Velocity Dispersion Research Articles

GD-1 Stellar Stream and Cocoon in the DESI Early Data Release

Abstract We present ∼115 new spectroscopically identified members of the GD-1 tidal stream observed with the 5000-fiber Dark Energy Spectroscopic Instrument (DESI). We confirm the existence of a “cocoon,” which is a broad (FWHM ∼ 2 . ° 932 ∼ 460 pc) and kinematically hot (velocity dispersion, σ ∼ 5–8 km s−1) component that surrounds a narrower (FWHM ∼ 0 . ° 353 ∼ 55 pc) and colder [(σ = 3.09 ± 0.76 km s−1)] thin stream component (based on a median per star velocity precision of 2.7 km s−1). The cocoon extends over at least a 30∘ segment of the stream observed by DESI. The thin and cocoon components have similar mean values of [Fe/H]: −2.54 ± 0.04 dex and −2.47 ± 0.06 dex, suggestive of a common origin. The data are consistent with the following scenarios for the origin of the cocoon. The progenitor of the GD-1 stream was an accreted globular cluster (GC) and: (a) the cocoon was produced by pre-accretion tidal stripping of the GC while it was still inside its parent dwarf galaxy; (b) the cocoon comprises debris from the parent dwarf galaxy; (c) an initially thin GC tidal stream was heated by impacts from dark subhalos in the Milky Way; (d) an initially thin GC stream was heated by a massive Sagittarius dwarf galaxy; or a combination of some of these. Future DESI spectroscopy and detailed modeling may enable us to distinguish between these possible origins.

Experimentally-informed numerical investigation of a high-power magnetically shielded Hall thruster operating on xenon and krypton

Abstract The operation of a 20 kW magnetically shielded Hall effect thruster, with xenon and krypton as propellants, is numerically studied with a 2D (axial-radial) axisymmetric hybrid (particle-in-cell/fluid) code. Thrust efficiency with Kr is 2 to 6% lower than with Xe. Kr exhibits smaller propellant utilization, voltage utilization and divergence efficiencies. Contrary to what is observed in previous works, the current efficiency is higher with Kr. Apart from the dissimilar mass and electronic properties of Xe and Kr atoms, the widening of the ionization and acceleration zones with Kr, with respect to the Xe discharge, plays an important role in the Xe-Kr differences in performance. The level of plume velocity dispersion with both propellants is found similar due to two opposing effects: on the one hand, the stronger overlap of the ionization and acceleration zones with Kr leads to a larger dispersion on a per species basis; and, on the other hand, the smaller fraction of doubly-charged ions with Kr reduces the overall dispersion efficiency. Magnetic shielding is effective with both Xe and Kr as propellants, with slightly higher ion impact energy on the walls in the Kr discharge.

Unraveling the Kinematic and Morphological Evolution of the Small Magellanic Cloud

Abstract We model the kinematics of the Small Magellanic Cloud (SMC) by analyzing the proper motions (PMs) from Gaia DR3 of nine different stellar populations, including young main-sequence (MS) stars (<2 Gyr), red giant branch stars, red clump stars, red giants with line-of-sight velocities, and three groups of star clusters. This analysis is carried out using a robust Markov Chain Monte Carlo method, to derive up to seven kinematic parameters. We trace the evolution from a nonrotating flattened elliptical system, as mapped by the old population, to a rotating highly stretched disk structure, as denoted by the young MS stars and clusters (<400 Myr). We estimate that the inclination i (∼58°–82°) decreases and the position angle Θ (∼180°–240°) increases with age. We estimate an asymptotic velocity of ∼49–89 km s−1 with a scale radius of ∼6–9 kpc for the young MS populations, with velocity dispersion of ∼11 km s−1, suggesting a rotation-supported disk structure. Our models estimate a line-of-sight extension of ∼30 kpc, in agreement with observations. We identify four regions of the SMC showing anomalies in the residual PM: the East Anomaly, the Southeast Anomaly (SEA), the South Anomaly, and the West Anomaly. The SEA appears like an infalling feature and is identified for the first time. The tidal imprints observed in the residual PM of the SMC suggest that its evolution is considerably shaped by the recent interaction with the Large Magellanic Cloud.

