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Physics Astrophysics Astrophysics of Galaxies

astro-ph.GA Watch Astrophysics of Galaxies

Friday 4 September

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

Deep H$\alpha$ Imaging Survey of IC 348 with the Hubble Space Telescope: I. Accretion Properties of Stellar and Substellar Objects

Lillian Yushu Jiang, Brendan P. Bowler, Yifan Zhou et al. (+5 more)

Accretion governs the growth of young stars and the early evolution of their circumstellar disks, yet population-level measurements of accretion are often hampered by heterogeneous diagnostics and by samples preferentially selected toward disk-bearing or accreting objects. This can impact the mass accretion rate-stellar mass ($\dot{M}$-$M_\star$) relation, particularly at substellar masses. We present a uniform analysis of accretion in the $\sim$2 Myr-old star-forming cluster IC 348 based on deep Hubble Space Telescope F656N imaging. Using H$\alpha$ excess as a single, homogeneous accretion tracer, we derive accretion rates and robust upper limits for 200 cluster members spanning the stellar to substellar mass regime ($3\ M_\odot$ to $4\ M_{\rm Jup}$). Accretion is detected in $37\pm3\%$ of the sample, with fractions of $34\pm4\%$ among stellar members and $46\pm6\%$ among substellar objects. For accretors alone, the inferred $\dot{M}$-$M_\star$ relation is consistent with those measured in similarly aged regions such as Lupus. In contrast, including weak accretors and non-detections increases the scatter and flattens the slope while lowering the intercept, demonstrating the strong influence of the low-accretion tail on population-level accretion relations. For free-floating planetary-mass objects in IC 348, extrapolating the accretors-only fit overpredicts $\dot{M}$ by approximately an order of magnitude compared to the fit that includes upper limits, which instead implies mass accretion rates of $<10^{-12}$ $M_{\odot}\ \mathrm{yr}^{-1}$. These results show that sample selection, specifically whether the sample is restricted to disk-bearing or accreting targets, or instead is drawn from a complete membership census, is a dominant factor shaping population-level accretion relations across a wide dynamic range in stellar mass.
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All authors Lillian Yushu Jiang, Brendan P. Bowler, Yifan Zhou, Adam L. Kraus, Sean M. Andrews, Lynne A. Hillenbrand, Michael J. Ireland, Zhaohuan Zhu

Astrophysics of Galaxies

Halo mass, star formation, and morphology in galaxies: A morphology-dependent connection between halo environment and star formation activity

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

Understanding the mechanisms responsible for quenching star formation in galaxies requires disentangling the roles of internal structure and large-scale environment. We investigate how galaxy morphology and specific star formation rate (sSFR) relate to the typical masses of host dark-matter halos (DMHs), and whether the halo–star formation connection depends on morphology. We analyse non-active galaxies in the VIPERS and Stripe 82 fields at $0.5 \leq z \leq 1.2$, combining DESI-based Sérsic classifications with stellar masses and SFRs from spectral energy distribution fitting. Galaxies are separated into disc- and bulge-dominated systems and divided into quantile-based sSFR bins designed to minimise stellar-mass differences between morphologies. Each bin is characterised by its median $\Delta{\rm MS}=\log {\rm SFR}-\log {\rm SFR}_{\rm MS}$, and characteristic DMH masses are inferred from projected galaxy cross-correlations. The halo-mass relation with star formation activity depends strongly on morphology: disc-dominated galaxies show little variation in halo mass across the probed $\Delta{\rm MS}$ range, whereas bulge-dominated galaxies occupy lower-mass halos at high $\Delta{\rm MS}$ and higher, mutually consistent halo masses toward lower $\Delta{\rm MS}$. These trends are unlikely to be driven mainly by stellar-mass differences. Typical halo mass alone therefore does not map cleanly onto quiescence; instead, the connection between halo environment and star formation activity is stronger in bulge-dominated than in disc-dominated galaxies.
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All authors G. Mountrichas, F. Shankar, F. J. Carrera, A. Georgakakis

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

Unresolved Binary Systems in the Rubin Era I: An Autoencoder Framework for Binary Identification Applied to 47 Tucanae

Tobias Géron, Alexander Laroche, Joshua S. Speagle et al. (+1 more)

Binary systems are extremely common and influence many areas of astrophysics, such as stellar evolution and cluster dynamics. In this work, we introduce flexAE, a framework that combines a classical autoencoder architecture with a dedicated classifier component. It is designed to distinguish between single stars and binary systems based on their broadband spectral energy distributions, using all available photometry simultaneously while accounting for observational uncertainties. We demonstrate the capability of flexAE by using it to identify unresolved main sequence binaries in the outskirts of the 47 Tucanae globular cluster (NGC 104) with photometric data from the Vera C. Rubin Observatory. We train the model on a simulated sample of single star and binary systems created with stellar atmosphere models. The model accurately reconstructs the photometric input features and achieves a classification accuracy of 0.85 on the simulated test set. We then apply the trained model to a sample of 1,424 cluster members found in Rubin DP1 with reliable $gri$ photometry, of which 32 are identified by the model as unresolved main sequence binaries. This corresponds to an observed binary fraction of $2.2^{+0.5}_{-0.3}$%. We estimate that the intrinsic main sequence binary fraction lies between $3.7^{+0.8}_{-0.5}$% and $7.5^{+1.5}_{-1.1}$%. The binary fraction stays constant between 18-36 arcmin (5.7-11.4 half-light radii) from the cluster center. We highlight that flexAE can be easily adapted for other use cases. This makes flexAE well-suited for large-scale binary classification in upcoming wide-field surveys, such as Rubin LSST. The code is publicly available at https://github.com/tobiasgeron/flexAE.
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All authors Tobias Géron, Alexander Laroche, Joshua S. Speagle, Maria R. Drout

Astrophysics of Galaxies

A direct measurement of the tilting rate of the Milky Way disc

Guillaume F. Thomas, Benoit Famaey, Alicia Rivero et al. (+2 more)

The Milky Way disc is typically described, at lowest order, as a flattened rotating stellar component with a fixed spin axis. However, interactions and mergers with smaller galaxies are expected to have an impact on the disc properties. Such interactions can leave long-lasting signatures within galactic discs, including warps and corrugation waves that have already been detected in the Milky Way, but also a slow reorientation of the disc known as tilting, which has never been directly measured up to now. Here, using the vertical proper motions of 110 million stars from the Gaia DR3 catalogue, we present the first-ever direct measurement of the tilting rate of our Galaxy's disc. The detected signal of 14 +/- 2 deg/Gyr around the Sun-Galactic Centre axis is statistically significant and is consistent with typical values found in cosmological simulations. By comparing our measurements with high-resolution tailored simulations, we infer that the observed tilting rate is likely the long-term consequence of a major accretion event that occurred around 8-10 billion years ago. The signal is consistent with the merger of a massive satellite galaxy on a retrograde orbit with a ratio of about 1:4, probably associated with the Gaia-Enceladus/Sausage event. These results provide independent evidence for a key episode in the Milky Way's formation history and introduce a new way to reconstruct the assembly histories of disc galaxies from their present-day motions.
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All authors Guillaume F. Thomas, Benoit Famaey, Alicia Rivero, Giuseppina Battaglia, Rodrigo Ibata

