arXiv subjects

Guang-Xing Li

Publications and source records attributed to Guang-Xing Li.

At least 37 records · Page 2Linked to original sources

arXiv2025

Volume Density Mapper: 3D Density Reconstruction Algorithm for Molecular Clouds

The interstellar medium (ISM) exhibits complex, multi-scale structures that are challenging to study due to their projection into two-dimensional (2D) column density maps. We present the Volume Density Mapper, a novel algorithm based on constrained diffusion to reconstruct three-dimensional (3D) density distributions of molecular clouds from 2D observations. This method decomposes the column density into multi-scale components, reconstructing a 3D density field that preserves key physical properties such as mean density, maximum density, and standard deviation along the line of sight. Validated against numerical simulations (FLASH and ENZO), the algorithm achieves high accuracy, with mean density estimates within 0.1 dex and dispersions of 0.2 to 0.3 dex across varied cloud structures. The reconstructed 3D density fields enable the derivation of critical parameters, including volume density, cloud thickness, and density probability distribution functions, offering insights into star formation and ISM evolution. The versatility of the method is demonstrated by applying diverse systems from galaxies (NGC 628) to protostellar disks. The code is available at https://github.com/gxli/volume-density-mapper.

astro-ph.IM↗

arXiv2025

Magnetic, Kinetic, and Transition Regime: Spatially-Segregated Structure of Compressive MHD Turbulence

Turbulence is a complex physical process prevalent in modern physics, particularly in ionized environments like interstellar gas, where magnetic fields play a dynamic role. However, the precise influence of magnetic fields in such settings remains unclear. We employ the Alfvén Mach number, ${M}_{\mathrm{A}} = \sqrt{E_{\mathrm{k}}/E_{\mathrm{B}}}$, to gauge the magnetic field's significance relative to turbulent motion, uncovering diverse interaction patterns. In the low-${M}_{\mathrm{A}}$ magnetic regime, the field is force-free, yet gas motion does not align with it. At intermediate ${M}_{\mathrm{A}}$ (magnetic-kinetic transition regime), velocity and magnetic fields show peak alignment, likely due to rapid relaxation. In the high-${M}_{\mathrm{A}}$ kinetic regime, both fields are irregular and unaligned. These regimes find observational counterparts in interstellar gas, highlighting the multifaceted nature of MHD turbulence and aiding future astrophysical interpretations.

astro-ph.GA↗

arXiv2025

Mapping correlations and coherence: adjacency-based approach to data visualization and regularity discovery

The development of science has been transforming man's view towards nature for centuries. Observing structures and patterns in an effective approach to discover regularities from data is a key step toward theory-building. With increasingly complex data being obtained, revealing regularities systematically has become a challenge. Correlation is a most commonly-used and effective approach to describe regularities in data, yet for complex patterns, spatial inhomogeneity and complexity can often undermine the correlations. We present an algorithm to derive maps representing the type and degree of correlations, by taking the two-fold symmetry of the correlation vector into full account using the Stokes parameter. The method allows for a spatially resolved view of the nature and strength of correlations between physical quantities. In the correlation view, a region can often be separated into different subregions with different types of correlations. Subregions correspond to physical regimes for physical systems, or climate zones for climate maps. The simplicity of the method makes it widely applicable to a variety of data, where the correlation-based approach makes the map particularly useful in revealing regularities in physical systems and alike. As a new and efficient approach to represent data, the method should facilitate the development of new computational approaches to regularity discovery.

physics.comp-ph↗

arXiv2025

Revealing hidden correlations from complex spatial distributions: Adjacent Correlation Analysis

