic effects do not play a significant role in producing the observed polarization. In particular, we find that, at most, returning disk radiation contributes only a minor amount.
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All authors A. Niedzwiecki, M. Szanecki, A. Veledina, A. A. Zdziarski, A. Chakraborty, J. Poutanen, P. Lubinski, A. Salganik

Astrophysics of Galaxies

Spectroscopic galaxy redshifts in the Peanut cluster – a massive nearly head-on cluster merger shortly after pericenter passage

I. Zaznobin, N. Lyskova, I. Bikmaev et al. (+13 more)

The Peanut cluster (SRGe J023820.8+200556, SRGe CL0238.3+2005, $z_{spec}$ = 0.42) has recently emerged as a candidate for a rare, massive merger, potentially analogous to the Bullet cluster. We present the results of optical identification and spectroscopic redshift measurements for 31 galaxies in the Peanut cluster, including 26 new redshifts obtained with the 6-m telescope BTA (Big Telescope Alt-azimuthal) at SAO RAS between October 2024 and January 2025. The derived distribution of line-of-sight velocities reveals the possible presence of two subclusters with a line-of-sight velocity difference of ~2000 km/s. However, statistical tests and the Dressler-Schectman test show that the hypothesis that the observed velocity distribution can be described by a normal distribution for a single cluster cannot be ruled out, and the evidence for the existence of two gravitationally bound substructures remains ambiguous. Assuming a single cluster with the normal velocity distribution, the estimated galaxy velocity dispersion is $\sigma_{los} = 1455 \pm 83$ km/s, corresponding to the total cluster mass of $M_{200} = 2 \times 10^{15} M_\odot$ based on the mass-velocity dispersion scaling relation. In either scenario – a single extremely massive cluster or an ongoing merger – the Peanut cluster appears to be a very rare and peculiar object, comparable to such extreme systems as the Bullet cluster (1E 0657-56) or El Gordo (ACT-CL J0102-4915).
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All authors I. Zaznobin, N. Lyskova, I. Bikmaev, R. Burenin, A. Arshinova, E. Churazov, S. Dodonov, M. Gilfanov, I. Khabibullin, I. Khamitov, S. Kotov, A. Moiseev, S. Sazonov, R. Sunyaev, M. Suslikov, R. Uklein

Solar and Stellar Astrophysics

Kinematics of Wolf-Rayet Stars in the LMC: Clues to Subtype Origins

Caden Burkhardt, Fiona Han, M. S. Oey et al. (+2 more)

We measure transverse proper motion velocities of LMC Wolf-Rayet (WR) stars using Gaia DR3 astrometry. The combined velocity distribution of WNh, O If*/WN, and WNL very massive stars ($>100\ M_\odot$; VMS) shows both slow, unejected objects ($v_\perp < 10$ $\rm km\ s^{-1}$) and stars dominated by fast, runaway velocities ($v_\perp > 24$ $\rm km\ s^{-1}$). This supports expectations that VMS ages are comparable to the dynamical ejection timescale ($\sim1.5$ Myr). These kinematics share similarities with those of lower-luminosity, classical WNh, O If*/WN, and WNL stars, as well as the SMC field OB stars, suggesting that dynamical ejections may also dominate these populations. In contrast, both single and binary WNE stars are ejected populations that show single-peaked velocity distributions, suggesting a different ejection mechanism(s). We speculate that single WNE stars might result from explosive mergers onto the shell-burning layer, thereby stripping the H envelope. Binary WC stars appear to be faster (median $v_\perp = 54$ $\rm km\ s^{-1}$) and have higher luminosities than singles (median $v_\perp = 38$ $\rm km\ s^{-1}$), suggesting that single WC stars are not descendants of the binaries. Thus, the binaries are probably stripped by mass transfer, while the WC singles likely originate from another process. The high velocities of binary WC stars are consistent with some predictions that lower mass clusters generate fast dynamical ejections. Single WC and WN3/O3 stars have ambiguous kinematics, but both show high $v_\perp$ (median $\sim 38$ $\rm km\ s^{-1}$), possibly linked to their lower masses.
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All authors Caden Burkhardt, Fiona Han, M. S. Oey, Natalia Ivanova, Mathieu Renzo