adsabs/SciX_UAT_keywords huggingface dataset:
Probing the Conditions for the H I-to-H2 Transition in the Interstellar Medium

We investigate the conditions for the H I-to-H2 transition in the solar neighborhood by analyzing H I emission and absorption measurements toward 58 Galactic lines of sight (LOSs) along with 12CO(1-0) (CO) and dust data. Based on the accurate column densities of the cold and warm neutral medium (CNM and WNM), we first perform a decomposition of gas into atomic and molecular phases, and show that the observed LOSs are mostly H I-dominated. In addition, we find that the CO-dark H2, not the optically thick H I, is a major ingredient of the dark gas in the solar neighborhood. To examine the conditions for the formation of CO-bright molecular gas, we analyze the kinematic association between H I and CO, and find that the CNM is kinematically more closely associated with CO than the WNM. When CNM components within CO line widths are isolated, we find the following characteristics: spin temperature < 200 K, peak optical depth > 0.1, CNM fraction of ~0.6, and V-band dust extinction > 0.5 mag. These results suggest that CO-bright molecular gas preferentially forms in environments with high column densities where the CNM becomes colder and more abundant. Finally, we confront the observed CNM properties with the steady-state H2 formation model of Sternberg et al. and infer that the CNM must be clumpy with a small volume filling factor. Another possibility would be that missing processes in the model, such as cosmic-rays and gas dynamics, play an important role in the H I-to-H2 transition.

Keywords: interstellar atomic gas; molecular clouds; interstellar dust extinction; interstellar molecules; interstellar medium; radio astronomy
The Activation of Galactic Nuclei and Their Accretion Rates Are Linked to the Star Formation Rates and Bulge-types of Their Host Galaxies

We use bulge-type classifications of 809 representative Sloan Digital Sky Survey galaxies by Gadotti to classify a large sample of galaxies into real bulges (classical or elliptical) and pseudobulges using Random Forest. We use structural and stellar population predictors that can easily be measured without image decomposition. Multiple parameters—such as the central-mass density with 1 kpc, concentration index, Sérsic index, and velocity dispersion—result in accurate bulge classifications when combined together. We classify ∼44,500 face-on galaxies above stellar mass of 1010 M⊙ and redshift 0.02 < z < 0.07 into real bulges or pseudobulges with 93% ± 2% accuracy. We show that ∼75%-90% of active galactic nuclei (AGNs) identified by the optical line ratio diagnostic are hosted by real bulges. The pseudobulge fraction significantly decreases with AGN signature as the line ratios change from indicating pure star formation (∼54% ± 4%), to composite of star formation and AGN (∼18% ± 3%), and to AGN-dominated galaxies (∼5% ± 3%). Using the dust-corrected [O III] luminosity as an AGN accretion indicator, and the stellar mass and radius as proxies for a black hole mass, we find that AGNs in real bulges have lower Eddington ratios than AGNs in pseudobulges. Real bulges have a wide range of AGN and star formation activities, although most of them are weak AGNs. For both bulge-types, their Eddington ratios are correlated with specific star formation rates (SSFR). Real bulges have a lower specific accretion rate but higher AGN fraction than pseudobulges do at similar SSFRs.

Keywords: galaxy bulges; active galactic nuclei; galaxy classification systems; random forests; galaxy quenching; black holes
Characterization of JWST NIRCam PSFs and Implications for AGN+host Image Decomposition

We present a detailed analysis of the point-spread function (PSF) of JWST NIRCam imaging in eight filters: F070W, F115W, F150W, F200W, F277W, F356W, F444W, and F480M, using publicly available data. Spatial variations in the PSF FWHM generally decrease with wavelength: the maximum and rms fractional variations are ∼20% and 5% in F070W, reduced to ∼3% and 0.6% in F444W. We compare three commonly used methods (SWarp, photutils, and PSFEx) to construct model PSFs and conclude that PSFEx delivers the best performance. Using simulated images of broad-line active galactic nuclei (AGNs), we evaluate the impact of PSF mismatches on the recoverability of host galaxy properties. Host fluxes are generally overestimated when adopting mismatched PSF models, with larger overestimation for more AGN-dominated systems. Broader PSFs tend to produce less concentrated hosts, while narrower PSFs tend to produce more concentrated and compact hosts. Systematic uncertainties in host measurements from PSF and model mismatches are generally larger than the formal fitting uncertainties for high signal-to-noise ratio data. Image decomposition can also lead to an artificial offset between the AGN and host centroids, which is common (e.g., >1σ [3σ] detection in ∼80% [∼20%–30%] of systems), and scales with the mean host surface brightness (SB). Near the SB limit, this artificial offset can reach as large as ∼80%, 26%, and 7% of R e in systems with R e = 0.″12, 0.″48, and 1.″92, respectively. We demonstrate our PSF construction and image decomposition methods with an example broad-line quasar at z = 1.646 in the CEERS field.

