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Physics Astrophysics Astrophysics of Galaxies

astro-ph.GA Watch Astrophysics of Galaxies

Wednesday 9 September

Astrophysics of Galaxies · Cross-listed from astro-ph.HE

Aarmed with Data: Bumps, Outflows, and Disk-like Emission in TDE 2025aarm

Aysha Aamer, Kate D. Alexander, Charlotte R. Angus et al. (+43 more)

The origin of the optical emission in tidal disruption events (TDEs) remains one of the major outstanding questions in the field, in part due to the limited number of nearby events with high-cadence monitoring to track their evolving photometric and spectroscopic properties. We present multi-wavelength observations of the nearby ($z=0.01368$) TDE\,2025aarm, including near-daily spectroscopic coverage prior to the optical peak. Its proximity makes it one of the brightest TDEs discovered, reaching a peak magnitude of $m_r\sim15.5$ ($M_r\sim-18$). The light curve deviates from a smooth evolution, exhibiting multiple rebrightening episodes visible in both the individual filter light curves and the bolometric luminosity. Blackbody modelling reveals that these rebrightenings are associated with an increase in temperature of $> 5,000-10,000$\,K, while the inferred photospheric radius remains approximately constant. Simultaneously, the H$\alpha$ line not only increases in blueshift but also broadens, suggesting a link between the continuum rebrightenings to changes in the kinematics of the line-forming gas. We identify a persistent absorption component at $\sim-3900$\,km\,s$^{-1}$ in multiple Balmer lines, providing further evidence for outflowing material. The H$\alpha$ profile also exhibits excess flux compared to a Gaussian on both sides of the line, inconsistent with simple scattering-dominated outflow models. Disk-profile modelling provides evidence for the emergence of a disk-like component least $\sim20$ days after peak, with substantial changes in the disk properties between $\sim50$ and 60 days. These observations highlight the complexity of TDE emission processes and demonstrate how dense multi-wavelength monitoring can disentangle the roles of accretion, reprocessing, and outflows in shaping TDE emission.
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All authors Aysha Aamer, Kate D. Alexander, Charlotte R. Angus, Panos Charalampopoulos, Megan Newsome, Iair Arcavi, Giorgos Leloudas, Matt Nicholl, Jennifer Andrews, Katie Auchettl, Thomas de Boer, K. Azalee Bostroem, Y.-Z. Cai, Kenneth C. Chambers, Collin T. Christy, Sara Faris, Noah Franz, Sebastian Gomez, Maider González-Bañuelos, Mariusz Gromadzki, Paul J. Groot, Claudia Gutiérrez, Brian Hsu, Mark Huber, Thomas L. Killestein, Chien-Cheng Lin, Thomas B. Lowe, Peter Lundqvist, Dylan Magill, Seppo Mattila, Francesca Onori, Gregory S. H. Paek, Jeniveve Pearson, Miika Pursiainen, Niko Pyykkinen, Conor Ransome, Sara Romagnoli, Jennifer Shi, Manisha Shrestha, Nathan Smith, Maximilian Stritzinger, Bhagya Subrayan, Richard Wainscoat, Ziyang Y. Wang, Simon de Wet, Hannah Wichern

Astrophysics of Galaxies

High Resolution Spectroscopic Follow-up Observation Results for 13 EMP Candidates Selected by Narrow-band Photometry

Huiling Chen, Yang Huang, Huawei Zhang et al. (+1 more)

Extremely metal-poor (EMP; [Fe/H] < -3.0) stars preserve key information about the earliest stages of Galactic chemical evolution. Based on a training set of spectroscopic metallicities, we constructed catalogs of more than 120,000 EMP candidates from large-scale narrow-band photometric surveys. To validate this approach, we obtained high-resolution CFHT/ESPaDOnS spectra for 13 candidates selected from the SkyMapper-based catalog. The follow-up observation confirms that the narrow-band photometric selection is effective in identifying very metal-poor stars and retaining a substantial EMP fraction: four targets are confirmed as EMP stars, and all targets remain very metal-poor with [Fe/H] < -2.4. The photometric metallicities are systematically lower than the spectroscopic values by 0.44 dex, with a relatively small scatter of 0.19 dex, indicating a systematic offset in the photometric metallicity scale that could be reduced through improved calibration. Abundances for more than 20 chemical species are derived, leading to the identification of a new potassium-enhanced r-II star, J2001-1215. By combining chemical abundances with orbital properties, five stars are found to be dynamically consistent with known Galactic substructures, including Gaia–Sausage–Enceladus, Thamnos, and Sequoia. These stars provide high-resolution chemical measurements for the extremely metal-poor regime of these Galactic substructures. These results underscore the role of narrow-band photometry in efficiently selecting EMP candidates for targeted high-resolution spectroscopic follow-up, thereby enabling detailed chemical and dynamical studies of the early Milky Way.
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All authors Huiling Chen, Yang Huang, Huawei Zhang, Zhen Guo

Astrophysics of Galaxies · Cross-listed from astro-ph.SR

Planetary Nebula Central Stars as Tracers of Planetary Nebula-Star Cluster Associations in the Galaxy

Vasiliki Fragkou, Luis Lomelí-Núñez, Dante Minniti et al. (+2 more)

Planetary nebulae (PNe) that are physically associated with star clusters are rare but crucial for stellar evolution studies because they allow their initial and final stellar properties to be determined independently, contributing to constraints on the initial-to-final-mass relation (IFMR), which traces stellar mass loss. Although the identification of such pairs is essential, most reported cases have been identified incidentally rather than through systematic searches. Here, we attempt to uncover new such associations in our Galaxy employing literature PN central stars (CSs) and star cluster catalogues. Exploring relevant angular positions on the sky and kinematic Gaia DR3 data, we identify 38 candidate CS-cluster pairs. The components of eight of these pairs present consistent interstellar extinctions, and their CS properties are compatible with stellar evolution models. Two of these candidate pairs are already reported in the literature as true associations. We evaluate separately the remaining six pairs using literature PN observational data, assessing the likelihood of each pair being a true association. The initial and final masses of our best candidates are mostly consistent with the latest IFMR predictions and confirmed cluster-PNe points. An additional 46 CS-cluster pairs have components with compatible angular positions but lack kinematic CS data and are thus presented separately. More data are needed for a precise estimation of the physical parameters of most of our candidates and to confirm these associations, which will be addressed in future studies.
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All authors Vasiliki Fragkou, Luis Lomelí-Núñez, Dante Minniti, Arianna Cortesi, Denise R. Gonçalves

Astrophysics of Galaxies

Time-Domain Dust Astrophysics. II. TransRAT: Time-Dependent Grain Alignment and Disruption by Cosmic Transients and Their Observational Signatures

