adsabs/SciX_UAT_keywords huggingface dataset:
Evolution of the Earth's Magnetosheath Turbulence: A Statistical Study Based on MMS Observations

Composed of shocked solar wind, the Earth's magnetosheath serves as a natural laboratory to study the transition of turbulence from low Alfvén Mach number, MA, to high MA. The simultaneous observations of magnetic field and plasma moments with unprecedented high temporal resolution provided by NASA's Magnetospheric Multiscale Mission (MMS) enable us to study the magnetosheath turbulence at both magnetohydrodynamics (MHD) and sub-ion scales. Based on 1841 burst-mode segments of MMS-1 from 2015 September to 2019 June, comprehensive patterns of the spatial evolution of magnetosheath turbulence are obtained: (1) from the subsolar region to the flanks, MA increases from <1 to >5. At MHD scales, the spectral indices of the magnetic-field and velocity spectra present a positive and negative correlation with MA. However, no obvious correlations between the spectral indices and MA are found at sub-ion scales. (2) From the bow shock to the magnetopause, the turbulent sonic Mach number, ${M}_{\mathrm{turb}}$ , generally decreases from >0.4 to <0.1. All spectra steepen at MHD scales and flatten at sub-ion scales, representing positive/negative correlations with Mturb. The break frequency increases by 0.1 Hz when approaching the magnetopause for the magnetic-field and velocity spectra, while it remains at 0.3 Hz for the density spectra. (3) In spite of minor differences, similar results are found for the quasi-parallel and quasi-perpendicular magnetosheath. In addition, such spatial evolution of magnetosheath turbulence is found to be independent of the upstream solar wind conditions, e.g., the averaged Z-component of the interplanetary magnetic field and solar wind speed.

Keywords: planetary magnetospheres; interplanetary turbulence
A Census of WISE-selected Dual and Offset AGNs Across the Sky: New Constraints on Merger-driven Triggering of Obscured AGNs

Pairs of galaxies hosting active galactic nuclei (AGNs) are powerful probes of merger-driven supermassive black hole (SMBH) growth as they can resolve individual AGNs and trace mergers over a large range of physical separations. To exploit this on a large scale for the first time for both obscured and unobscured AGNs, we use the photometric redshifts of AGNs selected by the Wide-field Infrared Survey Explorer (WISE) to find probabilistic pairs (<100 kpc separations) across the sky, along with a comparison sample of inactive galaxy pairs. Our final sample of integrated pair probabilities yields 198 AGN-AGN pairs (dual AGNs) and 2767 AGN-galaxy pairs (offset AGNs) with uniformly measured AGN and host galaxy physical properties. We find the fraction of galaxy pairs hosting WISE AGNs is dominated by offset AGNs and is significantly elevated above that of inactive galaxies for large host stellar masses. We show how the AGN merger fraction directly increases with AGN extinction for both offset and dual AGNs, with up to ~40% of heavily obscured AGNs found in galaxy pairs. Elevated AGN merger fractions coincide with increased host specific star formation rates that suggest merger-driven coevolution of galaxies and SMBHs. Among dual AGNs, the most rapid SMBH growth may occur within the less-massive galaxy. Relative to stochastic mechanisms, mergers produce an excess of AGNs at increasingly smaller separations, especially for obscured AGNs (up to a factor of ~5), and are augmented by correlated triggering. Finally, this excess is stronger than for lower-luminosity optically selected AGNs, regardless of the AGN obscuration level.

Keywords: active galactic nuclei; agn host galaxies; infrared galaxies; galaxy interactions; galaxy evolution; galaxy mergers; galaxy pairs
Powerful t-SNE Technique Leading to Clear Separation of Type-2 AGN and H II Galaxies in BPT Diagrams

Narrow emission-line galaxies can be distinguished in the well-known BPT diagrams through narrow emission-line properties. However, there are no boundaries visible to the naked eye between type-2 active galactic nuclei (AGN) and H II galaxies in BPT diagrams, besides the extreme dividing lines expected by theoretical photoionization models. Here, based on the powerful t-SNE technique applied to the local narrow emission-line galaxies in the Sloan Digital Sky Survey Data Release 15, type-2 AGN and H II galaxies can be clearly separated in the t-SNE determined two-dimensional projected map, and then the dividing lines can be mathematically determined in BPT diagrams, leading to charming harmonization of the theoretical expectations and the actual results from real observed properties. The results not only provide an interesting and robust method to determine the dividing lines in BPT diagrams through the powerful t-SNE technique, but also lead to further confirmation on previously defined composite galaxies more efficiently classified in the BPT diagram of [O III]/Hβ versus [N II]/Hα.

