Earth and Planetary Astrophysics
Incompatibility of iron in post-perovskite and the stability of basal magma oceans in super-Earths
Francis Dragulet and Lars Stixrude
Post-perovskite is expected to dominate much of the solid mantles of rocky
exoplanets, yet iron partitioning between post-perovskite and silicate melt, which controls the compositional
evolution and buoyancy of crystallizing magma oceans, is unconstrained at these pressures. We use first-principles molecular dynamics and thermodynamic integration to compute the Fe–Mg distribution coefficient $K_D$ between post-perovskite and (Mg,Fe)SiO$_3$
liquid at 150–600~GPa and 6000–10000~K. Iron is strongly incompatible in post-perovskite and becomes increasingly so with pressure. Combining $K_D$ with equations of state, we find that iron enrichment of residual
liquid reverses the solid–
liquid density contrast, causing post-perovskite to become buoyant. Basal magma oceans are therefore gravitationally stable in super-Earth
exoplanets up to 4 M$_{\oplus}$.
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Cosmology and Nongalactic Astrophysics
Supernova Cousins: The Intrinsic Dispersion and Environmental Dependence of Type Ia Supernovae in Galaxy Groups with TITAN
Allison Blum, Erik R. Peterson, David O. Jones et al. (+7 more)
At low
redshift, scatter in the Type Ia
supernova (SN Ia) Hubble diagram is increased by peculiar velocities (PVs) and correlations between SN distances and their host-
galaxy environments, limiting the precision of cosmological distance measurements. We investigate these effects using 1086 spectroscopically-confirmed SNe Ia from the Type Ia
Supernova Trove from ATLAS in the Nearby Universe (TITAN) sample in the
redshift range $0.01<z<0.05$. We identify 314 group-associated SNe, including 80 SN "cousins" occurring in different
galaxies within 32 common groups. We first examine whether using the group-averaged
redshifts of SNe in
galaxy groups can mitigate PV-induced scatter; however, we find that the Hubble residual scatter increases slightly from $0.184^{+0.017}_{-0.009}$ mag using individual host-
galaxy redshifts to $0.200^{+0.016}_{-0.015}$ mag using group-averaged
redshifts. For the sample of SN cousins, we next analyze pairwise differences in their distances and compare their scatter to the Hubble residual scatter of the full SN sample. For analysis variants that limit line-of-sight differences in distance modulus between cousins, we find $\sim$30-40% lower scatter from SN cousins at significances of 2.3-2.8$\sigma$. We also find tentative evidence that these differences in scatter may be driven by the youngest groups, with a 2$\sigma$ reduction in scatter for groups with high fractions of spiral
galaxies. These results suggest that SN cousins may provide a novel route for increasing SN Ia distance precision and understanding the role of host-
galaxy and group properties on SN Ia distance measurements.
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All authors Allison Blum, Erik R. Peterson, David O. Jones, R. Brent Tully, Elijah G. Marlin, Yukei S. Murakami, Jack W. Tweddle, Dillon Brout, Mitchell Dixon, Stephen J. Smartt
Earth and Planetary Astrophysics · Cross-listed from cs.LG
Operational Range Bounding in Spectroscopy: A Safety Cage Framework for Machine Learning Models
Nikki Grens, Luís F. Simões, Kai Hou Yip et al. (+1 more)
Ensuring the reliability of black-box machine learning models in safety-critical space missions remains a significant challenge, particularly when ground-truth is unavailable for validation. Although machine learning models offer a powerful means to augment standard pipelines by extracting transmission spectra from complex exoplanetary light curves, their susceptibility to unmodelled instrument anomalies, stellar activity, and domain shifts introduces unquantified risks. This study evaluates a modular safety cage architecture that operates as a parallel monitoring layer to assess the validity of a prediction without modifying the underlying
estimator. By monitoring different runtime indicators, including uncertainty quantification, out-of-domain detection, and influence functions, the framework constrains the model's operational domain to a verified region. A controlled evaluation is conducted under both in-domain and cross-domain conditions, using
datasets from the 2019 and 2021 editions of the Ariel Data Challenges. The results reveal that model failure is multifaceted and that no single indicator captures all failure modes, demonstrating the need for indicator fusion. The application of safety-driven rejection strategies shows that a modest 20% reduction in data coverage results in error reductions between 45% and 65% across different domains and evaluation metrics. Using a formalised coverage-risk framework, a systematic analysis of indicator combinations is performed to identify configurations that maximise risk-ranking accuracy and optimise the trade-off between data coverage and scientific performance. Safety cages provide a transparent mechanism for detecting unreliable predictions and represent a critical step towards the safe deployment of data-driven models in scientific applications, such as astrophysics, where ground truth is seldom available.
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All authors Nikki Grens, Luís F. Simões, Kai Hou Yip, Theresa Lueftinger
Instrumentation and Methods for Astrophysics
Photonic Integrated Circuits for the Habitable Worlds Observatory: Science Drivers, Material Platforms, Arrayed-Waveguide Spectrographs, and a Space-Qualification Roadmap
Kalaga Madhav
NASA's Habitable Worlds Observatory (HWO) will require ultraviolet, optical, and near-infrared instruments that are simultaneously compact, mechanically and thermally stable, high-throughput, and replicable at large channel counts. Space-qualified photonic integrated
circuits (PICs) and optical fibres are key platforms that can deliver these properties: they manipulate light at the diffraction limit within micron-scale single-mode waveguide
circuits. An astrophotonic instrument fully guides the starlight from focal plane to detector, eliminating scatter and ghosts and offering excellent stability with no moving parts. This paper examines how astrophotonics can address the principal science drivers of HWO through four topics: (I) the HWO observing modes best implemented with photonic instruments; (II) the waveguide and fibre materials that can span the demanding 100nm-2.5{\mu}m HWO wavelength range; (III) the potential of arrayed-waveguide-grating (AWG) spectrographs to reach the resolving powers and throughputs HWO science requires; and (IV) the steps needed to space-qualify PICs and fibres against radiation, thermal cycling,
vacuum, and launch loads. For each topic, concrete technical requirements are extracted and the current Technology Readiness Level (TRL) is assessed. The main contribution of this paper is to unify these science drivers, candidate-aperture
photon budgets, UV-to-near-infrared material platforms, AWG architectures, and space-qualification requirements within a single
photon-budget framework that pairs each claimed benefit with a measurable requirement, a TRL, and an environmental test that can retire it. A phased roadmap advances the critical photonic components from their present TRL~2 to -5, depending on platform and application, to the TRL~6 needed at HWO's instrument-definition gate.
