Low-density Gas Research Articles

Time-dependent metal ionization and the persistence of collisionally excited emission lines in the diffuse ionized gas of star forming galaxies

Abstract We extend our time-dependent hydrogen ionization simulations of diffuse ionized gas to include metals important for collisional cooling and diagnostic emission lines. The combination of heating from supernovae and time-dependent collisional and photoionization from midplane OB stars produces emission line intensities (and emission line ratios) that follow the trends observed in the Milky Way and other edge-on galaxies. The long recombination times in low density gas result in persistent large volumes of ions with high ionization potentials, such as O iii and Ne iii. In particular, the vertically extended layers of Ne iii in our time-dependent simulations result in [Ne III] 15μm/[Ne II] 12μm emission line ratios in agreement with observations of the edge-on galaxy NGC 891. Simulations adopting ionization equilibrium do not allow for the persistence of ions with high ionization states and therefore cannot reproduce the observed emission lines from low density gas at high altitudes.

First Detection of Molecular Gas in the Giant Low Surface Brightness Galaxy Malin 1

After over three decades of unsuccessful attempts, we report the first detection of molecular gas emission in Malin 1, the largest spiral galaxy observed to date, and one of the most iconic giant low surface brightness galaxies. Using Atacama Large Millimeter/submillimeter Array, we detect significant 12CO (J = 1–0) emission in the galaxy’s central region and tentatively identify CO emission across three regions on the disk. These observations allow for a better estimate of the H2 mass and molecular gas mass surface density, both of which are remarkably low given the galaxy’s scale. By integrating data on its H i mass, we derive a very low molecular-to-atomic gas mass ratio. Overall, our results highlight the minimal presence of molecular gas in Malin 1, contrasting sharply with its extensive, homogeneous atomic gas reservoir. For the first time, we position Malin 1 on the Kennicutt–Schmidt diagram, where it falls below the main sequence for normal spirals, consistent with previous upper limits but now with more accurate figures. These findings are crucial for constraining our understanding of star formation processes in environments characterized by extremely low molecular gas densities and for refining models of galaxy formation, thereby improving predictions concerning the formation, evolution, and distribution of these giant, elusive galaxies.

On the Physical Nature of Lyα Transmission Spikes in High-redshift Quasar Spectra

We investigate Lyman-alpha (Lyα) transmission spikes at 5.2 < z < 6.8 using synthetic quasar spectra from the “Cosmic Reionization on Computers” simulations. We focus on understanding the relationship between these spikes and the properties of the intergalactic medium (IGM). Disentangling the complex interplay between IGM physics and the influence of galaxies on the generation of these spikes presents a significant challenge. To address this, we employ Explainable Boosting machines, an interpretable machine learning algorithm, to quantify the relative impact of various IGM properties on the Lyα flux. Our findings reveal that gas density is the primary factor influencing absorption strength, followed by the intensity of background radiation and the temperature of the IGM. Ionizing radiation from local sources (i.e., galaxies) appears to have a minimal effect on Lyα flux. The simulations show that transmission spikes predominantly occur in regions of low gas density. Our results challenge recent observational studies suggesting the origin of these spikes in regions with enhanced radiation. We demonstrate that Lyα transmission spikes are largely a product of the large-scale structure, of which galaxies are biased tracers.

Relativistic Electrons from Vacuum Laser Acceleration Using Tightly Focused Radially Polarized Beams.

We generate a tabletop pulsed relativistic electron beam at 100Hz repetition rate from vacuum laser acceleration by tightly focusing a radially polarized beam into a low-density gas. We demonstrate that strong longitudinal electric fields at the focus can accelerate electrons up to 1.43MeV by using only 98GW of peak laser power. The electron energy is measured as a function of laser intensity and gas species, revealing a strong dependence on the atomic ionization dynamics. These experimental results are supported by numerical simulations of particle dynamics in a tightly focused configuration that take ionization into consideration. For the range of intensities considered, it is demonstrated that atoms with higher atomic numbers like krypton can favorably inject electrons at the peak of the laser field, resulting in higher energies and an efficient acceleration mechanism that reaches a significant fraction (≈14%) of the theoretical energy gain limit.