Physics has been transforming our view of nature for centuries. While combining physical knowledge with computational approaches has enabled detailed modeling of physical systems' evolution, understanding the emergence of patterns and structures remains limited. Correlations between quantities are the most reliable approach to describe relationships between different variables. However, for complex patterns, directly searching for correlations is often impractical, as complexity and spatial inhomogeneity can obscure correlations. We discovered that the key is to search for correlations in local regions and developed a new method, adjacent correlation analysis, to extract such correlations and represent them in phase space. When multiple observations are available, a useful way to study a system is to analyze distributions in phase space using the Probability Density Function (PDF). Adjacent correlation analysis evaluates vectors representing local correlations, which can be overlaid on the PDF plot to form the adjacent correlation plot. These correlation vectors often exhibit remarkably regular patterns and may lead to the discovery of new laws. The vectors we derive are equivalent to the vector field in dynamical systems on the attracting manifold. By efficiently representing spatial patterns as correlation vectors in phase space, our approach opens avenues for classification, prediction, parameter fitting, and forecasting.

physics.comp-ph↗

arXiv2025

Dispersion relation for the linear theory of relativistic Rayleigh Taylor instability in magnetized medium revisited

The Rayleigh-Taylor instability (RTI) arises at the interface between two fluids of different densities, notably when a heavier fluid lies above a lighter one in an effective gravitational field. In astrophysical systems with high velocities, relativistic corrections are necessary. We investigate the linear theory of relativistic Rayleigh-Taylor instability (R-RTI) in a magnetized medium, where fluids can move parallel to the interface at relativistic velocities. We chose an "intermediate frame" where fluids on each side of the interface move in opposite directions with identical Lorentz factors gamma_*. This symmetry facilitates analytical derivations and the study of relativistic effects on the instability's dynamics. We derive the correct version of the R-RTI. We find that the instability is activated when the Atwood number A = (rho1 h1 - rho2 h2) / (rho1 h1 + rho2 h2) > 0, where rho1 and rho2 are densities measured in the rest frame of the fluids, and that this criterion does not contain relativistic corrections. The relativistic effect is mostly contained in the Lorentz transformation of the gravitational acceleration g' = g / (gamma_*)^2, which, combined with time dilation, leads to a much slower growth of instability (omega' = omega0 / gamma_*), and a slightly elongated length of the unstable patch, due to weaker g in the intermediate frame. Taking time dilation into account, when viewed in the rest frame of the medium, we expect the instability to grow at a much reduced rate. The analytical results should guide further explorations of instability in systems such as microquasars (muQSOs), active galactic nuclei (AGNs), gamma-ray bursts (GRBs), and radio pulsars (PSRs), where the apparent stability of the jet can be attributed to either the intrinsic stability (e.g. the Atwood number) or the much prolonged duration through which R-RTI can grow.

astro-ph.HE↗

arXiv2025

Super-Jeans fragmentation in massive star-forming regions revealed by triangulation analysis

Understanding the fragmentation of the gas cloud and the formation of massive stars remains one of the most challenging questions of modern astrophysical research. Either the gas fragments in a Jeans-like fashion, after which the fragments grow through accretion, or the fragmentation length is larger than the Jeans length from the start. Despite significant observational efforts, a consensus has not been reached. The key is to infer the initial density distribution upon which gravitational fragmentation occurs. Since cores are the products of the fragmentation process, the distances between adjacent cores serve as a scale indicator. Based on this observation, we propose a Delaunay triangulation-based approach to infer the density structure before the fragmentation and establish the link between density distribution and gas fragmentation length. We find that at low density, the fragmentation is Jeans-like, and at high densities, the core separations are larger than the prediction of the Jeans fragmentation. This super-Jeans fragmentation is a key step toward the formation of massive stars.

astro-ph.GA↗

arXiv2025

Shear-gravity transition determines the steep velocity dispersion-size relation in molecular clouds: confronting analytical formula with observations