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Solar and Stellar Astrophysics
SDSS-IV MaStar: Determination of Stellar Parameters Using Bayesian Averaging
Yan-Ping Chen, Renbin Yan, Szabolcs Mészáros et al. (+12 more)
We present the stellar parameters for 59,266 high-quality spectra of 24,130 unique
stars from the MaNGA Stellar Library (MaStar) in the Sloan Digital Sky Survey (SDSS) Data Release 17 (DR17). The median signal-to noise ratio per pixel of the spectra is 96. We derive four stellar parameters, effective temperature (Teff), surface
gravity (log g),
metallicity ([M/H]), and {$\alpha$}-enhancement ratio ($\rm [\alpha/M]$), by comparing the data with BOSZ (ATLAS-9 based) and MARCS theoretical atmospheric models. We adopt a
Bayesian method and use color and absolute magnitude derived from Gaia to select a subset of theoretical models for each
star. We then perform full-spectrum fitting to estimate the likelihood of each model in the subset and then compute their likelihood-weighted mean parameters as the final parameters. We set stellar-parameter quality flags to facilitate the use of the derived stellar parameters. The MaStar stellar parameters derived herein span an effective temperature range of $\rm 2,600 \leq T_{eff} \leq 29,861 K$, a surface
gravity range of $0 \leq \log g \leq 5.5$, a
metallicity range of $-4.9 \leq \rm[M/H] \leq 1.0$, and an {$\alpha$}-abundance range of $\rm -0.98 \leq [\alpha/M] \leq 1.0$. We compare these parameters with those from APOGEE and Gaia for
stars in common, finding general consistency within the uncertainties. However, some artifacts and systematic differences are present, and we discuss their potential causes. These new stellar parameters are available through the MaStar SDSS-IV DR17 value-added catalog (https://www.sdss4.org/dr17/mastar/mastar-stellar-parameters/).
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All authors Yan-Ping Chen, Renbin Yan, Szabolcs Mészáros, Claudia Maraston, Daniel Thomas, Daniel Lazarz, Guy S. Stringfellow, Lewis Hill, Julie Imig, Joseph D. Gelfand, Jon A. Holtzman, Matthew Bershady, Dmitry Bizyaev, Niv Drory, Keivan G. Stassun
Astrophysics of Galaxies
Host Galaxy UV Emission and Dust Reddening in Little Red Dots
Emmanuel Durodola, Ryan C. Hickox, Jonathan Cohn et al. (+1 more)
Little Red Dots (LRDs) have been the subject of extensive analysis to place them within the context of
galaxy and black hole
evolution. Despite several proposed scenarios for the nature of the rest optical emission in LRDs, our understanding of the UV emission remains a matter of debate. Here, we aim to study the origin of the UV emission in LRDs. We conduct emission-line and continuum fitting analysis of thirteen LRDs using a combination of NIRSpec PRISM and medium-resolution G235M and G395M spectroscopy. We isolate and analyze the narrow-line emission arising from larger physical scales in LRDs. We observe that the narrow-line Balmer decrements are typically greater than those predicted for case B recombination and broadly correlated with both the UV slope and the break strength, providing clear evidence that the UV emission originates from
galaxy scales and its slope is significantly affected by dust reddening. SED fitting analysis further shows that the UV continuum of LRDs is consistent with a dust attenuated stellar template. Finally, we show through a comparison of the UV slope and the Balmer break strength, that LRDs can be further divided based on break strength, such that the canonical LRD spectral shape (strong break strength ($>$ 3) and flat rest UV continuum) represents the most extreme sub-population. Our results imply that for most LRDs, the UV emission originates primarily from the young host
galaxy, and that the characteristic optical emission may be due to a combination of dust reddening and intrinsically red
accretion processes.
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All authors Emmanuel Durodola, Ryan C. Hickox, Jonathan Cohn, David M. Alexander
Solar and Stellar Astrophysics
Sinking Silicates I: Characterizing the benchmark system containing the T0 brown dwarf CWISE J210640.16+250729.0 using JWST
Austin Rothermich, Jacqueline K. Faherty, Ben Burningham et al. (+16 more)
In this study, we present the full (97.3% complete) 0.8–12.5 $\mu$m spectral energy distribution (SED) of an L/T transition object, CWISE J210640.16+250729.0 (CW2106), using the James Webb Space
Telescope (JWST). We provide a full characterization of the host
star's elemental abundances and age. We empirically derive the bolometric luminosity ($L_{\rm bol}\approx-4.825$ $\textup{L}_\odot$) of CW2106, and obtain estimates of its mass (M$\approx50-62$ M$_{\rm Jup}$), radius (R$\approx0.83-0.87$ R$_{\rm Jup}$), effective temperature ($T_{\rm eff}$$\approx1213$ K), and surface
gravity ($\log~g$$\approx5.28$ dex). We find the near-infrared (near-IR) spectrum ($0.8-2.5 ~\mu$m) is best reproduced with cloudy atmospheric models while the mid-infrared (mid-IR) spectrum ($5-12.5 ~\mu$m) is best reproduced with cloudless models. This suggests a cloud layer restricted to only the deepest observable parts of the atmosphere and is qualified by the lack of a 9 $\mu$m silicate feature. Making use of the Mg/Si ratio of the primary, alongside thermochemical models, we predict the clouds in CW2106 to be composed primarily of enstatite (MgSiO$_3$), removing $\sim23\%$ of the bulk oxygen out of the atmosphere. Future
retrieval studies will be able to help investigate the existence and full impact of these cloud species.
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All authors Austin Rothermich, Jacqueline K. Faherty, Ben Burningham, Catherine Manea, Emily Calamari, Genaro Suárez, Daniella C. Bardalez Gagliuffi, John M. Brewer, Kelle L. Cruz, Josefine Gaarn, Jonathan Gagné, Eileen C. Gonzales, Marina E. Gemma, Viktória Kecskeméthy, Mark S. Marley, Caprice L. Phillips, Channon Visscher, Johanna M. Vos, Niall Whiteford
Earth and Planetary Astrophysics
The core-mantle mode of gravitational oscillation
Mathieu Dumberry
We present the characteristics of a mode of axial oscillation between Earth's mantle, fluid core and solid inner core that has not been previously reported. The mode involves a quasi-rigid rotation of the fluid core outside the tangent cylinder (TC) exchanging its angular momentum with the mantle via a three step process. First, by a
magnetic torque with the fluid inside the TC; second by a
magnetic torque between the latter and the inner core; and finally by a gravitational torque between the inner core and mantle. Although the gravitational torque is purely between the mantle and inner core, the mode involves an oscillation of the whole of the core, and we refer to it as the core-mantle gravitational (CMG) mode. This form of gravitational oscillation occurs when the
magnetic field within the core is sufficiently strong that the propagation time of Alfvén waves is shorter than the mode period, which is the case for Earth. We show how the period, quality factor $Q$ and structure of the CMG mode depends on the strength of the gravitational torque and viscous relaxation time $\tau_i$ of the inner core. For Earth, the CMG mode period should be in the range of 40 to 100 years, but viscous relaxation of the inner core likely implies a small $Q$, below 1 if $\tau_i<10$ years. Our results suggest that the CMG mode may act to amplify resonantly, though only modestly, multi-decadal changes in the length of day driven by zonal accelerations in the fluid core.
