Sazuna/UAT_keywords huggingface dataset:
Bar Instability and Formation Timescale across Toomre's Q Parameter and Central Mass Concentration: Slow Bar Formation or True Stability. We investigate the bar formation process using N-body simulations across the Toomre's parameter Q  and central mass concentration (CMC), focusing principally on the formation timescale. Of importance is that, as suggested by cosmological simulations, disk galaxies have a limited time of ~8 Gyr in the Universe's timeline to evolve secularly, starting when they became physically and kinematically steady to prompt the bar instability. By incorporating this time limit, bar-unstable disks are further subdivided into those that establish a bar before and after that time, namely, the normal and the slowly bar-forming disks. Simulations demonstrate that evolutions of bar strengths and configurations of the slowly bar-forming and the bar-stable cases are nearly indistinguishable prior to 8 Gyr, albeit dynamically distinct, while differences can be noticed afterward. Differentiating them before 8 Gyr is possible by identifying the proto-bar, a signature of bar development visible in kinematical maps such as the Fourier spectrogram and the angular velocity field, which emerges in the former group 1─2 Gyr before the fully developed bar, whereas it is absent in the latter group until 8 Gyr and such bar-stable disk remains unbarred until at least 10 Gyr. In addition, we find complicated interplays between Q  and CMC in regulating the bar formation. First, disk stabilization requires both high Q  and CMC. Either high Q  or high CMC only results in slow bar formation. Second, some hot disks can form a bar more rapidly than the colder ones in a specific range of Q  and CMC.

Keywords: Disk galaxies, Galaxy bars, N-body simulations
A Second Candidate Magnetic Helium Core White Dwarf and 3 Other Variable White Dwarfs in the Globular Cluster NGC 6397. Using archival Hubble Space Telescope observations, we report the discovery of four variable low-mass white dwarfs (WDs; 0.18 M⊙ ≤ M ≤ 0.5 M⊙) in the globular cluster NGC 6397. One source exhibits a periodic optical modulation of 5.21 ± 0.02 hr, which we interpret as potentially due to the rotation of a magnetic helium core WD (He WD). This makes this candidate the second magnetic He WD in NGC 6397, and one of the few He WDs with a measured rotation period. The other three candidates show aperiodic variability, with a change in magnitude ranging from ∼0.11 to 0.6. These discoveries highlight the importance of high-cadence photometric surveys in dense stellar environments. Follow-up spectroscopic observations are needed to confirm the nature of the variability of these systems.

Keywords: Globular star clusters, White dwarf stars, Stellar magnetic fields
ALMA/SCUBA-2 COSMOS Survey: Properties of X-Ray- and SED-selected Active Galactic Nuclei in Bright Submillimeter Galaxies. We investigate the properties of active galactic nuclei (AGNs) in the brightest submillimeter galaxies (SMGs) in the COSMOS field. We utilize the bright sample of the ALMA/SCUBA-2 COSMOS Survey (AS2COSMOS), which consists of 260 SMGs with S870 μm = 0.7–19.2 mJy at z = 0–6. We perform optical to millimeter spectral energy distribution (SED) modeling for the whole sample. We identify 24 AGN-host galaxies from the SEDs. Supplemented by 23 X-ray-detected AGNs (X-ray AGNs), we construct an overall sample of 40 AGN-host galaxies. The X-ray luminosity upper bounds indicate that the X-ray-undetected SED-identified AGNs are likely to be nearly Compton thick or have unusually suppressed X-ray emission. From visual classification, we identify  % of the SMGs without AGNs as major merger candidates. This fraction is almost consistent with the general galaxy population at z ∼ 2, suggesting that major mergers are not necessarily required for the enhanced star formation in SMGs. We also identify  % of the AGN hosts as major merger candidates, which is about twice as high as that in the SMGs without AGNs. This suggests that major mergers play a key role in triggering AGN activity in bright SMGs.