Keywords: active galactic nuclei; galaxy structure; supermassive black holes; agn host galaxies; galaxy evolution
Eccentric Early Migration of Neptune

The dynamical structure of the Kuiper Belt can be used as a clue to the formation and evolution of the solar system, planetary systems in general, and Neptune's early orbital history in particular. The problem is best addressed by forward modeling where different initial conditions and Neptune's orbital evolutions are tested, and the model predictions are compared to orbits of known Kuiper Belt objects (KBOs). It has previously been established that Neptune radially migrated, by gravitationally interacting with an outer disk of planetesimals, from the original radial distance r ≲ 25 au to its current orbit at 30 au. Here we show that the migration models with a very low orbital eccentricity of Neptune (eN ≲ 0.03) do not explain KBOs with semimajor axes 50 < a < 60 au, perihelion distances q > 35 au, and inclinations i < 10°. If eN ≲ 0.03 at all times, the Kozai cycles control the implantation process and the orbits with q > 35 au end up having, due to the angular momentum's z-component conservation, i > 10°. Better results are obtained when Neptune's eccentricity is excited to eN ≃ 0.1 and subsequently damped by dynamical friction. The low-e and low-i orbits at 50-60 au are produced in this model when KBOs are lifted from the scattered disk by secular cycles—mainly the apsidal resonance ν8—near various mean motion resonances. These results give support to a (mild) dynamical instability that presumably excited the orbits of giant planets during Neptune's early migration.

Keywords: kuiper belt; dynamical evolution; neptune
Black Hole Mass Function and Its Evolution—The First Prediction for the Einstein Telescope

Knowledge of the black hole mass function (BHMF) and its evolution would help us understand the origin of BHs and how BH binaries formed at different stages in the history of the universe. We demonstrate the ability of a future third-generation gravitational-wave (GW) detector - the Einstein Telescope (ET) - to infer the slope of the BHMF and its evolution with redshift. We perform a Monte Carlo simulation of the measurements of chirp signals from binary BH systems (BBH) that could be detected by ET, including the BH masses and their luminosity distances (dL). We use the mass of a primary black hole in each binary system to infer the BHMF as a power-law function with slope parameter α. Taking into account the bias that could be introduced by the uncertainty of measurements and by the selection effect, we carried out the numerical tests and found that only 1000 GW events registered by ET (∼1% of its yearly detection rate) could accurately infer the α with a precision of α ∼ 0.1. Furthermore, we investigate the validity of our method to recover a scenario where α evolves with redshift as α(z)= α0 + α1 {z}/{1+z}. Taking a thousand GW events and using dL as the redshift estimator, our tests show that one could infer the value of evolving parameter α1 accurately at the uncertainty level of ∼0.5. Our numerical tests verify the reliability of our method. The uncertainty levels of the inferred parameters can be trusted directly for several sets of the parameters we assumed, yet they should not be treated as general.

Keywords: gravitational waves; black holes
High-energy Emission from Tidal Disruption Events in Active Galactic Nuclei

Tidal disruption events (TDEs) taking place in active galactic nuclei (AGNs) are different from ordinary TDEs. In these events, the returning tidal debris stream drills through the preexisting AGN accretion disk near the stream pericenter, destroying the inner disk in the process, and then intersects with the disk a second time at radii ranging from a few times to hundreds of times the pericenter distance. The debris dynamics of such TDEs, and hence their appearance, are distinct from those of ordinary TDEs. Here we explore the observational signatures of this "second impact" of the stream with the disk. Strong shocks form as the dilute stream is stopped by the denser disk. Compton cooling of the shocked material produces hard X-rays and even soft γ-rays, with most of the energy emitted between ~10 keV and 1 MeV. The luminosity follows the mass-return rate, peaking between ~1042 and 1044 erg s-1. The X-ray hardness and the smoothness of the light curve provide possible means for distinguishing the second impact from ordinary AGN flares, which exhibit softer spectra and more irregular light curves.