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Astrophysics of Galaxies
Mapping parameters of idealised hydrodynamic galaxy simulations to bar properties: a case study with the RAMSES code
Srikanth T. Nagesh, Benoit Famaey, Jonathan Freundlich et al. (+4 more)
Cosmological simulations with high spatial resolution often struggle to reproduce the observed fraction of galactic bars across
redshift and may produce bars that are too short. To investigate these issues from a purely Newtonian dynamical perspective, we run a grid of idealised hydrodynamic simulations of disc
galaxies with stellar masses of $\sim10^{10},{\rm M}_\odot$ using RAMSES. Without cooling,
star formation, or feedback, we vary the resolution, gas mass, stellar velocity dispersion, bulge mass,
halo mass, and concentration to identify the conditions that inhibit bar formation. We test whether initially axisymmetric discs form bars over the typical time elapsed between $z\sim1$ and $z\sim0.2$. We find that most diagnostics proposed in the literature are too simplistic to reliably predict bar formation. However, a region of parameter space that strongly inhibits bar formation is identified: a high Romeo-Falstad stability parameter, with a threshold that decreases quadratically with bulge mass, combined with a high generalised Efstathiou-Lake-Negroponte (ELN) parameter that accounts for the bulge. Lower gas fractions and larger numbers of dark matter particles also tend to reduce the bar growth rate. We argue that if a bar is destroyed, for example by bulge formation, after the
galaxy enters this bar-inhibiting region, re-formation may be difficult. This suggests that a modern version of the angular momentum catastrophe may persist in large-volume cosmological simulations. Finally, we confirm that only baryon-dominated discs lying away from the stellar-to-
halo-mass relation expected from abundance matching can form bars sufficiently large relative to their corotation radius.
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All authors Srikanth T. Nagesh, Benoit Famaey, Jonathan Freundlich, Yves Revaz, Giacomo Monari, Arnaud Siebert, Rodrigo Ibata
High Energy Astrophysical Phenomena
Gas absorption of soft X-rays strongly impacts the redshift distribution of dark Fast X-ray Transients
Javier Sánchez-Sierras, Peter G. Jonker, Jonathan Quirola-Vásquez et al. (+25 more)
The progenitors of many Fast X-ray Transients (FXTs) and those of $\gamma$-ray bursts (GRBs) are strongly linked. Given that "dark" GRBs are typically found to be events suffering from enhanced extinction in the host
galaxy, we investigate the nature of "dark" FXTs. However, unlike $\gamma$-rays, soft X-rays are strongly affected by absorption, implying that dark FXTs discovered by Einstein Probe's Wide-field X-ray
Telescope (EP-WXT) have to be emitted at energies $>$2–3 keV restframe hence at a
redshift $z\gtrsim2$. To illustrate this we present two dark FXTs, EP241103a and EP260409a, discovered by EP-WXT. For EP241103a, the high extinction precluded the detection of the optical counterpart, while for a probable source
redshift of $\sim2.5$, the rest-frame X-ray
photons are not severely affected. For EP260409a no counterpart is detected in the optical, but we do detect a near-infrared counterpart. A plausible scenario for EP260409a is that it lies at a
redshift $z \gtrsim 4$. Additionally, we conduct simulations of FXTs observed with EP-WXT, showing that the detected counts for absorbed events ($N_{\mathrm{H}}\gtrsim10^{22}$~cm$^{-2}$) strongly decrease at low
redshifts, hindering their detection. These results support the theoretical prediction that dark FXTs have an intermediate to high
redshift and a different selection function from those of dark GRBs.
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All authors Javier Sánchez-Sierras, Peter G. Jonker, Jonathan Quirola-Vásquez, Agnes P. C. van Hoof, Andrew J. Levan, Maria E. Ravasio, Joyce N. D. van Dalen, Franz E. Bauer, Jia-Ying Cao, Francesco Carotenuto, Jennifer A. Chacón, Ashley A. Chrimes, Laura Cotter, Gregory Corcoran, Rob A. J. Eyles-Ferris, Guoli Huang, Shuai-Qing Jiang, Zexi Li, Yifang Liang, He-Yang Liu, Daniele B. Malesani, Antonio Martin-Carrillo, Daniel Mata Sánchez, Nikhil Sarin, Manuel A. P. Torres, Qinyu Wu, Tong Zhao, Wen-Da Zhang
Solar and Stellar Astrophysics
Classical Cases of Non-solar Mixing Length in Low Mass Stars
Rebecca Woody, Charlie Conroy, Phillip A. Cargile et al. (+1 more)
The treatment of convection is one of the largest remaining uncertainties in models of low mass
stars. The Mixing Length Theory (MLT) for super-adiabatic convection has come under particular scrutiny. Evidence is mounting that a single value of the mixing length parameter ($\alpha_\mathrm{MLT}$) should not be applied universally across stellar model grids, although simulations and asteroseismic calibrations disagree on how it should vary with stellar parameters. A variable $\alpha_\mathrm{MLT}$ causes critical systematic changes to the ages of low mass
stars, the key age tracers for galactic and exoplanetary studies. In this work we use classical (non-seismic)
geometric radii and specialized MIST isochrones with a variable $\alpha_\mathrm{MLT}$ to measure mixing length in detached eclipsing binaries (DEBs) and interferometrically resolved
stars, finding plenty of evidence for non-solar $\alpha_\mathrm{MLT}$. In particular we find that standard evolutionary models overpredict the radii of metal-poor interferometric
stars and that a sub-solar $\alpha_\mathrm{MLT}$/$\alpha_{\mathrm{MLT},\odot}\sim0.50-0.75$ can neatly explain this discrepancy, consistent with other empirical calibrations and in stark contrast to simulations. We infer low convective efficiency in cool, rapidly rotating lower main-sequence
stars, consistent with the known phenomenon of magnetically-induced radius inflation. We also identify several slowly rotating DEBs whose measurements cannot be reproduced by standard isochrones, and we propose these systems as useful case studies of non-solar mixing length in
stars outside of the Sun. However, we find no definitive population level correlations between $\alpha_\mathrm{MLT}$ and stellar parameters among the DEBs, calling into question the existence of a ubiquitous mixing length calibration in low mass
stars.
