Dense Gas Clouds Research Articles

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.

Possibilities of Using a Virtual Source Model to Refine the Prediction of the Impact of Injurious Effects in Massive Releases of Toxic Gases

Massive leaks of toxic substances occur not only during accidents connected with the operation of industrial enterprises, but also during their transfers by the means of transport. These events pose a serious threat to both people and the environment. Particularly dangerous are situations where dense gas clouds are formed after the release of the given substance. These spread very quickly, while they tend to remain at the earth's surface for a relatively long time and flow into various depressions. In a few minutes, the toxic gas can reach a large area, as confirmed by the conclusions from the Jack Rabbit field tests. Knowledge of the behavior of heavy gas, as well as knowledge of events influencing its dispersion in real conditions, thus provide important information needed for effective management of the resulting accident. The key data is the extent of the harmful effects of the given hazardous substance, which can be obtained by simulating the predicted emergency situation using modeling software (e.g. ALOHA). However, the fundamental shortcoming of this approach is that these software usually generate overestimated results. The fact that dangerous concentrations in the real environment do not reach such distances from the source of leakage has been repeatedly proven by past accidents. The reason for this discrepancy is that the computer programs used do not allow to model with sufficient accuracy all the simultaneous physico-chemical processes that are applied during the dispersion of dense gas clouds. This task is far too complicated and only a part of it can be solved with the necessary precision. However, for the needs of emergency planning, these inaccuracies represent a relatively fundamental limitation. One way how to deal with this problem is to use a virtual resource (sometimes called a virtual point) model. It is successfully used in various areas where it is necessary to simplify the otherwise demanding physical modeling of the temporal and spatial distribution of matter emitted from a surface or volume source. The main idea of a virtual source is that the real primary emission source is approximated to an imaginary source that is located at a different location but acts and appears outwardly as if it were a real source of leakage. The key task of this solution is to determine the parameters of this source, because this is the only way to obtain results with significantly higher reliability during the subsequent simulation of the considered accident scenario.

Hydrodynamic shielding in radiative multicloud outflows within multiphase galactic winds

ABSTRACT Stellar-driven galactic winds regulate the mass and energy content of star-forming galaxies. Emission- and absorption-line spectroscopy show that these outflows are multiphase and comprised of dense gas clouds embedded in much hotter winds. Explaining the presence of cold gas in such environments is a challenging endeavour that requires numerical modelling. In this paper, we report a set of 3D hydrodynamical simulations of supersonic winds interacting with radiative and adiabatic multicloud systems, in which clouds are placed along a stream and separated by different distances. As a complement to previous adiabatic, subsonic studies, we demonstrate that hydrodynamic shielding is also triggered in supersonic winds and operates differently in adiabatic and radiative regimes. We find that the condensation of warm, mixed gas in between clouds facilitates hydrodynamic shielding by replenishing dense gas along the stream, provided that its cooling length is shorter than the cloud radius. Small separation distances between clouds also favour hydrodynamic shielding by reducing drag forces and the extent of the mixing region around the clouds. In contrast, large separation distances promote mixing and dense-gas destruction via dynamical instabilities. The transition between shielding and no-shielding scenarios across different cloud separation distances is smooth in radiative supersonic models, as opposed to their adiabatic counterparts for which clouds need to be in close proximity. Overall, hydrodynamic shielding and re-condensation are effective mechanisms for preserving cold gas in multiphase flows for several cloud-crushing times, and thus can help understand cold gas survival in galactic winds.

