Sazuna/UAT_keywords huggingface dataset:
Little Red Dots and Their Progenitors from Direct Collapse Black Holes. The James Webb Space Telescope (JWST) has discovered a new population of objects, the Little Red Dots (LRDs), characterized by V-shaped spectra indicative of strong breaks around the Balmer limit and a compact morphology that gave them their name. A popular explanation is that they are a subpopulation of active galactic nuclei (AGN) and/or supermassive black holes (SMBHs) predominantly found in the high-redshift Universe (z ≳ 3). Similarly, direct collapse black holes (DCBHs), theorized to form from collapsing massive, extremely metal-poor gas clouds, have been invoked to explain high-redshift quasars, the most massive AGN subpopulation. Here, we employ the semianalytical code A-SLOTH to produce a population of DCBHs and compare them against observed LRD demographics and properties. Specifically, we compare the DCBH-seeded SMBH population against the standard stellar-remnant seeds and find that the DCBH models agree better with the observed LRD population statistics and host halo properties. Furthermore, for the most extreme and earliest LRD detections, interpreted to be systems with an AGN but small stellar component, DCBHs are able to reproduce the observed spectral shape and properties under multiple scenarios—high dust attenuation or AGN surrounded by dense gas—that have been proposed to explain the unique shape of LRD spectra. Even when super-Eddington accretion, invoked previously to explain the nature of LRDs, is enforced on stellar-remnant seeds, the spectral characteristics of extreme LRDs cannot be reproduced. We emphasize the importance of gas-metallicity observations as an additional dimension besides the widely used SMBH to stellar mass ratios to further constrain the progenitors of LRDs.

Keywords: Early universe, Galaxy formation, Supermassive black holes, Theoretical models, Active galactic nuclei
Magnetic Field Amplification and Particle Acceleration in Weakly Magnetized Transrelativistic Electron─Ion Shocks. We investigate the physics of quasi-parallel transrelativistic shocks propagating in weakly magnetized plasmas by means of long-duration two-dimensional particle-in-cell simulations. The structure of the shock precursor is shaped by a competition between the Bell instability and the Weibel (filamentation) instability. The Bell instability is dominant at relatively high magnetizations (σ ≳ 10−3), whereas the Weibel instability prevails at lower magnetizations (σ ≲ 10−4). Shocks with precursors shaped by Bell modes efficiently accelerate ions, converting a fraction ∊i ∼ 0.2 of the upstream flow energy into downstream nonthermal ion energy. The maximum energy of nonthermal ions exhibits a Bohm scaling in time, as  Emax∝t . A much smaller fraction ∊e ≪ 0.1 of the upstream flow energy goes into downstream nonthermal electrons in the Bell regime. On the other hand, when the precursor is dominated by Weibel modes, the shock efficiently generates both nonthermal ions and electrons with ∊i ∼ ∊e ∼ 0.1, albeit with a slower scaling for the maximum energy,  Emax∝t1/2 . Our results are applicable to a wide range of transrelativistic shocks, including the termination shocks of extragalactic jets, the late stages of gamma-ray burst afterglows, and shocks in fast blue optical transients.

Keywords: High energy astrophysics, Plasma astrophysics, Non-thermal radiation sources, Shocks, Cosmic rays
MESA-QUEST: Tracing the Formation of Direct Collapse Black Hole Seeds via Quasi-stars. The origin of the first supermassive black holes (SMBHs) observed at redshifts z ≥ 9 remains one of the most challenging open questions in astrophysics. Their rapid emergence suggests that massive "heavy seeds" must have formed early, possibly through the direct collapse of pristine gas clouds in the first galaxies. We present Modules for Experiments in Stellar Astrophysics (MESA)-QUEST, a new framework built on the MESA code, designed to model the structure and evolution of quasi-stars—massive, radiation-supported envelopes hosting accreting black holes (BHs) at their cores—believed to be the progenitors of direct collapse BH seeds. Our implementation introduces flexible boundary conditions representing both Bondi accretion and saturated convection regimes and explores the impact of several stellar wind and mass-loss prescriptions, including Reimers, Dutch, and super-Eddington radiation-driven winds. We find that quasi-stars can grow central BHs to ≥103 M⊙ under favorable conditions, with saturated convection models yielding black-hole-to-total mass ratios up to 0.55M*—five times higher than Bondi-limited cases. However, strong radiation-driven winds can dramatically curtail growth, potentially quenching heavy-seed formation unless balanced by sustained envelope accretion. Our results delineate the physical limits under which quasi-stars can remain stable and produce heavy seeds capable of evolving into the earliest SMBHs detected by JWST and Chandra. Future extensions will incorporate rotation, magnetic fields, and general relativistic radiation hydrodynamics to refine accretion physics and constrain the viability of the quasi-star pathway for early SMBH formation.