Exploring Magnetic Fields in Molecular Clouds through Denoising Diffusion Probabilistic Models

Abstract Accurately measuring magnetic field strength in the interstellar medium, including giant molecular clouds, remains a significant challenge. We present a machine learning approach using denoising diffusion probabilistic models (DDPMs) to estimate magnetic field strength from synthetic observables such as column density, orientation angles of the dust continuum polarization vector, and line-of-sight (LOS) nonthermal velocity dispersion. We trained three versions of the DDPM model: the 1-channel DDPM (using only column density), the 2-channel DDPM (incorporating both column density and polarization angles), and the 3-channel DDPM (which combines column density, polarization angles, and LOS nonthermal velocity dispersion). The code and trained model are available on GitHub at https://github.com/xuduo117/DDPM_Bmag. We assessed the models on both synthetic test samples and new simulation data that were outside the training set's distribution. The 3-channel DDPM consistently outperformed both the other DDPM variants and the power-law fitting approach based on column density alone, demonstrating its robustness in handling previously unseen data. Additionally, we compared the performance of the Davis–Chandrasekhar–Fermi (DCF) methods, both classical and modified, to the DDPM predictions. The classical DCF method overestimated the magnetic field strength by approximately an order of magnitude. Although the modified DCF method showed improvement over the classical version, it still fell short of the precision achieved by the 3-channel DDPM.

Spatiotemporal optical vortex reconnections of loop vortices

Abstract Reconnections of spatiotemporal optical vortices have been shown to occur between line vortices. Here, we show that reconnections also occur between spatiotemporal loop vortices in optical waves. As optical loop vortices propagate in a media with spatial diffraction and material group velocity dispersion, unique reconnections occur. The birth and death of loops can occur, with certain loop vortices emerging from or collapsing to a single point while interacting with others. As certain parameters are varied in the model, complex arrangements of loops form in space-time from simple initial fields.

Galaxies Lighting Up: Discovery of Seventy New Turn-on Changing-look Active Galactic Nuclei

“Changing-look active galactic nuclei” (CL AGN) show dramatic, rapid changes in optical/UV continuum and broad-line emission. The majority of CL AGN have been found dimming as “turn-off” CL AGN because most selection methods start from samples of spectroscopically confirmed quasars. We present here a sample of 82 spectroscopically confirmed “turn-on” CL AGN, 70 of which are newly identified. The turn-on CL AGN are selected from spectroscopically classified galaxies with subsequent significant and dramatic variability in both the optical and mid-infrared bands, indicating a mechanism of changing accretion rate of the supermassive black holes (BHs) rather than variable obscuration. Based on their bright state Eddington ratios, turn-on CL AGN are associated with lower accretion rates compared to turn-off CL AGN or typical Sloan Digital Sky Survey quasars with similar redshift and magnitude distributions, even though turn-on CL AGN have lower BH masses. Most turn-on CL AGN reside in host galaxies that follow local relations between the central BH mass and host galaxy properties, such as stellar mass and velocity dispersion. However, their host galaxies have higher mass than normal inactive galaxies, with star formation rates more similar to hosts of Type 2 AGN than to the overall galaxy population.

Dataset of microscale atmospheric flow and pollutant concentration large-eddy simulations for varying mesoscale meteorological forcing in an idealized urban environment.