Keywords: Submillimeter astronomy, Active galaxies, High-redshift galaxies, X-ray active galactic nuclei
The Hidden Magnetic Structures of a Solar Intermediate Filament Revealed by the Injected Flare Material. Solar filaments are spectacular objects in the solar atmosphere, consisting of accumulations of cool, dense, and partially ionized plasma suspended in the hot solar corona against gravity. The magnetic structures that support the filament material remain elusive, partly due to the lack of high-resolution magnetic field measurements in the chromosphere and corona. In this study, we reconstruct the magnetic structures of a solar intermediate filament using EUV observations and two different methods, to follow the injection of hot material from a B-class solar flare. Our analysis reveals the fine-scale magnetic structures of the filament, including a compact set of mutually wrapped magnetic fields encasing the cool filament material, two groups of helical magnetic structures intertwining with the main filament, and a series of arched magnetic loops positioned along the filament. Additionally, we find that the northern footpoints of the helical structures are rooted in the same location, while their southern footpoints are rooted in different areas. The results obtained in this study offer new insights into the formation and eruption mechanisms of solar filaments.

Keywords: Solar flares, Solar prominences, Solar physics, Solar active regions, Solar filaments, Solar activity
Hydrogen Column Density Variability in a Sample of Local Compton-thin AGN II. We present the multiepoch analysis of 13 variable, nearby (z ≲ 0.1), and Compton-thin (1022 &lt; NH &lt; 1.5 × 1024 cm−2) active galactic nuclei (AGN) selected from the 105-month BAT catalog. Analyzing all available archival soft and hard X-ray observations, we investigate the line-of-sight hydrogen column density (NH,los) variability on timescales ranging from a few days to approximately 20 yr. Each source is analyzed by simultaneously modeling the data with three physical torus models, providing tight constraints on torus properties, including the covering factor, the cloud dispersion, and the torus average hydrogen column density (NH,av). Our final sample includes 27 variable, Compton-thin AGN after implementing another 14 AGN analyzed in our previous work. We find that all sources require either flux or NH,los variability. Based on the degree of variability, we classify 37% of them as "NH,los Variable," 44% as "Nonvariable in NH,los," and 19% as "Undetermined." Noticeably, there is no discernible difference between geometrical and intrinsic properties among the three variability classes, suggesting no intrinsic differences between the NH,los-variable and nonvariable sources within the considered sample. We measure the median variation in NH,los between any observation pair of the same source to be 25% with respect to the lowest NH,los measure in the pair. Furthermore, 48% of the analyzed sources require the inclusion of a Compton-thick reflector in the spectral fitting. Among these, 30% exhibit recorded 22 GHz water megamaser emission, suggesting a potential shared nature between the two structures.

Keywords: X-ray active galactic nuclei, Active galaxies, Active galactic nuclei, High energy astrophysics
Segue 2 Recently Collided with the Cetus-Palca Stream: New Opportunities to Constrain Dark Matter in an Ultra-faint Dwarf. Stellar streams in the Milky Way are promising detectors of low-mass dark matter (DM) subhalos predicted by ΛCDM. Passing subhalos induce perturbations in streams that indicate the presence of the subhalos. Understanding how known DM-dominated satellites impact streams is a crucial step toward using stream perturbations to constrain the properties of dark perturbers. Here, we cross-match a Gaia Early Data Release 3 and SEGUE member catalog of the Cetus-Palca stream (CPS) with H3 for additional radial velocity measurements and fit the orbit of the CPS using this six-dimensional (6D) data. We demonstrate for the first time that the ultra-faint dwarf Segue 2 had a recent (77 ± 5 Myr ago) close flyby (within the stream's 2σ width) with the CPS. This interaction enables constraints on Segue 2's mass and density profile at larger radii (  kpc) than are probed by its stars (  pc). While Segue 2 is not expected to strongly affect the portion of the stream covered by our 6D data, we predict that if Segue 2's mass within ∼ 6 kpc is 5 × 109 M⊙, the CPS's velocity dispersion will be ∼ 40 km s‑1 larger at ϕ1 &gt; 20° than at ϕ1 &lt; 0°. If no such heating is detected, Segue 2's mass cannot exceed 109 M⊙ within ∼ 6 kpc. The proper motion distribution of the CPS near the impact site is mildly sensitive to the shape of Segue 2's density profile. This study presents a critical test for frameworks designed to constrain properties of dark subhalos from stream perturbations.