A new measurement of the Galactic 12C/13C gradient from sensitive HCO+ absorption observations

We present a new constraint on the Galactic 12C/13C gradient with sensitive HCO+ absorption observations against strong continuum sources. The new measurements suffer less from beam dilution, optical depths, and chemical fractionation, allowing us to derive the isotopic ratios precisely. The measured 12C/13C ratio in the solar neighborhood (66±5) is consistent with those obtained from CH+. Two measurements toward the GC are 42.2±1.7 and 37.5±6.5. Though the values are a factor of two to three higher than those derived from dense gas tracers (e.g., H2CO, complex organic molecules) toward Sagittarius (Sgr) B2 regions, our results are consistent with the absorption measurements from c-C3H2 toward Sgr B2 (~40) and those from CH+ toward Sgr A* and Sgr B2(N) (&gt;30). We have calculated a new Galactic 12C/13C gradient of (6.4±1.9)RGC/kpc+(25.9±10.5) and found an increasing trend of the 12C/13C gradient obtained from high-density to low-density gas tracers, suggesting that opacity effects and chemical fractionation may have a strong impact on the isotopic ratios observed in high-density regions.

Comprehensive Evaluation of the Massively Parallel Direct Simulation Monte Carlo Kernel “Stochastic Parallel Rarefied-Gas Time-Accurate Analyzer” in Rarefied Hypersonic Flows—Part A: Fundamentals

The Direct Simulation Monte Carlo (DSMC) method, introduced by Graeme Bird over five decades ago, has become a crucial statistical particle-based technique for simulating low-density gas flows. Its widespread acceptance stems from rigorous validation against experimental data. This study focuses on four validation test cases known for their complex shock–boundary and shock–shock interactions: (a) a flat plate in hypersonic flow, (b) a Mach 20.2 flow over a 70-degree interplanetary probe, (c) a hypersonic flow around a flared cylinder, and (d) a hypersonic flow around a biconic. Part A of this paper covers the first two cases, while Part B will discuss the remaining cases. These scenarios have been extensively used by researchers to validate prominent parallel DSMC solvers, due to the challenging nature of the flow features involved. The validation requires meticulous selection of simulation parameters, including particle count, grid density, and time steps. This work evaluates the SPARTA (Stochastic Parallel Rarefied-gas Time-Accurate Analyzer) kernel’s accuracy against these test cases, highlighting its parallel processing capability via domain decomposition and MPI communication. This method promises substantial improvements in computational efficiency and accuracy for complex hypersonic vehicle simulations.

Probing the physics of star formation (ProPStar)

Context. Turbulence is a key component of molecular cloud structure. It is usually described by a cascade of energy down to the dissipation scale. The power spectrum for subsonic incompressible turbulence is ∝k−5/3, while for supersonic turbulence it is ∝k−2. Aims. We determine the power spectrum in an actively star-forming molecular cloud, from parsec scales down to the expected magnetohydrodynamic (MHD) wave cutoff (dissipation scale). Methods. We analyzed observations of the nearby NGC 1333 star-forming region in three different tracers to cover the different scales from ∼10 pc down to 20 mpc. The largest scales are covered with the low-density gas tracer 13CO (1–0) obtained with a single dish, the intermediate scales are covered with single-dish observations of the C18O (3–2) line, while the smallest scales are covered in H13CO+ (1–0) and HNC (1–0) with a combination of NOEMA interferometer and IRAM 30m single-dish observations. The complementarity of these observations enables us to generate a combined power spectrum covering more than two orders of magnitude in spatial scale. Results. We derive the power spectrum in an active star-forming region spanning more than 2 decades of spatial scales. The power spectrum of the intensity maps shows a single power-law behavior, with an exponent of 2.9 ± 0.1 and no evidence of dissipation. Moreover, there is evidence that the power spectrum of the ions to have more power at smaller scales than the neutrals, which is opposite to the theoretical expectations. Conclusions. We show new possibilities for studying the dissipation of energy at small scales in star-forming regions provided by interferometric observations.

Dynamics and kinetic theory of hard spheres under strong confinement.