Slow and steady does the trick: Slow outflows enhance the fragmentation of molecular clouds

Most massive galaxies host a supermassive black hole at their centre. Matter accretion creates an active galactic nucleus (AGN), forming a relativistic particle wind. The wind heats and pushes the interstellar medium, producing galactic-wide outflows. Fast outflows remove the gas from galaxies and quench star formation, and while slower ($v<500$ km s$^ $) outflows are ubiquitous, their effect is less clear but can be both positive and negative. We wish to understand the conditions required for positive feedback. We investigated the effect that slow and warm-hot outflows have on the dense gas clouds in the host galaxy. We aim to constrain the region of outflow and cloud parameter space, if any, where the passage of the outflow enhances star formation. We used numerical simulations of virtual `wind tunnels' to investigate the interaction of isolated turbulent spherical clouds ($10^ $ M$_ sun $) with slow outflows ($10$ km s$^ out 400$ km $) spanning a wide range of temperatures ($10^ $ K). We modelled 57 systems in total. We find that warm outflows compress the clouds and enhance gas fragmentation at velocities $ leq 200$ km s$^ $, while hot ($T_ out = 10^6$ K) outflows increase fragmentation rates even at moderate velocities of $400$ km s$^ $. Cloud acceleration, on the other hand, is typically inefficient, with dense gas only attaining velocities of $ 0.1 v_ out We suggest three primary scenarios where positive feedback on star formation is viable: stationary cloud compression by slow outflows in low-powered AGN, sporadic enhancement in shear flow layers formed by luminous AGN, and self-compression in fragmenting AGN-driven outflows. We also consider other potential scenarios where suitable conditions arise, such as compression of galaxy discs and supernova explosions. Our results are consistent with current observational constraints and with previous works investigating triggered star formation in these disparate domains.

Velocity-resolved Ionization Mapping of Broad Line Region. I. Insights into Diverse Geometry and Kinematics

Broad emission lines of active galactic nuclei (AGNs) originate from the broad-line region (BLR), consisting of dense gas clouds in orbit around an accreting supermassive black hole. Understanding the geometry and kinematics of this region is crucial for gaining insights into the physics and evolution of AGNs. Conventional velocity-resolved reverberation mapping may face challenges in disentangling the degeneracy between intricate motion and geometry of this region. To address this challenge, new key constraints are required. Here, we report the discovery of an asymmetric BLR using a novel technique: velocity-resolved ionization mapping, which can map the distance of emitting gas clouds by measuring Hydrogen line ratios at different velocities. By analyzing spectroscopic monitoring data, we find that the Balmer decrement is anticorrelated with the continuum and correlated with the lags across broad emission line velocities. Some line ratio profiles deviate from the expectations for a symmetrically virialized BLR, suggesting that the redshifted and blueshifted gas clouds may not be equidistant from the supermassive black hole (SMBH). This asymmetric geometry might represent a formation imprint, provide new perspectives on the evolution of AGNs, and influence SMBH mass measurements.

Great Balls of FIRE

Context. Despite the nearly hundred gravitational-wave detections reported by the LIGO-Virgo-KAGRA Collaboration, the question of the cosmological origin of merging binary black holes (BBHs) remains open. The two main formation channels generally considered are from isolated field binaries or via dynamical assembly in dense star clusters. Aims. Here we focus on understanding the dynamical formation of merging BBHs within massive clusters in galaxies of different masses. Methods. To this end, we applied a new framework to consistently model the formation and evolution of massive star clusters in zoom-in cosmological simulations of galaxies. Each simulation, taken from the FIRE project, provides a realistic star formation environment, with a unique star formation history, that hosts realistic giant molecular clouds that constitute the birthplace of star clusters. Combined with the code for star cluster evolution CMC, we are able to produce populations of dynamically formed merging BBHs across cosmic time in different environments. Results. As the most massive star clusters preferentially form in dense massive clouds of gas, we find that, despite their low metallicities favouring the creation of black holes, low-mass galaxies contain few massive clusters and therefore make a limited contribution to the global production of dynamically formed merging BBHs. Furthermore, we find that massive clusters can host hierarchical BBH mergers with clear, identifiable physical properties. Looking at the evolution of the BBH merger rate in different galaxies, we find strong correlations between BBH mergers and the most extreme episodes of star formation. Finally, we discuss the implications for future LIGO-Virgo-KAGRA gravitational wave observations.