Keywords: Supermassive black holes, Black holes, Stellar evolutionary models, Active galactic nuclei, Galaxies
Mutual Orbit Alignment in Resolved Triple Systems. A sample of 278 triple systems with outer separations under 300 au and resolved inner pairs is studied, focusing on the mutual alignment between the inner and outer orbits. The degree of alignment increases with (i) decreasing outer separation, (ii) decreasing ratio of outer and inner separations, (iii) decreasing mass of the inner primary component, and (iv) increasing inner mass ratio. There is no dependence on the outer mass ratio. The average mutual inclination is ∼40° for the full sample and ∼10° for 38 triples with primary components less massive than 1 M⊙ and outer separations below 50 au. Inner eccentricities in aligned triples are smaller compared to misaligned ones. In another sample of 371 hierarchies with known outer orbits and inner eclipsing subsystems, only 22% show mutual alignment within 20°, while the rest are aligned randomly. These findings match qualitatively the current understanding of the formation of hierarchical systems, where the N-body dynamics dominates at large scales, while the accretion and migration shape systems closer than ∼100 au. Fragmentation of isolated cores apparently produces approximately aligned low-mass hierarchies.

Keywords: Eclipsing binary stars, Trinary stars, Multiple stars, Speckle interferometry
Dynamical Origins of Azimuthal Metallicity Variations in the Galactic Disk: Insights from Kinematic Ridges with Gaia. Kinematic and spectroscopic studies in the past few years have revealed coherent azimuthal metallicity variations across the disk of the Milky Way (MW) that may be the result of dynamical process associated with nonaxisymmetric features of the Galaxy. At the same time, stellar kinematics from Gaia have uncovered ridgelike features in the velocity space, raising the question of whether these chemical and dynamical substructures share a common origin. Using a sample of disk stars from Gaia Data Release 3, we find that azimuthal metallicity variations are correlated with kinematic ridges in the Vϕ─R plane, suggesting a shared origin. We utilize a suite of MW test-particle simulations to assess the role of transient spiral arms, the bar, and interactions with a Sagittarius (Sgr)-like dwarf galaxy in simultaneously shaping both chemical and kinematic substructures. Among the physical mechanisms explored, bar and spiral arm interactions are the ones that consistently reproduce both the chemokinematic features and alignment observed in the Gaia data. While our model of an interaction with a Sgr-like dwarf galaxy can also induce kinematic and metallicity substructure, the amplitude of the azimuthal metallicity variations is too weak, suggesting this is likely not the dominant influence. Although additional contributing processes cannot be ruled out, the azimuthal metallicity variations observed in Gaia are best explained by a dynamical origin. Our results support the view that azimuthal metallicity variations in the Galaxy are driven by similar dynamical mechanisms responsible for generating the kinematic ridges and comoving groups.

Keywords: Galactic archaeology, Milky Way dynamics, Spiral arms, Milky Way disk, Galactic bar
A Multiband Photometric Selection of Period Bouncer Candidates in the Solar Neighborhood with Gaia, GALEX, WISE, eROSITA, and XMM-Newton Data. As cataclysmic variables (CVs) that have evolved past their minimum orbital period, period bouncers have been proposed to be a substantial fraction of the CV population, yet the observed number is significantly lower, posing the "missing bouncer" problem. In this work, we present a multiband photometric selection method to search for new period bouncer candidates. We identify a specific locus for confirmed bouncers in the optical-UV-IR space. Using this locus, we selected 430 candidates within 500 pc. Crossmatching with eROSITA and XMM-Newton catalogs reveals 10 candidates (seven of which are new) with X-ray emission, indicative of accretion. X-ray spectral analysis of these candidates confirmed plasma temperatures consistent with confirmed bouncers. Based on these findings, we estimate a local space density of high-likelihood period bouncer candidates of approximately 4.1 × 10−7 pc−3. The nature of these candidates requires confirmation through follow-up observations.