Astrophysics of Galaxies

Discovery of Very Minor Merger-Induced Star-forming Clumps at $z=3.33$ with JWST

Alexander de la Vega and Bahram Mobasher

High-redshift galaxies often contain ”clumps:” compact, $\sim$kpc-scale regions of intense star formation. These substructures make up significant fractions of the masses and star-formation rates of their host galaxies and are expected to affect the morphologies of their hosts, such as by building up bulges. Studying clumps can therefore inform theoretical models of galaxy evolution. Two main mechanisms have been proposed for their formation: gas fragmentation from disk instabilities and compression of gas during gravitational interactions with other galaxies. In the latter, only mergers with mass ratios greater than 1:10 have been considered. Here, we report James Webb Space Telescope spectroscopy and imaging of JADES 1034862, a galaxy at $z_{\textrm{spec}} = 3.33$, which hosts a clump that formed during an interaction with a companion with a stellar mass that is $\approx28$ times lower than the host. From fits to the spectral energy distributions of the clump and the companion, it is found that both of their star-formation histories rose sharply in the most recent 100-300 Myr, during which the clump formed $\sim10^{7.2} \ M_{\odot}$ in stars and currently makes up $\approx10\%$ of the total star-formation rate of its host. Given the higher frequency of mergers with low mass ratios ($<1:10$) to those with higher ratios, we infer that a substantial fraction of high-redshift clumps could have formed through weak gravitational interactions.
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Astrophysics of Galaxies · Cross-listed from astro-ph.SR

The youngest white dwarf companion to a millisecond pulsar: Insights from NGC 362D

Greta Ettorre, Emanuele Dalessandro, Mario Cadelano et al. (+7 more)

We report on the identification of the optical counterpart to the recently discovered millisecond pulsar (MSP) NGC362D in the Galactic globular cluster NGC362 based on deep, multi-band, and multi-epoch Hubble Space Telescope observations. Our analysis robustly shows that this object is a very low-mass ($\sim0.18 M_{\odot}$) He white dwarf (WD) still in the pre-cooling phase. Interestingly, a detailed comparison with updated binary evolution models indicates that this object completed the mass-transfer phase only recently ($\sim0.6$ Gyr ago), thus strongly suggesting that COM-NGC362D is the youngest WD companion to a MSP identified to date. Remarkably, in this respect, the photometric properties of NGC362D show, for the first time in this class of objects, significant wavelength-dependent variations that are consistent with the presence of residual circumstellar material. This system therefore provides a valuable test case to directly probe the immediate aftermath of the MSP recycling process and to constrain the early evolutionary stages of proto-WD companions and their radio and optical properties. Our results suggest that residual material can produce radio and optical signatures that mimic those of systems with non-degenerate companions, potentially leading to the misclassification of young MSPs.
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All authors Greta Ettorre, Emanuele Dalessandro, Mario Cadelano, Alessandro Ridolfi, Cristina Pallanca, Paulo C. C. Freire, Vivek Venkatraman Krishnan, Maurizio Salaris, Franca D'Antona, Rouhin Nag

Astrophysics of Galaxies

Baryon-Accelerated Core Collapse in SIDM Halos and its Imprint on Galactic Disks

Zeineb Mezghanni, Elliot Y. Davies, Adriana Dropulic et al. (+3 more)

The gravitational coupling between dark matter (DM) halos and the baryonic structures they host is one of the most powerful windows into the particle nature of DM. Self-interacting dark matter (SIDM) presents a minimal, well-motivated extension to the dark sector with dramatic consequences for the structure of galaxies and their halos. However, the impact of baryons on SIDM halo evolution and the resulting galactic structure has been underexplored in Milky Way (MW)-size galaxies. In this paper, we demonstrate that the inclusion of a baryonic component in a MW-size galaxy causes accelerated core collapse to begin within the MW's lifetime for a cross section as low as $\sigma/m = 1 \, \rm{cm}^2/\rm{g}$. We present a suite of $N$-body simulations of cold dark matter and SIDM MW-size galaxies with and without a baryonic component for cross sections $\sigma/m =[1.0, 2.5, 5.0]$ cm$^2$/g. We find numerically, and semi-analytically, that the presence of a stellar disk and bulge shortens the predicted core collapse timescales from the DM only simulations by a factor of $\sim 40$. Further, as the core collapse begins within the lifetime of the galaxy, the subsequent density increase strengthens the mid-plane restoring force exerted on stellar disk orbits, leading the disk to flare. This work quantifies both directions of the baryon–SIDM coupling: baryons accelerate core collapse in MW-sized halos, and the resulting halo evolution reshapes the disk through thinning and flaring. Both processes open new observational windows into DM.
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All authors Zeineb Mezghanni, Elliot Y. Davies, Adriana Dropulic, Gonzalo Herrera, Abdelaziz Hussein, Lina Necib

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

Exploring the dark sector through galaxy dynamics in clusters

Andrea Biviano, Lorenzo Pizzuti and Antonaldo Diaferio

Galaxy clusters are among the most powerful laboratories to probe fundamental physics, from the formation of cosmic structures to the nature of dark matter, dark energy, and gravity. Realizing their full potential requires accurate and unbiased determinations of their mass profiles, a longstanding challenge in cluster cosmology. In this context, the kinematics of member galaxies in clusters has become a key tool to reconstruct the mass distribution and to investigate the dynamical properties of clusters. The advent of large spectroscopic datasets has enabled the application of increasingly sophisticated models and robust techniques, which make kinematic analyses competitive with other probes of cluster dynamics. In this review, we provide a comprehensive overview of cluster kinematics, covering the main methods to infer dynamical masses and mass profiles, their physical assumptions, and the impact of systematics such as deviation from dynamical equilibrium, triaxiality, and substructures. We discuss the complementarity with other techniques to investigate the structure of clusters and highlight recent applications to tests cosmological models. We focus in particular on the role of kinematical studies in constraining the dark sector (dark matter and dark energy) and in providing novel insights into the behavior of gravity at Mpc scales. Finally, we outline current limitations and future prospects, emphasizing that the combination of improved modeling and forthcoming spectroscopic surveys will establish cluster kinematics as a fundamental probe in the era of precision cosmology.
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Astrophysics of Galaxies · Cross-listed from astro-ph.SR

Metallicity dependence of Wolf-Rayet binaries using detailed binary models: An absence of long-period systems at low metallicity

Max M. Briel, Eirini Kasdagli, Tassos Fragos et al. (+6 more)

Observations of Wolf-Rayet (WR) stars in binaries in the SMC and LMC suggest a preference for short period ($P\lesssim30$ days) orbits, with an apparent absence of WRs in long-period systems. The Galactic population does extend to longer periods but shows a deficit in long-period WR binaries compared to their progenitors, the O-star population. Across these populations a nearly constant binary fraction has been observed. We aim to characterize the population of WR binaries across a range of metallicity and determine the metallicity dependence of the formation of these systems, specifically focusing on the period distribution. We use detailed binary evolution models from POSYDON to predict the population of WR binaries at 0.01, 0.1, 0.2, 0.45, and $1Z_\odot$, analyzing the resulting period distribution and formation channels, while comparing them against the observed population in the SMC, LMC, and Milky Way. We find that the absence of wider WR binaries in the SMC, and potentially in the LMC, can be explained by stable, Case-B mass transfer only partially stripping the donor star, combined with WR winds at low metallicity being insufficiently strong to strip the remaining envelope. As a result, the long-period peak from Case-B mass transfer, present at Zsun, disappears at low metallicity. We additionally find that stable mass transfer (SMT) produces a short-period peak through Case-A mass transfer (P~5-10 days) that closely matches observations across metallicity. Furthermore, SMT and non-interacting systems are the dominant formation channels of WR binaries at all metallicities, with their relative contribution showing no metallicity dependence. This result implies that the SMT channel has the same metallicity-dependence as isolated WR formation. At the same time, we find that common envelope evolution primarily produces short-period ($P<1$ day) WR binaries with black-hole companions.
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All authors Max M. Briel, Eirini Kasdagli, Tassos Fragos, Tomer Shenar, Jeff J. Andrews, Denzel Goh, Abhishek Chattaraj, Seth Gossage, Philipp Srivastava