Simulations of early structure formation: Properties of halos that host primordial star formation

Context.Population III (pop III) stars were born in halos characterised by a pristine gas composition. In such a halo, once the gas density reachesnH ∼ 1 cm−3, molecular cooling leads to the collapse of the gas and the birth of pop III stars. Halo properties, such as the chemical abundances, mass, and angular momentum can affect the collapse of the gas, thereby leading to the pop III initial mass function (IMF) of star formation.Aims.We want to study the properties of primordial halos and how halos that host early star formation differ from other types of halos. The aim of this study is to obtain a representative population of halos at a given redshift hosting a cold and massive gas cloud that enables the birth of the first stars.Methods.We investigated the growth of primordial halos in a ΛCDM Universe in a large cosmological simulation. We used the hydrodynamic code RAMSESand the chemical solver KROMEto study halo formation with non-equilibrium thermochemistry. We then identified structures in the dark and baryonic matter fields, thereby linking the presence or absence of dense gas clouds to the mass and the physical properties of the hosting halos.Results.In our simulations, the mass threshold for a halo for hosting a cold dense gas cloud is ≃7 × 105 M⊙and the threshold in the H2mass fraction is found to be ≃2 × 10−4. This is in agreement with previous works. We find that the halo history and accretion rate play a minor role. Here, we present halos with higher HD abundances, which are shown to be colder, as the temperature in the range between 102 − 104 cm−3depends on the HD abundance to a large extent. The higher fraction of HD is linked to the higher spin parameter that is seen for the dense gas.

The Radcliffe Wave is oscillating.

Our Sun lies within 300 parsecs of the 2.7-kiloparsecs-long sinusoidal chain of dense gas clouds known as the Radcliffe Wave1. The structure's wave-like shape was discovered using three-dimensional dust mapping, but initial kinematic searches for oscillatory motion were inconclusive2-7. Here we present evidence that the Radcliffe Wave is oscillating through the Galactic plane while also drifting radially away from the Galactic Centre. We use measurements of line-of-sight velocity8 for 12CO and three-dimensional velocities of young stellar clusters to show that the most massive star-forming regions spatially associated with the Radcliffe Wave (including Orion, Cepheus, North America and Cygnus X) move as though they are part of an oscillating wave driven by the gravitational acceleration of the Galactic potential. By treating the Radcliffe Wave as a coherently oscillating structure, we can derive its motion independently of the local Galactic mass distribution, and directly measure local properties of the Galactic potential as well as the Sun's vertical oscillation period. In addition, the measured drift of the Radcliffe Wave radially outwards from the Galactic Centre suggests that the cluster whose supernovae ultimately created today's expanding Local Bubble9 may have been born in the Radcliffe Wave.

Zooming in on the circumgalactic medium with GIBLE: Resolving small-scale gas structure in cosmological simulations

ABSTRACT We introduce Project GIBLE (Gas Is Better resoLved around galaxiEs), a suite of cosmological zoom-in simulations where gas in the circumgalactic medium (CGM) is preferentially simulated at ultra-high numerical resolution. Our initial sample consists of eight galaxies, all selected as Milky Way-like galaxies at z = 0 from the TNG50 simulation. Using the same galaxy formation model as IllustrisTNG, and the moving-mesh code arepo, we re-simulate each of these eight galaxies maintaining a resolution equivalent to TNG50-2 (mgas ∼ 8 × 105 M⊙). However, we use our super-Lagrangian refinement scheme to more finely resolve gas in the CGM around these galaxies. Our highest resolution runs achieve 512 times better mass resolution (∼103 M⊙). This corresponds to a median spatial resolution of ∼75 pc at 0.15 R200, c, which coarsens with increasing distance to ∼700 pc at the virial radius. We make predictions for the covering fractions of several observational tracers of multiphase CGM gas: H i, Mg ii, C iv, and O vii. We then study the impact of improved resolution on small scale structure. While the abundance of the smallest cold, dense gas clouds continues to increase with improving resolution, the number of massive clouds is well converged. We conclude by quantifying small scale structure with the velocity structure function and the autocorrelation function of the density field, assessing their resolution dependence. The GIBLE cosmological hydrodynamical simulations enable us to improve resolution in a computationally efficient manner, thereby achieving numerical convergence of a subset of key CGM gas properties and observables.