Keywords: Cataclysmic variable stars, Binary stars, X-ray sources
Repeating versus Nonrepeating Fast Radio Bursts: A Deep Learning Approach to Morphological Characterization. We present a deep learning approach to classify fast radio bursts (FRBs) based purely on morphology as encoded on recorded dynamic spectrum from Canadian Hydrogen Intensity Mapping Experiment (CHIME)/FRB Catalog 2. We implemented transfer learning with a pretrained ConvNext architecture, exploiting its powerful feature extraction ability. ConvNext was adapted to classify dedispersed dynamic spectra (which we treat as images) of the FRBs into one of the two subclasses, i.e., repeater and nonrepeater, based on their various temporal and spectral properties and the relation between the subpulse structures. Additionally, we also used a mathematical model representation of the total intensity data to interpret the deep learning model. Upon fine-tuning the pretrained ConvNext on the FRB spectrograms, we were able to achieve high classification metrics while substantially reducing training time and computing power as compared to training a deep learning model from scratch with random weights and biases without any feature extraction ability. Importantly, our results suggest that the morphological differences between repeating and nonrepeating CHIME events persist in Catalog 2 and the deep-learning model leveraged these differences for classification. The fine-tuned deep-learning model can be used for inference, which enables us to predict whether an FRB's morphology resembles that of repeaters or nonrepeaters. Such inferences may become increasingly significant when trained on larger datasets that will exist in the near future.

Keywords: Neural networks, Convolutional neural networks, Radio astronomy, Radio transient sources
Detection of Dark Companions via The Combination of Eclipse Timing Variation, Hipparcos, and/or Gaia Astrometry: The Cases of V Puppis and CY Ari. The third body is expected to shape the formation and evolution of close binary systems. In this work, we develop a method to detect and characterize the tertiary companion around eclipsing binaries through the combined analysis of eclipse timing variation, Hipparcos, and/or Gaia astrometry. This method allows us to determine both the true mass and the inclination of the tertiary body that inferred from light-travel time effect. For the massive B-type binary V Pup, we do not confirm the previously reported 5.47 yr signal; instead, we identify a longer period of 14 yr. The orbital semimajor axis and mass of the outer body are revised to  aC=17.88−0.15+0.15  au and  MC=7.73−0.14+0.14M⊙ , confirming it as a promising stellar-mass black-hole candidate for further follow-up study. For the tertiary of the contact binary CY Ari, we obtain  PC=5.406−0.016+0.017 yr,  eC=0.526−0.027+0.032 ,  IC=85.6−6.5+7.8  °, and a true mass of  MC=0.640−0.029+0.029M⊙ , supporting the white dwarf hypothesis proposed in previous study. The orbits of both systems are nearly edge-on (I = 90°), implying that they may form in a coplanar environment. We highlight the advantages of our method for detecting dark companions in binary and triple systems.

Keywords: Eclipsing binary stars, Astrometry
Formation of an Optically Thick Shocked Shell in the Very Fast Nova V1674 Herculis: The Origin of Superbrightness. V1674 Her is the fastest (t2 ∼ 1 day) classical nova in our Galaxy, and its absolute V peak of  MV,max∼−10.2  is 1 mag brighter than typical very fast novae. Such a nova is sometimes called a superbright nova. Using our fully self-consistent nova outburst model combined with the optically thick winds on a 1.35 M⊙white dwarf (WD) with a mass accretion rate of 1 × 10−11 M⊙ yr−1, we have clarified that a strong reverse shock arises 0.3 day after the outburst, which is just after the maximum expansion of the WD photosphere. The shocked shell is optically thick and expanding with a velocity of ∼3500 km s−1. Its V brightness reaches a maximum of  MV,max=−10.2  when the shocked shell expands to Rshell ∼ 300 R⊙ on day ∼0.7. After that, the shocked shell turns optically thin and becomes fainter than the brightness of free─free emission from the nova wind. In chronological order, the optical brightness of free─free emission reaches a maximum of MV = −9 on day 0.3. However, it is overtaken on days 0.5─0.7 by the ∼1 mag brighter luminosity of the optically thick shocked shell. The GeV gamma-ray flux reaches a maximum on day 0.4 because the gamma-rays are emitted by the shock that arises on day 0.3. Our model consistently explains both the superbrightness and chronological order that the gamma-ray peak precedes substantially before the optical V peak. We also present a similar light-curve model for another superbright nova, V1500 Cyg.