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All authors Rebecca Woody, Charlie Conroy, Phillip A. Cargile, Aaron Dotter
Astrophysics of Galaxies
The GMRT CAT$z$1-COSMOS Survey: HI 21 cm emission from star-forming galaxies at $z\approx1$ in the COSMOS field
Balpreet Kaur, Nissim Kanekar, Aditya Chowdhury et al. (+1 more)
We report a 216-hr upgraded Giant Metrewave
Radio Telescope (GMRT) Band-4 $550-850$ MHz observation of the COSMOS field, using 7 GMRT pointings to cover an area of $2.4$ sq. deg., with on-source times of $67$ hrs in the central pointing, and $89$ hrs distributed across the other six pointings. We stacked, at a spatial resolution of 90 kpc, the HI 21 cm emission signals from 8,122
star-forming
galaxies with accurate spectroscopic
redshifts and lying within the FWHM of the GMRT primary beam at their redshifted HI 21 cm frequencies. We detect the stacked HI 21 cm signal from the 8,122
galaxies at $4.2\sigma$ significance, obtaining an average HI mass of $\langle{M_{\scriptsize{\textrm{HI}}}}\rangle=(15.5\pm3.7)\times10^9 \textrm{M}_\odot$. This is only the second
galaxy population at $z\approx1$ with a measurement of the average HI content; the other being the blue
star-forming
galaxies in the DEEP2 fields. We also detected the median 1.4 GHz
radio continuum emission signal from the 8,122
galaxies by stacking the GMRT 680 MHz continuum images, obtaining a weighted-median
star-formation rate (SFR) of ${8.90\pm0.92}~M_\odot$ yr$^{-1}$. We measure a characteristic HI-to-stellar mass ratio of $\langle{M_{\scriptsize{{\textrm{HI}}}}}\rangle/\langle{M_\star}\rangle=(1.15\pm0.27)$ and a characteristic HI depletion timescale of $\langle{M_{\scriptsize{{\textrm{HI}}}}}\rangle/\textrm{SFR}=(1.74\pm0.45)$ Gyr for
star-forming
galaxies at $\langle{z}\rangle=1.081$ in COSMOS, both consistent with earlier measurements for the blue,
star-forming DEEP2
galaxies at $z\approx1$. Our results support the evidence from the DEEP2 survey that the observed decline in the SFR density at $z<1$ is due to insufficient
accretion of atomic gas from the intergalactic medium to replenish the gas reservoir consumed in the process of
star-formation.
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All authors Balpreet Kaur, Nissim Kanekar, Aditya Chowdhury, Jayaram N. Chengalur
Astrophysics of Galaxies
Chemo-dynamical Analysis of a CNO-Enhanced Ultra Metal-poor Star ($\rm[Fe/H] < -4$): Insights into Early Enrichment by Faint Population III Supernova
Guilherme Limberg, Vinicius M. Placco, Alexander P. Ji et al. (+10 more)
We report an independent identification of the ultra metal-poor (UMP)
star ($\rm[Fe/H] =-4.06$) $Gaia$ DR3 $\texttt{source_id}$ 4795913112968206720 (GDR3_479591) in the $Gaia$ mission's Blue and Red Photometer 'XP' spectro-photometric catalog. We combine multi-band photometry, astrometry, and high-resolution spectroscopy to confirm GDR3_479591 as a red giant-branch (RGB)
star located at ${\sim} 6\,{\rm kpc}$ from the Sun. Abundance analysis under local thermodynamic
equilibrium reveals significant enhancements, relative to the solar level, in carbon ($\rm[C/Fe] = +1.54$, after evolutionary correction for depletion in the RGB), nitrogen ($\rm[N/Fe] = +2.64$), and oxygen ($\rm[O/Fe] = +2.62$). The CNO-enhanced GDR3_479591 is thus one of only 5 UMP
stars with a detected oxygen abundance. The CNO excess is accompanied by enhancements in several other light elements, such as Na, Mg, Al, and Si. We demonstrate that the chemical pattern of GDR3_479591 can be reproduced by the yields of a single Population~III 'faint'
supernova with progenitor mass of $\sim$21-to-28$\,M_\odot$ and a low explosion energy of ($0.3 \leq E_{\rm SN}/(10^{51}\,{\rm erg}) \leq 0.9$). Additionally, we use literature metal-poor
stars to show that, contrary to recent propositions for high-
redshift galaxies, a mild enhancement in [C/O] does not automatically translate to the high [C/Fe] typically observed in UMP
stars in the Milky Way and its satellites. GDR3_479591 could not be dynamically associated with any of the most relevant accreted substructures in the Galactic
halo, and we speculate that it was formed in an ultra-faint dwarf
galaxy environment that later merged with the Milky Way.
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All authors Guilherme Limberg, Vinicius M. Placco, Alexander P. Ji, Yupeng Yao, Friedrich Anders, Wendy Q. Sun, Anna de Graaff, Rohan P. Naidu, Harley Katz, Pierre N. Thibodeaux, Anirudh Chiti, Mohammad K. Mardini, Anna Frebel
High Energy Astrophysical Phenomena
Spinning Between Models: Continuum and Reflection Constraints in the Intermediate States of GRS 1716-249 and GRS 1739-278
Tiffany Tausch, Paul A. Draghis, Erin Kara et al. (+7 more)
Measurements of stellar-mass black hole
spin probe
accretion under strong
gravity and constrain black hole formation and
evolution. Intermediate states, in which thermal disk, coronal, and reflection emission all contribute significantly, are particularly complex environments for
spin measurements and useful for probing spectral-modeling systematics. We investigate these effects using paired, non-simultaneous Swift/XRT and NuSTAR observations of GRS 1716-249 and GRS 1739-278. We jointly model the spectra with kerrbb and relxillCp, linking or separately varying their
spin parameters, and explore the parameter space with a Markov Chain Monte Carlo analysis. Within the adopted model, GRS 1716 strongly favors a high
spin, whereas the GRS 1739 data allow high- and low-
spin solutions with nearly identical fit statistics. Parameter-stepping experiments show that coordinated changes among parameters allow substantially different physical configurations to produce nearly indistinguishable spectra. For these
datasets and within the adopted model, energy-band tests show that the Fe band provides the strongest direct sensitivity to the inferred
spin, while the Compton hump helps constrain reflection parameters and the soft X-ray band characterizes the underlying continuum. When both the disk continuum and reflection components are adequately constrained, their associated
spin parameters can favor a common solution within the joint model. With weaker constraints, the model permits statistically comparable solutions spanning nearly the full range of
spins. Robust joint continuum and reflection
spin inference requires broadband coverage, independent binary constraints, physically self-consistent models, and thorough exploration of parameter space.