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

Ion abundances in the plasma tail of 3I/ATLAS show that it is N2-rich

Lea Ferellec, Cyrielle Opitom and Colin Snodgrass

Interstellar objects allow us to compare the early planetary formation process in the Solar System and around other stars. The second and third interstellar objects, 2I/Borisov and 3I/ATLAS, showed visible signs of activity, making it possible to assess their composition via spectroscopic analysis of their gas coma. While neutral volatiles surrounding comets are easily probed by common spectrographs, ion emissions emanate mostly from the tail, making them easier to detect with integral-field units. We present WHT/WEAVE-LIFU observations of 3I/ATLAS, performed post-perihelion on 2025 November 30 and 2025 December 02. We simultaneously detect the ions N2+, CO+, CO2+, H2O+ and CH+ located in an anti-solar tail. Using common fluorescence factors, we determine abundance ratios of these species in the tail. In particular, the N2+/CO+ ratio allows us to constrain the lower-limit N2/CO>0.023+/-0.001, indicating that 3I/ATLAS is N2-rich compared to Solar System comets, a property which can be linked to cold formation conditions. Abundance ratios between other ions cannot directly be used to constrain abundances of neutrals but seem consistent with the fact that 3I/ATLAS is enriched in hypervolatiles. We also investigate whether these ratios vary along the tail, and only detect a marginal decrease of CH+ with cometocentric distance.
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Astrophysics of Galaxies

Rapid quenching and early gas depletion in the core of a galaxy protocluster at z=2.2

C. D'Eugenio, H. Dannerbauer, A. Naufal et al. (+6 more)

Understanding how galaxy evolution is affected in forming high-z overdensities is a major open question in modern astrophysics. Here, we present the average properties of the quiescent galaxy population recently identified in the core of the Spiderweb protocluster at z = 2.16. By stacking observations from the Hubble Space Telescope (HST; F160W imaging and G141 grism spectroscopy), the James Webb Space Telescope (JWST; PaBeta narrowband imaging), and the Atacama Large Millimeter/submillimeter Array (ALMA; 1.2 mm dust continuum imaging), we constrained their ensemble star formation history (SFH), residual dust-obscured star formation, and cold gas mass, respectively. The inferred average SFH indicates steady stellar mass growth, with progenitor SFRs consistent with those of massive Halpha emitters (HAEs) in the same structure. This is followed by rapid quenching, on average 500 Myr prior to observation. No significant age differences are found between active galactic nucleus (AGN) hosts and non-AGN systems. We find weak PaBeta emission for 80% of the sample, yet with dust-obscured star formation rates consistent with quiescence in all but one source (ID 443) excluded from the sample. Our galaxies remain individually and globally undetected in the dust continuum, yielding an average gas fraction fgas= Mmol/Mstar<7%, consistent with previously reported gas depletion trends with stellar mass observed in neighboring HAEs. We thus find the Spiderweb protocluster to host a sizable population of gas-depleted, massive quenched galaxies in its core, likely descending from main-sequence galaxy progenitors and building the first elements of the red sequence in this high-z forming cluster.
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All authors C. D'Eugenio, H. Dannerbauer, A. Naufal, J. M. Pérez-Martínez, Y. Koyama, P. G. Pérez-González, Y. H. Zhang, T. Kodama, K. Daikuhara

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

ROCKETS I: Investigating the Impact of the Rocket Effect on Two Nearby Young Open Clusters Encased in Infrared Bubbles

R. Arun, G. Maheswar, Annapurni Subramaniam et al. (+5 more)

We introduce ROCKETS (Rocket-driven Cluster Kinematics & Triggered Star-formation), a Gaia based programme investigating kinematic fingerprints of stellar feedback or rocket effect in young star forming regions. We analyse two young clusters with O-type stars enclosed by mid-infrared bubbles, Collinder 69 and IC 1396 as a methodological demonstration on two nearby benchmark regions. Co-moving candidates are selected within an astrometric ellipse defined by the median and median absolute deviation of Class I/II YSOs with Gaia data. For each star, we compute a relative proper-motion angle (RPMA) relative to the ionising source. In both regions the RPMA distribution shows a strong excess at RPMA $<15^{\circ}$, corresponding to sources moving away from the respective ionising sources. We quantify this using the Rocket Effect Index (REI), finding $\mathrm{REI}\simeq 1.1$ for Collinder 69 and $\mathrm{REI}\simeq 1.7$ for IC 1396. When considering only Class I/II YSOs, the REI of both regions increases to $\mathrm{REI}\gtrsim 10$. Velocity-structure analysis of the sample shows that the RPMA $<15^\circ$ subset has stronger positive pairwise expansion than the full co-moving population, providing additional support for the rocket effect. The outward-moving stars are not uniformly distributed; instead, their position angles are strongly anisotropic and preferentially aligned towards the bright-rimmed clouds. Gaia colour-magnitude diagrams show no strong age offset between sources moving outward and literature cluster members. The observed kinematic signatures support feedback-driven acceleration of gas and triggered star formation preferentially towards higher density regions of expanding bubbles due to the rocket effect.
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All authors R. Arun, G. Maheswar, Annapurni Subramaniam, Piyali Saha, T.P. Sudheesh, B. Shridharan, Blesson Mathew, Akhil Krishna R

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

Enabling Metallicity Measurements of M-dwarf Microlensing Lenses out to the Galactic Bulge

Jiyuan Zhang, Gregory Green, Weicheng Zang et al. (+2 more)

We demonstrate that metallicity becomes identifiable for M-dwarf microlensing lenses out to bulge distances when multi-band lens photometry is combined with the angular Einstein radius, thereby defining the photometric–microlensing metallicity method. The cool atmospheres of M dwarfs contain abundant molecules, whose broad absorption bands make their positions in an optical–NIR color–absolute magnitude diagram sensitive to metallicity. The angular Einstein radius provides the mass–distance constraint needed to infer the lens absolute magnitude, while the intrinsic M-dwarf locus and reddening vector are non-parallel in color–color space, allowing the lens extinction and intrinsic color to be inferred simultaneously from its multi-band photometry, thereby recovering the lens intrinsic position in the metallicity-sensitive color–absolute magnitude diagram. The method requires three-band lens photometry spanning roughly the R, Z, and K bands, making it particularly well suited to Roman through its high-resolution F062, F087, and F213 imaging. With the planned Roman Galactic Bulge Time-Domain Survey (GBTDS) observations plus an additional $\sim$8 hr of F062 imaging per field ($\sim$40 hr in total) roughly a decade after Roman launch, host metallicities could be measured for $\sim$150 planetary systems under the GBTDS yield forecast, with a typical $1\sigma$ precision of $\sim$0.25 dex from our mock-recovery analysis. Applied homogeneously to GBTDS microlensing events with and without detected planets, the method would enable the first measurement of the occurrence–metallicity relation for cold low-mass planets beyond the snow line, constrain the low-metallicity cutoff for their formation, and extend occurrence–metallicity studies into the inner Galaxy.
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All authors Jiyuan Zhang, Gregory Green, Weicheng Zang, Hongjing Yang, Shude Mao