Thiem Hoang

We introduce TransRAT, a self-consistent time-domain framework that follows the coupled dynamical response of dust to transient irradiation and predicts its observable signatures. We apply TransRAT to a Type~IIP supernova illuminating a one-zone molecular cloud. Radiative heating raises the grain temperature, modifying the magnetic susceptibility and Larmor precession, while the enhanced radiation field accelerates radiative precession and can switch the alignment axis from the magnetic field (B-RAT) to the radiation direction (k-RAT). Simultaneously, strong RATs align grains faster than gas randomization, extending fast alignment to smaller grain sizes, and disrupt large grains through RAT disruption (RAT-D), irreversibly modifying the grain size distribution. As the transient fades, the alignment axis returns to ${\bf B}$ at a rate controlled by the magnetic susceptibility, whereas the enhanced aligned-grain population and RAT-D-modified grain size distribution can persist long after the radiation has faded- referred to as physical memory effects. The time-dependent evolution of grain alignment and disruption produces distinctive observational signatures: a flare in extinction and thermal dust polarization followed by a RAT-D-induced dip; a blueward shift of the polarization peak $\lambda_{\max}$; a polarization-angle rotation equal to the projected ${\bf B}$–${\bf k}$ separation ($45^{\circ}$ for our adopted geometry) when RAT-D removes large high-$J$ aligned grains and leaves low-$J$ grains aligned with kRAT followed by angle reverberation as alignment returns to ${\bf B}$; and a non-monotonic evolution of $R_V$. The persistent observational manifestations of the transient irradiation, including enhanced polarization, blueshifted $\lambda_{\max}$, modified $R_V$, and polarization-angle evolution, constitute fossil imprints of past cosmic transients.
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Tuesday 8 September

Astrophysics of Galaxies

SPHEREx 3.3 Micron Aromatic Emission in the Magellanic Clouds: Separating Dust Column, Excitation, and Environmental Suppression

Cheng Li

Bright emission in the 3.3um unidentified infrared band, often described as an aromatic infrared band, can trace dust column, radiative excitation, or the survival and emissivity of its carriers. We combine SPHEREx maps of the Magellanic Clouds with HERITAGE dust columns, a TIR heating proxy, Br\alpha, H\alpha, H II-region catalogs, and SAGE 8um imaging. We measure continuum-subtracted 3.3um feature excess, X_{3.3}, and Br\alpha excess, X_{Br\alpha}, in matched cells. In each Cloud, a quiescent diffuse baseline depending on dust surface density, \Sigma_d, and TIR/\Sigma_d predicts X_{3.3} with R^2=0.628 in the LMC and 0.288 in the SMC. Relative to this expectation, LMC H II interiors are under-luminous by -0.223^{+0.035}_{-0.040} dex; local-background measurements give -0.153^{+0.014}_{-0.043} dex despite a total-X_{3.3} enhancement of +0.749 dex. SMC H II-region residuals are consistent with zero over its lower-X_{Br\alpha} regime. A matched comparison with stellar-subtracted 8um dust intensity, I_{8,dust}, shows that column and heating predict I_{8,dust} more tightly than X_{3.3}, and that the high-X_{Br\alpha} LMC deficit is stronger for X_{3.3} than for I_{8,dust}. The deficit is therefore selective to the isolated 3.3um feature, not uniform in all aromatic-band-dominated emission. This result suggests that the small aromatic/aliphatic carbonaceous carriers traced by the isolated 3.3um feature are more susceptible to processing in active regions than the larger or differently emitting carbonaceous-grain population dominating the 8um aromatic complexes.
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Astrophysics of Galaxies

Phase-Spirals Across Galactic Disks II: Using large-scale "macro-spirals" in phase-spiral amplitude to derive perturbation times

Kiyan Tavangar, Kathryn V. Johnston, Jason A.S. Hunt et al. (+3 more)

Phase-space spirals in the Milky Way disk are a key observable remnant of recent perturbations to the Galaxy. They provide insight into the disk's potential, dynamical evolution, past interactions, and even substructure. However, the complex dynamics of phase-spiral formation and evolution have made clear interpretations challenging. For example, recent work has shown that the "winding time" – measured from how tightly wound a phase-spiral is – is a biased estimate of the true time since the inciting perturbation due to the complex effects of self-gravity. In this paper series, we present an alternative approach by looking at correlations in phase-spiral morphology across the Galactic disk. Here we show that following a localized perturbative event, the ridgeline connecting the largest amplitude phase-spiral at each radius winds up into a "macro-spiral" at the rate expected for differential rotation. This means the macro-spiral can 1) be unwound to constrain the origin – time and location – of correlated phase-spiral properties and 2) indicate the delay in individual phase-spiral winding as a novel diagnostic of disk dynamical properties. Applying these ideas to the Milky Way's phase-spirals, we estimate a perturbation time of $\simeq 1$ Gyr ago, consistent with the penultimate passage of the Sagittarius dwarf galaxy through the disk, and delay times of up to 800 Myr for phase-spirals in the inner disk. While the current application of this method to Gaia DR3 data is limited by the available radial velocities, future Gaia data releases will enable stronger constraints using a much larger area of the Galactic disk.
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All authors Kiyan Tavangar, Kathryn V. Johnston, Jason A.S. Hunt, Axel Widmark, Vandana G. Kaushik, Adrian M. Price-Whelan

Astrophysics of Galaxies · Cross-listed from astro-ph.HE

First Detection of Radio Polarization During Jet Formation in the Changing-Look AGN 1ES 1927+654

Onic I. Shuvo, Eileen T. Meyer, Sibasish Laha et al. (+10 more)

We present a multiwavelength radio study of the changing-look AGN 1ES 1927+654 following its renewed X-ray brightening since mid-2022, combining VLBA imaging with multi-band and time-resolved VLA observations. Our main results are: (i) VLBA observations continue to reveal the bipolar jet structure first reported in Meyer et al. (2025), with new X-band detections confirming the morphology at larger separation and K-band imaging revealing a bridge of emission consistent with a continuous outflow; (ii) the VLBA core exhibits a GHz-peaked spectrum, and VLA/VLITE observations from 340 MHz to 45 GHz show emission from an inhomogeneous synchrotron source, self-absorbed below ~3 GHz and optically thin above; (iii) VLA and VLBA flux densities are consistent above 8 GHz, indicating emission dominated by a compact core; (iv) linear polarization is detected above ~3 GHz, with fractional polarization increasing with frequency from ~0.2% to ~6% and EVPAs exhibiting non-linear lambda^2 behavior; (v) long-term VLA X-band monitoring reveals a large, smooth rotation of the EVPA of ~137 degrees over ~81 days in 2026, followed by a partial return, at stable total intensity and suppressed fractional polarization <~1.4%, consistent with a propagating disturbance crossing the compact emitting region; and (vi) circular polarization is not detected at any epoch. These observations demonstrate that 1ES 1927+654 has transitioned from an X-ray-dominated, non-jetted state to a radio-loud AGN (Laha et al. 2025) hosting a newly launched relativistic jet, a unique laboratory for studying jet birth and early magnetic field evolution.
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All authors Onic I. Shuvo, Eileen T. Meyer, Sibasish Laha, Agniva Roychowdhury, Dev R. Sadaula, Tracy E. Clarke, Wendy M. Peters, Mitchell C. Begelman, Markos Georganopoulos, Rostom Mbarek, Amelia M. Hankla, Alexander Philippov, Navin Sridhar

Astrophysics of Galaxies

JWST Spectra Conclusively Show an Excess of Neutral Gas Outflows in Quiescent Galaxies at z=2-5

Rion Oh, Gourav Khullar, Arianna S. Long et al. (+15 more)