The velocity dispersion-size relation ($σ_{\rm v}\sim R^β$) is a crucial indicator of the dynamic properties of interstellar gas, where the slope is considered as $β\sim 0.5$. Recent observations reveal a steep velocity dispersion-size relation with the slope $β> 0.6$, which cannot be explained by a single mechanism with only gravity ($β\sim0.5$) or shear ($β\sim 1$). We present a two-component model $σ_{\rm v_{total}} = λ_1 σ_{\rm v_{g}} + λ_2 σ_{\rm v_{shear}} = A[(GM/R)^{\frac{1}{2}} + f(R/t_{\rm shear})]$ to explain the steep velocity dispersion-size relation for clouds larger than several parsecs in observations from e.g. Miville-Deschênes et al. (2017), Zhou et al. (2022) and Sun et al. (2024). We find that, above several parsecs, the velocity dispersion of small clouds is mainly caused by self-gravity, while large clouds are primarily affected by shear, and these two regimes are linked by a gradual transition with a transition scale $\sim100$ pc -- the scale height of the Galactic molecular gas disk. The variation of cloud velocity dispersion-size relation with the Galactocentric distance results from the variation of both cloud internal density structure and Galactic shear rate. Our two-component model captures how the dynamics of the molecular gas can be affected by both internal and external factors, and we expect it to be applied to data from galaxies with different physical conditions to reveal the physics.

astro-ph.GA↗

arXiv2025

Evolution of molecular clouds on galaxy-cloud scale revealed by gravitational network analysis : High-mass clouds may deplete nearby gas via accretion or merging

Observations show that molecular gas in spiral galaxies is organized into a network of interconnected systems through the gravitational coupling of multi-scale hub-filament structures. Building on this picture, we model molecular gas in the galaxy NGC 628 as a gravitational network, where molecular clouds are represented as nodes. Through analyzing this network, we can characterize both the gravitational interactions and the physical properties of the clouds using geometry-based network metrics. A strong correlation is observed between the geometric and physical properties of the nodes (clouds). High-mass clouds tend to exhibit less clustering and greater average separations, suggesting that they generally have fewer neighbors. During their formation and evolution, high-mass clouds may deplete nearby gas via accretion or merging, leading to more isolated characteristics within the network. This aligns with observations showing a decrease in the virial ratio of molecular clouds as their mass increases. For clouds at different evolutionary stages, less evolved clouds with lower mass are typically found in tighter gravitational subnetworks, with closer proximity to neighboring clouds. As a result, they are more prone to accretion or merging during evolution.

astro-ph.GA↗

arXiv2024

Assessing the accuracy of the star formation rate measurements by direct star count in molecular clouds

Star formation estimates based on the counting of YSOs is commonly applied to nearby star-forming regions in the Galaxy. With this method, the SFRs are measured using the counts of YSOs in a particular protostellar Class, a typical protostellar mass, and the lifetime associated with this Class. However, the assumptions underlying the validity of the method such as that of a constant star formation history (SFH) and whether the method is valid for all protostellar Classes has never been fully tested. In this work, we use Monte Carlo models to test the validity of the method. We build synthetic clusters in which stars form at times that are randomly drawn from a specified SFH. The latter is either constant or time-dependent with a burst like behavior. The masses of the protostars are randomly drawn from an IMF which can be either similar to that of the Milky Way field or be variable . For each star in every cluster, the lifetimes associated with the different protostellar classes are also randomly drawn from Gaussian distribution functions centered around their most likely value as suggested by the observations. We find that only the SFR derived using the Class 0 population can reproduce the true SFR at all epochs, and this is true irrespective of the shape of the SFH. For a constant SFH, the SFR derived using the more evolved populations of protostars (Classes I, F, II, and III) reproduce the real SFR only at later epochs which correspond to epochs at which their numbers have reached a steady state. For a time-dependent burst-like SFH, all SFR estimates based on the number counts of the evolved populations fail to reproduce the true SFR. We also show how the offsets between Class I and Class II based SFRs and the true SFR plotted as a function of the number ratios of Class I and Class II versus Class III YSOs can be used in order to constrain the SFH of observed molecular clouds.