By 2050, two-thirds of the world's population will live in urban areas under climate change, exacerbating the environmental and public health risks associated with poor air quality and urban heat island effects. Assessing these risks requires the development of microscale meteorological models that quickly and accurately predict wind velocity and pollutant concentration with high resolution, as the heterogeneity of urban environments leads to complex wind patterns and strong pollutant concentration gradients. Computational Fluid Dynamics (CFD) has emerged as a powerful tool to address this challenge by providing obstacle-resolved flow and dispersion predictions. However, CFD models are very expensive and require intensive computing resources, which can hinder their systematic use in practical engineering applications. They are also subject to significant uncertainties, particularly those arising from the mesoscale meteorological forcing and the internal variability of the atmospheric boundary layer, some of which are aleatory and thereby irreducible. Given these issues, the construction of CFD datasets that account for uncertainty would be an interesting avenue of research for microscale atmospheric science. In this context, we present the PPMLES (Perturbed-Parameter ensemble of MUST Large-Eddy Simulations) dataset, which consists of 200 large-eddy simulations (LES) characterizing the complex interactions between the turbulent airflow, the tracer dispersion, and an idealized urban environment. These simulations reproduce the canonical MUST dispersion field campaign while perturbing the model's mesoscale meteorological forcing parameters. PPMLES includes time series at human height within the built environment to track wind velocity and pollutant release and dispersion over time. PPMLES also includes complete 3-D fields of first- and second-order temporal statistics of the wind velocity and pollutant concentration, with a sub-metric resolution. The uncertainty of the fields induced by the internal variability of the atmospheric boundary layer is also provided. The computation of PPMLES required significant resources, consuming 6 million CPU core hours, equivalent to the emission of approximately 10 tCO2eq of greenhouse gases. This significant computational effort and associated carbon footprint motivates the sharing of the data generated. The added value of the PPMLES dataset is twofold. First, the perturbed-parameter ensemble of LES enables to quantify and understand the effects of the mesoscale meteorological forcing and the internal variability of the atmospheric boundary layer, which has been identified as a major challenge in predicting atmospheric flow and pollutant dispersion in urban environments. Secondly, PPMLES reference data can be used to benchmark models of different levels of complexity, and to extract key information about the physical processes involved to inform more operational modeling approaches, for example through learning surrogate models.

The role of shocks and the velocity gradient in the relative orientation of the magnetic field and dense gas clouds

Magnetic fields are known to exhibit different relative orientations with density structures in different density regimes. However, the physical mechanisms behind these relative orientations remain unclear. We investigate the role of the flow features on the relative orientation between the magnetic field and cold neutral medium (CNM) clouds, as well as that of molecular clouds (MCs) as a corollary. We performed three- and two-dimensional (3D+2D) magnetohydrodynamic (MHD) simulations of warm gas streams in the thermally bistable atomic interstellar medium (ISM) colliding with velocities of the order of the velocity dispersion in the ISM to form CNM clouds. In these simulations, we followed the evolution of magnetic field lines to identify and elucidate the physical processes behind their evolution. The collision produces a fast MHD shock, as well as a condensation front roughly one cooling length behind it, on each side of the collision front. A compressive, decelerating velocity field arises between the shock and the condensation fronts, and a cold dense layer forms behind the condensation front. The magnetic field lines, initially oriented parallel to the flow direction, are perturbed by the fast MHD shock, across which the magnetic field fluctuations parallel to the shock front are amplified. The downstream perturbations of the magnetic field lines are further amplified by the compressive downstream velocity gradient between the shock and the condensation front caused by the settlement of the gas onto the dense layer. This process causes the magnetic field to become progressively aligned with the dense layer, leading to the formation of a shear flow around it, due to the field's backreaction on the flow. By extension, we suggest that a tidal stretching velocity gradient, such as that produced in gas infalling into a self-gravitating structure, must straighten the field lines along the accretion flow, orienting them perpendicular to the density structures. We also find that the initially super-Alfv'enic upstream flow becomes trans-Alfv'enic between the shock and the condensation front, and then sub-Alfv'enic inside the condensation. Finally, in 2D simulations with a curved collision front, the presence of the magnetic field inhibits the generation of turbulence by the shear around the dense layer. Our results provide a feasible physical mechanism for orienting the magnetic field parallel to CNM clouds through the action of fast MHD shocks and compressive velocity fields.