Galaxies exhibit a broad range of star formation activity, from actively star-forming to quiescent systems. Yet, the mechanisms responsible for the rapid shutdown and continued suppression of star formation remain poorly understood, particularly for the quiescent galaxies recently uncovered by JWST at $z>3$. Neutral gas outflows provide a direct tracer of gas removal and regulation in quiescent galaxies. We therefore present a NaID $\lambda\lambda5891,5897$ absorption–line stacking analysis of 274 galaxies at $z=2$–5 drawn from the DAWN JWST Archive (DJA) and the EMBER JWST program, using JWST/NIRSpec medium-resolution grating spectroscopy and NIRCam photometry. NaID absorption is detected in all quiescent galaxy stacks spanning $z=2$–5, providing the first statistical evidence that neutral gas outflows are a ubiquitous feature of quiescent galaxies in the early Universe. Compared to the non-quiescent galaxy stacks, the quiescent stacks show mass loading factors higher by 2–4 dex, with mass outflow rates $\dot{M}_{\rm out} \sim 9$–$30\,M_\odot\,{\rm yr}^{-1}$ elevated by $\sim0.5$–$1$ dex and outflow velocities $v_{\rm out} \sim 310$–$570\,{\rm km\,s}^{-1}$ higher by a factor of $\sim2$. The extreme mass loading factor values and declining star formation histories strongly indicate that these outflows cannot be driven by current star formation alone. In the statistically reliable quiescent redshift bins at $2 \leq z < 3$ and $3 \leq z < 4$, the elevated $[\mathrm{N\,II}]/\mathrm{H}\alpha$ and $[\mathrm{O\,III}]/\mathrm{H}\beta$ ratios are consistent with a possible AGN contribution, suggesting that non-stellar feedback may play a role in sustaining quiescence in galaxies at these epochs.
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All authors Rion Oh, Gourav Khullar, Arianna S. Long, Julissa Sarmiento, Hollis B. Akins, Caitlin M. Casey, Yingjie Cheng, Andreas L. Faisst, Maximilien Franco, Ghassem Gozaliasl, Micheaela Hirschmann, Erini Lambrides, Jacqueline E. McCleary, Tiara Anderson, David C. Andrews, Dylan Berry, Nguyen Binh, Elaine Gammon

Astrophysics of Galaxies

Characterization of Orbits in Bars in Disc Galaxies Using Fourier Frequencies

A. Silva-Castro, I. Puerari and D. Valencia-Enríquez

Bars are among the most prominent dynamical structures in disc galaxies, yet their long-term evolution and the orbital families that sustain them remain only partially understood. In this work, we introduce a new methodology based on frozen potentials combined with Fourier analysis of particle trajectories to identify regular and sticky orbits in self-consistent $N$-body simulations. This approach allows us to isolate particles belonging to the $x_1$ family, directly associated with the bar, and the $x_2$ family, linked to a secondary perpendicular structure. Tracking these orbits across multiple snapshots enables us to quantify the time evolution of key bar properties, including its mass fraction, semi-axes, and density profile. Compared with established used diagnostics, our method provides the closest match to the temporal behaviour of the Fourier bar-strength parameter $A_2$, while offering a direct dynamical interpretation and full three-dimensional characterization that Fourier-based approaches cannot provide. Our analysis further reveals that the indices $n_x$, $n_y$, and $n_z$ describing the bar density distribution are anisotropic and rarely equal to the canonical $n=2$ assumed in Ferrers profiles, highlighting the limitations of standard analytical approximations. We also find strong correlations between bar growth, orbital chaoticity, and angular momentum redistribution, with the bar acting both as a sink and as a driver of angular momentum transfer. A comparison between the two models analysed shows that rapidly forming bars are weaker and less stable than those that grow more gradually. Altogether, our methodology provides a robust framework for linking orbital structure to the dynamical evolution of barred galaxies.
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Astrophysics of Galaxies

TDCOSMO XXX: Spatially resolved kinematics of the deflectors in time-delay lens systems B1608+656 and SDSSJ1206+4332 from JWST-NIRSpec observation

Pritom Mozumdar, Shawn Knabel, Anowar J. Shajib et al. (+3 more)

We present spatially resolved stellar kinematics of the deflector galaxies in two time-delay lens systems, B1608+656 and SDSS J1206+4332, measured from JWST NIRSpec integral field spectroscopy. B1608+656 is a quadruply imaged quasar lensed by a pair of tidally interacting galaxies, the primary deflector (G1) and a close companion (G2), for which we quantified G2's tidal influence on G1 and define a criterion identifying the region where this perturbation is negligible and standard Jeans-equation modeling remains valid. SDSS J1206+4332 is a doubly imaged quasar whose extended host galaxy is itself quadruply imaged by a foreground massive galaxy (G0), accompanied by a fainter perturber (G1). For both systems, we report the first spectroscopic redshift and velocity dispersion measurements of the perturber galaxies. For G2 in B1608+656, we measure {\sigma}_v = 188 +/- 7 (stat.) +/- 2 (sys.) km/s at z = 0.6307, identical to the redshift of the main deflector G1. For G1 in SDSS J1206+4332, we find {\sigma}_v = 100 +/- 21 (stat.) +/- 2 (sys.) km/s at z = 0.7403, slightly lower than the primary deflector G0. The resolved velocity maps of both primary deflectors show clear rotation signatures and we visually classify them as fast or regular rotators. The data products will be made publicly available and will be combined with time delays, lens models, and line-of-sight convergence to measure cosmological parameters in the upcoming TDCOSMO 2026 milestone publication.
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All authors Pritom Mozumdar, Shawn Knabel, Anowar J. Shajib, Tommaso Treu, Michele Cappellari, Aymeric Galan

Astrophysics of Galaxies · Cross-listed from astro-ph.IM

The future of high-resolution UV spectroscopy: Science with a UV Échelle spectrograph on the Habitable Worlds Observatory, or a dedicated mission

Annalisa De Cia, David Cont, Kevin Heng et al. (+39 more)

High-resolution UV spectroscopy serves a diversity of science cases, from small bodies to planets, stars, and galaxies, but is currently limited to the Hubble Space Telescope and bright targets. Major advances require increasing sensitivity by at least one order of magnitude. Here we present the UV science cases for PEGASUS (Planets, Earths, Galaxies, And Stars UV Spectrograph), a UV Échelle high-resolution spectrograph concept, with $R = \lambda/\delta\lambda \sim 100\,000$ (full range 10 000-140 000) and covering 90–400 nm, with a foreseen extension to at least 800 nm. PEGASUS is ideally suited for the Habitable Worlds Observatory (HWO), enabling transformative science across the UV/optical wavelength ranges. PEGASUS will be unique in high sensitivity (effective area) and high spectral resolution – an uncharted territory – as well as robustness, thanks to the simplicity of its design. Its UV science cases include: I) Formation and evolution of planets and their habitability: properties of exoplanets and atmospheres, protoplanetary disks, Solar System bodies; II) Stellar lives and deaths at their extremes: the first stars and the origin of the elements, compact and massive stars, Supernovae; III) Gas and metals in the baryon cycle of galaxies: the interstellar, circumgalactic, and intergalactic medium and their roles in galaxy growth. These are essential for the Astro Decadal 2020 Survey, Voyage 2050, and HWO. While this paper focuses on high-impact science enabled by UV high-resolution spectroscopy, PEGASUS will extend into the optical regime and lower spectral resolution, making it a multi-purpose, widely used, workhorse spectrograph for HWO.
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All authors Annalisa De Cia, David Cont, Kevin Heng, Boris Gänsicke, Frank Grupp, Ralf Bender, Helena Lamprecht, Frances H. Cashman, Hsiao-Wen Chen, Valentina D'Odorico, Kevin France, Andrew Fox, Jens Hoeijmakers, Masahiro Ikoma, Wynn Jacobson-Galán, Sean D. Johnson, Mansi Kasliwal, Shingo Kameda, Varsha Kulkarni, Carlo F. Manara, Antonija Oklopčić, Cyrielle Opitom, Lidia Oskinova, Anna F. Pala, Ian U. Roederer, P. Christian Schneider, Peter Scicluna, Daniel E. Welty, Sascha Zeegers, Abel de Burgos Sierra, Miriam García, Avery Kim, Christina Konstantopoulou, Roberto Maiolino, Daniel Pauli, Tanita Ramburuth-Hurt, Philipp Richter, Andreas A.C. Sander, Tomer Shenar, Sergio Simón-Díaz, O. Grace Telford, Anna Velichko