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All authors Tiffany Tausch, Paul A. Draghis, Erin Kara, Abderahmen Zoghbi, Laura Brenneman, Anna Ogorzalek, John A. Tomsick, Mark Reynolds, Javier Garcia, Riley Connors
Cosmology and Nongalactic Astrophysics
Diffraction of gravitational waves by extended dark objects
Nikita Blinov and J. Leo Kim
Many dark sector models predict the formation of dark objects, such as
solitons and dark
stars, which can be searched for through their gravitational
lensing of gravitational waves (GWs). Although these objects vary widely in size and compactness, most GW
lensing studies have focused on point-like lenses. The finite size of an extended lens is important when it exceeds its Einstein radius. We evaluate the frequency-dependent effects on GW waveforms from extended lenses and demonstrate that they can be probed with current and future GW data. While finite-size effects can weaken the bounds on the fraction of dark matter in these objects compared to the point-like limit, current GW data provides complementary constraints to other gravitational tests for lens masses between $10^2M_\odot$ and $10^5M_\odot$; future LIGO-Virgo-KAGRA data and next-generation observatories will surpass their sensitivity. As expected, we find that the finite-size effects are important when the lens radius exceeds $\sim \text{few} \times 10^{6}R_\odot (M_L/100 M_\odot)^{1/2}$. Interpreting these constraints within specific models (a simple dissipative dark sector and
axion stars), we show that current and future observations probe significant portions of the microphysical parameter space.
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Astrophysics of Galaxies
Reigniting the FUSE II: O VI Emission in Massive Elliptical Galaxies
Michael Reefe, Michael McDonald, Greg L. Bryan et al. (+6 more)
This is the second paper in a series where we examine archival far-ultraviolet spectroscopy of 29 massive elliptical
galaxies from the Far Ultraviolet Spectroscopic Explorer (FUSE) with modern methods. In the first paper, we performed SED modeling with aperture-matched photometry and analyzed the young and old stellar populations in these
galaxies. In this paper, we report extinction-corrected O VI $\lambda\lambda$1032,1038 $\mathring{\rm A}$ fluxes and upper limits for each
galaxy, which we use to infer the rate at which gas is cooling through $10^{5.5}$ K via multiphase condensation of the
galaxy's CGM ($\dot{M}_{\rm O\,VI}$) under the assumption of steady radiative cooling. By considering both Galactic and intrinsic extinction, we overwhelmingly find $\dot{M}_{\rm O\,VI}$ to be larger than values found previously in the literature by a median of $\sim 80\%$. Additionally, we quantify correlations between $\dot{M}_{\rm O\,VI}$ and
star formation rates, classical luminosity-based X-ray cooling rates, and X-ray spectroscopic cooling rates. The results suggest that the efficiency of cooling from the $\gtrsim 10^7$ K circumgalactic medium evolves with host mass, being suppressed by a factor of $\sim 4$-$5$ more at the cluster scale than it is at the group and elliptical scales. In contrast, under the steady cooling assumption, cooling from $10^{5.5}$ K to the molecular phase appears to be suppressed by an order of magnitude, independent of mass, requiring turbulent mixing layers, thermal conduction, multiphase recycling, a bottom-heavy initial mass function, or some other additional physics to explain.
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All authors Michael Reefe, Michael McDonald, Greg L. Bryan, Megan Donahue, Andrew C. Fabian, Massimo Gaspari, Eric Miller, Valeria Olivares, G. Mark Voit
Cosmology and Nongalactic Astrophysics
Complete Second-Order Relativistic Derivation of the Observed Pulsar Timing Modulations
Matteo Magi and Jaiyul Yoo
We present a fully nonlinear relativistic description of the observed
pulsar timing modulations, defined through the ratio of the proper-time intervals between two successive emissions of
radio pulses and their observations by a
pulsar timing array. Working in the limit of a vanishing emission interval, we derive the observed
pulsar timing modulation to second order in relativistic perturbation theory around a Minkowski background, without fixing a gauge and retaining the full scalar, vector, and tensor content of the metric perturbations. Although the derivation naturally introduces several gauge-dependent intermediate quantities, we explicitly demonstrate that they combine into a coordinate-independent expression. Moreover, we explicitly show that the second-order timing modulation from two neighboring
radio pulses coincides with the observed
redshift, defined as the fractional change in wavelength along a single geodesic. The agreement represents a nontrivial second-order realization of the exact nonlinear equivalence proven in Magi & Yoo 2026.
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Cosmology and Nongalactic Astrophysics
The Loud Tail of the Supermassive Black Hole Binary Population: Multimessenger Candidates and Prospects for SKAO
Juhan Raidal, Ville Vaskonen, Juan Urrutia et al. (+1 more)
We assess whether electromagnetically identified supermassive black hole (SMBH) binary candidates populate the rare, high-amplitude tail of the gravitational wave (GW) amplitude distribution. Using an SMBH binary population model fitted to the NANOGrav 15-year data, we find that 3C~66B and Mrk~501 are likely to stand out from the GW background, making them promising targets for individual-source searches. Furthermore, we show that individual detections can probe the binary hardening mechanism, as models with strong environmental hardening predict a higher abundance of individually resolvable binaries than purely GW-driven models. Applying the same population fits and accounting for the confusion noise from the unresolved population, we forecast that the Square Kilometre Array Observatory (SKAO) will resolve tens of individual binaries, with 3C~66B robustly detectable.
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All authors Juhan Raidal, Ville Vaskonen, Juan Urrutia, Hardi Veermäe
Cosmology and Nongalactic Astrophysics
Big Bang For Your Helium Buck
Marilena Loverde, Murali M. Saravanan and Zachary J. Weiner
We study the implications of a recent measurement of the primordial helium fraction from the Large Binocular
Telescope for cosmological inference from the
cosmic microwave background. We show that LBT establishes the robustness of cosmological parameters to theoretical assumptions about big bang nucleosynthesis: its empirical calibration of the helium fraction enables constraints on cosmology and inflation that are agnostic to BBN but as precise as those that instead enforce standard BBN predictions. Future CMB surveys require at most a marginal improvement in precision over LBT to maximize their BBN-agnostic constraining power when the radiation density is free (and no more than a factor of two improvement across all cases we consider). We then apply the LBT measurement to a number of scenarios that feature new physics in BBN and the CMB. First, we constrain nonstandard radiation sectors (interacting and free-streaming) and search for
evolution of the radiation abundance between nucleosynthesis and recombination. We then test models that alleviate the Hubble tension and the tension between CMB and baryon acoustic oscillation data, including self-interacting light relics and varying fundamental constants; LBT precludes most but not all of the models we consider via their effect on BBN. Finally, we use the LBT measurement as an indirect but independent test of the
neutron lifetime anomaly, inferring values that are consistent with "bottle" experiments and $2.4 \sigma$ below "beam" experiments.