Astrophysics of Galaxies

Stellar Population and Dynamical Modeling of Galaxies with Kinematically Misaligned Components: Age and Metallicity of Structural Components

V. S. Goradzhanov, K. O. Parfenov, D. D. Mishurin et al. (+1 more)

We present stellar population and orbit modeling of two galaxies with kinematically misaligned components, PGC 35706 and LEDA 2220522, extending previous Schwarzschild dynamical models. Using spatially resolved age and metallicity maps from NBursts full spectral fitting of MaNGA data with the E-MILES grid, we solve a regularized bounded linear inverse problem that assigns light-weighted SSP-equivalent age, metallicity, and mass-to-light ratio to orbital cells in the $(R,\lambda_z)$ plane. Component means and radial profiles use projected mass within the actual MaNGA field of view; profile segments with local component fractions below $1\%$ are treated as weakly constrained. In LEDA 2220522, the counter-rotating disk is younger than the co-rotating disk, $1.32\pm0.04$ versus $1.61\pm0.06$ Gyr, and more metal-rich, $[\mathrm{M/H}]=-0.450\pm0.043$ versus $-0.613\pm0.024$. Its rotation agrees with that of the H$\alpha$ ionized gas, supporting late supply of chemically enriched retrograde gas without requiring multiple accretion episodes with alternating angular momentum. In PGC 35706, the counter-rotating disk contains $8.8\%$ of the mass in the observed field and is formally older than the co-rotating disk by $1.36\pm0.51$ Gyr and more metal-poor by $0.130\pm0.046$ dex. These differences are preliminary because the projected data provide limited support. Fifty Monte Carlo realizations confirm stability of the projected maps against statistical noise, but not uniqueness in individual phase-space cells. The stellar population and orbit approach is therefore applicable to MaNGA data when local projected component fractions and systematics from regularization and the dynamical model are explicitly considered.
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All authors V. S. Goradzhanov, K. O. Parfenov, D. D. Mishurin, O. K. Sil'chenko

Astrophysics of Galaxies

The dispersal law of young star clusters: free expansion – A Gaia/eROSITA census of Orion and Sco–Cen

João Alves, Stefan Meingast and Alena Rottensteiner

We measure how young star clusters disperse over time in the two nearest star-forming complexes. Our sample comprises 44 SigMA-Gaia-defined clusters in Orion and 31 in Sco–Cen, with isochronal ages of 2–25\,Myr. We extend each cluster's census with eROSITA-selected young stars that share its parallax and motion and lie within the distance they could have traveled ballistically since birth. By 10–30\,Myr, about one in three former cluster stars lies outside the cluster's cataloged boundary. Across both complexes and the full age range, we find that (1) the half-member radius increases linearly with age, $\rfifty \simeq \sigbirth t$, (2) the velocity dispersion shows no measurable evolution, and (3) all studied clusters are unbound. These results support free expansion. Young clusters in Orion and Sco-Cen emerge unbound from their birth clouds and coast at their birth velocity dispersion without significant relaxation or a characteristic dispersal timescale. We further show that memberships defined by density contrast preferentially omit the fastest stars, making the same clusters appear to expand more slowly ($\propto t^{0.6}$), cool with age, and approach equilibrium.
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Astrophysics of Galaxies

PPN–spin degeneracies in mock S62-like stellar-orbit inference

Shant Khlghatyan

We investigate the degeneracies between black hole (BH) spin effects and parametrized post-Newtonian (PPN) parameters in the relativistic orbital dynamics of S2-like and S62-like stars orbiting Sagittarius A$^{\ast}$. Using a 1PN+SO Hamiltonian framework and synthetic astrometric and radial velocity datasets, we perform Bayesian parameter inference. For current baseline observational precisions, the dominant relativistic observable–the Schwarzschild periapsis advance–allows the recovery of the effective precession parameter $\Upsilon$, while leaving the individual PPN parameters $\gamma$ and $\beta$ degenerate. Assuming microarcsecond-level astrometric precision, the spin-induced Lense-Thirring signal becomes partially detectable; fixing the PPN sector to General Relativity allows the BH spin magnitude to be constrained to an uncertainty of $\sim10^{-2}$. However, simultaneously varying PPN and spin parameters reveals a strong, approximately linear covariance between $\Upsilon$ and the dimensionless spin parameter $\chi$. To overcome this limitation, we demonstrate that joint multi-star inference can disentangle the degeneracy by combining a wider-orbit star, which constrains the dominant 1PN sector, with a compact relativistic orbit that is sensitive to Lense-Thirring frame dragging.
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Astrophysics of Galaxies · Cross-listed from astro-ph.CO

Euclid. A two-point correlation approach to diagnosing star-related systematics in the Euclid spectroscopic survey

Euclid Collaboration: I. Risso, B. R. Granett, E. Branchini et al. (+297 more)