Astrophysics of Galaxies · Cross-listed from astro-ph.CO

JZ-FMM: GPU-native differentiable N-body simulations with the Fast Multipole Method

Jens Stücker and Oskar Foldal

N-body simulations are an essential tool for modelling the evolution of gravitating systems. Differentiable N-body simulations allow solving complicated reconstruction problems by connecting observations of evolved systems to simple informative priors on their initial conditions. While so far most reconstruction efforts focus on mildly non-linear large scale dynamics, the internal dynamics of galaxies and clusters should exhibit rich information about their formation. The exploration of reconstruction possibilities in this highly non-linear regime naturally requires highly performant differentiable N-body simulations with small scale forces. Here, we present JZ-FMM, a GPU-native differentiable implementation of the fast multipole method for N-body simulations. We show that the GPU oriented design of tree structure, dual tree traversal and multipole translation operators improves performance over established CPU-native and hybrid N-body codes by notably more than an order of magnitude. Further, we show that gradients can be evaluated efficiently and accurately so that JZ-FMM can be used as an essential building block in future field-level reconstruction efforts. We apply the new code to the reconstruction problem of a tidally stripped satellite to show that deeply non-linear problems can indeed be solved efficiently with simulation gradients, although appropriate care must be taken to navigate the complicated optimization landscape.
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Astrophysics of Galaxies

Little Dots: the ULX Analogy

Andrew King

I consider recent observations of the Little Dots (LDs) observed in high–redshift ($z\sim 7$) galaxies. I have suggested that the central black holes in these objects are probably fed mass at very super–Eddington (factors $\sim 50$) rates. In physical terms, this idea makes them supermassive analogues of the (stellar–mass) ultraluminous X–ray sources (ULXs), whose hard X–ray emission is strongly anisotropic (`beamed'). In this paper I argue that the recent discovery of extended rest–frame hard X–ray ionization cones from an LD gives strong quantitative support to this view, as its geometry is what one would expect from a ULX–like object viewed not along one of its X–ray beams, but instead `from the side', very similar to the extreme Galactic system SS433. This in turn agrees in quantitative terms with recent suggestions that LDs resemble supermassive versions of SS433–like systems. I emphasize that in contrast, the rest–frame soft X–ray emission from LDs must be roughly isotropic, and presumably power the torus emission observed from these galaxies.
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Astrophysics of Galaxies

Exploring the AGN population in protoclusters: results from the TNG300 simulation and comparison with observations

A. Traina, F. Vito, A. Pillepich et al. (+7 more)

In this work, we investigate the physical properties and cosmic evolution of galaxies and AGN in a sample of 280 protoclusters identified in the TNG300 simulation, selected to end up in $z = 0$ clusters with $M_{200, \rm c} > 10^{14} \, {\rm M_{\odot}}$. Our goal is to provide the first statistical view of AGN activity enhancement in a uniformly defined sample of overdense environments, and to identify the physical mechanisms driving it. We identify protoclusters as the progenitors of present-day galaxy clusters through merger-tree reconstruction and compare their galaxy and AGN populations with a control sample of field galaxies across the redshift range $0 \leq z \leq 6$. We investigate galaxy and SMBH demographics, AGN fractions, bolometric luminosity functions, and the SMBH accretion rate density, consistently applying homogeneous selection criteria in all environments. We find that TNG300 protoclusters host systematically more massive galaxies and SMBHs than the field by up to $1 \, {\rm dex}$ at all redshifts, with signatures of accelerated galaxy evolution already visible at $z \sim 3-4$. The AGN fraction increases with stellar mass in both environments and, at fixed host-galaxy stellar mass, is broadly consistent between protoclusters and field galaxies, indicating no strong environmental triggering of SMBH accretion. However, when analysed as a function of redshift, protoclusters exhibit a significant enhancement of AGN activity (by a factor $> 2$), particularly at high luminosities and early cosmic times. We show that this enhancement primarily arises from differences in the stellar-mass distributions of galaxies in overdense regions, where massive systems assemble earlier than in the field. Consistently, protoclusters dominate the bright end of the AGN bolometric luminosity function and contribute up to $\sim 50\%$ of the total SMBH accretion rate density at $z \sim 6$.
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All authors A. Traina, F. Vito, A. Pillepich, A. Kapahtia, O. Cucciati, R. Gilli, F. Arrigoni-Battaia, C. Vignali, M. N. Isla Llave, V. L. Cavicchi

Astrophysics of Galaxies

Chasing Cosmic Reionization: An Extremely Faint Highly Magnified Source at $z=5.66$ with high $\xi_{\rm ion}$

Sofía Rojas-Ruiz, Matteo Messa, Eros Vanzella et al. (+5 more)

We present JWST NIRSpec IFU spectroscopic measurements of one of the faintest (M$_{UV}>-13.6$) known ionizing sources at $z=5.66$, dubbed Small And Lensed Source Arc, SALSA. This source is highly magnified ($\mu>100$) by the lensing galaxy cluster Abell 2744, providing a unique opportunity to investigate the physical properties of faint sources in the Epoch of Reionization. We characterize SALSA's nebular emission using rest-frame UV and optical emission lines and investigate the relationship between its ionizing efficiency, nebular excitation, and chemical enrichment. We robustly detect H$\alpha$ emission and use it to predict the H$\beta$ flux assuming negligible dust attenuation. We also measure the [OIII]$\lambda$5007 emission and estimate oxygen abundance resulting in a high R3 index $2.82_{-0.25}^{+0.34}$ and relatively low metallicity $\mathrm{12+log(O/H)}= 7.43\pm0.09$. SALSA presents a high ionizing production efficiency log$(\xi_{ion})=25.49_{-0.08}^{+0.09}$ Hz erg $^{-1}$, consistent with theoretical models from very massive stars and constant star formation rate, and a high Ly$\alpha$ escape fraction $f_{\mathrm{esc}}^{\mathrm{Ly}\alpha}=0.39\pm0.14$. These properties place SALSA among the most extreme star-forming sources known at this epoch.
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All authors Sofía Rojas-Ruiz, Matteo Messa, Eros Vanzella, Tommaso Treu, Pietro Bergamini, Giorgia Di Rosa, Marco Lombardi, Piero Rosati