The kinetic theory description of a low-density gas of hard spheres or disks, confined between two parallel plates separated at a distance smaller than twice the diameter of the particles, is addressed starting from the Liouville equationof the system. The associated Bogoliubov, Born, Green, Kirkwood, and Yvon hierarchy of equationsfor the reduced distribution functions is expanded in powers of a parameter measuring the density of the system in the appropriate dimensionless units. The Boltzmann level of description is obtained by keeping only the two lowest orders in the parameter. In particular, the one-particle distribution function obeys a couple of equations. Contrary to what happens with a Boltzmann-like kinetic equationthat has been proposed for the same system on a heuristic basis, the kinetic theory formulated here admits stationary solutions that are consistent with equilibrium statistical mechanics, in both the absence and presence of external fields. In the latter case, the density profile is rather complex due to the coupling between the inhomogeneities generated by the confinement and by the external fields. The general theory formulated provides a solid basis for the study of the properties of strongly confined dilute gases.

Supercritical retrograde solubility in R-14 and the second law of thermodynamics

A closed transcritical power cycle is described that uses a positive excess enthalpy of solution reaction differential between the reaction in the cycle’s high-density liquid state and low-density expanded gas state to internally transfer heat energy. The heat energy input to satisfy the solution reaction near the cycle’s low temperature is returned as heat during supercritical retrograde solubility near the cycle’s high temperature before that heat energy affects gas expansion. The power cycle format is used to frame this heat energy transfer so that Carnot efficiency can be used to calculate the maximum second law allowed amount of heat energy that can be transferred. If the amount of heat transfer exceeds the determined minimum threshold, the cycle’s Q efficiency will surpass the cycle’s T efficiency. The process demonstrates that the maximum positive excess enthalpy differential allowed by the second law is irreconcilably low.

Kinetic Simulation of Nozzle Flow in a Micronewton-Class Cold Gas Thruster

Many scientific space missions need highly precise attitude and orbit control or ultrafine drag compensation, which relies on the variable-thrust propulsion technology operating at the micronewton level. Cold gas thruster (CGT) is a very promising solution, mainly because of its high reliability. One of the keys to the success of micronewton variable-thrust CGT is to understand the flow in its nozzle, whose configuration is much more complex than traditional CGT nozzles. This paper applies kinetic-based multiscale models to investigate the gas flow in the complex nozzle of a micronewton variable-thrust CGT. The simulations reveal that the flow simultaneously experiences all kinds of regimes from continuum to free molecular. The continuum breakdown is likely to occur near the throat region due to large gradients of flow variables and in the expander due to low gas density. Frictional choking is observed, and the nozzle length can be optimized to improve the thruster performance. Nozzle performance measures such as thrust, discharge coefficient, and thrust efficiency are found to change only with the throat Knudsen number Knt. The performance curves can be divided into two sections at Knt≃0.1, and thereby an empirical piecewise formula for thrust prediction is proposed.

A pilot study of Galactic radio recombination lines using FAST: Identification of diffuse ionized gas clumps and off-arm star-forming regions

Observing low-frequency decimeter hydrogen radio recombination lines (RRLs) with large single-dish telescopes, such as the Five-hundred-meter Aperture Spherical radio Telescope (FAST) in the L band, is a unique method for probing massive star formation on scales of hundreds of parsecs. This approach is particularly effective for detecting relatively weak and extended emissions from low-density gas ionized by massive stars. Deep, unbiased decimeter or centimeter RRL surveys with large single-dish telescopes can significantly enhance our understanding of the diffuse ionized gas along the Galactic plane. This, in turn, will improve our knowledge of the life cycle of matter in the interstellar medium and the dynamics of the Galaxy. In this context, we present a pilot project for such a blind L-band RRL survey targeting the Galactic plane and conducted using FAST. The results include the detection of RRL clumps and the identification of an off-arm active massive star-forming region near the Sagittarius-Carina arm. The ongoing and upcoming massive star formation in this region may be associated with the kink in the Sagittarius-Carina arm near 23$^ circ $ azimuth.