On the transfer of dense gases from a forest sub-canopy into the wind field above

The need to anticipate high risk of exposure following the generation of a cloud of cold and dense gas beneath a forest canopy introduces a need to examine the processes facilitating exchange with the air above the canopy. It is the concentrations in this above-canopy air that are used to initialize many dispersion models and these concentrations are lower than would be expected if no canopy were present. In the lack of direct experimental studies of trans-canopy dilution in a forested environment, results from a variety of related experiments are used here to illustrate the complexity of the problem and to derive a first-order assessment of the extent of expected dilution. The focus is on the dense gas clouds following accidents involving liquid chlorine, ammonia and carbon dioxide. It is concluded that the dilution would result in above-canopy concentrations between 2% and 50% of average sub-canopy levels, depending on site-specific circumstances. It is also concluded that the relative importance of the various contributing processes is such that detailed incorporation of them in dispersion models would constitute an unjustified complexity unless definitive experimental observations are available. Instead, the consequences of the issues now considered might best be accommodated by reducing predicted downwind concentrations by a factor of about ten with a corresponding extension of the duration of the dispersion event.

Formation of first star clusters under the supersonic gas flow – I. Morphology of the massive metal-free gas cloud

ABSTRACT We performed 42 simulations of first star formation with initial supersonic gas flows relative to the dark matter at the cosmic recombination era. Increasing the initial streaming velocities led to delayed halo formation and increased halo mass, enhancing the mass of the gravitationally shrinking gas cloud. For more massive gas clouds, the rate of temperature drop during contraction, in other words, the structure asymmetry, becomes more significant. When the maximum and minimum gas temperature ratios before and after contraction exceed ∼10, the asymmetric structure of the gas cloud prevails, inducing fragmentation into multiple dense gas clouds. We continued our simulations until 105 yr after the first dense core formation to examine the final fate of the massive star-forming gas cloud. Among the 42 models studied, we find the simultaneous formation of up to four dense gas clouds, with a total mass of about $2254\, \mathrm{ M}_\odot$. While the gas mass in the host halo increases with increasing the initial streaming velocity, the mass of the dense cores does not change significantly. The star formation efficiency decreases by more than one order of magnitude from ϵIII ∼ 10−2 to 10−4 when the initial streaming velocity, normalized by the root mean square value, increases from 0 to 3.

The circumgalactic medium of Milky Way-like galaxies in the TNG50 simulation – II. Cold, dense gas clouds and high-velocity cloud analogs

ABSTRACT We use the TNG50 simulation of the IllustrisTNG project to study cold, dense clouds of gas in the circumgalactic media (CGM) of Milky Way-like galaxies. We find that their CGM is typically filled with of order one hundred (thousand) reasonably (marginally) resolved clouds, possible analogs of high-velocity clouds (HVCs). There is a large variation in cloud abundance from galaxy to galaxy, and the physical properties of clouds that we explore – mass, size, metallicity, pressure, and kinematics – are also diverse. We quantify the distributions of cloud properties and cloud-background contrasts, providing cosmological inputs for idealized simulations. Clouds characteristically have subsolar metallicities, diverse shapes, small overdensities (χ = ncold/ nhot ≲ 10), are mostly inflowing, and have sub-virial rotation. At TNG50 resolution, resolved clouds have median masses of ∼ $10^6\, \rm {M_\odot }$ and sizes of ∼10 kpc. Larger clouds are well converged numerically, while the abundance of the smallest clouds increases with resolution, as expected. In TNG50 MW-like haloes, clouds are slightly (severely) underpressurized relative to their surroundings with respect to total (thermal) pressure, implying that magnetic fields may be important. Clouds are not distributed uniformly throughout the CGM but are clustered around other clouds, often near baryon-rich satellite galaxies. This suggests that at least some clouds originate from satellites, via direct ram-pressure stripping or otherwise. Finally, we compare with observations of intermediate and high velocity clouds from the real Milky Way halo. TNG50 shows a similar cloud velocity distribution as observations and predicts a significant population of currently difficult-to-detect low velocity clouds.