Astrophysics of Galaxies

Deus Ex Statistica: A Statistical Solution to the binary-binary Outcome of the Chaotic, Non-Hierarchical Four-Body Problem

Yoav Zack, Hagai B. Perets and Yonadav Barry Ginat

We present an analytical, statistical solution to the binary-binary (2+2) outcome of the chaotic non-hierarchical four-body problem. The solution is based on the density-of-states formulation pioneered by J. J. Monaghan. The method skips the computationally expensive integration of the equations of motion, and instead samples the outcome from the chaotic phase-space, subject to conservation of energy, momentum, and angular momentum. From the joint distribution, we extract marginal distributions of several key parameters using Monte-Carlo integration, and numerically verify them by comparing to an identical ensemble of scattering experiments produced by the FEWBODY code. From the comparison, we identify a regime not represented by the density-of-states formulation: the hard binary regime, where one binary is much harder then the four-body energy scale, and the system acts as an effective three-body system. We hypothesize that at this regime the system's probability distribution spreads over an effective reduced three-body chaotic phase space. The process of transfer from four-body to three-body phase-space is still not understood, and represents the next natural extension of density-of-states methods.
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Astrophysics of Galaxies

Overmassive No More: The Case for Little Red Dots Hosting Black Hole Seeds as Massive as Single Supermassive Stars

Wendy Q. Sun, Rohan P. Naidu, Hanpu Liu et al. (+17 more)

Little Red Dots (LRDs) display singular properties unlike any known class of AGN or galaxies, motivating novel mass estimators for their central engines. Inspired by their similarities to stellar phenomena, here we interpret the LRD continuum as being produced by a pseudo-photosphere. We fit tailored stellar atmosphere models to host-subtracted LRD central engines ("black hole stars," BH*s) represented by stacks of $117$ objects. Typical BH* continuum spectra are well fit by models in a narrow range of temperatures ($T_{\rm eff}\approx4200-4800$ K), with bolometric luminosities $\approx10^{43-45}$ erg s$^{-1}$, implying pseudo-photospheric radii $\approx700-2000$ au. Based on these parameters, we explore four different approaches to deriving BH* masses: 1) using the surface gravity from atmosphere models; 2) appealing to the resemblance to super-Eddington phenomena; 3) approximating the escape velocity from the outflowing material; and 4) exploiting the lack of variability to bound the dynamical time. For the typical BH*, all of these methods yield remarkably consistent masses of $\approx10^{4-5}\,M_\odot$, implying a highly super-Eddington luminosity of $L_{\rm{bol}}/L_{\rm{Edd}}\sim5-50$. These mass estimates place BH*s within the scatter of the local scaling relation between black hole mass and host galaxy stellar mass, providing a self-consistent alternative to "overmassive" black holes that lie $2-3$ dex above it. Crucially, our derived masses are consistent with BH*s arising from single supermassive stars (SMSs), whose masses cannot exceed $\approx10^{5-6}\,M_\odot$ due to general relativistic instabilities. Furthermore, for our derived $L_{\rm bol}/L_{\rm Edd}$, the sharp cutoff of the LRD luminosity function matches the maximum theoretical mass of an SMS. With LRDs, we may therefore be directly observing the birth of heavy black hole seeds.
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All authors Wendy Q. Sun, Rohan P. Naidu, Hanpu Liu, Anna de Graaff, Jenny E. Greene, Jorryt Matthee, Chris Ashall, John Chisholm, Anna-Christina Eilers, Qinyue Fei, Kasper E. Heintz, Daichi Hiramatsu, Vasily Kokorev, Joel Leja, Zhaoran Liu, Priyamvada Natarajan, Pascal A. Oesch, Robert A. Simcoe, Alberto Torralba, Andrea Weibel

Astrophysics of Galaxies

The Ashes of Supermassive Stars: Globular Cluster-like Aluminum Enhancement in Little Red Dots

V. Kokorev, J. Chisholm, R. P. Naidu et al. (+16 more)

The relative abundances of elements in galaxies serve as fossil records of the physical conditions and processes by which they were forged. While the Big Bang produced only the lightest elements, subsequent stellar nucleosynthesis imprinted characteristic abundance patterns onto the surrounding gas, set initially by the temperatures reached inside stars and subsequently shaped by how the processed material was mixed and released. Globular clusters - dense, ancient groups of stars - provide a striking unique example. Some contain stars depleted in magnesium and enriched in aluminum, showing that they formed from gas exposed to exceptionally hot hydrogen burning. The stars responsible remain unknown. Little Red Dots may provide this missing engine. These compact, luminous objects formed at cosmic epochs similar to those associated with globular-cluster formation and are enshrouded by dense gas whose chemical composition can be measured with the James Webb Space Telescope. Here, using deep spectroscopy from the SPURS program, we show that this abundance pattern characterizes the LRD central engine: magnesium-depleted and aluminum-enhanced gas with a metallicity only 1% that of the Sun. This pattern is not produced by ordinary massive stars at these redshifts and cannot be mimicked by ionization, gas geometry or dust. Instead, it is reproduced by hot hydrogen burning in fully convective supermassive stars, with the measured abundances implying masses of at least 10,000 solar masses - approximately 100 times larger than any star observed in the present-day Universe. Little Red Dots may therefore reveal supermassive stars during their brief lives or in the immediate aftermath of their direct collapse, simultaneously identifying the long-sought source of globular cluster abundance anomalies and a formation pathway for massive black hole seeds.
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All authors V. Kokorev, J. Chisholm, R. P. Naidu, M. Gieles, S. Finkelstein, D. Berg, H. Akins, A. Taylor, S. Fujimoto, L. J. Furtak, J. Greene, A. de Graaff, K. Hawkins, T. Hsiao, D. Nandal, J. Matthee, S. Monty, P. Rinaldi, M. Boylan-Kolchin

Astrophysics of Galaxies

RIGEL: Ultra-faint dwarf galaxy diversity shaped by inhomogeneous cosmic reionization

Yunwei Deng, Hui Li, Ewald Puchwein et al. (+7 more)

Ultra-faint dwarf galaxies (UFDs) are among the smallest and oldest galaxies in the Universe and are widely regarded as relics of cosmic reionization. To investigate how reionization quenches star formation and shapes the diversity of UFDs, we present a suite of eight cosmological zoom-in simulations of isolated UFDs with present-day halo masses of $\sim10^9\,{\rm M}_\odot$. The simulations are performed with the radiation-magnetohydrodynamic galaxy formation framework Realistic ISM modeling in Galaxy Evolution and Lifecycles (RIGEL), coupled to realistic large-scale radiation fields extracted from the THESAN reionization simulation. Despite residing in similar $z=0$ halos, the simulated galaxies span nearly two orders of magnitude in stellar mass and broadly reproduce the observed luminosities, sizes, metallicities, and stellar kinematics of Local Group UFDs. We find that reionization quenches star formation through a two-stage process. The arrival of the ionization front rapidly photoionizes the diffuse circumgalactic and intergalactic gas, suppressing further gas accretion onto the galaxy. Star formation nevertheless continues for several hundred Myr using the surviving self-shielded gas reservoir and ceases only after this gas is consumed or dispersed. Within 500 Myr after reionization, less than 40% of the initial gas mass remains in the halo, with photoevaporation constituting the dominant gas-loss channel. We further show that the halo mass at the time of reionization is a key parameter governing the subsequent evolution of UFDs. Galaxies residing in more massive halos at reionization retain gas for longer periods and undergo more extended chemical enrichment. Consequently, the halo mass at reionization strongly correlates with the final stellar mass, stellar age spread, and chemical evolution of the galaxy.
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All authors Yunwei Deng, Hui Li, Ewald Puchwein, Boyuan Liu, Aaron Smith, Rahul Kannan, Federico Marinacci, Greg L. Bryan, Kung-Yi Su, Mark Vogelsberger