Keywords: X-ray astronomy, High energy astrophysics, Fast novae, Shocks, Gamma-ray astronomy, Classical novae
Kinematics of Distant Milky Way Halo RR Lyrae Stars out to 160 kpc. We present a kinematical study of the outer halo (rGC ∼ 60─160 kpc) of the Milky Way (MW) based on spectroscopy of 55 RR Lyrae stars using the Echelle Spectrograph and Imager instrument on the Keck II telescope. Our spectroscopic targets were selected from three photometric surveys: NGVS, DES, and Pan-STARRS 1. We derive center-of-mass radial velocities with uncertainties of 6─35 km s−1. The halo velocity dispersion measured with our sample is 70 ± 7 km s−1. The velocity field shows a possible dipole-like structure, with redshifted northern and blueshifted southern hemispheres. Fitting a MW─Large Magellanic Cloud (LMC) dipole perturbation model yields a weak/marginal dipole signal, with an amplitude of  −30−20+16  km s−1 and an apex direction  (l,b)=(−38.2−31.5+42.4,−41.3−23.8+27.9)  deg, along with a bulk compression velocity of −16 ± 11 km s−1. While limited by sky coverage and sample size, our results are consistent with the presence of LMC-induced disequilibrium in the distant halo beyond 100 kpc. Aside from the 55 RR Lyrae stars, our spectroscopic analysis reveals that 10 additional phometrically selected RR Lyrae candidates are, in fact, quasar/blazar contaminants; this provides a cautionary tale about the presence of such contaminants in sparsely sampled photometric surveys. Our study demonstrates that single-epoch spectroscopy of RR Lyrae stars is a viable method for probing the kinematics of the outer halo, and future surveys like Rubin/LSST and the Dark Energy Spectroscopic Instrument (DESI-II) have the potential to significantly advance this effort.

Keywords: Radial velocity, RR Lyrae variable stars, Milky Way stellar halo
Spider-webb: Spatially Resolved Evidence of Inside-out Quenching in the Spiderweb Protocluster at z ∼ 2. We present a spatially resolved analysis of galaxy quenching within the Spiderweb Protocluster at z ∼ 2.16, combining deep imaging from the James Webb Space Telescope (JWST) and the Hubble Space Telescope. Utilizing pixel-by-pixel spectral energy distribution fitting, we derive maps of stellar mass, star formation rate (SFR), specific SFR (sSFR), and rest-frame UVJ colors. Quiescent galaxies, predominantly found at  log(M*/M⊙)≥10.5 , exhibit clear mass-dependent inside-out quenching, with central sSFR approximately an order of magnitude lower than outer regions, while lower-mass star-forming galaxies show flat sSFR profiles. Central star formation activity fundamentally anticorrelates with Sérsic index, indicating reduced activity in bulge-dominated systems. Spatially resolved UVJ colors reveal heterogeneous internal star formation, distinguishing star-forming regions in quiescent hosts from those in globally star-forming systems. These findings demonstrate that quenching mechanisms were effectively operating by z ∼ 2, with the observed inside-out patterns and morphological correlations consistent with active galactic nuclei-driven feedback processes. Our study provides key observational constraints on galaxy evolution during this critical epoch.