Fractional analysis of unsteady magnetohydrodynamics Jeffrey flow over an infinite vertical plate in the presence of Hall current

The impact of Hall current on the multiphase thermal transfer of an incompressible electrically conductive Jeffrey flow over an infinitely vertical plate when heat absorption and chemical reaction are present has been examined. Partial differential equations have been used to describe the process, accounting for heat and mass transfer effects. This study uses extended Fourier's and Fick's laws together with the recently announced constant proportional Caputo (CPC) fractional operator. The fractional model is converted into a nondimensional form by applying some appropriate quantities. The nondimensional produced fractional model for momentum, heat, and diffusion equations based on the CPC fractional operator has been calculated semi‐analytically by applying the Laplace method. The Mathcad 15 software to sketch the graphs for several factors, like the Grashof number, mass Grashof number, Schmidt number, Prandtl number, Hall, and magnetic field parameters, is used to describe the velocity profile. Additionally, a graphical explanation is provided for the influence of the appeared parameters, particularly the effect of the fractional parameters. It is concluded that the result of the fluid model developed by the generalized constitutive relations is more accurate and generalized than the results of the artificially contracted fractional model. A fractional derivative is therefore the ideal option to achieve controlled concentration, temperature, and velocity. The current study is immediately relevant to geophysical, cosmically fluid dynamics, medical, biological, and any other processes that are significantly enhanced by a low gas density and a high magnetic field.

Formation of Methanol via Fischer-Tropsch Catalysis by Cosmic Iron Sulphide.

Chemical reactions in the gas phase of the interstellar medium face significant challenges due to its extreme conditions (i. e., low gas densities and temperatures), necessitating the presence of dust grains to facilitate the synthesis of molecules inaccessible in the gas phase. While interstellar grains are known to enhance encounter rates and dissipate energy from exothermic reactions, their potential as chemical catalysts remain less explored. Here, we present mechanistic insights into the Fischer-Tropsch-type methanol (FTT-CH3OH) synthesis by reactivity of CO with H2 and using cosmic FeS surfaces as heterogeneous catalysts. Periodic quantum chemical calculations were employed to characterize the potential energy surface of the reactions on the (011) and (001) FeS surfaces, considering different Fe coordination environments and S vacancies. Kinetic calculations were also conducted to assess catalytic capacity and allocate reaction processes within the astrochemical framework. Our findings demonstrate the feasibility of FeS-based astrocatalysis in the FTT-CH3OH synthesis. The reactions and their energetics were elucidated from a mechanistic standpoint. Kinetic analysis demonstrates the temperature dependency of the simulated processes, underscoring the compulsory need of energy sources considering the astrophysical scenario. Our results provide insights into the presence of CH3OH in diverse regions where current models struggle to explain its observational quantity.

Overview Results of JWST Observations of Star-forming Clusters in the Extreme Outer Galaxy

The extreme outer Galaxy (EOG), which we define as the region of the Milky Way with a galactocentric radius of more than 18 kpc, provides an excellent opportunity to study star formation in an environment significantly different from that in the solar neighborhood because of its lower metallicity and lower gas density. We carried out near- and mid-infrared (NIR and MIR) imaging observations toward two star-forming clusters located in the EOG using JWST NIRCam and MIRI with nine filters: F115W, F150W, F200W, F350W, F405N, F444W, F770W, F1280W, and F2100W. In this paper, we present an overview of the observations, data reduction, and initial results. The NIR sensitivity is approximately 10–80 times better than our previous observation with the Subaru 8.2 m telescope. Accordingly, the mass detection limit reaches to about 0.01–0.05 M ⊙, which is about 10 times better than the previous observations. At MIR wavelengths, the high sensitivity and resolution data enable us to resolve individual young stellar objects in such a distant region for the first time. The mass detection limit at MIR F770W filter reaches about 0.1–0.3 M ⊙. With these new observations, we have identified components of the clusters that previous surveys did not detect, including class 0 candidates, outflow/jet components, and distinctive nebular structures. These data will enable us to investigate the properties of star formation in the EOG at the same depth of detail as previous observations of star formation in the solar neighborhood.