astro-ph.GA↗

arXiv2024

Slope of Magnetic Field-Density Relation as An Indicator of Magnetic Dominance

The electromagnetic field is a fundamental force in nature that regulates the formation of stars in the universe. Despite decades of efforts, a reliable assessment of the importance of the magnetic fields in star formation relations remains missing. In star-formation research, our acknowledgment of the importance of magnetic field is best summarized by the Cruther+ 2010 B-rho relation. The relation is either interpreted as proof of the importance of a magnetic field in the collapse, or the result of self-similar collapse where the role of the magnetic is secondary to gravity. Using simulations, we find a fundamental relation, ${\cal M}_{\rm A}$-k$_{B-ρ}$(slope of $B-ρ$ relation) relation. This fundamental B-$ρ$-slope relation enables one to measure the Alfvénic Mach number, a direct indicator of the importance of the magnetic field, using the distribution of data in the B-$ρ$ plane. It allows us to drive the following empirical $B-ρ$ relation \begin{equation} \frac{B}{B_c} = {\rm exp}\left(\left(\fracγ{\cal K}\right)^{-1}\left( \fracρ{ρ_c}\right)^\fracγ{\cal K}\right)\nonumber, \end{equation} which offers an excellent fit to the Cruther et al. data, where we assume ${\cal M}_{\rm A}-ρ$ relation. The foundational ${\cal M}_{\rm A}-{\rm k}_{B-ρ}$ relation provides an independent way to measure the importance of magnetic field against the kinematic motion using multiple magnetic field measurements. Our approach offers a new interpretation of Cruther+2010, where a gradual decrease in the importance of B at higher densities is implied.

astro-ph.GA↗

arXiv2024

Little time for oscillation: Fast disruption of the Radcliffe Wave by Galactic motions

The Radcliffe wave \cite{2020Natur.578..237A} is a 2.7 kpc long, 100 pc wide-like structure in the Galactic disk with a wave-like velocity structure \cite{2022MNRAS.517L.102L,2024arXiv240212596K}. A referent Nature paper \cite{2024arXiv240212596K} treated the Wave as a solid body in the disk plane, modeled its oscillation along the vertical direction, and derived the local Galactic mass distribution from the oscillation pattern. In reality, Galactic shear can stretch gas through differential rotation, whereas gas clouds experience epicyclic motions. We simulate the 3D evolution of the local interstellar gas and find shear and encyclic motion stretches the Radcliffe wave to almost twice its current length at the timescale of 45 Myr, within which only half a cycle of the proposed vertical oscillation occurs. The simulation also reveals the formation of new filaments and filament-filament mergers. Treating the Radcliffe wave as a solid body in the Galactic disk and an oscillating structure in the vertical direction is thus an oversimplification. Our data-driven simulation reveals the 3D evolution of the local interstellar gas with several processes at play, strengthening the role of the Solar Neighborhood as a unique test ground for theories of interstellar gas evolution.

astro-ph.GA↗

arXiv2024

Inverted EEMD: a robust method to identify narrow absorption features form spectral data and cubes

Extracting information from complex data is a challenge shared by multiple frontiers of modern astrophysical research. Among those, analyzing spectra cubes, where the emission is mapped in the position-position-velocity space is a difficult task given the vast amount of information contained within. The cubes often contain a superposition of emissions and absorptions, where extracting absorption signatures is often necessary. One example is the extraction of narrow absorption structures in HI 21 cm emission spectra. These HI self-absorption (HISA) clouds trace the cold HI gas in interstellar space. We introduce an automatic and robust method called the \emph{inverted EEMD} method to extract narrow features from spectral cubes. Our method is based on the EEMD method, an established method to decompose 1d signals. The method is robust and parameter-free, making it useful in analyzing spectral cubes containing localized absorption signals of different types. The inverted-EEMD method is suitable for the analysis of spectral cubes where it can produce a cube containing the absorption signal and one containing the unabsorbed signal, where cold clouds can be identified as coherent regions in the absorption map. A Python implementation of the method is available at \url{https://github.com/zhenzhen-research/inverted_eemd_map}.