Keywords: emission line galaxies; active galactic nuclei; h ii regions
Supernova Hydrodynamics: A Lab-scale Study of the Blast-driven Instability Using High-speed Diagnostics

A novel experimental approach to study the blast-driven instability at a nondiffuse, gaseous interface with a density gradient is presented. Under Euler similarity, this approach enables study of dissipative-scale hydrodynamics relevant to many astrophysical and laboratory high energy density phenomena in a well-resolved manner. The instability is initiated by passing a Taylor-Sedov blast wave originating from a controlled detonation through a perturbed and stably stratified interface between two gases. The facility and driving blast wave are characterized to obtain repeatable conditions and capture large ensembles of time-resolved Mie scattering imaging that show consistent hydrodynamic development. We analyze the instability evolution between different gas pairs to demonstrate the wide range of development and turbulent behavior that may occur between different supernova layers. The mean evolution of the hydrodynamic instability is compared to a buoyancy-drag model that is frequently used to estimate perturbation growth in supernova mixing research. We propose a time delay to this model in order to reproduce the measured instability behavior and demonstrate model robustness in handling flows driven by a time-varying acceleration.

Keywords: supernova dynamics; hydrodynamics; shocks; experimental techniques; laboratory astrophysics; supernovae; core-collapse supernovae; ejecta; astrophysical fluid dynamics; interdisciplinary astronomy
Reduced Late Bombardment on Rocky Exoplanets around M Dwarfs

Ocean-vaporizing impacts of chemically reduced planetesimals onto the early Earth have been suggested to catalyze atmospheric production of reduced nitrogen compounds and trigger prebiotic synthesis despite an oxidized lithosphere. While geochemical evidence supports a dry, highly reduced late veneer on Earth, the composition of late-impacting debris around lower-mass stars is subject to variable volatile loss as a result of their hosts' extended pre-main-sequence phase. We perform simulations of late-stage planet formation across the M-dwarf mass spectrum to derive upper limits on reducing bombardment epochs in Hadean-analog environments. We contrast the solar system scenario with varying initial volatile distributions due to extended primordial runaway greenhouse phases on protoplanets and the desiccation of smaller planetesimals by internal radiogenic heating. We find a decreasing rate of late-accreting reducing impacts with decreasing stellar mass. Young planets around stars ≤0.4 M ⊙ experience no impacts of sufficient mass to generate prebiotically relevant concentrations of reduced atmospheric compounds once their stars have reached the main sequence. For M-dwarf planets to not exceed Earth-like concentrations of volatiles, both planetesimals, and larger protoplanets must undergo extensive devolatilization processes and can typically emerge from long-lived magma ocean phases with sufficient atmophile content to outgas secondary atmospheres. Our results suggest that transiently reducing surface conditions on young rocky exoplanets are favored around FGK stellar types relative to M dwarfs.

Keywords: astrobiology; pre-biotic astrochemistry; exoplanet atmospheres; extrasolar rocky planets; planet formation; atmospheric composition
An Improved and Physically Motivated Scheme for Matching Galaxies with Dark Matter Halos

The simplest scheme for predicting real galaxy properties after performing a dark matter simulation is to rank order the real systems by stellar mass and the simulated systems by halo mass and then simply assume monotonicity-that the more massive halos host the more massive galaxies. This has had some success, but we study here if a better motivated and more accurate matching scheme is easily constructed by looking carefully at how well one could predict the simulated IllustrisTNG galaxy sample from its dark matter computations. We find that using the dark matter rotation curve peak velocity, vmax, for normal galaxies reduces the error of the prediction by 30% (18% for central galaxies and 60% for satellite systems)-following expectations from the physics of monolithic collapse. For massive systems with halo mass >1012.5 M⊙ hierarchical merger driven formation is the better model and dark matter halo mass remains the best single metric. Using a new single variable that combines these effects, φ = vmax/ ${v}_{\max ,12.7}$ + Mpeak/(1012.7 M⊙) allows further improvement and reduces the error, as compared to ranking by dark matter mass at z = 0 by another 6% from vmax ranking. Two parameter fits-including environmental effects produce only minimal further impact.