Keywords: Quenched galaxies, Galaxy properties, High-redshift galaxy clusters, Protoclusters, Galaxy structure, Galaxy evolution
An Improved UV─Optical Diagnostic for Rejuvenating Galaxies in the Local Universe and Implications for Galaxy Evolution. Rejuvenating galaxies are important probes of galaxy evolution, yet identifying them observationally is challenging, as constraining recent star formation histories requires both photometric and spectroscopic data. We present a method for identifying rejuvenating galaxies in the local Universe using ultraviolet (UV) imaging and optical spectroscopy, building on a recent selection that identifies a system as rejuvenating if it is quenched in the near-UV (NUV; tracing ∼100 Myr timescales) but star-forming in Hα (tracing ∼10 Myr timescales). Shortly after a star formation episode, however, the NUV is dominated by the same massive stars that power Hα, so these indicators do not always trace distinct timescales. To address this, we derive a relation that predicts the NUV emission associated with the ionizing O-star population traced by Hα, enabling us to isolate the NUV contribution from longer-lived stars (primarily B/A stars with M ≲ 20 M⊙). Subtracting the predicted O-star NUV from the dust-corrected NUV yields a more reliable rejuvenation diagnostic. Using this method, we identify ∼104 rejuvenating galaxies in a sample of Sloan Digital Sky Survey galaxies (∼4.5%). These galaxies have intermediate stellar masses and are found primarily in lower-density environments, becoming increasingly rare toward the centers of groups and clusters. Rejuvenating galaxies also exhibit systematically lower gas-phase metallicities, consistent with fueling by the accretion of metal-poor gas.

Keywords: Observational astronomy, Galaxy environments, Star formation, Galaxy evolution, Galaxies
Spectrally Resolved Gas Kinematics in Cygnus A: XRISM Detects AGN Jet-induced Velocity Dispersion in Multitemperature Gas. We report spectral analysis on a 170 ks XRISM Resolve exposure of the core of Cygnus A. Analyzing the full field of view spectrum in the 1.7─12.0 keV band, we find evidence for two-temperature cluster gas. The hotter (kT = 5.53 ± 0.13 keV) gas has a velocity dispersion of 261 ± 13 km s−1 and a bulk velocity of 120 ± 20 km s−1 with respect to the central galaxy. The cooler gas ( kT=2.0−0.3+0.4  keV) has an even broader velocity dispersion of 440 ± 130 km s−1, with a systematic uncertainty of 120 km s−1. The relative line-of-sight velocity between the hotter and cooler gas can be as high as 450 ± 140 km s−1. We interpret the high velocity dispersions as a combination of turbulence and bulk motion due to the cocoon shock. The upper limit on the nonthermal pressure fraction for the hotter gas is 7.7% ± 0.7%. We associate the cooler gas with the central region (&lt;35 kpc) and the hotter phase with the gas surrounding it (35─100 kpc). The total energy due to the kinetic motion is 5.1 × 1060 erg, consistent with the energy associated with the central radio source. The kinetic energy injection rate is 6.9 × 1044─7.4 × 1045 erg s−1 under varying assumptions of injection timescales. The range of injection power is higher than the cooling luminosity, and thus the heating and cooling rates in Cygnus A are unbalanced.

Keywords: Galaxy clusters, Jets, Active galactic nuclei, Shocks, Intracluster medium
A Novel Technique for Long-term Timing of Redback Millisecond Pulsars. We present timing solutions spanning nearly two decades for five redback (RB) systems found in globular clusters (GCs), created using a novel technique that effectively "isolates" the pulsar. By accurately measuring the time of passage through periastron (T0) at points over the timing baseline, we use a piecewise-continuous binary model to get local solutions of the orbital variations that we pair with long-term orbital information to remove the orbital timing delays. The isolated pulse times of arrival can then be fit to describe the spin behavior of the millisecond pulsar (MSP). The results of our timing analyses via this method are consistent with those of conventional timing methods for binaries in GCs as demonstrated by analyses of NGC 6440D. We also investigate the observed orbital phase variations for these systems. Quasiperiodic oscillations in Terzan 5P's orbit may be the result of changes to the gravitational quadruple moment of the companion as prescribed by the Applegate model. We find a striking correlation between the standard deviation of the phase variations as a fraction of a system's orbit ( σ∆T0 ) and the MSP's spin frequency, as well as a potential correlation between  σ∆T0  and the binary's projected semimajor axis. While long-term RB timing is fraught with large systematics, our work provides a needed alternative for studying systems with significant orbital variations, especially when high-cadence monitoring observations are unavailable.

Keywords: Globular star clusters, Millisecond pulsars