astro-ph.IM↗

arXiv2024

The Correlation Between Dust and Gas Contents in Molecular Clouds

Molecular clouds are regions of dense gas and dust in space where new stars and planets are born. There is a strong correlation between the distribution of dust and molecular gas in molecular clouds. The present work focuses on the three-dimensional morphological comparisons between dust and gas within 567 molecular clouds identified in a previously published catalog. We confirm a sample of 112 molecular clouds, where the cloud morphology based on CO observations and dust observations displays good overall consistency. There are up to 334 molecular clouds whose dust distribution might be related to the distribution of gas. We are unable to find gas structures that correlate with the shape of the dust distribution in 24 molecular clouds. For the 112 molecular clouds where the dust distribution correlates very well with the distribution of gas, we use CO observational data to measure the physical properties of these molecular clouds and compare them with the results derived from dust, exploring the correlation between gas and dust in the molecular clouds. We found that the gas and dust in the molecular clouds have a fairly good linear relationship, with a gas-to-dust ratio of $\mathrm{GDR}=(2.80_{-0.34}^{+0.37})\times10^{21}\mathrm{\,cm^{-2}\, mag^{-1}}$. The ratio varies considerably among different molecular clouds. We measured the scale height of dust-CO clouds exhibiting strong correlations, finding $h_{Z} = 43.3_{-3.5}^{+4.0}\mathrm{\,pc}$.

astro-ph.GA↗

arXiv2024

Morphology of molecular clouds at kiloparsec scale in the Milky Way: Shear-induced alignment and vertical confinement

The shape of the cold interstellar molecular gas is determined by several processes, including self-gravity, tidal force, turbulence, magnetic field, and galactic shear. Based on the 3D dust extinction map derived by Vergely et al., we identify a sample of 550 molecular clouds (MCs) within $3\hspace{0.2em}\rm kpc$ of the solar vicinity in the Galactic disk. Our sample contains clouds whose size ranges from pc to kiloparsec, which enables us to study the effect of Galactic-scale processes, such as shear, on cloud evolution. We find that our sample clouds follow a power-law mass-size relation of $M\propto 32.00\hspace{0.2em}{R_{\rm{max}}}^{1.77}$, $M\propto 20.59\hspace{0.2em}{R_S}^{2.04}$ and $M\propto 14.41\hspace{0.2em}{R_V}^{2.29}$, where $R_{\rm{max}}$ is the major axis-based cloud radius, $R_S$ is the area-based radius, and $R_V$ is the volume-based radius, respectively. These clouds have a mean constant surface density of $\sim 7 \hspace{0.2em} \rm {M_{\odot}pc^{-2}}$, and follow a volume density-size relation of $ρ\propto 2.60\hspace{0.2em}{R_{\rm{max}}}^{-0.55}$. As cloud size increases, their shapes gradually transition from ellipsoidal to disk-like to bar-like structures. Large clouds tend to have a pitch angle of $28^{\circ} - 45^{\circ}$, where the angle is measured concerning the Galactic tangential direction. These giant clouds also tend to stay parallel to the Galactic disk plane and are confined within the Galactic molecular gas disk. Our results show that large molecular clouds in the Milky Way can be shaped by Galactic shear and confined in the vertical direction by gravity.

astro-ph.GA↗

arXiv2024

Updated kinematics of the Radcliffe Wave: non-synchronous, dipole-like vertical oscillations