Keywords: galaxies; galaxy dark matter halos
Prebiosignature Molecules Can Be Detected in Temperate Exoplanet Atmospheres with JWST

The search for biosignatures on exoplanets connects the fields of biology and biochemistry to astronomical observation, with the hope that we might detect evidence of active biological processes on worlds outside the solar system. Here we focus on a complementary aspect of exoplanet characterization connecting astronomy to prebiotic chemistry: the search for molecules associated with the origin of life, prebiosignatures. Prebiosignature surveys in planetary atmospheres offer the potential to both constrain the ubiquity of life in the galaxy and provide important tests of current prebiotic syntheses outside of the laboratory setting. Here, we quantify the minimum abundance of identified prebiosignature molecules that would be required for detection by transmission spectroscopy using JWST. We consider prebiosignatures on five classes of terrestrial planets: an ocean planet, a volcanic planet, a post-impact planet, a super-Earth, and an early-Earth analog. Using a novel modeling and detection test pipeline, with simulated JWST noise, we find the detection thresholds of hydrogen cyanide ( $\mathrm{HCN}$ ), hydrogen sulfide (H2S), cyanoacetylene (HC3N), ammonia (NH3), methane (CH4), acetylene (C2H2), sulfur dioxide (SO2), nitric oxide (NO), formaldehyde (CH2O), and carbon monoxide (CO) in a variety of low-mean-molecular-weight (<5) atmospheres. We test the dependence of these detection thresholds on an M dwarf target star and the number of observed transits, finding that a modest number of transits (1-10) are required to detect prebiosignatures in numerous candidate planets, including TRAPPIST-1e with a high-mean-molecular-weight atmosphere. We find that the Near Infrared Spectrograph G395M/H instrument is best suited for detecting most prebiosignatures.

Keywords: astrobiology; exoplanet astronomy; planetary atmospheres; pre-biotic astrochemistry
Obliquity Constraints on an Extrasolar Planetary-mass Companion

We place the first constraints on the obliquity of a planetary-mass companion outside of the solar system. Our target is the directly imaged system 2MASS J01225093-2439505 (2M0122), which consists of a 120 Myr 0.4 M⊙ star hosting a 12-27 MJ companion at 50 au. We constrain all three of the system's angular-momentum vectors: how the companion spin axis, the stellar spin axis, and the orbit normal are inclined relative to our line of sight. To accomplish this, we measure projected rotation rates (v sin i) for both the star and the companion using new near-infrared high-resolution spectra with NIRSPEC at Keck Observatory. We combine these with a new stellar photometric rotation period from TESS and a published companion rotation period from Hubble Space Telescope to obtain spin-axis inclinations for both objects. We also fitted multiple epochs of astrometry, including a new observation with NIRC2/Keck, to measure 2M0122b's orbital inclination. The three line-of-sight inclinations place limits on the true de-projected companion obliquity and stellar obliquity. We find that while the stellar obliquity marginally prefers alignment, the companion obliquity tentatively favors misalignment. We evaluate possible origin scenarios. While collisions, secular spin-orbit resonances, and Kozai-Lidov oscillations are unlikely, formation by gravitational instability in a gravito-turbulent disk -- the scenario favored for brown dwarf companions to stars -- appears promising.