Faraday rotation measure variations of repeating fast radio burst sources

ABSTRACT Recently, some fast radio burst (FRB) repeaters were reported to exhibit complex, diverse variations of Faraday rotation measures (RMs), which implies that they are surrounded by an inhomogeneous, dynamically evolving, magnetized environment. We systematically investigate some possible astrophysical processes that may cause RM variations of an FRB repeater. The processes include (1) a supernova remnant (SNR) with a fluctuating medium; (2) a binary system with stellar winds from a massive/giant star companion or stellar flares from a low-mass star companion; (3) a pair plasma medium from a neutron star (including pulsar winds, pulsar wind nebulae, and magnetar flares); (4) outflows from a massive black hole. For the SNR scenario, a large relative RM variation within a few years requires that the SNR is young with a thin and local anisotropic shell, or the size of dense gas clouds in interstellar/circumstellar medium around the SNR is extremely small. If the RM variation is caused by the companion medium in a binary system, it is more likely from the stellar winds of a massive/giant star companion. The RM variation contributed by stellar flares from a low-mass star is disfavored, because this scenario predicts an extremely large relative RM variation during a short period of time. The scenarios invoking a pair plasma from a neutron star can be ruled out due to their extremely low RM contributions. Outflows from a massive black hole could provide a large RM variation if the FRB source is in the vicinity of the black hole.

The Integral Dense-gas Dispersion Model (IDDM) and comparison with Jack Rabbit II data

The Integral Dense-gas Dispersion Model (IDDM) is presented and used to predict the maximum concentrations and spread of a dense-gas cloud from a short-term release. IDDM is based on integrated conservation equations and includes spreading due to source momentum and density effects, cloud-top air entrainment, height changes with time, and wind transport. The equations are solved numerically but some useful analytical results are found showing the dependence of cloud radius and height on source conditions, entrainment, and time. IDDM comparisons with the Jack Rabbit (JR) II experiments are made for two smooth-terrain Trials (6, 7) and one with a Mock Urban Array (MUA) of Conex containers (Trial 1). For the smooth-terrain cases, predicted concentrations (C) follow the observed JR II data trends with distance x, and for x>1 km are consistent with a -5/3 empirical correlation. In the near field, analytical results for C show excellent agreement with the IDDM numerical solution. For Trial 1, an internal boundary layer (IBL) model is added to obtain the modified winds, friction velocity, and IBL depth due to the MUA. With the IBL and a mean depth (10 m) included, IDDM concentrations agree with the data. Over all Trials (1,6,7), IDDM exhibits good statistical performance with 83% of the predictions within a factor of 2 of the observations. An elliptical horizontal cloud geometry is added to estimate the lateral cloud width given an anisotropy ratio (ani) of along-wind (x) to lateral (y) cloud width. With constant ani (=3), the agreement between predicted and observed width is good with a geometric mean (GM) of predicted-to-observed width of 1.02 and a geometric standard deviation (GSD) of 1.35. However, with a trial-specific ani (=0.5−3) and longitudinal wind shear included, the mean model agreement (GM) is about the same, but the model-data scatter is reduced to a GSD of ≃1.1.