Keywords: high energy astrophysics; black hole physics; gravitation; accretion; gamma-ray transient sources; x-ray transient sources; hydrodynamical simulations; active galactic nuclei; tidal disruption
Near- and Mid-infrared Observations in the Inner Tenth of a Parsec of the Galactic Center Detection of Proper Motion of a Filament Very Close to Sgr A*

We analyze the gas and dust emission in the immediate vicinity of the supermassive black hole Sgr A* at the Galactic center (GC) with the ESO Very Large Telescope (Paranal/Chile) instruments SINFONI and VISIR. The SINFONI H+K data cubes show several emission lines with related line map counterparts. From these lines, the Brγ emission is the most prominent one and appears to be shaped as a bar extending along the north-south direction. With VISIR, we find a dusty counterpart to this filamentary emission. In this work, we present evidence that this feature could most be likely connected to the minispiral and potentially influenced by the winds of the massive stars in the central cluster or an accretion wind from Sgr A*. To this end, we coadd the SINFONI data between 2005 and 2015. The spectroscopic analysis reveals a range of Doppler-shifted emission lines. We also detect substructures in the shape of clumps that can be investigated in the channel maps of the Brγ bar. In addition, we compare the detection of the near-infrared Brγ feature to PAH1 mid-infrared observations and published 226 GHz radio data. These clumps show a proper motion of about 320 km s-1 that are consistent with other infrared continuum-detected filaments in the GC. Deriving a mass of $2.5\times {10}^{-5}\,{M}_{\odot }$ for the investigated Brγ feature shows an agreement with former derived masses for similar objects. Besides the north-south Brγ bar, we find a comparable additional east-west feature. Also, we identify several gas reservoirs that are located west of Sgr A* that may harbor dusty objects.

Keywords: galactic center; interstellar atomic gas; interstellar clouds; interstellar dust extinction; interstellar dust
1991T-Like Type Ia Supernovae as an Extension of the Normal Population

Type Ia supernovae (SNe) remain poorly understood despite decades of investigation. Massive computationally intensive hydrodynamic simulations have been developed and run to model an ever-growing number of proposed progenitor channels. Further complicating the matter, a large number of subtypes of Type Ia SNe have been identified in recent decades. Due to the massive computational load required, inference of the internal structure of Type Ia SNe ejecta directly from observations using simulations has previously been computationally intractable. However, deep-learning emulators for radiation transport simulations have alleviated such barriers. We perform abundance tomography on 40 Type Ia SNe from optical spectra using the radiative transfer code TARDIS accelerated by the probabilistic DALEK deep-learning emulator. We apply a parametric model of potential outer ejecta structures to comparatively investigate abundance distributions and internal ionization fractions of intermediate-mass elements (IMEs) between normal and 1991T-like Type Ia SNe in the early phases. Our inference shows that the outer ejecta of 1991T-like Type Ia SNe is underabundant in the typical intermediate mass elements that heavily contribute to the spectral line formation seen in normal Type Ia SNe at early times. Additionally, we find that the IMEs present in 1991T-like Type Ia SNe are highly ionized compared to those in the normal Type Ia population. Finally, we conclude that the transition between normal and 1991T-like Type Ia SNe appears to be continuous observationally and that the observed differences come out of a combination of both abundance and ionization fractions in these SNe populations.

Keywords: type ia supernovae; bayesian statistics; spectroscopy; neural networks
Systematic Search for and Study of Short-timescale Flare Structures in BL Lac Object Gamma-Ray Emission