FAST observations of neutral hydrogen in the interacting galaxies NGC 3395/3396

ABSTRACT We report on high-sensitivity neutral hydrogen observations toward the gas-rich interacting galaxies NGC 3395/3396 with the Five-hundred-meter Aperture Spherical radio Telescope (FAST). Compared to previous observations carried out by the Very Large Array (VLA) and the Westerbork Synthesis Radio Telescope (WSRT), a more extended H i envelope around this system has been detected. The total H i gas mass of the NGC 3395/3396 system is estimated to be 7.8 × 109 M$_\odot$. This value is 2.7 times more than that reported based on the VLA interferometric maps. Previous observations found a large H i tail extending to the south-west and a minor tail emerging from the north of this peculiar galaxy pair. Based on the high-sensitivity observations of FAST, an extended H i plume to the north-west and a gas plume to the north-east have been detected for the first time. Neutral hydrogen of the two smaller galaxies IC 2604 and IC 2608 in the south of the system have also been detected. We discuss the origins of these extra gas and possible tidal interactions between these galaxies. The most prominent tidal feature of NGC 3395/3396, the south-west tail combined with the new detected north-west plume, behaves like a large ring. We suggest the ring might be formed by the previous fly-by interaction between NGC 3395 and 3396, which happened 500 Myr ago. Our study shows that high-sensitivity H i observations are important in revealing low column density gas, which is crucial to a deeper understanding of this interacting system.

Possible origins of anomalous H I gas around MHONGOOSE galaxy, NGC 5068

The existing reservoirs of neutral atomic hydrogen gas (H I) in galaxies are insufficient to have maintained the observed levels of star formation without some kind of replenishment. This refuelling of the H I reservoirs is likely to occur at column densities an order of magnitude lower than previous observational limits (NH I, limit ∼ 1019 cm−2 at a 30″ resolution over a linewidth of 20 km s−1). In this paper, we present recent deep H I observations of NGC 5068, a nearby isolated star-forming galaxy observed by MeerKAT as part of the MHONGOOSE survey. With these new data, we were able to detect low column density H I around NGC 5068 with a 3σ detection limit of NH I = 6.4 × 1017 cm−2 at a 90″ resolution over a 20 km s−1 linewidth. The high sensitivity and resolution of the MeerKAT data reveal a complex morphology of the H I in this galaxy – a regularly rotating inner disk coincident with the main star-forming disk of the galaxy, a warped outer disk of low column density gas (NH I &lt; 9 × 1019 cm−2), in addition to clumps of gas on the north-western side of the galaxy. We employed a simple two disk model that described the inner and outer disks, which enabled us to identify anomalous gas that deviates from the rotation of the main galaxy. The morphology and the kinematics of the anomalous gas suggest a possible extra-galactic origin. We explore a number of possible origin scenarios that may explain the anomalous gas, and conclude that fresh accretion is the most likely scenario.

Aerodynamic performance optimization of a UAV's airfoil at low-Reynolds number and transonic flow under the Martian carbon dioxide atmosphere

Unmanned aerial vehicles (UAV) on Mars operate in a thin atmosphere of carbon dioxide, where the flow Reynolds number and local sound velocity are significantly reduced compared to the Earth's atmosphere because of low gas density and temperature. Therefore, a Mars UAV with a rotary-wing design can operate in a state of low-Reynolds number (Re = 1-5 × 104) and transonic flow (Matip = 0.7–1.2), where flow separation and shock waves occur simultaneously. The interaction between flow separation and shock waves not only makes the entire flow field more complex but also seriously affects the aerodynamic performance of the airfoil, thereby reducing the payload of UAVs, which previous aircraft designs based on the thick atmospheric environment of the Earth have not considered. Therefore, in this study, a numerical simulation method is used to conduct airfoil optimization research on a characteristic airfoil under low-Reynolds number transonic flow (Re = 33200, Ma = 0.85) conditions to acquire additional insight, improve aerodynamic performance of the airfoil, and enhance the payload. The results were compared with the optimization results of subsonic flow (Re = 19500, Ma = 0.50). The Hicks-Henne bump functions method was selected for airfoil parameterization, and non-dominated sorting genetic algorithms (NSGA)-Ⅱ were used for optimization. Through an analysis of the pressure coefficient (Cp), surface friction coefficient (Cf), and density gradient, the number and intensity of oblique shock waves on the upper and lower surfaces were observed to significantly affect the aerodynamic performance of airfoils under low-Reynolds number flow. The changes in flow separation and transition positions do not directly affect the aerodynamic performance but weaken the oblique shock wave intensity induced by the separation point or directly transform strong shock waves into weaker compression waves because of the absence of flow separation. The optimization results for both subsonic and transonic flow point to a thinner overall shape with a flatter surface with the drag greatly reduced due to the disappearance of shock waves on the lower surface, which significantly improves aerodynamic performance. Compared with the original airfoil, at Ma = 0.85, this design can increase the lift coefficient (Cl) by at least 124 % and lift drag ratio (Cl/Cd) by 177 %.