Keywords: exoplanet systems; high resolution spectroscopy; direct imaging; photometry
Relics of Supermassive Black Hole Seeds: The Discovery of an Accreting Black Hole in an Optically Normal, Low Metallicity Dwarf Galaxy

The detection and characterization of supermassive black holes (SMBHs) in local low mass galaxies is crucial to our understanding of the origins of SMBHs. This statement assumes that low mass galaxies have had a relatively quiet cosmic history, so that their black holes have not undergone significant growth and therefore can be treated as relics of the original SMBH seeds. While recent studies have found optical signatures of active galactic nuclei (AGNs) in a growing population of dwarf galaxies, these studies are biased against low metallicity and relatively merger-free galaxies, thus missing precisely the demographic in which to search for the relics of SMBH seeds. Here, we report the detection of the [Si VI]1.963 μm coronal line (CL), a robust indicator of an AGN in the galaxy SDSS J160135.95+311353.7, a nearby (z = 0.031) low metallicity galaxy with a stellar mass approximately an order of magnitude lower than the LMC (M* ≈ 108.56 M⊙) and no optical evidence for an AGN. The AGN bolometric luminosity implied by the CL detection is ≈1042 erg s-1, precisely what is predicted from its near-infrared continuum emission based on well-studied AGNs. Our results are consistent with a black hole of mass ≈105 M⊙, in line with expectations based on its stellar mass. This is the first time a near-infrared CL has been detected in a low mass, low metallicity galaxy with no optical evidence for AGN activity, providing confirmation of the utility of infrared CLs in finding AGNs in low mass galaxies when optical diagnostics fail. These observations highlight a powerful avenue of investigation to hunt for low mass black holes in the James Webb Space Telescope era.

Keywords: dwarf galaxies; active galactic nuclei
Cosmic-Ray Tracks in Astrophysical Ices: Modeling with the Geant4-DNA Monte Carlo Toolkit

Cosmic rays are ubiquitous in interstellar environments, and their bombardment of dust-grain ice mantles is a possible driver for the formation of complex, even prebiotic molecules. Yet, critical data that are essential for accurate modeling of this phenomenon, such as the average radii of cosmic-ray tracks in amorphous solid water (ASW) remain unconstrained. It is shown that cosmic-ray tracks in ASW can be approximated as a cylindrical volume with an average radius that is mostly independent of the initial particle energy. Interactions between energetic ions and both low-density amorphous (LDA) and high-density amorphous (HDA) ice targets are simulated using the Geant4-DNA Monte Carlo toolkit, which allows for tracking secondary electrons down to subexcitation energies in the material. We find the peak track-core radii, rcyl, for LDA and HDA ices to be 9.9 nm and 8.4 nm, respectively—somewhat less than double the value of 5 nm often assumed in astrochemical models.

Keywords: cosmic rays; astrochemistry; molecular clouds; molecular physics; laboratory astrophysics
Double Neutron Star Mergers: Are Late-time Radio Signals Overestimated?

The coalescence of binary neutron stars can yield the expulsion of a fast-moving, quasi-isotropic material, which may induce thermal radiation and give rise to kilonova emission. Moreover, the interaction between the ejected material and the surrounding environment generates an external shock, which can result in a long-lasting radio signal that persists for several decades following the merger. In contrast to supernova ejecta, the kilonova ejecta exhibits a relatively lesser mass and higher velocity, and its expansion may ultimately result in the ejecta density becoming so low that the medium particles can freely pass through the ejecta. It would thereby lead to a kind of incomplete sweeping on the interstellar medium. Employing a toy model, our investigation reveals that such incomplete sweeping may considerably diminish the late-time radio radiation power, irrespective of whether the binary neutron star merger results in the formation of a black hole or a neutron star. Our findings thus imply that the previously reported radio upper limits for certain short gamma-ray bursts (GRBs) may not necessarily place stringent constraints on the presence of a long-lived magnetar remnant in these short GRBs.