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Earth and Planetary Astrophysics
JWST/MIRI Imaging Search for Kinematically Detected Protoplanetary Candidates
G. Cugno, M. Benisty, R. Teague et al. (+32 more)
Kinematic perturbations observed with ALMA in CO line emission provide evidence for a population of embedded giant protoplanets shaping the structure of protoplanetary disks. We present JWST/MIRI F1140C ($\lambda = 11.3~\mu$m) coronagraphic observations of five protoplanetary disks, HD163296, RXJ1615.3-3255, RXJ1842.9-3532, SY Cha, and LkCa 15, with the goal of directly detecting candidate protoplanets orbiting at $\gtrsim$ 70 au previously inferred from gas kinematics. The data were analyzed using a bespoke methodology that combines reference PSF subtraction with forward modeling of partially resolved inner disk emission, which otherwise dominates the diffraction pattern in the images. This approach improves the sensitivity to young companions at small separations. No point source consistent with an embedded protoplanet is detected in any of the systems. Instead, in three systems we detect extended emission at $11.3~\mu$m tracing the outer disk out to radii comparable to those probed by CO. Injection tests indicate upper mass limits of roughly $3-20$ M$_J$ at separations of a few hundred au, assuming no additional thermal contribution from circumplanetary environment. Even with space-based observations, these limits remain mostly above the $\sim1-5$ M$_J$ masses inferred from disk kinematics, largely due to the limitations imposed by emission (and/or scattered light) contributions from both the inner and outer disk. These observations highlight the challenges of observing protoplanets embedded in their forming environment at large separation with JWST/MIRI. Lessons learned can inform future studies with the Extremely Large
Telescope, which will probe separations where the occurrence rate of gas giants is expected to be higher.
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All authors G. Cugno, M. Benisty, R. Teague, A. Boccaletti, L. Pueyo, M. Perrin, M. Mâlin, J. Girard, V. Christiaens, P. Patapis, K. Lawson, S. M. Andrews, J. Bae, M. Barraza-Alfaro, M. J. Bonse, M. Courtoux, S. Facchini, M. Fukagawa, G. Guidi, R. Helled, T. Henning, J. Huang, J. Kammerer, C. Law, G. Lodato, F. Long, F. Ménard, M. Meyer, D. J. Price, C. Rab, I. Rebollido Vazquez, L. M. Stapper, T. Stolker, D. Wilner, A. J. Winter
Astrophysics of Galaxies
The Nature of Small-Scale Perturbation Modes in a Nonstationary Self-Gravitating Disk
K.T.Mirtadjieva, J.M.Ganiev and S.N.Nuritdinov
In this work, we investigate the nature of gravitational instability of small-scale perturbation modes during the early nonstationary stages of the
evolution of disk-like
galaxies. To this end, we study the gravitational instability of small-scale horizontal perturbation modes, such as $(m;N) = (12;20)$, $(14;20)$, $(16;20)$, and $(18;20)$, in the framework of a nonlinear nonstationary anisotropic model of a self-gravitating disk. The corresponding nonstationary analogues of the dispersion equation for these perturbation modes are derived, and their numerical analysis is performed. The results are presented in the form of critical dependences of the initial virial ratio on the disk rotation parameter. The nature of the instability of small-scale perturbation modes in a nonlinear nonstationary self-gravitating disk is determined, and a comparative analysis of their instability increments is carried out.
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Instrumentation and Methods for Astrophysics
Spectro-polarimetry of HAbitable Planet Earth (SHAPE) on Chandrayaan-3: Instrument characteristics, calibration and onboard performance
Bhavesh Jaiswal, Swapnil Singh, Ravishankar B T et al. (+20 more)
The orbiter of the Chandrayaan-3 mission of the Indian Space Research Organisation (ISRO) carries an experimental payload called SHAPE (Spectro-polarimetry of HAbitable
Planet Earth). This payload makes disc-integrated observations of Earth as an
exoplanet, from the Moon as well as from the high altitude Earth orbit. The instrument consists of an Acousto-Optic Tunable Filter (AOTF) based near-infrared spectro-polarimeter making the measurements in the two orthogonally polarized directions using a pair of Indium-Gallium-Arsenide (InGaAs) detectors. The laboratory characterization of the flight model of the instrument included a relative measurement of the response of the two channels of the instrument. The field measurements of the instrument revealed a non-uniformity in the field response. In the light of non-identical response of the two channels and the non-uniformity of the field response, a theoretical model of such a polarimeter is developed to gain insights into the performance of a polarimeter. Analysis of lunar observations obtained within the SHAPE field of view indicates that the transmission ratio between the two polarized channels lies in the range 0.8-1.2. Such deviations from unity can introduce offsets in the measured polarization of up to about 10%. The suitability of SHAPE for studying the band polarization, defined as the relative polarization within a spectral absorption band, is studied. Using the Moon observations and the theoretical instrumental model, the suitability of measuring the relative band polarization is demonstrated. A study of systematic biases in the band polarization demonstrates a maximum offset of less than 1% in the measured value of the band polarization. The initial results of Earth observations, spectra and flux measurements across phase angles, and the methodology for retrieving the band polarization from these observations are also discussed.