Jet Cloud–Star Interaction as an Interpretation of Neutrino Outburst from the Blazar TXS 0506+056

A neutrino outburst between September 2014 and March 2015 was discovered from the blazar TXS 0506+056 by an investigation of 9.5 years of IceCube data, while the blazar was in a quiescent state during the outburst with a gamma-ray flux of only about one-fifth of the neutrino flux. In this work, we give a possible interpretation of the abnormal feature by proposing that the neutrino outburst originated from the interaction between a relativistic jet and a dense gas cloud formed via the tidally disrupted envelope of a red giant being blown apart by the impact of the jet. Gamma-ray photons and electron/positron pairs produced through the hadronuclear interactions, correspondingly, will induce electromagnetic cascades and then make the cloud ionized and thermalized. The EM radiation from jet cloud–star interaction is mainly contributed by the relatively low-energy relativistic protons which propagate in the diffusion regime inside the cloud due to magnetic deflections, whereas the observed high-energy neutrinos (≳100 TeV) are produced by the relatively high-energy protons which can continue to beam owing to the weak magnetic deflections, inducing a much higher flux of neutrinos than electromagnetic radiation. The observed low-energy electromagnetic radiations during the neutrino outburst period are almost the same as that in the quiescent state of the source, so it may arise mainly as the same state as the generally quiescent. As a result, due to the intrusion of a dense cloud, the neutrino outburst can be expected, and, in the meantime, the accompanying electromagnetic radiations from hadronic processes will not cause any enhancement in the blazar’s electromagnetic flux.

Cosmological nanolensing by dense gas clouds

ABSTRACT We study the influence of a cosmological population of dense gas clouds on distant sources, with an emphasis on quasar optical variability. In addition to gravitational lensing, such clouds affect flux measurements via refraction in the neutral gas and via dust extinction, leading to a variety of possible light curves even in the low optical depth limit. We classify and illustrate the types of light curves that can arise. For sources as large as quasars, we show that gravitational lensing and extinction are the dominant effects, with gas refraction playing only a minor role. We find that clouds with mass $\sim 10^{-4.5\pm 0.5}\, \mathrm{M}_\odot$ can reproduce the observed distribution of quasar variation amplitudes, but only if such clouds make up a large fraction of the closure density. In that case, there may also be substantial extinction of distant optical sources, which can, in principle, be constrained by data on ‘standard candles’ such as type Ia supernovae. Unfortunately, that extinction is essentially grey, even when the material opacity is strongly wavelength dependent, making it difficult to distinguish from the influence of the background geometry. We propose a novel statistical test of the origin of quasar variability, based on the angular structure of the variation timescale for a large number of quasars distributed all over the sky. If quasar variability is primarily due to nanolensing, then angular structure is expected to include a quadrupole term of amplitude $\sim 5{{\ \rm per\ cent}}$, which ought to be measurable with future data from the Gaia mission.

On the low star formation rate in Saggitarius C

The Central Molecular Zone (CMZ) contains approximately 10% of the total molecular gas in the Galaxy concentrated in turbulent and dense molecular clouds. The fact that the star formation rate is low in CMZ is an open question, since there is a large amount of dense gas clouds where star formation could have started. Sagittarius (Sgr) C is the only known star formation region located in the western side of the CMZ. The main goal of our study was to find out the dominant physical conditions of Sgr C and how they could affect the process of star formation in the CMZ. We studied the dust emission toward Sgr C using infrared data. Using radio data we also studied the kinematic temperature, star formation rate surface density and cloud morphology of Sgr C. Our study showed a dust temperature T d of approximately 19 K toward Sgr C, which is in agreement with values estimated in other clouds of the CMZ and the Galactic plane. A high kinetic temperature higher than 58 K was determined toward Sgr C, which plays an important role in the cloud fragmentation and subsequent star formation. Based on our radio analysis we found a star formation rate surface density of ∑ SFR = 4 × 10-3 M ⊙yr-1 kpc -2 which is lower than in others molecular clouds of the Galaxy. Finally, the HCO + emission reveals the presence of cavities and shells in regions where there is no evidence of star formation. These shells could be related to cloud shocks believed to take place in Sgr C. It is thought that cloud shocks may have triggered star formation in other sites of the Galaxy, which is not observed in Sgr C. A comparison between physical conditions in Sgr C with those of high star formation sources could allow to find the parameters that suppress the birth of stars in this region of the Galactic Center.