We present here the first systematic search of short-timescale γ-ray flares from 29 high Galactic latitude BL Lac objects over 14 yr of Fermi Large Area Telescope data. Using a combined Bayesian Blocks and HOP algorithm, we identified seven high-quality orbital timescale flare segments from three sources and quantified 24 short-timescale flare structures. We then performed a comprehensive analysis of flare symmetry, power spectral density (PSD) of variability, and flux–photon index relation. The main results are as follows. (1) The flare symmetry parameter A shows a "U-shaped" distribution. Short-timescale flares are symmetric while long-timescale flares are asymmetric. The number of fast-rise, slow-decay and slow-rise, fast-decay type flares are equal. No correlation is found between A and peak/integral flux. No parameter evolution is seen between consecutive flares, either. The observations support a scenario where longer-timescale flares originate from the superposition of short, symmetric subhour flares. (2) PSD from yearly to hourly timescales is modeled using the CARMA process. At lower frequencies, the PSD follows the typical broken power-law form. The high-frequency region of the PSD exhibits a continuous power-law shape, indicating that γ-ray variability originates from a single physical process across all probed timescales. (3) The flux–photon index distribution shows a pattern of "harder-when-brighter" or "softer-when-brighter," but becomes flat above a certain critical flux, with Γ ≈ 2. This behavior cannot be simply explained by a two-component or blazar sequence model, and we speculate it may be related to complex interplay between electron acceleration and cooling.

Keywords: blazars; bl lacertae objects
Constraints on the Assembly History of the Milky Way's Smooth, Diffuse Stellar Halo from the Metallicity-dependent, Radially Dominated Velocity Anisotropy Profiles Probed with K Giants and BHB Stars Using LAMOST, SDSS/SEGUE, and Gaia

We analyze the anisotropy profile of the Milky Way's smooth, diffuse stellar halo using the Sloan Digital Sky Survey (SDSS) SEGUE blue horizontal branch stars and the SDSS/SEGUE and LAMOST K giants. These intrinsically luminous stars allow us to probe the halo to approximately 100 kpc from the Galactic center. Line-of-sight velocities, distances, metallicities, and proper motions are available for all stars via SDSS/SEGUE, LAMOST, and Gaia, and we use these data to construct a full 7D set consisting of positions, space motions, and metallicity. We remove substructure from our samples using integrals of motion based on the method of Xue et al. We find radially dominated kinematic profiles with nearly constant anisotropy within 20 kpc, beyond which the anisotropy profile gently declines but remains radially dominated to the farthest extents of our sample. Independent of star type or substructure removal, the anisotropy depends on metallicity, such that the orbits of the stars become less radial with decreasing metallicity. For -1.7 < [Fe/H] < -1, the smooth, diffuse halo anisotropy profile begins to decline at Galactocentric distances ~20 kpc, from $\beta \sim 0.9$ to 0.7 for K giants and from $\beta \sim 0.8$ to 0.1 for blue horizontal branch stars. For [Fe/H] < -1.7, the smooth, diffuse halo anisotropy remains constant along all distances with $0.2\lt \beta \lt 0.7$ depending on the metallicity range probed, but independent of star type. These samples are ideal for estimating the total Galactic mass as they represent the virialized stellar halo system.

Keywords: milky way stellar halo
Particle Acceleration by Magnetic Reconnection in Relativistic Jets: The Transition from Small to Large Scales

Several MHD works, and, in particular, the recent one by Medina-Torrejón et al. based on three-dimensional MHD simulations of relativistic jets, have evidenced that particle acceleration by magnetic reconnection driven by the turbulence in the flow occurs from the resistive up to the large injection scale of the turbulence. Particles experience Fermi-type acceleration up to ultrahigh energies, predominantly of the parallel velocity component to the local magnetic field, in the reconnection layers in all scales due to the ideal electric fields of the background fluctuations (V × B, where V and B are the velocity and magnetic field of the fluctuations, respectively). In this work, we show MHD-particle-in-cell (MHD-PIC) simulations following the early stages of the particle acceleration in the relativistic jet, which confirm these previous results, demonstrating the strong potential of magnetic reconnection driven by turbulence to accelerate relativistic particles to extreme energies in magnetically dominated flows. Our results also show that the dynamical time variations of the background magnetic fields do not influence the acceleration of the particles in this process.