A new method for determining the onset times of solar energetic particles and their uncertainties: Poisson-CUSUM bootstrap hybrid method

Solar energetic particle (SEP) events are a type of space weather phenomena in which highly energetic charged particles are released from the Sun into interplanetary space by violent and eruptive phenomena, such as solar flares and coronal mass ejections. In order to assess the origin of SEPs, an accurate timing of their arrival at spacecraft is of utmost importance. Several methods for determining the starting time of an SEP event at an observer exist, but the uncertainty of this starting time is not assessed in a systematic way by the vast majority of studies. Employing a newly developed hybrid method of Poisson-CUSUM combined with bootstrapping, we show that the uncertainty related to the onset of an event in any particular energy range is often more than the mere time resolution of the measuring apparatus, and furthermore, it is not necessarily symmetric with respect to the past and future of the determined onset. In addition, we provide a software tool to the scientific community that applies the presented method and automates the determination of SEP event onset times and their related uncertainties, and it finally allows one to easily perform a velocity dispersion analysis. By applying the Poisson-CUSUM method coupled with statistical bootstrapping to SEP event observations, we demonstrate the effectiveness of the method on synthetic and real data, and we compare them to an analysis conducted using a classical approach in which the uncertainty is assumed based on the time resolution of the data. In the example case, the inferred SEP path length and injection time related to the event, acquired by the velocity dispersion analysis, differ from what is obtained without properly assessing the uncertainty related to the onset times in varying energies. We also present the software package, PyOnset, that automates many steps of the method along with providing powerful data-visualization methods and analysis tools. We release the code to the scientific community as open-source software.

A compact group lens modeled with GIGA-Lens: Enhanced inference for complex systems

Context. In the era of large-scale astronomical surveys, the fast modeling of strong lens systems has become increasingly vital. While significant progress has been made for galaxy-scale lenses, the development of automated methods for modeling larger systems, such as groups and clusters, is not as extensive. Aims. Our study aims to extend the capabilities of the GIGA-Lens code, enhancing its efficiency in modeling multi-galaxy strong lens systems. We focus on demonstrating the potential of GPU-accelerated Bayesian inference in handling complex lensing scenarios with a high number of free parameters. Methods. We employed an improved inference approach that combines image position and pixelated data with an annealing sampling technique to obtain the posterior distribution of complex models. This method allowed us to overcome the challenges of limited prior information, a high number of parameters, and memory usage. We validated our process through the analysis of the compact group lens system DES J0248-3955 and we present the relevant VLT/X-shooter spectra. Results. We measured a redshift of z = 0.69 ± 0.04 for the group, and z = 1.2722 ± 0.0005 for one of the extended arcs. Our enhanced method successfully constrained a lens model with 29 free parameters and lax priors in a remarkably short time. The mass of the lens is well described by a single dark-matter halo with a velocity dispersion of σv = (690 ± 30) km s−1. The model predicts the presence of a second source at the same redshift and a third source at approximately z ~ 2.7. Conclusions. Our study demonstrates the effectiveness of our lens modeling technique for dealing with a complex system in a short time using ground-based data. This presents a considerable prospect within the context of large surveys, such as LSST, in the future.

Global and Local Infall in the ASHES Sample (GLASHES). I. Pilot Study in G337.541

Abstract Recent high-angular-resolution observations indicate the need for core growth to form high-mass stars. To understand the gas dynamics at the core scale in the very early evolutionary stages before being severely affected by feedback, we have conducted Atacama Large Millimeter/submillimeter Array (ALMA) observations toward a 70 μm dark massive clump, G337.541-00.082 as part of the Global and Local infall in the ASHES sample (GLASHES) program. Using dense gas tracers such as N2H+ (J = 1–0) and HNC (J = 3–2), we find signs of infall from the position–velocity diagram and more directly from the blue asymmetry profile in addition to the clump-scale velocity gradient. We estimate infall velocities from intermediate and low-mass cores to be 0.28–1.45 km s−1, and infall rates to be on the order of 10−4–10−3 M ⊙ yr−1, both are higher than those measured in low-mass star-forming regions by more than a factor of 5 and an order of magnitude, respectively. We find a strong correlation between the infall velocity with the nonthermal velocity dispersion, suggesting that infall may contribute significantly to the observed line width. Consistent with clump-fed scenarios, we show that the mass infall rate is larger for larger core masses and shorter distances to the clump center. Such high infall rates in cores embedded in IRDCs can be considered as strong signs of core growth, allowing high-mass star formation from intermediate-mass cores that would not initially form high-mass stars at their current mass.