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All authors Bhavesh Jaiswal, Swapnil Singh, Ravishankar B T, Anand Jain, Smrati Verma, Reenu Palawat, Brajpal Singh, Bijoy Raha, Sathyanarayana Raju, Bhavesh Mendhekar, Srinivasa Rao Kondapi, Priyanka Das, Rahul Waghmare, Supratik Bose, Supriya Verma, Yogesh Prasad K R, Praloy Karmakar, Abhishek Kumar Singh, Honey Gupta, Balaraman Prabakaran, Motamarri Srikanth, Sankarasubramanian Kasiviswanathan, Anuj Nandi
Earth and Planetary Astrophysics
The atomic C/O ratio of KELT-9b
Stefan Pelletier, Nicholas W. Borsato, Jacob L. Bean et al. (+12 more)
The carbon-to-oxygen (C/O) ratio of a giant
planet's atmosphere has long been theorised to hold compositional information that can be traced back to its formation history. Typically, the C/O ratio of an exoplanetary atmosphere is inferred from abundance measurements of major C- and O-bearing molecules such as CO, CO$_2$, H$_2$O, and OH. However, some
exoplanets have such elevated temperatures that molecules can be nearly completely thermally dissociated at the pressure levels that observations probe, making it difficult to measure their C/O ratio via these traditional tracers. Here, rather than using molecules, we aim to retrieve the C/O ratio of KELT-9b (T$_\mathrm{eq} = 4000$K) directly from atomic C and O, which are detected in its atmosphere. We analysed two transits of KELT-9b observed with the MAROON-X high-resolution spectrograph, finding absorption cross-correlation signals from atomic C and O as well as refractory metals. From this, we inferred the vertical temperature structure, the relative proportions of volatile and refractory species, and the C/O ratio of KELT-9b using a 1D local thermodynamic
equilibrium atmospheric
retrieval framework applied only to spectral regions mostly unaffected by non-local thermodynamic
equilibrium effects. We measure the atomic C/O ratio of the atmosphere of KELT-9b to be $0.20_{-0.07}^{+0.13}$, which is slightly lower than the stellar value of $0.38\pm0.15$ and significantly below the solar value of $0.59\pm0.07$. We otherwise confirm previous investigations of the terminator region, finding KELT-9b's atmosphere to be thermally inverted and slightly metal-rich. We measure the volatile-to-refractory ratio, a proxy for the ice-to-rock ratio, to be $1.24_{-0.78}^{+1.93}$ $\times$ solar ($[$M$_{\mathrm{vol}}$/M$_{\mathrm{ref}}]$ = $0.09_{-0.20}^{+0.19}$), which is consistent with both KELT-9 and the Sun.
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All authors Stefan Pelletier, Nicholas W. Borsato, Jacob L. Bean, Andreas Seifahrt, David Kasper, Julian Stürmer, Ana Rita Costa Silva, David Ehrenreich, Luca Fossati, Jens Hoeijmakers, Bibiana Prinoth, Michal Steiner, Brian Thorsbro, Valentina Vaulato, Daniel B. Zucker
Astrophysics of Galaxies
High-redshift supermassive black hole population from core-collapse in self-interacting dark matter halos
Sambo Sarkar and Ujjal Kumar Dey
Self-interactions between dark matter (DM) particles facilitate the inter-particle redistribution of energy within the central region of DM
halos. Recent studies of dark matter spikes around massive black holes, and the diversity in rotation curves of dark matter rich low-mass
galaxies motivate the exploration of self-interaction cross-section $\mathcal{O}(10)\,\rm cm^2/gm$. At such high scattering rates, DM can lead to the formation of supermassive black hole seeds, through the gravothermal collapse of
halo cores, under certain cosmological conditions.
Quasars near
cosmic dawn are powerful probes for examining the formation scenario of high
redshift supermassive black holes, and their connection to structure formation. In this work we point out the favorable initial cosmological conditions that are likely to provide the black hole seeds resulting in supermassive black holes, mediated by core-collapse in self-interacting dark matter
halos. Employing a semi-analytic prescription of core formation, and realistic mass
accretion and
halo merger histories, we compare the existing observations of high-
redshift supermassive black holes with those predicted for our core-collapse framework. We find the median values of velocity-dependent self-interacting dark matter parameter space $\sigma/m_{\chi}\sim 56\, \rm cm^2/gm$, and $\omega=101\,\rm km/s$, assuming an Eddington
accretion rate of unity. Sub-leading values are also presented for sub and super-Eddington
accretion rates. We also report the derived self-interacting dark matter model parameters to account for the observed binned supermassive black hole mass functions at high-
redshifts.
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High Energy Astrophysical Phenomena
An efficient approach to resistive GRMHD simulations of binary neutron star mergers
Miquel Miravet-Tenés, Ian Hawke, Rahime Matur et al. (+1 more)
Astrophysical
plasmas are often highly conducting, but finite resistivity can nevertheless influence the
evolution of strongly magnetised systems. Capturing these effects in general relativistic magnetohydrodynamics is computationally challenging because high conductivity leads to stiff source terms in the
evolution equations. Standard numerical approaches rely on implicit-explicit (IMEX) time integration to maintain stability, but this substantially increases the complexity and computational cost of the simulations. Building upon the previously developed Resistive Extension upGrade for Ideal MagnEtohydrodynamics (REGIME), we present a new fluid-frame formulation that simplifies the incorporation of leading-order resistive effects by avoiding
matrix inversions required in previous approaches. The method is implemented in a general relativistic magnetohydrodynamics code and validated through one-dimensional current-sheet tests and simulations of isolated magnetised
neutron stars and binary
neutron star mergers. We find that the new formulation reproduces the expected resistive behaviour with accuracy comparable to IMEX-based methods, while substantially reducing the computational cost and complexity of the simulations. In binary
neutron star mergers, finite resistivity leads to measurable differences in the post-merger gravitational-wave signal and remnant structure, showing that resistive effects can produce measurable changes in the post-merger dynamics even in the high-conductivity regime.
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All authors Miquel Miravet-Tenés, Ian Hawke, Rahime Matur, Alexander J. Wright
Astrophysics of Galaxies
Magnetized interstellar molecular clouds - III. Filament Collisions and Core Formation: Insights into Substructures and Evolution
Pak Shing Li, Christopher F. McKee and Richard I. Klein
We present the results of a driven, ideal magnetohydrodynamics simulation of a molecular cloud to study the formation of filamentary structures in molecular clouds and their fragmentation into dense cloud cores. Shock compression results in the formation of sub-parsec long filamentary structures. Large-scale supersonic flows push these small filaments together and form massive filamentary clouds. The main filament that forms stretches across the full 4.4 pc length of the simulation and has a median FWHM of 0.075 pc, similar to that observed in many molecular clouds. Subfilaments inside the main filament have a median FWHM of 0.03-0.04 pc, similar to the “fibers" observed in molecular lines. The collision and merger of subcritical subfilaments is an important mechanism for the formation of prestellar cores. Colliding subfilaments and the resulting core at their intersection is a hub-filament system similar to that associated with massive
star formation, but on a smaller scale. Core mergers can be significant in the growth of pre-stellar cores and may play a role in the formation of binary
stars.
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Instrumentation and Methods for Astrophysics · Cross-listed from gr-qc
cWB-space: A time-frequency transient-search pipeline for LISA
Shubhanshu Tiwari, Yumeng Xu and Giovanni A. Prodi
Gravitational-wave observations can reveal sources whose signals are not known in advance. Searches that do not require a predicted waveform are therefore an important complement to searches for specific source classes. For the
Laser Interferometer Space
Antenna (LISA), the challenge is to find such signals among overlapping astrophysical sources and disturbances in the instrument. We present cWB-space, which identifies candidate signals by following how their power is distributed in time and frequency, and then reconstructs parts of their waveforms without imposing a source model. Simulations of merging massive black holes, other burst signals, and instrumental disturbances test both candidate identification and the information carried by the detector's response. The response comparison estimates its background from the observed data. Tests of disturbances in several
laser measurements examine where a similar response can arise from an instrumental origin. Searching the Sangria simulated LISA data, we recover all six injected massive-black-hole binaries as the six highest-ranked candidates. Their reconstructed waveforms agree well with the injected signals over the intervals studied. The results demonstrate another route to investigating transients, while leaving the rate of false detections and general discrimination of instrumental disturbances to future validation.