Keywords: Dwarf galaxies, Dark matter, Stellar streams, Milky Way dynamics
Elevated Rates of Tidal Disruption Events in Active Galactic Nuclei. Advances in time domain astronomy have produced a growing population of flares from galactic nuclei, including both tidal disruption events (TDEs) and flares in active galactic nuclei (AGN). Because TDEs are uncommon and AGN variability is abundant, large-amplitude AGN flares are usually not categorized as TDEs. While TDEs are normally channelled by the collisional process of two-body scatterings over a relaxation timescale, the quadrupole moment of a gas disk alters the stellar orbits, allowing them to collisionlessly approach the central massive black hole (MBH). This leads to an effectively enlarged loss cone, the loss wedge. Earlier studies found a moderate enhancement, up to a factor ~2–3, of TDE rates   for a static axisymmetric perturbation. Here, we study the loss wedge problem for an evolving AGN disk, which can capture large number of stars into the growing loss wedge over much shorter times. The rates   of collisionless TDEs produced by these time-evolving disks are much higher than the collisional rates   in a static loss wedge. We calculate the response of a stellar population to the axisymmetric potential of an adiabatically growing AGN disk and find that the highest rates of collisionless TDEs are achieved for the largest (i) MBH masses M• and (ii) disk masses Md. For M• ~ 107M⊙ and Md ~ 0.1M•, the rate enhancement can be up to a factor  . The orbits of collisionless TDEs sometimes have a preferred orientation in apses, carrying implications for observational signatures of resulting flares.

Keywords: AGN host galaxies, Black hole physics, Tidal disruption, Stellar dynamics, Galaxy nuclei, Active galactic nuclei
Deep Swift/UVOT Observations of GOODS-N and the Evolution of the Ultraviolet Luminosity Function at 0.2 &lt; z &lt; 1.2. We present Swift Ultraviolet Optical Telescope (UVOT) observations of the deep field GOODS-N in four near-UV filters. A catalog of detected galaxies is reported, which will be used to explore galaxy evolution using UV emission. Swift/UVOT observations probe galaxies at z ≲ 1.5 and combine a wide field of view with moderate spatial resolution; these data complement the wide-field observations of Galaxy Evolution Explorer and the deep, high-angular-resolution observations by Hubble Space Telescope. Using our catalog of detected galaxies, we calculate the UV galaxy number counts as a function of apparent magnitude and compute the UV luminosity function (LF) and its evolution with redshift. From the LF fits in various redshift bins, we calculate the star formation rate density as a function of redshift and find evolution consistent with past works. We explore how different assumptions such as dust attenuation corrections can dramatically change how quickly the corrected star formation rate density changes with redshift. At these low redshifts, we find no trend between UV attenuation and redshift or absolute magnitude with significant scatter in the UV spectral slope β. This data set will complement the extensive observations of GOODS-N already in the literature.

Keywords: Interstellar dust, Galaxy evolution, Near ultraviolet astronomy
Chemical Abundances in the Nuclear Star Cluster of the Milky Way: Alpha-element Trends and Their Similarities with the Inner Bulge. Chemical characterization of the Galactic center is essential for understanding its formation and structural evolution. Trends of alpha (α) elements, such as magnesium, silicon, and calcium, serve as powerful diagnostic tools, offering insights into star formation rates and gas-infall history. However, high extinction has previously hindered such studies. In this study, we present a detailed chemical abundance analysis of M giants in the Milky Way's nuclear star cluster (NSC), focusing on α-element trends with metallicity. High-resolution, near-infrared spectra were obtained using the Immersion GRating INfrared Spectrograph on the Gemini South telescope for nine M giants. Careful selection of spectral lines, based on a solar-neighborhood control sample of 50 M giants, was implemented to minimize systematic uncertainties. Our findings show enhanced α-element abundances in the predominantly metal-rich NSC stars, consistent with trends in the inner bulge. The NSC stars follow the high-[α/Fe] envelope seen in the solar vicinity's metal-rich population, indicating a high star formation rate. The α-element trends decrease with increasing metallicity, also at the highest metallicities. Our results suggest the NSC population likely shares a similar evolutionary history with the inner bulge, challenging the idea of a recent dominant star formation burst. This connection between the NSC and the inner-disk sequence suggests that the chemical properties of extragalactic NSCs of Milky Way─type galaxies could serve as a proxy for understanding the host galaxies' evolutionary processes.