Unveiling the kinematics of a central region in the triple-AGN host NGC 7733-7734 interacting group

We present a detailed study of the interacting triple active galactic nuclear system, NGC 7733-34, focusing on stellar kinematics, ionised gas characteristics, and star formation within the central region and stellar bars of both galaxies. We performed a comprehensive analysis using archival data from MUSE, HST/ACS, and DECaLS, complemented with observations from UVIT and IRSF. We identified a disc-like bulge in both NGC 7733 and NGC 7734 through a 2D decomposition. A central nuclear structure, with a semi-major axis of ∼1.113 kpc, was detected in NGC 7733 via a photometric and kinematic analysis, confirmed by the strong anti-correlation between V/σ and h_3, indicative of circular orbits in the centre. NGC 7734 lacks a distinct nuclear structure. The presence of a disc-like bulge results in an anti-correlation between V/σ and h_3, along with diffuse light. However, it does show higher central velocity dispersion, possibly attributed to an interaction with a smaller clump, which is likely a fourth galaxy within the system. Both galaxies demonstrate ongoing star formation, evidenced by FUV and Hα observations. NGC 7734 shows recent star formation along its bar, while NGC 7733 experiences bar quenching. The star formation rate (SFR) analysis of NGC 7734 reveals that the bar region's SFR dominates the galaxy's overall SFR. Conversely, in NGC 7733, the lack of star formation along the bar and the presence of a Seyfert 2 active galactic nucleus (AGN) at the galaxy centre suggest the possibility of link among both. However, this would not affect the galaxy's overall star formation. Our findings provide valuable insights into the stellar and gas kinematics, star formation processes, and AGN feedback mechanisms in interacting galaxies hosting stellar bars.

Causal Discovery in Astrophysics: Unraveling Supermassive Black Hole and Galaxy Coevolution

Abstract Correlation does not imply causation, but patterns of statistical association between variables can be exploited to infer a causal structure (even with purely observational data) with the burgeoning field of causal discovery. As a purely observational science, astrophysics has much to gain by exploiting these new methods. The supermassive black hole (SMBH)–galaxy interaction has long been constrained by observed scaling relations, which is low-scatter correlations between variables such as SMBH mass and the central velocity dispersion of stars in a host galaxy's bulge. This study, using advanced causal discovery techniques and an up-to-date data set, reveals a causal link between galaxy properties and dynamically measured SMBH masses. We apply a score-based Bayesian framework to compute the exact conditional probabilities of every causal structure that could possibly describe our galaxy sample. With the exact posterior distribution, we determine the most likely causal structures and notice a probable causal reversal when separating galaxies by morphology. In elliptical galaxies, bulge properties (built from major mergers) tend to influence SMBH growth, while, in spiral galaxies, SMBHs are seen to affect host galaxy properties, potentially through feedback in gas-rich environments. For spiral galaxies, SMBHs progressively quench star formation, whereas, in elliptical galaxies, quenching is complete, and the causal connection has reversed. Our findings support theoretical models of hierarchical assembly of galaxies and active galactic nuclei feedback regulating galaxy evolution. Our study suggests the potentiality for further exploration of causal links in astrophysical and cosmological scaling relations, as well as any other observational science.