Astrophysics of Galaxies

SHELLQs. Black Hole Mass and Eddington Ratio Distributions of Intermediate-Luminosity Quasars at 6<z<7

Masafusa Onoue, John D. Silverman, Yoshiki Matsuoka et al. (+31 more)

We present near-infrared spectroscopy of 21 quasars at 6.07 < z < 6.90 obtained with JWST/NIRSpec and Subaru/MOIRCS. These targets have absolute ultraviolet magnitudes of -25 < M_1450 < -22 and are drawn from a sample of z>6 quasars identified in the Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) project. These quasars occupy the intermediate-luminosity regime between luminous quasars and the faint high-redshift AGNs uncovered by JWST. We detect broad Balmer emission lines and MgII 2798 with underlying continua from the NIRSpec and MOIRCS targets, respectively. The virial black hole masses of this sample span a wide range of 7.2 < log M_BH/M_sun < 9.4, with Eddington ratios of -1.3 < log L_bol/L_Edd < 0.4. Combining these measurements with our previous mass estimates for other SHELLQs quasars, we construct a sample of 27 quasars with M_BH estimates at 6 < z < 7 and derive the distributions of M_BH and L_bol/L_Edd. When compared with luminosity-matched quasars at z ~ 1.3, we find median offsets of delta log M_BH/M_sun = -0.4 and Delta log L_bol/L_Edd = 0.2 in the high-redshift sample. In addition, 11% of the high-redshift quasars are accreting near or above the Eddington limit. As a systematic check, adopting the Eddington-ratio-dependent calibration of single-epoch M_BH estimates increases the inferred super-Eddington fraction to 22%. These results indicate that a representative population of distant supermassive black holes undergo a rapid accretion phase, although not as extreme as that seen in the most luminous quasar population, consistent with the anti-hierarchical growth scenario of supermassive black holes.
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All authors Masafusa Onoue, John D. Silverman, Yoshiki Matsuoka, Xuheng Ding, Camryn L. Phillips, Michael A. Strauss, Junya Arita, Takuma Izumi, Mahoshi Sawamura, Nobunari Kashikawa, Irham Andika, Kentaro Aoki, Shunsuke Baba, Anna-Christina Eilers, Seiji Fujimoto, Tomotsugu Goto, Masatoshi Imanishi, Kohei Inayoshi, Kazushi Iwasawa, Knud Jahnke, Yuki Kaneko, Toshihiro Kawaguchi, Kotaro Kohno, Chien-Hsiu Lee, Alessandro Lupi, Tohru Nagao, Dragan Salak, Malte Schramm, Yoshiki Toba, Hideki Umehata, Marta Volonteri, Fabian Walter, Feige Wang, Jinyi Yang

Astrophysics of Galaxies

A Spectral Framework for Testing the Quasi-Star Hypothesis in Little Red Dots I: Weighing LRDs by Their Super-Eddington Luminosity Ratios—No Signs of Overmassive Black Holes

Olivia Curtis, Nikko J. Cleri, Jakob M. Helton et al. (+2 more)

We present a spectral test of the quasi-star hypothesis for Little Red Dots (LRDs) whereby a black hole grows inside a stellar-like envelope. We use Prospector to fit host galaxies and TLUSTY photospheres to 5 LRDs that show strong molecular or atomic absorption. We approximate each object's electron-scattered Eddington luminosity ratio, $\phi \equiv \kappa_{\rm es}\sigma T_{\rm eff}^{4}/(gc)$, and we use MESA-QUEST to simulate their envelopes. All 5 are super-Eddington at $\phi = {87}$–$299$, with the caveat that the largest sources sit at the edge of our atmosphere grid and beyond our simulations. We derive envelope masses between $700$–${15{,}000}\,M_\odot$, where quasi-star theory requires the black hole to be less than a third of that. GN-28074 falls 5 decades below its published virial mass estimate, alleviating the overmassive black hole problem. Their black holes double every $\sim0.05$–${0.2}$~Myr and can produce intermediate-mass black holes in $\lesssim30$~Myr. Our 3 water-absorbing objects have cold components that are $2$–$3$~dex denser than their hot components, which we interpret as the water forming in cold dense clouds. We then extend our measurements to 82 archival LRDs, finding that the population has super-Eddington photospheres and wind speeds that increase with $\phi$, which implies that LRDs evolve from massive sources with slow winds to having more eruptive winds as they mature and shed their outer envelopes. We thus constrain the Eddington ratios and masses of LRDs and show self-consistently that quasi-stars may be the central engines powering LRDs.
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All authors Olivia Curtis, Nikko J. Cleri, Jakob M. Helton, Joel Leja, Jason T. Wright

Astrophysics of Galaxies

Virgo Filaments VII: MeerKAT HI-imaging of the VirgoIII filament

M. Ramatsoku, G. Nagaraj, P. Jablonka et al. (+11 more)

We present MeerKAT HI observations of galaxies in the nearby Virgo III filament. At a median distance of $\sim30$ Mpc, the filament extends over $\approx16$ Mpc across $\sim56$ deg$^2$. Our survey comprises 15 targeted MeerKAT pointings centred on HI deficient galaxies. Multi-resolution HI imaging at 8-90 arcsec with an rms of $\sim0.3$ mJy beam$^{-1}$ reaches column densities of $\sim4\times10^{20}$ to $\sim4\times10^{18}$ atoms cm$^{-2}$. This probes HI in galaxy discs at $\sim1.2$ kpc resolution and their outskirts at $\sim13.5$ kpc. We detect HI emission from 80 sources within the 500-3000 km/s range; most are optically detected galaxies, while 10 per cent lack confirmed counterparts. We also identify three HI clouds without optical counterparts near an interacting system, possibly displaced during the interaction. Galaxies in the filament lie below the field HI gas-fraction scaling relations at fixed stellar mass. Crucially, $\sim40$ per cent of galaxies not selected for HI deficiency fall more than $1\sigma$ below these relations, with an even greater fraction lying under the mean relation itself, indicating that the low HI gas fractions are widespread in the filament rather than confined to the pre-selected deficient population. Quantifying HI morphologies using an asymmetry parameter, we find a wide range of disturbances without a clear trend with projected distance to the filament spine or local density, suggesting a combination of local interactions and the filament itself. This paper presents the survey description, catalogue, and HI atlas enabling investigations of gas removal mechanisms in Virgo III galaxies.
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All authors M. Ramatsoku, G. Nagaraj, P. Jablonka, R. A. Finn, B. Vulcani, G. Rudnick, F. Combes, G. De Lucia, G. Castignani, D. Zaritsky, Y. Bahè, R. A. Koopmann, D. Zakharova, K. Conger