Keywords: High resolution spectroscopy, Galactic center, Galactic archaeology, Chemical abundances, Late-type giant stars, M giant stars, Near infrared astronomy
Fuzzy Galaxies or Cirrus? Decomposition of Galactic Cirrus in Deep Wide-field Images. Diffuse Galactic cirrus, or diffuse Galactic light (DGL), can be a prominent component in the background of deep wide-field imaging surveys. The DGL provides unique insights into the physical and radiative properties of dust grains in our Milky Way, and it also serves as a contaminant on deep images, obscuring the detection of background sources such as low surface brightness galaxies. However, it is challenging to disentangle the DGL from other components of the night sky. In this paper, we present a technique for the photometric characterization of Galactic cirrus based on (1) extraction of its filamentary or patchy morphology and (2) incorporation of color constraints obtained from Planck thermal dust models. Our decomposition method is illustrated using a ~10 deg2 imaging data set obtained by the Dragonfly Telephoto Array, and its performance is explored using various metrics that characterize the flatness of the sky background. As a concrete application of the technique, we show how removal of cirrus allows low surface brightness galaxies to be identified on cirrus-rich images. We also show how modeling the cirrus in this way allows optical DGL intensities to be determined with high radiometric precision.

Keywords: Interstellar dust, Sky surveys, Astronomy data reduction, Astronomy image processing, Low surface brightness galaxies
A Pride of Satellites in the Constellation Leo? Discovery of the Leo VI Milky Way Satellite Ultra-faint Dwarf Galaxy with DELVE Early Data Release 3. We report the discovery and spectroscopic confirmation of an ultra-faint Milky Way satellite in the constellation of Leo. This system was discovered as a spatial overdensity of resolved stars observed with Dark Energy Camera (DECam) data from an early version of the third data release of the DECam Local Volume Exploration (or DELVE) survey. The low luminosity (  ;  ), large size (  pc), and large heliocentric distance (  kpc) are all consistent with the population of ultra-faint dwarf galaxies (UFDs). Using Keck/DEIMOS observations of the system, we were able to spectroscopically confirm nine member stars, while measuring a tentative mass-to-light ratio of   and a nonzero metallicity dispersion of  , further confirming Leo VI's identity as a UFD. While the system has a highly elliptical shape,  , we do not find any conclusive evidence that it is tidally disrupting. Moreover, despite the apparent on-sky proximity of Leo VI to members of the proposed Crater-Leo infall group, its smaller heliocentric distance and inconsistent position in energy–angular momentum space make it unlikely that Leo VI is part of the proposed infall group.

Keywords: Surveys, Dwarf galaxies
What Rogue Hydrogen Clouds Tell Us about Galactic Magnetic Fields. Observations of the Milky Way and external galaxies support the idea that large-scale magnetic fields are concentrated in galactic disks, with halo magnetic fields at least an order of magnitude weaker. However, very little is known about the transition between the two. We present the discovery of linearly polarized radio emission at the interface between interacting shells of gas within a well-known grouping of high-velocity clouds (HVCs), the anticenter shell. Faraday rotation of diffuse emission and of background extragalactic compact sources demonstrates an enhancement of the field at the interface. This is the clearest observed example of an HVC altering the large-scale magnetic field at the disk–halo interface and is the first image of magnetic field effects in an HVC. These results demonstrate the possibility of future three-dimensional reconstruction of the Galactic magnetic field and showcase the versatility of the Synthesis Telescope at the Dominion Radio Astrophysical Observatory as one of the few existing telescopes that can exploit this new method of probing Galactic magnetism.