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Solar and Stellar Astrophysics
Late-Time Evolution of the Massive Stellar Merger M101 OT2015-1
Marco A. Gómez-Muñoz, Nadejda Blagorodnova, Tomasz Kamiński et al. (+6 more)
Luminous red novae (LRNe) are astronomical transients arising from unstable mass transfer and common envelope phases in close binary systems, culminating in stellar mergers. Their cold, expanding ejecta provide the ideal conditions for molecule and dust formation. We investigate the late-time dust and molecular
evolution of the LRN M101 OT2015-1, a high-mass LRN progenitor ($18\pm 1$M$_\odot$), using multi-band near-infrared (NIR) and mid-infrared (MIR) observations spanning up to ~5 years post-outburst. Archival and unpublished photometry from NEOWISE, Spitzer, Keck NIRC2 and MOSFIRE, are interpolated via 2D Gaussian process regression. Spectral energy distributions (SEDs) are modelled with 1D spherical radiative transfer models to track the
evolution of dust mass, dust temperature, and central source properties. At +150d post-peak, the SED is well-fit by a pure stellar blackbody ($T_{\rm eff}=3330$K) with negligible dust (log$M_{\rm dust}/$M$_\odot<-6.36$). Rapid dust condensation begins around day +200 ($T_{\rm dust}=1591$K), with dust mass increasing to log$M_{\rm dust}/$M$_\odot =-3.65$ and optical depth reaching $\tau_V =64$ by +1300d. A prominent NIR re-brightening at ~600d ($M_K=-12.04$), accompanied by a dust reheating bump ($T_{\rm dust}=1567$K), highlights a second phase of dust nucleation likely driven by shock interactions. Low-resolution NIR MOSFIRE spectra are analysed for line identification and also using an LTE isothermal slab model to characterise CO molecular features. The MOSFIRE spectra reveal an O-rich environment with photospheric expansion velocities of ~$430-530$km s$^{-1}$, as measured in the Pa$\beta$ emission line, and high CO column densities (log$N_{\rm CO}$/cm$^{-2}=20.9$ and $20.7$ at +142d and +197d, respectively). These results confirm that LRNe from massive progenitors are prolific producers of
cosmic dust and are efficient molecular factories.
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All authors Marco A. Gómez-Muñoz, Nadejda Blagorodnova, Tomasz Kamiński, Gerard Garcia-Moreno, Maxime Wavasseur, Hugo Tranin, Grace Katusiime, Jacob E. Jencson, Mansi M. Kasliwal
Instrumentation and Methods for Astrophysics
XMST: An Extended Minimum Spanning Tree Framework with Objective Fracture-Scale Selection
Mark Gallaway
We present XMST, an extended minimum spanning tree framework for identifying spatially coherent stellar structures using an objective, data-driven fracture-scale criterion. XMST combines percolation analysis with Jenks Natural Breaks optimisation, restricting the MST edge-length distribution to the subcritical regime before determining the final fracture scale. The method was validated using 1000 controlled Monte Carlo realisations containing eight empirical stellar-association templates, including a deliberately overlapping pair. The Percolation-Jenks fracture scale was highly reproducible, with a median of 17.028 pc and a between-realisation standard deviation of 0.066 pc. The injected structures were recovered with a mean completeness of 0.9996 and mean purity of 0.7413, while alternative CDF and Mean Edge criteria selected substantially larger fracture scales and produced markedly lower purities. Propagation of the published distance uncertainties through 10,000 additional XMST reconstructions produced a median absolute fracture-scale change of only 0.282 pc. Under these perturbations, mean completeness was 0.8605 and mean purity 0.7126, while 99.30 per cent of injected-template cases continued to satisfy the adopted detection criterion. Individual membership was less stable, with peripheral and locally sparse members showing lower persistence than structural cores. The deliberately overlapping pair was resolved spatially in only 15 of 1000 realisations; a reddening-based post-processing diagnostic increased this to 446, although with a corresponding completeness-purity trade-off. XMST therefore provides a robust method for identifying candidate stellar structures and their stable cores, while detailed outer membership and strongly overlapping populations require additional astrophysical information.
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Astrophysics of Galaxies
The Infrared Glow of Galactic Outskirts: A New Window with NASIM
Elham Saremi, Ignacio Trujillo, Mohammad Akhlaghi et al. (+5 more)
The outskirts of
galaxies retain fossil records of their assembly history and are dominated by old stellar populations shaped by tidal interactions,
accretion events, and environmental processes. Observing these low-surface-brightness (LSB) regions in the near-infrared (NIR), particularly in the $K_s$ band, is crucial for tracing evolved
stars and minimising the effects of dust. Yet, ground-based NIR studies of faint extended structures remain challenging because of the bright and variable sky background and complex detector systematics. To overcome these limitations, we developed NASIM, the Near-infrared Automated low Surface brightness reduction In Maneage pipeline, a fully automated and reproducible framework designed to recover faint structures in VISTA/VIRCAM data. Using GNU Astronomy Utilities (Gnuastro), NASIM corrects large-scale flat-field patterns and instrumental signatures across the VIRCAM detectors while preserving LSB emission that can be strongly suppressed in conventional NIR reductions. We apply NASIM to $K_s$-band imaging of the Euclid Deep Field South through the KEDFS programme, a key region also targeted by Rubin/LSST and Euclid. The final reduction reaches a surface-brightness limit of $\mu_{K_s} \simeq 27.7$ mag arcsec$^{-2}$, measured at $3\sigma$ over 100 arcsec$^2$, approximately 67 times deeper than 2MASS and 11 times deeper than VHS in surface-brightness sensitivity. Direct comparisons with publicly available VISTA products show that NASIM preserves diffuse emission without substantially compromising compact-source depth. These results open a deep NIR view of faint
galaxy outskirts, LSB
galaxies, and intracluster light, providing a reproducible ground-based foundation for future studies of the faint Universe.
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All authors Elham Saremi, Ignacio Trujillo, Mohammad Akhlaghi, Zohreh Ghaffari, Johan H. Knapen, Manda Banerji, Helmut Dannerbauer, Sébastien Comerón