Astrophysics of Galaxies · Cross-listed from astro-ph.CO

Sizing the Universe with DESI Galaxy Sizes: Plain Fundamentals of Fundamental-Plane Lensing

Kun Xu, Jiaqi Wang, Ravi K. Sheth et al. (+44 more)

Weak gravitational lensing provides a powerful way to map cosmic structure, but most current measurements rely on galaxy shape distortions from deep imaging surveys and are affected by systematics such as intrinsic alignments, photometric-redshift uncertainties and shape-measurement biases. Here we present a spectroscopic galaxy-galaxy lensing magnification measurement using Fundamental-Plane (FP) size residuals, $\delta_r\equiv\Delta\log_{10}R_\mathrm{e}$, from 3.26 million DESI luminous red galaxy (LRG) sources behind DESI Bright Galaxy Survey lenses. The FP-like relation predicts the intrinsic sizes of LRGs from lensing-invariant quantities, including velocity dispersion $\sigma_0$ and surface brightness $I_\mathrm{e}$, with a scatter of about 0.06-0.07 dex. Lensing magnifies LRG sizes, giving the direct convergence response $\delta_r(\kappa)=\kappa/\ln 10$, but we show that the full lensing response is modified by magnification bias, because fitting $R_\mathrm{e}$ with $I_\mathrm{e}$ inevitably induces a magnitude dependence in $\bar{\delta}_r(m)$. After calibrating this response, we recover surface-density profiles with uncertainties comparable to those from individual Stage-III shear surveys using 5-20 million higher-redshift sources. The corresponding excess surface-density profiles agree with shear-based measurements. We further show that the estimator is robust to size-measurement uncertainties, with a convergence multiplicative bias only $\simeq -0.2$ times the size bias. FP lensing therefore provides a clean, spectroscopic and complementary probe of cosmic structure.
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All authors Kun Xu, Jiaqi Wang, Ravi K. Sheth, J. Aguilar, S. Ahlen, F. Beutler, D. Bianchi, D. Brooks, A. Carnero Rosell, T. Claybaugh, A. de la Macorra, P. Doel, J. E. Forero-Romero, E. Gaztañaga, G. Gutierrez, K. Honscheid, C. Howlett, M. Ishak, J. Jimenez, S. Juneau, R. Kehoe, D. Kirkby, O. Lahav, M. Landriau, L. Le Guillou, M. Manera, A. Meisner, R. Miquel, J. Moustakas, S. Nadathur, J. A. Newman, W. J. Percival, F. Prada, I. Pérez-Ràfols, C. Ravoux, G. Rossi, R. Ruggeri, E. Sanchez, C. Saulder, D. Schlegel, M. Schubnell, H. Seo, J. Silber, M. Siudek, G. Tarlé, B. A. Weaver, H. Zou

Astrophysics of Galaxies

The fading hierarchy of Galactic open clusters

Guimei Liu, João Alves, Yu Zhang et al. (+7 more)

Galactic open clusters provide a record of both hierarchical star formation and the subsequent dynamical evolution of the Milky Way disk. We use the two-point correlation function to characterize the spatial and kinematic clustering of open clusters in the solar neighborhood, considering their three-dimensional (3D) distributions and their projections onto the Galactic plane, as well as subsamples divided by age, mass, and survey volume. We quantify the clustering through fractal dimensions and characteristic correlation scales. We find that the youngest clusters have fractal dimensions comparable to those measured in the interstellar medium, suggesting that they retain part of the hierarchical structure of their natal molecular clouds. The clustering strength then decreases systematically with age and becomes weak on a characteristic timescale of order 100 Myr. This evolution is scale dependent: the excess correlation at tens of parsecs fades more rapidly than the weaker correlation at hundreds of parsecs. By contrast, we find only a weak dependence of the clustering properties on cluster mass and volume. Using the same cluster sample, we derive an empirical mapping between projected and 3D fractal dimensions, providing an observational calibration for comparing the Milky Way with studies of external galaxies, where only projected measurements are available. The open-cluster population exhibits characteristic spatial and kinematic clustering scales of approximately 370 pc and 11 km/s, respectively. These results support a picture in which young open clusters inherit a spatial hierarchy from star formation and progressively lose their spatial correlations through subsequent evolution.
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All authors Guimei Liu, João Alves, Yu Zhang, Emily L. Hunt, Ruqiu Lin, Josefa E. Großschedl, Efrem Maconi, Nora Wagner, Lilly Kormann, Cameren Swiggum

Astrophysics of Galaxies · Cross-listed from astro-ph.CO

TILING I: Field-level Bayesian reconstruction of cosmological initial conditions during the epoch of reionization

Nikolaos Triantafyllou, Andrei Mesinger, Steven Murray et al. (+1 more)

Reconstructing the initial conditions (ICs) of the matter field for an observed volume gives a complete picture of that region's temporal evolution. IC reconstruction is common at low redshifts but largely unexplored during the epoch of reionization (EoR; $z\gtrsim 5$), partly due to difficulties modeling inhomogeneous cosmic radiation fields. Yet the EoR spans half the observable Universe, offering unmatched potential for astrophysics and cosmology. Here we quantify how well upcoming galaxy and 21cm observations can constrain ICs during the EoR. We develop $\tt TILING$ ($\bf T$omographic $\bf I$nference of $\bf L$inear $\bf I$Cs via $\bf N$etwork $\bf G$rafting): a hybrid machine learning pipeline that first produces a point estimate of the ICs, which then improves a score-based diffusion network for generating posterior samples. We train $\tt TILING$ on mock galaxy maps at varying UV magnitude limits, plus corresponding 21cm maps at varying noise levels and foreground "wedge" contamination. For fiducial survey choices, we achieve accurate IC reconstruction (posterior mean cross-correlation coefficients >0.8 and power spectrum errors under a few percent) down to $\text{k}\lesssim 0.2 \text{cMpc}^{-1}$. While 21cm interferometry aids recovery of IC power spectra, most constraining power, especially for IC Fourier phases, comes from galaxy maps, underscoring the need for complementary observations when interpreting the 21cm signal. We show how $\tt TILING$ can recover 21cm power spectrum modes excised by foreground contamination. Our framework can also: (i) guide follow-up observations of sub-volumes of interest; (ii) reconstruct galaxy evolution and reionization morphology for specific volumes; and (iii) isolate the contribution to reionization from the vast majority of galaxies unobservable by optical/IR telescopes like $\it JWST$. Our code is publicly available.
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All authors Nikolaos Triantafyllou, Andrei Mesinger, Steven Murray, Samuel Gagnon-Hartman

Astrophysics of Galaxies

Small-Scale Clustering of Primordial Black Holes: The Little Red Dot Mass Function and the High-Redshift Galaxy Tension