X-ray surface brightness fluctuations in smooth galaxy cluster atmospheres

Abstract We measure surface brightness fluctuations in Chandra X-ray images of the cores of the galaxy clusters Abell 2029, Abell 2151, Abell 2107, RBS0533, and RBS0540. Their relatively structureless X-ray atmospheres exhibit the thermodynamic properties of cool cores including short central cooling times and low entropy. However, unlike typical cool-core clusters, molecular gas, star formation, and bubbles associated with radio jets are faint or absent near their central galaxies. Four clusters show typical gas density fluctuation amplitudes of ∼ 10percnt on the scales probed, apart from RBS0540, which exhibits lower amplitudes, suggesting that its gas is mildly disturbed. Under the assumption that gas density fluctuations are indicative of random gas velocities, we estimate scale-dependent velocity amplitudes of gas motions across all studied clusters, which range from 100 $\rm km~s^{-1}$ to 200 $\rm km~s^{-1}$ in Abell 2029, Abell 2151, and Abell 2107. These velocity estimates are comparable to the atmospheric velocity dispersion in the Perseus cluster measured by the Hitomi X-ray Observatory. The turbulent heating rates implied by our measurements are of the same order as the radiative cooling rates. Our results suggest that atmospheric sloshing and perhaps turbulent motion may aid radio jets in stabilizing atmospheric cooling.

Segue 2 Recently Collided with the Cetus-Palca Stream: New Opportunities to Constrain Dark Matter in an Ultra-faint Dwarf

Abstract 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 ( O ( 1 ) kpc) than are probed by its stars ( O ( 10 ) 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 ϕ 1 > 20° than at ϕ 1 < 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.

Discovery and Spectroscopic Confirmation of Aquarius III: A Low-mass Milky Way Satellite Galaxy

We present the discovery of Aquarius III, an ultra-faint Milky Way satellite galaxy identified in the second data release of the DECam Local Volume Exploration survey. Based on deeper follow-up imaging with DECam, we find that Aquarius III is a low-luminosity ( MV=−2.5−0.5+0.3; LV=850−260+380L⊙ ), extended ( r1/2=41−8+9 pc) stellar system located in the outer halo (D ⊙ = 85 ± 4 kpc). From medium-resolution Keck/DEIMOS spectroscopy, we identify 11 member stars and measure a mean heliocentric radial velocity of vsys=−13.1−0.9+1.0kms−1 for the system and place an upper limit of σ v < 3.5 km s−1 (σ v < 1.6 km s−1) on its velocity dispersion at the 95% (68%) credible level. Based on calcium-triplet metallicities of the six brightest red giant members, we find that Aquarius III is very metal-poor ([Fe/H]= − 2.61 ± 0.21) with a statistically significant metallicity spread ( σ[Fe/H]=0.46−0.14+0.26 dex). We interpret this metallicity spread as strong evidence that the system is a dwarf galaxy as opposed to a star cluster. Combining our velocity measurement with Gaia proper motions, we find that Aquarius III is currently situated near its orbital pericenter in the outer halo (r peri = 78 ± 7 kpc) and that it is plausibly on first infall onto the Milky Way. This orbital history likely precludes significant tidal disruption from the Galactic disk, notably unlike other satellites with comparably low velocity dispersion limits in the literature. Thus, if further velocity measurements confirm that its velocity dispersion is truly below σ v ≲ 2 km s−1, Aquarius III may serve as a useful laboratory for probing galaxy formation physics in low-mass halos.

Direct N-body simulations of NGC 6397 and its tidal tails

ABSTRACT We have performed a series of direct N-body simulations that study the evolution of the Galactic globular cluster NGC 6397 under the combined influence of two-body relaxation, stellar evolution, and the Milky Way’s tidal field. Our simulations follow the evolution of the cluster over the last several Gyr up to its present-day position in the Milky Way in order to allow us to derive present-day cluster parameters and the distribution of its extra-tidal stars. We have also determined a new density profile of NGC 6397 by selecting stars from Gaia Data Release 3 (DR3) using Gaia DR3 proper motions, parallaxes, and photometry to discriminate cluster members from non-members. This allows us to derive the surface density profile of NGC 6397 and the location of its tidal tails up to $10^{\circ }$ of the cluster centre, well beyond the tidal radius of NGC 6397. Our results show that the current state of NGC 6397 in terms of surface density, velocity dispersion profile, and stellar mass function can be matched by a cluster model evolving from a standard initial mass function and does not require an additional central cluster of dark remnants. We also find good agreement in the location and absolute number of the extra-tidal stars between our simulations and the observations, making it unlikely that NGC 6397 is surrounded by a dark matter halo.