The kinematic information of the Radcliffe Wave (RW) is essential for determining its existence and gaining insights into its origin and evolution. In this work, we present an accurate measurement of the vertical velocity ($V_Z$) of RW by incorporating the radial velocity (RV) measures through two methods, which is crucial but was neglected previously. First, the velocities are measured towards young stars, using their RV measurements from APOGEE-2 and proper motion measurements from Gaia DR3. Second, we combine RV measurements toward clouds with proper motion measurements of associated Young Stellar Objects (YSOs) to determine the vertical velocities of the clouds. The results reveal that the oscillations in $V_Z$ are not synchronous with the vertical coordinate $Z$, which differs from the conclusions of previous studies. Instead, we find a 5 km$\cdot$s$^{-1}$$\cdot$kpc$^{-1}$ gradient in $V_Z$ along the RW, exhibiting a dipole-like pattern. Consequently, the kinematic arrangement does not show a corresponding coherence with the spatial arrangement, bringing the Radcliffe Wave model into question.

astro-ph.GA↗

arXiv2024

What role of gravity, turbulence and magnetic fields in high-mass star formation clouds?

To explore the potential role of gravity, turbulence and magnetic fields in high-mass star formation in molecular clouds, this study revisits the velocity dispersion--size ($σ$--$L$) and density--size ($ρ$--$L$) scalings and the associated turbulent energy spectrum using an extensive data sample. The sample includes various hierarchical density structures in high-mass star formation clouds, across scales of 0.01 to 100 pc. We observe $σ\propto L^{0.26}$ and $ρ\propto L^{-1.54}$ scalings, converging toward a virial equilibrium state. A nearly flat virial parameter--mass ($α_{\rm vir}-M$) distribution is seen across all density scales, with $α_{\rm vir}$ values centered around unity, suggesting a global equilibrium maintained by the interplay between gravity and turbulence across multiple scales. Our turbulent energy spectrum ($E(k)$) analysis, based on the $σ$--$L$ and $ρ$--$L$ scalings, yields a characteristic $E(k) \propto k^{-1.52}$. These findings indicate the potential significance of gravity, turbulence, and possibly magnetic fields all in regulating dynamics of molecular clouds and high-mass star formation therein.

astro-ph.GA↗

arXiv2024

Modification to the Jeans criterion by external tides: Anisotropic fragmentation and formation of filaments

The Jeans criterion sets the foundation of our understanding of gravitational collapse. Jog studied the fragmentation of gas under external tides and derived a dispersion relation $$ l' = l_{\rm Jeans} \frac{1} {(1 + λ_0' / 4 πG ρ_0)^{1/2}} \;. $$ She further concludes that the Jeans mass is $m_{\rm incorrect}'=m_{\rm Jeans} ( 1/(1 + λ_0' / 4 πG ρ_0)^{3/2})$. We clarify that due to the inhomogeneous nature of tides, this characteristic mass is incorrect. Under weak tides, the mass is $m \approx ρ\, l_1 l_2 l_3$, where the modifications to Jeans lengths along all three dimensions need to be considered; when the tide is strong enough, collapse can only occur once 1 or 2 dimensions. In the latter case, tides can stretch the gas, leading to the formation of filaments.

astro-ph.GA↗

arXiv2024

Gas content and evolution of a sample of YSO associations at $d \lesssim$ 3.5 kpc from the Sun

Young Stellar Objects (YSO) are newly formed stars from molecular clouds. They stay close to where they were born and serve as good tracers to study gas and star formation. During cloud evolution, young massive stars can disrupt the surrounding gas through stellar feedback, changing the gas distribution. We study the distribution of the gas around a sample of YSO associations located at $d \lesssim 3.5 \;\rm kpc$ from the Sun by comparing the location and morphology between $^{12}$CO (J = 1$-$0) emission, Planck 870 $μ$m maps and YSO associations. Based on the spatial distribution of the gas compared to that of the YSOs, we classify the YSO associations into six types: direct, close, bubble, complex, diffuse, and clean associations. The complex associations are large structures consisting of both gas-rich and gas-poor segments. We study the velocity dispersion-size relation toward different association types. From the ratio between different types, we estimate a feedback time of $\approx$ 1.7 Myr in the solar neighborhood. The sample sets a solid foundation to explore the relationship between interstellar medium evolution, star formation, and Galaxy structure.

astro-ph.GA↗