Borui Zhang, Wei-Xiang Feng and Haipeng An

Supermassive black holes (SMBHs) in "little red dots" (LRDs) discovered by the James Webb Space Telescope (JWST) may result from runaway mergers of primordial black holes (PBHs) in clusters—through long-short mode coupling on small scales in the early Universe. In this framework, we derive the SMBH mass function, together with the compactness and overmassive features of LRDs. We also estimate that the dense gas residing in PBH clusters is consistent with LRD observations. In addition, SMBHs formed from PBH clusters can help accelerate galaxy formation at high redshifts, thus alleviating tension with $\Lambda$CDM cosmology.
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Astrophysics of Galaxies

Active galactic nucleus activity in brightest cluster galaxies at intermediate redshifts in Sunyaev-Zel'dovich–selected clusters

N. Hatamkhani, R.E. Skelton, J. Delhaize et al. (+4 more)

Context. Brightest cluster galaxies (BCGs) host powerful active galactic nuclei (AGNs) and help regulate the heating–cooling balance of the intracluster medium. While radio-mode feedback is well established locally, its prevalence at intermediate redshifts remains poorly constrained. Aims. We investigate the incidence, accretion state, and host-galaxy properties of AGNs in BCGs in Sunyaev–Zel'dovich (SZ)-selected clusters at $0.3 < z < 0.8$, testing dependences on cosmic epoch, cluster mass, and dynamical state. Methods. We analyse 171 BCGs from the Atacama Cosmology Telescope SZ catalogue, combining Southern African Large Telescope spectroscopy, WISE mid-infrared photometry, and Rapid ASKAP Continuum Survey radio data with X-CIGALE modelling to derive stellar masses, star formation rates, and Eddington-scaled accretion rates. Results. Thirty-eight BCGs (22%) host radio-loud AGNs. Only two ($\sim$5%) are high-excitation radio galaxies; the remainder are low-excitation systems. Most AGNs have low Eddington ratios ($-3 \lesssim \log_{10}\lambda_{\mathrm{Edd}} \lesssim -1$), consistent with radiatively inefficient accretion. Accretion efficiency increases with redshift, while correlations with cluster mass and dynamical state are weak. Radio luminosity is independent of cluster mass. Compared to field radio AGNs, BCG radio-loud AGNs have similar radio powers but are hosted by galaxies $\sim0.8$ dex more massive. Conclusions. BCG radio-loud AGNs at $0.3 < z < 0.8$ are predominantly low-excitation, radiatively inefficient systems, indicating that maintenance-mode accretion was widespread in massive clusters ($M_{500c} \simeq 3.2 \times 10^{14},M_\odot$) by $z \sim 1$. Accretion appears regulated primarily by fuel availability and cosmic evolution rather than global cluster properties, while the cluster environment mainly sets the host-galaxy mass scale.
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All authors N. Hatamkhani, R.E. Skelton, J. Delhaize, M. Hilton, S.I. Loubser, N. Gupta, K. M. Mogotsi

Astrophysics of Galaxies · Cross-listed from astro-ph.HE

Detection of episodic Ultra-fast Outflows in the Low-luminosity quasar Mrk 205 using XMM-Newton data

César Ivan Victoria-Ceballos, Anna Lia Longinotti, Yair Krongold et al. (+3 more)

Ultra-fast outflows (UFOs) from Active Galactic Nuclei (AGN) are winds which reach velocities up to 0.3c, and show mass outflow rates of 0.01-1 $M_{\odot} yr^{-1}$ and kinetic energies of $10^{42-45}$ erg/s. Their extreme properties allow UFOs to carry substantial kinetic power, sufficient to drive AGN feedback by injecting energy and momentum into the interstellar medium, regulating the host galaxy evolution and the supermassive black hole growth. In this work, we present a detailed analysis of XMM-Newton X-ray spectra of the low-luminosity quasar Mrk205. We find evidence of multi-component ultra-fast outflows, traced by two components detected in the high-resolution data, showing velocities in the range of $\sim$0.07c to $\sim$0.1c. In addition, we marginally detected an ultra-fast outflow feature in the Fe K band. According to our analysis, Mrk 205 shows episodes of wind launches, rather than persistent winds. We further discuss whether the energy carried by the UFOs in Mrk 205 is capable of producing significant feedback on its host galaxy.
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All authors César Ivan Victoria-Ceballos, Anna Lia Longinotti, Yair Krongold, Giorgio Lanzuisi, Miriam Gudiño, Armando Lara-DI

Astrophysics of Galaxies · Cross-listed from astro-ph.SR

Open Clusters as Laboratories for Cool Star Evolution: Highlights from the Cool Stars 23 Splinter Session

Deepak Chahal, Beatrice Caccherano, Edward Gillen et al. (+16 more)

Open clusters remain among the most powerful laboratories for calibrating how fundamental stellar properties such as rotation, magnetic activity, surface chemistry, and internal structure evolve over the pre-main-sequence to main-sequence and post-main-sequence stellar lifetime. Because cluster members share a common age, galactic environment, and initial composition, they provide empirical anchors upon which age-dating techniques such as gyrochronology, activity-age relations, and chemical clocks are built. This paper summarizes the Cool Stars 23 splinter session - Open Clusters as Laboratories for Cool Star Evolution, held on 15 June 2026 in Tokyo, Japan. The session comprised one invited review, ten contributed talks, and ten poster pop-up presentations, organized around three themes: the evolution of rotation, magnetic activity, and chemical abundances (Li-depletion, [C/N], [Y/Mg], and neutron-capture elements). We close with a summary of the open questions identified in the panel discussion and the observational and theoretical work that new surveys (e.g. Gaia DR4, 4MOST, WEAVE) and missions (e.g. PLATO, Roman) will enable over the coming years. Together, these efforts will help assess the current state of the field and shape its future directions.
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All authors Deepak Chahal, Beatrice Caccherano, Edward Gillen, Dario J. Fritzewski, Robin D. Jeffries, Kevin R. Covey, Andrew Boyle, Lyra Cao, Federica Chiti, Alexander Hughes, Leslie Moranta, Natalie R. Myers, Emily K. Pass, Phil Van-Lane, Fábio C. Wanderley, Mackenna L. Wood, Stephanie T. Douglas, David Montes, Loredana Prisinzano

Astrophysics of Galaxies

Closed-Form of the Local Galactic Potential and Stellar Distribution Function from Gaia DR3

Indranil Das, Adam Kamoski, Dora Demiri et al. (+3 more)

The local dark matter density determines the strength of the signal expected in direct-detection experiments, yet published estimates from stellar motions disagree by more than their errors, and the most recent machine-learning analysis of Gaia data finds a local density consistent with zero. According to Jeans' theorem, a distribution function built from integrals of motion satisfies the collisionless Boltzmann equation (CBE) trivially for any choice of potential, so a search that simultaneously fits the distribution function and the potential to the CBE identifies neither. Our pipeline instead estimates the distribution function in isolation, linearizing the equation in terms of accelerations and allowing for direct measurement of the local force field, and then fits closed forms to that field via symbolic regression. Throughout, we find that the usable information lies not in the CBE residual but in the stellar number counts, the observable most distorted by survey selection. Along the vertical profile, our recovered potential agrees with the classical self-gravitating isothermal disc.
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All authors Indranil Das, Adam Kamoski, Dora Demiri, Brianna Isola, Hanieh Karimi, Dmitrii S. Zagorulia