The Euclid spectroscopic survey will measure galaxy clustering with unprecedented precision, requiring stringent control of observational and instrumental systematics. Star-related effects may contaminate spectroscopic images through photometric persistence and imperfect masking of stars. We characterized their impact on galaxy clustering analyses, focusing on angular features identifiable in the data. We used angular and spatial auto- and cross-correlation statistics. For galaxies, we used mock spectroscopic catalogues from the EuclidLargeMocks in a 330 deg2 region of the Euclid Wide Survey (EWS). For stars, we used Gaia and 2MASS catalogues in the same area. To simulate photometric persistence, we implemented a simplified detector-level model calibrated on spectroscopic measurements and varied its strength to introduce different interloper fractions. For stellar masking, we modelled inconsistencies between the mask applied to the data and to the random catalogue. We measured star-star, galaxy-galaxy, and star-galaxy angular correlation functions using the Landy–Szalay estimator, and quantified deviations from the expected null star-galaxy correlation. We also evaluated the large-scale impact of such systematics through the three-dimensional two-point correlation function (2PCF). Photometric persistence produces a characteristic feature in the star-galaxy angular cross-correlation at scales of order 100", corresponding to the Euclid dithering pattern and grism dispersion geometry, and a spurious positive signal approximately constant up to 1{\deg}. Star-galaxy cross-correlation can detect residual persistence at contamination levels as low as 10% in a Data Release 1 spectroscopic catalogue. In contrast, stellar-mask mismatches produce strong small-scale angular signatures but have negligible impact on the large-scale 2PCF under realistic EWS conditions.
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All authors Euclid Collaboration: I. Risso, B. R. Granett, E. Branchini, A. Veropalumbo, B. Kubik, P. Monaco, J. Comparat, A. Hall, N. Aghanim, B. Altieri, A. Amara, S. Andreon, N. Auricchio, C. Baccigalupi, M. Baldi, A. Balestra, S. Bardelli, P. Battaglia, A. Biviano, M. Bolzonella, M. Brescia, S. Camera, G. Cañas-Herrera, V. Capobianco, C. Carbone, J. Carretero, F. J. Castander, M. Castellano, G. Castignani, S. Cavuoti, A. Cimatti, C. Colodro-Conde, G. Congedo, L. Conversi, Y. Copin, F. Courbin, H. M. Courtois, H. Degaudenzi, S. de la Torre, G. De Lucia, H. Dole, F. Dubath, X. Dupac, S. Dusini, S. Escoffier, M. Farina, R. Farinelli, S. Ferriol, F. Finelli, N. Fourmanoit, M. Frailis, E. Franceschi, M. Fumana, L. Gabarra, S. Galeotta, K. George, W. Gillard, B. Gillis, C. Giocoli, J. Gracia-Carpio, A. Grazian, F. Grupp, L. Guzzo, S. V. H. Haugan, S. Hemmati, W. Holmes, F. Hormuth, A. Hornstrup, K. Jahnke, M. Jhabvala, B. Joachimi, S. Kermiche, A. Kiessling, M. Kümmel, M. Kunz, H. Kurki-Suonio, A. M. C. Le Brun, S. Ligori, P. B. Lilje, V. Lindholm, I. Lloro, M. Magliocchetti, G. Mainetti, O. Mansutti, O. Marggraf, M. Martinelli, N. Martinet, F. Marulli, R. J. Massey, N. Mauri, E. Medinaceli, M. Melchior, M. Meneghetti, E. Merlin, G. Meylan, A. Mora, M. Moresco, C. Moretti, L. Moscardini, R. Nakajima, C. Neissner, S.-M. Niemi, J. W. Nightingale, C. Padilla, S. Paltani, F. Pasian, W. J. Percival, V. Pettorino, A. Pezzotta, S. Pires, G. Polenta, M. Poncet, L. A. Popa, F. Raison, A. Renzi, J. Rhodes, G. Riccio, F. Rizzo, E. Romelli, M. Roncarelli, B. Rusholme, R. Saglia, Z. Sakr, A. G. Sánchez, D. Sapone, B. Sartoris, P. Schneider, T. Schrabback, M. Scodeggio, A. Secroun, E. Sihvola, C. Sirignano, G. Sirri, L. Stanco, P. Tallada-Crespí, A. N. Taylor, I. Tereno, N. Tessore, S. Toft, R. Toledo-Moreo, F. Torradeflot, I. Tutusaus, J. Valiviita, T. Vassallo, Y. Wang, J. Weller, G. Zamorani, F. M. Zerbi, E. Zucca, M. Ballardini, E. Bozzo, C. Burigana, R. Cabanac, M. Calabrese, A. Cappi, F. Caro, T. Castro, J. A. Escartin Vigo, J. García-Bellido, T. Gasparetto, J. Macias-Perez, R. Maoli, R. B. Metcalf, A. A. Nucita, M. Pöntinen, E. Sarpa, V. Scottez, M. Sereno, M. Tenti, M. Tucci, M. Viel, M. Wiesmann, J. A. Acevedo Barroso, Y. Akrami, I. T. Andika, S. Anselmi, M. Archidiacono, G. Aricò, F. Atrio-Barandela, M. Baes, L. Bazzanini, J. Bel, D. Bertacca, M. Bethermin, F. Beutler, L. Blot, M. Bonici, M. L. Brown, S. Bruton, B. Camacho Quevedo, P. Carrilho, C. S. Carvalho, F. Cogato, T. E. Collett, S. Conseil, S. Contarini, A. R. Cooray, P. Corcho-Caballero, B. Csizi, O. Cucciati, S. Davini, T. de Boer, F. De Paolis, G. Desprez, A. Díaz-Sánchez, S. Di Domizio, J. M. Diego, V. Duret, A. Enia, Y. Fang, A. Farina, A. Finoguenov, A. Franco, Y. Fu, K. Ganga, E. Gaztanaga, Z. Ghaffari, F. Giacomini, F. Gianotti, G. Gozaliasl, A. Gruppuso, M. Guidi, C. M. Gutierrez, H. Hildebrandt, J. Hjorth, M. Jauzac, J. J. E. Kajava, Y. Kang, V. Kansal, D. Karagiannis, J. Kim, C. C. Kirkpatrick, A. Kovács, I. Kovačić, K. Koyama, S. Kruk, M. C. Lam, F. Leclercq, L. Legrand, M. Lembo, G. Leroy, G. F. Lesci, J. Lesgourgues, T. I. Liaudat, L. Linke, S. J. Liu, F. Mannucci, F. R. Marleau, C. J. A. P. Martins, M. Miluzio, G. Morgante, S. Nadathur, K. Naidoo, A. Navarro-Alsina, S. Nesseris, L. Nicastro, F. Oppizzi, F. Pace, D. Paoletti, G. Parimbelli, F. Passalacqua, K. Paterson, L. Patrizii, C. Pattison, R. Paviot, A. Pisani, D. Potter, G. W. Pratt, S. Quai, M. Radovich, G. Rodighiero, W. Roster, S. Sacquegna, M. Sahlén, D. B. Sanders, A. Schneider, D. Sciotti, E. Sellentin, S. Serjeant, L. C. Smith, J. G. Sorce, I. Szapudi, M. Talia, K. Tanidis, C. Tao, F. Tarsitano, G. Testera, R. Teyssier, S. Tosi, A. Troja, C. Uhlemann, C. Valieri, A. Venhola, D. Vergani, G. Verza, S. Vinciguerra, M. von Wietersheim-Kramsta, N. A. Walton, L. Wang, A. H. Wright

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

Can isolated binaries form unequal-mass binary black-hole mergers with a high-spin primary black hole?

Xiao-Tian Xu, Dong Lai and Bin Liu

GWTC-5 has revealed a subpopulation of merging binary black holes (BBHs) with a high-spin primary black hole (BH) and possibly unequal BH masses. GW241110 additionally exhibits a large spin-orbit misalignment, which suggests a hierarchical-merger origin. However, other formation scenarios are possible or even likely, especially for events without constraints on spin-orbit misalignment. As an alternative to hierarchical mergers, we investigate whether binary evolution can produce unequal-mass BBH mergers with a high-spin primary BH. Rather than performing comprehensive population-synthesis calculations, we examine the evolutionary pathways of forming merging BBHs and assess their uncertainties. We identify two possible pathways for producing unequal-mass BBHs with a high-spin primary. In initially wide binaries, mass-ratio reversal can make the tidally spun-up second-born BH both more massive and more rapidly rotating than the first-born BH; alternatively, in an initially close, unequal-mass binary, the primary star may evolve chemically homogeneously, while the secondary star evolves normally, producing a high-spin first-born BH that is more massive than its companion. Generally, large spin-orbit misalignment can be produced by large natal kicks or tertiary-induced nodal precession and/or Zeipel-Lidov-Kozai oscillations. We conclude that hierarchical mergers are not uniquely required to produce unequal-mass BBHs with a high-spin primary BH, although each isolated-binary pathway faces important theoretical constraints. Future detections of more merger events with primary BH spins around 0.7 would discriminate between binary evolution and hierarchical merger origin.
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Astrophysics of Galaxies