Keywords: high energy astrophysics; plasma astrophysics; active galactic nuclei; jets
TOI 560: Two Transiting Planets Orbiting a K Dwarf Validated with iSHELL, PFS, and HIRES RVs

We validate the presence of a two-planet system orbiting the 0.15-1.4 Gyr K4 dwarf TOI 560 (HD 73583). The system consists of an inner moderately eccentric transiting mini-Neptune (TOI 560 b, $P={6.3980661}_{-0.0000097}^{+0.0000095}$ days, $e={0.294}_{-0.062}^{+0.13}$ , $M={0.94}_{-0.23}^{+0.31}{M}_{\mathrm{Nep}}$ ) initially discovered in the Sector 8 Transiting Exoplanet Survey Satellite (TESS) mission observations, and a transiting mini-Neptune (TOI 560 c, $P={18.8805}_{-0.0011}^{+0.0024}$ days, $M={1.32}_{-0.32}^{+0.29}{M}_{\mathrm{Nep}}$ ) discovered in the Sector 34 observations, in a rare near-1:3 orbital resonance. We utilize photometric data from TESS Spitzer, and ground-based follow-up observations to confirm the ephemerides and period of the transiting planets, vet false-positive scenarios, and detect the photoeccentric effect for TOI 560 b. We obtain follow-up spectroscopy and corresponding precise radial velocities (RVs) with the iSHELL spectrograph at the NASA Infrared Telescope Facility and the HIRES Spectrograph at Keck Observatory to validate the planetary nature of these signals, which we combine with published Planet Finder Spectrograph RVs from the Magellan Observatory. We detect the masses of both planets at >3σ significance. We apply a Gaussian process (GP) model to the TESS light curves to place priors on a chromatic RV GP model to constrain the stellar activity of the TOI 560 host star, and confirm a strong wavelength dependence for the stellar activity demonstrating the ability of near-IR RVs to mitigate stellar activity for young K dwarfs. TOI 560 is a nearby moderately young multiplanet system with two planets suitable for atmospheric characterization with the James Webb Space Telescope and other upcoming missions. In particular, it will undergo six transit pairs separated by <6 hr before 2027 June.

Keywords: radial velocity
Expansion of Hot Plasma with Kappa Distribution into Cold Plasma

The X-ray emission of coronal flare sources can be explained by considering the kappa electron distribution. Motivated by this fact, we study the problem of how hot plasma with the kappa distribution of electrons is confined in these sources. For comparison, we analyze the same problem, but with the Maxwellian distribution. We use a 3D particle-in-cell code, which is large in one direction and thus effectively only one-dimensional, but describe all electromagnetic effects. In the case with the Maxwellian distribution, and in agreement with the previous studies, we show a formation of the double layer at the hot-cold transition region that suppresses the flux of hot electrons from hot plasma into the cold one. In the case with the kappa distribution, contrary to the Maxwellian case, we found that there are several fronts with the double layers in the hot-cold transition region. It is caused by a more extended tail in the kappa case than in the Maxwellian one. The electrons from the extended tail freely escape from the hot plasma into a cold one. They form a beam that generates the return current and also Langmuir turbulence, where Langmuir waves accumulated at some locations. At these locations, owing to the ponderomotive force, Langmuir waves generate density depressions, where the double layers with the thermal fronts that suppress the hot electron flux, are formed. We also show how protons accelerate in these processes. Finally, we compare the Kappa and Maxwellian cases and discuss how these processes could be observed.

Keywords: plasma astrophysics; space plasmas; solar flares; computational methods; radio bursts; solar radio emission; solar x-ray emission
Shortest Microlensing Event with a Bound Planet: KMT-2016-BLG-2605

With a planet-host mass ratio q = 0.012 ± 0.001, KMT-2016-BLG-2605 has the shortest Einstein timescale, tE = 3.41 ± 0.13 days, of any planetary microlensing event to date. This prompts us to examine the full sample of seven short (tE < 7 days) planetary events with good q measurements. We find that six have clustered Einstein radii θE = 115 ± 20 μas and lens-source relative proper motions μrel ≃ 9.5 ± 2.5 mas yr-1. For the seventh, these two quantities could not be measured. These distributions are consistent with a Galactic bulge population of very low mass (VLM) hosts near the hydrogen-burning limit. This conjecture could be verified by imaging at first adaptive optics light on next-generation (30 m) telescopes. Based on a preliminary assessment of the sample, "planetary" companions (i.e., below the deuterium-burning limit) are divided into "genuine planets," formed in their disks by core accretion, and VLM brown dwarfs, which form like stars. We discuss techniques for expanding the sample, which include taking account of the peculiar "anomaly-dominated" morphology of the KMT-2016-BLG-2605 light curve.

Keywords: gravitational microlensing; gravitational microlensing exoplanet detection