The nature of diffuse ionized gas in star-forming galaxies

ABSTRACT We present an analysis of the diffuse ionized gas (DIG) in a high-resolution simulation of an isolated Milky Way-like galaxy, incorporating on-the-fly radiative transfer and non-equilibrium thermochemistry. We utilize the Monte-Carlo radiative transfer code colt to self-consistently obtain ionization states and line emission in post-processing. We find a clear bimodal distribution in the electron densities of ionized gas ($n_{\rm e}$), allowing us to define a threshold of $n_{\rm e}=10\, \mathrm{cm}^{-3}$ to differentiate DIG from ${\rm H\, {\small II}}$ regions. The DIG is primarily ionized by stars aged 5 – 25 Myr, which become exposed directly to low-density gas after ${\rm H\, {\small II}}$ regions have been cleared. Leakage from recently formed stars ($\lt 5$ Myr) is only moderately important for DIG ionization. We forward model local observations and validate our simulated DIG against observed line ratios in [${\rm S\, {\small II}}$]/H$\alpha$, [${\rm N\, {\small II}}$]/H$\alpha$, [${\rm O\, {\small I}}$]/H$\alpha$, and [${\rm O\, {\small III}}$]/H$\beta$ against $\Sigma _{\rm H\alpha }$. The mock observations not only reproduce observed correlations, but also demonstrate that such trends are related to an increasing temperature and hardening ionizing radiation field with decreasing $n_{\rm e}$. The hardening of radiation within the DIG is caused by the gradual transition of the dominant ionizing source with decreasing $n_{\rm e}$ from 0 to 25 Myr stars, which have progressively harder intrinsic ionizing spectra primarily due to the extended Wolf–Rayet phase caused by binary interactions. Consequently, the DIG line ratio trends can be attributed to ongoing star formation, rather than secondary ionization sources, and therefore present a potent test for stellar feedback and stellar population models.

Density and porosity analyses of porous hybrid microparticles containing gas in closed pores using centrifugal liquid sedimentation-dynamic light scattering combined analytical method.

Porous hybrid microparticles are characterized by their densities and porosities. Consequently, the evaluation for density and porosity of porous hybrid microparticles in liquids is crucial for predicting the transport of particles in the atmosphere, human body, and industrial processes. However, direct measurement of the density and porosity of porous hybrid microparticles in liquids remains a challenge. In this study, we investigated the centrifugal sedimentation of polystyrene-silica hybrid microparticles with and without gas-containing closed pores. A centrifugal liquid sedimentation-dynamic light scattering combined analytical method was employed to determine the apparent densities of hybrid microparticles with and without gas-containing closed pores. The porosity of the hybrid microparticles with gas-containing closed pores was elucidated based on the inner buoyancy, which is a centrifugal force generated by the presence of low-density gas inside numerous closed pores. Further, the inner gas buoyancy was analyzed to estimate the particle porosity in liquids. The results obtained in this study confirmed the feasibility of utilizing the proposed method to determine the apparent density and porosity of porous hybrid microparticles in liquids.

Absorption in the Spectrum of a Background AGN by C ii* by the Supernova Remnant G354-33

A number of very large, very faint supernova remnants (SNRs) have been discovered in the past few years. Those in the Galactic halo occur in low density gas, so they might be more easily observed in absorption than in emission. The SNR G354-33 is a shell of faint UV and optical filaments about 10° in diameter. Here we point out that a spectrum of the QSO J2017-4516 used study the galaxy cluster J2016-4517 shows an absorption line from the excited fine structure level of C ii at −50 km s−1. Absorption features of Si ii and Si iii are also seen at this velocity. The observed column density compares well with shock wave models. Doppler shifted C ii* absorption features provide an excellent means to find and characterize SNR shocks because they arise from compressed gas.