Corona Australis 151: an extremely young protostar

E. Redaelli, S. Spezzano, P. Caselli et al. (+9 more)

Context. Prestellar cores are the birthplace of stars and planetary systems, but they are short-lived objects, since the initial stages of dense core evolution, collapse, and the formation of a protostellar seed are fast. In an effort to build a catalogue of bona-fide prestellar cores in the Solar neighbourhood, we used APEX observations to identify dynamically evolved cores among dense cores observed with Herschel. One of them, Corona Australis 151, stood out because of its centrally peaked structure, with densities above 1e7cm-3 in the central 500-1000 au, and high deuteration levels (N2D+/N2H+~0.5), suggestive of an evolved, prestellar stage. Aims. Corona Australis 151 appears to be an evolved prestellar core, but the presence of broad wings in some of the detected lines and a tentative 70micron detection with Herschel partially challenges its prestellar stage. We intend to assess its evolutionary stage. Methods. We analyse new ALMA data of the continuum emission and of several line tracers (including typical outflow tracers) at a resolution of~150-200au, tracing the envelope. Results. We unveil the presence of a compact and young outflow (projected dynamical age:~500yr), traced by SiO, CO, and H2CO emission. The continuum emission traces an envelope structure of size~1000 au and mass 0.33M_sun(assuming Tdust=20 K). Conclusions. Corona Australis 151 is an extremely young protostar, possibly one of the youngest known in the Solar neighbourhood, and hence a new laboratory to study the chemical and dynamical evolution at the dawn of star formation.
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All authors E. Redaelli, S. Spezzano, P. Caselli, J. Harju, D. Arzoumanian, O. Sipilä, A. Belloche, F. Wyrowski, J. E. Pineda, S. Jensen, Y. Misugi, G. Majerczyk

Astrophysics of Galaxies

Rubin J122659.4+090236: An Extremely Low Surface Brightness Galaxy Candidate Discovered in the Rubin LSST Early Data Preview 2

Dipanjan Mitra, Kanak Saha, Sugata Kaviraj et al. (+28 more)

We report the serendipitous discovery of an exceptionally low surface brightness galaxy (LSBG) candidate, Rubin J122659.4+090236, in Rubin Observatory imaging of the interacting NGC 4410 system, identified in the Cosmic Treasure Chest public release. 2D Sérsic modelling of the Rubin g, r, and i images reveals a nearly round system with a shallow profile (n ~ 0.4), an effective radius of R_e ~ 6”, and central surface brightnesses of $\mu_{0,g}=27.52\pm0.04$, $\mu_{0,r}=27.62\pm0.07$, and $\mu_{0,i}=27.04\pm0.08$ mag arcsec$^{-2}$. EAZY photo-z fitting favours an intermediate-z solution at z~0.3, while a low-redshift solution at z~0.028, consistent with the NGC 4410 system, is also permitted by a restricted EAZY fit over 0<z<0.1 without imposing a redshift prior. These alternatives imply substantially different physical interpretations, ranging from a diffuse dwarf-like system to an exceptionally extended background LSBG. This discovery demonstrates Rubin's sensitivity to extremely diffuse galaxies and highlights the potential of the LSST survey to uncover large samples of such elusive systems across wide areas, enabling systematic studies of the LSBG population and its role in galaxy evolution.
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All authors Dipanjan Mitra, Kanak Saha, Sugata Kaviraj, Alister W. Graham, Sandro Tacchella, Ricardo Demarco, Dieu D. Nguyen, Elham Saremi, hernandez toledo, hector., Jose Benavides, Natanael M. Cardoso, Samuel Boissier, Dan S. Taranu, Bililign Dullo, Patricia B. Tissera, Matias Blana, Hao Fu, Priyanka Chakraborty, Kevin A. Pimbblet, Tayyaba Zafar, Anirban Dutta, Aidan P Cotter, Darko Donevski, Katarzyna Malek, Rossella Ragusa, Agnieszka Pollo, Unnikrishnan Sureshkumar, Aritra Ghosh, Antonio Vanzanella, Hareesh Thuruthipilly

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

Multi-Wavelength Post-Perihelion Polarimetry of Interstellar Comet 3I/ATLAS

Zuri Gray, Galin Borisov, Ludmilla Kolokolova et al. (+15 more)

We present new post-perihelion polarimetric observations of the third discovered interstellar object, 3I/ATLAS (C/2025 N1; hereafter 3I), obtained using FORS2 at the Very Large Telescope, ALFOSC at the Nordic Optical Telescope, and FoReRo2 at the 2 m Ritchey-Chrétien-Coudé telescope. The observations span phase angles 1-30$^{\circ}$, providing the most extensive polarimetric phase angle coverage obtained for an interstellar object to date. The post-perihelion measurements reveal that the unusual polarimetric properties of 3I persist across perihelion and match recent independent post-perihelion observations, indicating no significant evolution in the polarimetric properties. The increased phase angle coverage allowed us to further constrain the minimum polarisation to $-2.9\%$ at phase angle $5.5^{\circ}$. Multi-band BVRI observations reveal a wavelength dependence of polarisation, with the negative polarisation branch becoming deeper and shifted toward smaller phase angles at longer wavelengths. The polarimetric colour is predominantly red and increases with phase angle, consistent with behaviour observed in other comets. Imaging and polarimetric maps show a substantially more extended coma after perihelion, while the polarisation distribution itself remains smooth and spatially homogeneous. Numerical modelling suggests that the unusually deep polarisation phase curve originates from moderately porous dust aggregates consisting of weakly absorbing sub-micron monomers, resulting in much brighter dust than that in typical solar system comets.
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All authors Zuri Gray, Galin Borisov, Ludmilla Kolokolova, Johannes Markkanen, Yuna G. Kwon, Stefano Bagnulo, Alberto Cellino, Rosemary C. Dorsey, Grigori Fedorets, Mikael Granvik, Eric MacLennan, Antti Penttilä, Karri Muinonen, Olga Muñoz, Philippe Bendjoya, Maxime Devogèle, Simone Ieva, Francisco J. García-Izquierdo

Astrophysics of Galaxies

Quantifying collision-driven mass loss in supermassive star formation: the role of stellar structure and accretion

P. A. Solar, B. Reinoso, D. R. G. Schleicher et al. (+1 more)

Observations of high-redshift galaxies with JWST have renewed interest in scenarios where supermassive stars form via runaway stellar collisions in dense clusters, yet the impact of collision-driven mass loss on their growth remains uncertain. In this work, we perform a post-processing analysis of 3D hydrodynamical simulations of the formation of a supermassive star, applying an analytic mass-loss prescription to stellar collisions while exploring different assumptions for the internal stellar structure. We consider a polytropic main-sequence model, a semi-analytic accreting protostar model, and structures derived from stellar evolution calculations. We find that the cumulative mass-loss fraction depends sensitively on the adopted stellar structure, ranging from $\lesssim 10-25\%$ for more compact configurations to $\gtrsim 30-40\%$ for more extended protostellar models. The importance of mass loss further depends on the dynamical state of the system, including the ratio of stellar velocity dispersion to the stellar surface escape velocity. We find significant uncertainty depending on the prescription used. As a result, collision-driven mass loss could significantly limit the growth of the central object, at least in some cases. Overall, our results indicate that uncertainties in the internal structure of rapidly accreting protostars represent a major source of systematic uncertainty and must be better constrained to robustly assess the viability of runaway-collision pathways for forming massive black hole seeds.
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All authors P. A. Solar, B. Reinoso, D. R. G. Schleicher, R. Banerjee