Keywords: gamma-ray bursts; gravitational waves; neutron stars; interstellar medium
Energetic Particles Associated with a Coronal Mass Ejection Shock Interacting with a Convected Magnetic Structure

On 2020 November 30, Parker Solar Probe (PSP) was crossed by a coronal mass ejection (CME)-driven shock, which we suggest was also crossing a convected, isolated magnetic structure (MS) at about the same time. By analyzing PSP/FIELDS magnetic field measurements, we find that the leading edge of the MS coincided with the crossing of the shock, while its trailing edge, identified as a crossing of a current sheet, overtook PSP about 7 minutes later. Prior to the arrival of the shock, the flux of 30 keV-3 MeV ions and electrons, as measured by PSP/Integrated Science Investigation of the Sun (ISOIS)/Energetic Particle Instrument (EPI-Lo), increased gradually, peaking at the time of the shock passage. However, during the crossing of the MS downstream of the shock, the energetic-ion flux dropped dramatically, before recovering at about the time of the crossing of the trailing edge of the MS. Afterwards, the ion fluxes remained approximately constant within the sheath region of the CME shock. We interpret this depletion of energetic ions within the MS as the result of insufficient time to accelerate particles at the shock within the MS, given that the structure moves along the shock surface owing to its advection with the solar wind. We present results from a quantitative numerical model of the interaction of an idealized MS with a shock, which supports this interpretation.

Keywords: solar energetic particles; interplanetary discontinuities; interplanetary shocks
The DESI One-Percent Survey: A Concise Model for the Galactic Conformity of Emission-line Galaxies

Galactic conformity is the phenomenon whereby a galaxy of a certain physical property is correlated with its neighbors of the same property, implying a possible causal relationship. The observed auto correlations of emission-line galaxies (ELGs) from the highly complete DESI One-Percent Survey exhibit a strong clustering signal on small scales, providing clear evidence for the conformity effect of ELGs. Building upon the original subhalo abundance-matching (SHAM) method developed by Gao et al., we propose a concise conformity model to improve the ELG-halo connection. In this model, the number of satellite ELGs is boosted by a factor of ~5 in the halos whose central galaxies are ELGs. We show that the mean ELG satellite number in such central halos is still smaller than 1 and that the model does not significantly increase the overall satellite fraction. With this model, we can well recover the ELG auto correlations to the smallest scales explored with the current data (i.e., r p > 0.03 Mpc h -1 in real space and at s > 0.3 Mpc h -1 in redshift space), while the cross correlations between luminous red galaxies and ELGs are nearly unchanged. Although our SHAM model has only eight parameters, we further verify that it can accurately describe the ELG clustering in the entire redshift range from z = 0.8 to 1.6. We therefore expect that this method can be used to generate high-quality ELG lightcone mocks for DESI.

Keywords: emission line galaxies; redshift surveys; galaxy dark matter halos; dark energy; observational cosmology
False-positive Self-lensing Events: TESS Observing Asteroid-crossing Events in Disguise

We report observations of four asteroid-crossing events in Transiting Exoplanet Survey Satellite light curves masquerading as self-lensing pulses from binary systems containing main-sequence stars and black hole or neutron-star companions. The observed changes in flux and the durations of the events appear to be consistent with self-lensing pulses provided that (a) the compact-object mass is greater than 2 solar masses, and (b) the transit is not a perfect alignment, i.e., the center of the lens is not passing directly in front of the center of the source. We examine the relationship between the physical characteristics of these asteroid crossings and the derived parameters of our self-lensing model fits to the data sets. As the search for new self-lensing systems continues, we caution observers about such false-positive signals imitating real self-lensing pulses.

Keywords: asteroids; compact binary stars; gravitational microlensing
Electron Dynamics in Strong Turbulence within the Diffusion Region during Magnetopause Magnetic Reconnection

Magnetic reconnection has been proposed to play an important role in energy dissipation in space plasma. The diffusion region is an essential place for generation of energetic electrons. However, the mechanism responsible for the generation of these energetic electrons in such a confined region remains elusive. Here a diffusion region of asymmetric reconnection is observed at the turbulent magnetopause. The diffusion region is a rather structured region where two dynamical filamentary currents at subion scale were observed. Intense electron flow and nonideal electric field inside the filamentary currents induced significant energy dissipation. Concurrently, the electron parallel temperature between the current layers increases from 58 to 80 eV. Direct evidence indicates that the electrons inside the three-dimensional diffusion region are accelerated by the parallel electric fields inside the current layers and are effectively heated by the turbulence between them.

Keywords: space plasmas; magnetic fields