Toward the fabric of the Milky Way. I. The density of disk streams from a local 250^3 pc^3 volume

We studied 12 disk streams found in a 250^3 pc^3 volume in the solar neighborhood, which we define as coeval and comoving stellar structures with aspect ratios greater than 3:1. Using Gaia Data Release 3 data and the advanced clustering algorithms SigMA and Uncover we identified and characterized these streams beyond the search volume, doubling, on average, their known populations. We estimate the number density of disk streams to be ≈ 820 objects,/,kpc^3 (for $|Z| &lt; 100$,pc), or surface densities of ≈ 160 objects,/,kpc^2. These estimates surpass N-body estimates by one to two orders of magnitude and challenge the prevailing understanding of their destruction mechanisms. Our analysis reveals that these 12 disk streams are dynamically cold with 3D velocity dispersions between 2 and 5,km,s^-1, exhibit narrow sequences in the Hertzsprung-Russell diagram, and are highly elongated with average aspect ratios of 7:1, extending up to several hundred parsecs. We find evidence suggesting that one of the disk streams, currently embedded in the Scorpius-Centaurus association, is experiencing disruption, likely due to the primordial gas mass of the association.

Dispersion and attenuation characteristics of Lamb waves in multilayered piezoelectric semiconductor plates with imperfect interfaces

Lamb waves in piezoelectric semiconductor multilayered plates with imperfect interfaces are investigated using the generalized linear spring model. The Legendre polynomial method with analytical integration, accounting for imperfect interfaces, is used to derive field quantities for each layer, corresponding to displacement and traction at the plate interfaces. The complex wave partial differential equations are converted into a generalized eigenvalue problem. The study of Lamb waves in ZnO/GaN bilayer piezoelectric semiconductor plates with imperfect interfaces elucidates the influence of carrier concentration, interface coefficients, plate thickness, and bias electric fields on phase velocity dispersion and attenuation curves.

The Age–Velocity Dispersion Relations of the Galactic Disk as Revealed by the LAMOST-Gaia Red Clump Stars

Abstract Using nearly 230,000 red clump stars selected from LAMOST and Gaia, we conduct a comprehensive analysis of the stellar age–velocity dispersion relations (AVRs) for various disk populations, within 5.0 ≤ R ≤ 15.0 kpc and ∣Z∣ ≤ 3.0 kpc. The AVRs of the whole red clump sample stars are accurately described as σ v = σ v,0 (τ + 0.1) β v , with β R , β ϕ , and β Z displaying a global exponential decreasing trend with R, which may point to the difference in spatial distributions of various disk heating mechanisms. The measurements of β–R for various disks suggest that the thin disk exhibits a radial dependence, with a global exponential decreasing trend in β R –R and β Z –R, while β ϕ remains a nearly constant value (around 0.20 ∼ 0.25) within 8.5 ≤ R ≤ 11.5 kpc. The thick disk displays a global increasing trend in β R –R, β ϕ –R, and β Z –R. These results indicate that the thin disk stars are likely heated by long-term heating from giant molecular clouds and spiral arms, while thick disk stars are likely heated by some violent heating process from merger and accretion, and/or formed by the inside-out and upside-down star formation scenarios, and/or born in the chaotic mergers of gas-rich systems and/or turbulent interstellar medium. Our results also suggest that the disk perturbation by a recent minor merger from Sagittarius may have occurred within 3.0 Gyr.