Astrophysics of Galaxies

Membership Determination of 45 Open Clusters Beyond 3 kpc

Qian Cui, Zhihong He, Yangping Luo et al. (+1 more)

Context. Open clusters (OCs) are fundamental tracers for studying the structure and chemical evolution of the Galactic disc. Reliable identification of cluster members, particularly at the faint end of the main sequence, remains challenging owing to severe field-star contamination and the declining photometric completeness for distant, obscured systems. Aims. We aim to identify and characterise distant OC candidates at heliocentric distances $d \gtrsim 3$ kpc using Gaia DR3 astrometry, with emphasis on improving the recovery of faint candidate members in highly contaminated stellar fields. Methods. We apply a probabilistic membership method based on Gaia DR3 proper motions and parallaxes. Candidate members are selected through combined astrometric criteria and refined via spatial filtering using radial density profiles, with photometric data serving as a consistency check. For clusters affected by significant extinction, near-infrared photometry from 2MASS is incorporated to compensate for the limitations of optical data. Results. We analyse a sample of 45 targets, comprising 30 previously reported OCs and 15 newly identified candidates, all located at $d \gtrsim 3$ kpc. The method yields more compact astrometric distributions and improves the recovery of faint candidate members in several systems. However, residual field contamination remains non-negligible for the most obscured clusters projected against the Galactic plane. Conclusions. Gaia DR3 astrometry provides an effective basis for the exploratory identification and first-order characterisation of distant OC candidates. Dedicated near-infrared follow-up observations will be essential for robust membership assessment in high extincted regions.
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All authors Qian Cui, Zhihong He, Yangping Luo, Yan Jiang

Thursday 3 September

Astrophysics of Galaxies

A Spatio-Temporal Self-Propagating Log-Gaussian Cox-Hawkes Process for Star Formation Modelling

Qihan Zou

Stochastic self-propagating star formation models describe galactic structure through local triggering and feedback but are commonly formulated using discrete spatial cells and time steps. We extend the spatio-temporal log-Gaussian Cox-Hawkes framework to obtain the Spatio-Temporal Self-Propagating Log-Gaussian Cox-Hawkes Process, a continuous point process model for the locations and times of star-forming events. The model combines spontaneous formation, correlated environmental effects, outwardly propagating excitation, local inhibition, differential rotation, and saturation. An observation layer transforms the conditional event intensity into idealised maps of instantaneous star-forming arm emissivity and recent young stellar surface brightness. For the selected parameter setting, the displayed realisation exhibits transient flocculent spiral-like patterns without any deterministic spiral geometry being imposed. The Monte Carlo experiment finds similar event production with and without rotation, whereas the stationary-front case produces fewer events. The displayed maps further suggest that differential rotation contributes to the winding and spatial arrangement of activity. The proposed framework provides a basis for future statistical inference from spatially and temporally resolved observations of star formation.
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Astrophysics of Galaxies

Star Formation Evolution in Galaxy Pairs: Constraints from Morphological Disturbances and Recent Star Formation Histories

Shaoze Geng, Shuai Feng, Shuhao Chen et al. (+8 more)

Galaxy interactions can enhance star formation, but how star formation evolves during galaxy interactions remains poorly constrained by observations. We combine pair separation, morphological disturbance, and recent star formation history to study this evolution. We measure morphological disturbance with the shape asymmetry parameter ($A_{\rm shape}$) using deep images from the DESI Legacy Surveys, and use Galaxy Zoo DESI classifications as an independent test. We infer recent star formation histories by comparing SFR and $\mathrm{EW}(\mathrm{H}\alpha)$, which trace current star formation, with $\mathrm{EW}(\mathrm{H}\delta_A)$ and $D_n4000$, which are sensitive to stellar populations formed over longer timescales. At small projected separations, strongly disturbed galaxies show the strongest enhancements in SFR and $\mathrm{EW}(\mathrm{H}\alpha)$, indicating that close encounters trigger strong star formation. At intermediate separations ($d_{\rm p}\sim100\,\mathrm{kpc}$), their current star formation is only moderately enhanced, but their $\mathrm{EW}(\mathrm{H}\delta_A)$ enhancement is the strongest. This indicates a larger contribution from intermediate-age stars formed during stronger star formation in the past $\sim0.1$–$1\,\mathrm{Gyr}$. The TNG100 analysis shows that this pattern results from rapid changes in SFR around close passage. SFR rises sharply during the encounter and declines as the galaxies move apart, while tidal disturbances remain visible. As short-lived massive stars disappear and intermediate-age A-type stars begin to dominate the spectrum, the $\mathrm{EW}(\mathrm{H}\delta_A)$ enhancement peaks later than the SFR enhancement. Our results provide important observational evidence for the evolution of SFR along the merger sequence.
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All authors Shaoze Geng, Shuai Feng, Shuhao Chen, Yaotian Duan, Shiyin Shen, Cailu Shi, Linlin Li, Yijin Niu, Bingxi Huo, Wenyuan Cui, Guozhen Hu

Astrophysics of Galaxies

KKR 18: A Late Arrival Galaxy?

R. Brent Tully, Valentina E. Karachentseva, Igor D. Karachentsev et al. (+8 more)

KKR 18 is a dwarf irregular galaxy in the Local Void at a distance of 9.46 Mpc. It is gas rich and was experiencing vigorous star formation 20 - 100 Myr ago. Its metallicity is 4% of Solar from an emission spectrum, qualifying is as extremely metal poor (XMP). Its most remarkable characteristic is an extremely deficient Red Giant Branch. This feature is shared with a small number of other dwarf galaxies and it is inferred that they lack ancient stellar components. It is suggested that these are late arrival galaxies (LAG).
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All authors R. Brent Tully, Valentina E. Karachentseva, Igor D. Karachentsev, Gagandeep S. Anand, Maksim I. Chazov, Serafim S. Kaisan, Ehsan Kourkchi, Lidia N. Makarova, Simon A. Pustilnik, Edward J Shaya, Arina L. Tepliakova

Astrophysics of Galaxies

Linking orbital history to the quenching of isolated dwarf galaxies

Sophia Um, Devontae C. Baxter and Ethan O. Nadler

Recent discoveries of isolated dwarf galaxies (${M}_{\star}\sim 10^{7-9}~{\rm M}_{\odot}$) with no signs of ongoing star formation have challenged the prevailing notion that these solitary systems are exclusively star-forming. Investigating the origins of isolated quenched dwarfs provides important clues into the dominant processes driving galaxy quenching as a function of environment and stellar mass. In this study, we identify central dwarf galaxies in the Illustris-TNG50 cosmological simulation with ${M}_{\star} \sim 10^{7-9}~{\rm M}_{\odot}$, evaluate their star formation and orbital histories, and investigate how their dark matter halo properties correlate with quenching and interactions with massive neighbors. We find that $R_{\rm{peri}}$, defined as the closest separation ever attained between a central dwarf and a galaxy more massive than ${M}_{\star}> 10^{10}~{\rm M}_{\odot}$, is a reliable metric for inferring the star formation history of central dwarfs in TNG50. We also show that isolated quenched dwarf galaxies separate into backsplash and non-backsplash subpopulations as a function of $R_{\rm{peri}}$, and that non-backsplash galaxies are consistent with being quenched by cosmic web stripping or internal feedback mechanisms. Furthermore, we find that $R_{\rm{peri}}$ correlates strongly with dark matter halo mass and star formation proxies based on halo maximum circular velocity, which are commonly used in empirical galaxy-halo connection models. These predictions can be tested with data from upcoming deep and wide surveys (e.g., Rubin LSST and Roman) that are anticipated to significantly increase the cosmic census of dwarf galaxies.
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Astrophysics of Galaxies