Keywords: Interstellar magnetic fields, High-velocity clouds, Interstellar clouds, Interstellar medium
Constraints on Primordial Black Holes from N<SUB>eff</SUB>: Scalar-induced Gravitational Waves as an Extra Radiation Component. In 2023 June, multiple pulsar timing array collaborations provided evidence for the existence of a stochastic gravitational-wave background. Scalar-induced gravitational waves (SIGWs), as one of the most likely sources of stochastic gravitational waves, have received widespread attention. When primordial curvature perturbations on small scales are sufficiently large, primordial black holes (PBHs) inevitably form, concurrently producing SIGWs with significant observable effects. These SIGWs can serve as an additional radiation component, influencing the relativistic degrees of freedom Neff. Taking into account primordial non-Gaussianity, we study the energy density spectrum of SIGWs up to the third order and use the current observational data of Neff to constrain small-scale primordial curvature perturbations and the abundance of PBHs.

Keywords: Cosmology, Cosmic microwave background radiation, Cosmological parameters from large-scale structure, Non-Gaussianity, Gravitational waves, Primordial black holes
Electron Acceleration by Multiple Fast Fermi Reflections at Earth's Bow Shock. The generation of energetic particles in the Universe is an important topic in astrophysical and space physics. Although the diffusive shock acceleration has been acknowledged to play an essential role in particle acceleration, the injection problem has not been well resolved yet. Using measurements from the Magnetospheric Multiscale spacecraft, we report unexpected spiky enhancements of energetic electron flux in the upstream region of the Earth's bow shock. Such enhancements are then demonstrated to result from the accelerated electron population by shock reflection. Meanwhile, the magnetic field exhibits no variations except an increase in radius of curvature. Based on the results above, we uncover the underlying acceleration scenario. When the interplanetary magnetic field with a proper curvature approaches the bow shock, the two ends of a magnetic field line may touch the shock front earlier than its middle part. Magnetic mirror points form at the two field-line-shock intersections. The solar wind electrons are therefore trapped and bounce between two mirror points. Eventually, these electrons undergo fast Fermi reflection several times and gain substantial energies. The spacecraft observation of accelerated electrons is reproduced with a numerical method based on the reflection model. Such an acceleration process may be a potential solution to the injection problem.

Keywords: Shocks, Interplanetary particle acceleration, Solar wind, Planetary bow shocks
Multiple Episodes of Remote Brightenings Driven by a Coronal Extreme-ultraviolet Jet on the Sun. Remote brightening (RB) is compact brightening at footpoints of magnetic loops, which are remotely connecting to and confining an eruption in the solar atmosphere. Here, we report on observations of an RB resulting from an EUV jet with a speed of about 90 km s‑1. The loops connecting the RB and the jet have an apparent length of about 59 Mm. Intriguingly, the RB exhibits at least two episodes of brightenings, as characterized by two peaks in its lightcurve. The energies that sustain the first and second peaks of the RB are 6.3 × 1026 erg and 8.4 × 1026 erg, respectively, and comprise a significant proportion of the total energy. The first peak of the RB brightenings coincides with the jet's peak with a time delay of 12 s, while the second peak lags behind by 108 s. Besides the flows of the ejecta, we have identified two additional flows originating from the eruption site. One is relatively cool with a temperature of  –6.1 and a speed of about 275 ± 15 km s‑1. The other is hot with a temperature of  –7.3 and a much greater speed of about 750 ± 70 km s‑1. We attribute the second peak of RB directly to this hot flow, which our numerical experiments suggest is the result of a slow shock wave. Considering the minimal time delay between the first peak of RB and the eruption, we infer that this first episode is due to heating by nonthermal electrons. Our research demonstrates that the dynamics in an RB can offer vital insights into the nature of the corresponding eruption and help understand how energy is distributed throughout the solar atmosphere.

Keywords: Solar atmosphere, Solar coronal transients, Solar magnetic reconnection, Solar coronal loops, Solar coronal heating, Solar corona, Solar magnetic fields