Hint of bimodal Mg-Al anticorrelation in the metal-poor Globular Cluster NGC 4372

Camila Beltrand, Macarena Uribe, Sandro Villanova et al. (+1 more)

We present a detailed chemical abundance analysis of seven red giant branch stars in the very metal-poor globular cluster NGC~4372, based on high-resolution UVES spectra. Our study aims to characterize the chemical evolution of NGC~4372 and investigate the presence of multiple populations. We derived abundances for 17 elements, including light, iron-peak, $\alpha$, and heavy elements, using both equivalent width and spectral synthesis methods. We find a mean cluster metallicity of [Fe/H]=$-$2.36 $\pm$ 0.03 dex, with no evidence of an intrinsic iron spread. The cluster exhibits a typical $\alpha$-enhancement of [$\alpha$/Fe]= 0.33 $\pm$ 0.05 dex and a well defined Na-O anticorrelation, confirming that NGC 4372 hosts at least two distinct stellar populations. Furthermore, the measured ratio of [Ba/Eu] =$-0.47 \pm 0.04$ indicates that the synthesis of heavy elements was dominated by $r$-process, consistent with other very metal-poor environments. We also identify a potential bimodal Mg-Al anticorrelation, which aligns with the patterns observed in GCs of similar mass and metallicity. These results reinforce the status of NGC 4372 as a cornerstone for understanding the early chemical enrichment of the Galactic Halo.
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All authors Camila Beltrand, Macarena Uribe, Sandro Villanova, César Muñoz

Astrophysics of Galaxies

Extending the Stellar-to-Halo Mass Relation to Dwarf Galaxies with DESI DR1

Z. Shao, Y. Zu, A. Salcedo et al. (+51 more)

Constraining the dark matter halos of the smallest galaxies offers fundamental insights into the nature of dark matter and stellar feedback. Using the Dark Energy Spectroscopic Instrument (DESI) Data Release 1, we infer the stellar-to-halo mass relation (SHMR) down to the dwarf scale ($M_\star < 10^9\,M_\odot$), without extrapolation from the higher mass range. Leveraging the unprecedented depth of the DESI Bright Galaxy Survey at $0.01 < z < 0.2$, we construct 12 samples spanning nearly four orders of magnitude in stellar mass, and measure their projected clustering $w_p$, galaxy-galaxy lensing $\Delta\Sigma$, as well as a novel observable: satellite occupation number $N_{\rm sat}$. The addition of $N_{\rm sat}$ enables robust subtraction of satellite contributions to both $w_p$ and $\Delta\Sigma$ across the 12 individual halo occupation distribution analyses, yielding an average halo-to-stellar mass relation (HSMR) of $\log \langle M_h(M_\star) \rangle = 12.06 + 0.58\log(M_\star/10^{11}) + (M_\star/10^{11})^{0.73}$. Combining this HSMR with an observed stellar mass function, we constrain the SHMR across five orders of magnitude in halo mass, with the power-law slope steepening from $0.32 \pm 0.06$ above the Milky Way mass to $2.08 \pm 0.21$ in the dwarf regime. Interestingly, the scatter about the SHMR grows from $0.17 \pm 0.02$ dex at Milky Way-like scales to $0.68_{-0.33}^{+0.21}$ dex for systems comparable to the Large Magellanic Cloud, suggesting that smaller galaxies follow increasingly diverse evolutionary paths. Our work highlights the power of DESI in probing the galaxy-halo connection within the dwarf regime, offering an exciting avenue to bridge the gap between large-scale and near-field cosmologies in the future.
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All authors Z. Shao, Y. Zu, A. Salcedo, Y. Lin, Z. Chen, X. Xu, J. Hua, Z. Zhai, J. Aguilar, S. Ahlen, F. Beutler, D. Bianchi, D. Brooks, A. Carnero Rosell, F. J. Castander, T. Claybaugh, A. de la Macorra, Biprateep Dey, Z. Ding, J. E. Forero-Romero, E. Gaztañaga, Satya Gontcho A Gontcho, G. Gutierrez, C. Hahn, S. Juneau, R. Kehoe, A. Kremin, O. Lahav, A. Lambert, M. Landriau, L. Le Guillou, M. Manera, P. Martini, A. Meisner, R. Miquel, J. Moustakas, S. Nadathur, E. Paillas, W. J. Percival, F. Prada, I. Pérez-Ràfols, C. Ravoux, G. Rossi, R. Ruggeri, M. Saraf, L. Samushia, E. Sanchez, C. Saulder, D. Schlegel, J. Silber, M. Siudek, G. Tarlé, B. A. Weaver, H. Zou

Astrophysics of Galaxies

H I Absorbers as Beacons of Hidden Structure at $z \sim 3$: Multi-Component, Metal-Rich Absorption System near a Protocluster

Kentaro Koretomo, Nobunari Kashikawa, Toru Misawa et al. (+12 more)

H I gas traces the large-scale structure and provides the primary fuel for star formation. High-$z$ protoclusters are ideal laboratories to study how H I gas is accreted and consumed during the formation of the most massive structures in the Universe. However, much remains unknown about the distribution and physical state of their H I gas. We examine a rare configuration in which a protocluster candidate is located in front of a quasar at $z=3.09$. Our spectroscopic campaign confirms a protocluster at $z=3.079$: however, no corresponding strong H I absorption is found in the background quasar spectrum. Instead, we serendipitously discover a prominent H I absorption feature at $z\sim 3.01$, offset by $\sim 60$ cMpc from the centre of the protocluster. Spanning an exceptionally broad velocity range of $\sim 2000$ km s$^{-1}$ ($\sim 40$ cMpc), this absorption is decoupled from the confirmed member galaxies. Detailed kinematic modelling reveals this absorption comprises five distinct components rather than a single cloud. Moreover, one of these components exhibits a super-solar metallicity ($[\mathrm{O/H}] = +1.19^{+0.91}_{-0.78}$). We propose two physical scenarios for this unique system: (1) an additional, hidden massive protocluster along the line of sight, and/or (2) metal-rich outflows and metal-poor inflows driven by a single massive galaxy. The discovery highlights that while protoclusters are not universally associated with strong H I absorption, targeting the strong H I absorbers may serve as a beacon for uncovering massive, metal-rich protoclusters or complex gas kinematics in the early Universe.
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All authors Kentaro Koretomo, Nobunari Kashikawa, Toru Misawa, Jun Toshikawa, Yoshihiro Takeda, Hisakazu Uchiyama, Junya Arita, Shunta Shimizu, Ryo Emori, Kei Ito, Satoshi Kikuta, Mariko Kubo, Yoshiaki Ono, Marcin Sawicki, Rhythm Shimakawa