Hadronuclear interpretation of the possible neutrino emission from PKS B1424-418, GB6 J1040+0617 and PKS 1502+106

In addition to neutrino event IceCube-170922A which is observed to be associated with a γ-ray flare from blazar TXS 0506+056, there are also several neutrino events that may be associated with blazars. Among them, PKS B1424-418, GB6 J1040+0617 and PKS 1502+106 are low synchrotron peaked sources, which are usually believed to have the broad line region in the vicinity of the central black hole. They are considered as counterparts of IceCube event 35, IceCube-141209A and IceCube-190730A, respectively. By considering the proton-proton (pp) interactions between the dense gas clouds in the broad line region and the relativistic protons in the jet, we show that the pp model that is applied in this work can not only reproduce the multi-waveband spectral energy distribution but also suggest a considerable annual neutrino detection rate. We also discuss the emission from the photopion production and Bethe-Heitler pair production with a sub-Eddington jet power that is suggested in our model and find that it has little effect on the spectrum of total emission for all of three sources.

Stratified flows and associated shear instabilities modelling over an inclined plan

We investigate isothermal continuous gravity currents down on an incline of 5°. The front region with strong mixing and the shallower layer behind it are modelled using computational fluid dynamics in order to discuss the dilution process in accidental releases. The simulations were performed using two different customised packages for compressible and a non-compressible flows. The heavy fluid dilution was analysed considering RANS (Reynolds Average Navier Stokes) approach and three turbulence models k−ε, k−ω SST and RNG k−ε. The simulated gravity currents were compared with experimental by means of the density distribution in the flow and volume of the current. As far as dense gas cloud prediction is concerned, the numerical findings agree with the experimental data and there seems to be good indication that the solvers are suitable for consequence analysis. Shear instabilities caused by the flow of two fluids near the interface zone are well captured by RNG k−ε turbulence model. The simulations show that the modelling of small scale turbulence and associated rate of deformation are important to mimic the wavy instabilities and curling of the interface region of the released and ambient fluid. Such process is of paramount importance when predicting the mixing and dilution of the released material. Analysis of the results shows that better agreement is observed when the proper modelling of the shear instabilities is considered as well as the extra source of turbulence due to the effects of buoyancy.

Evolution of giant molecular clouds across cosmic time

ABSTRACT Giant molecular clouds (GMCs) are well studied in the local Universe, however, exactly how their properties vary during galaxy evolution is poorly understood due to challenging resolution requirements, both observational and computational. We present the first time-dependent analysis of GMCs in a Milky Way-like galaxy and an Large Magellanic Cloud (LMC)-like dwarf galaxy of the FIRE-2 (Feedback In Realistic Environments) simulation suite, which have sufficient resolution to predict the bulk properties of GMCs in cosmological galaxy formation self-consistently. We show explicitly that the majority of star formation outside the galactic centre occurs within self-gravitating gas structures that have properties consistent with observed bound GMCs. We find that the typical cloud bulk properties such as mass and surface density do not vary more than a factor of 2 in any systematic way after the first Gyr of cosmic evolution within a given galaxy from its progenitor. While the median properties are constant, the tails of the distributions can briefly undergo drastic changes, which can produce very massive and dense self-gravitating gas clouds. Once the galaxy forms, we identify only two systematic trends in bulk properties over cosmic time: a steady increase in metallicity produced by previous stellar populations and a weak decrease in bulk cloud temperatures. With the exception of metallicity, we find no significant differences in cloud properties between the Milky Way-like and dwarf galaxies. These results have important implications for cosmological star and star cluster formation and put especially strong constraints on theories relating the stellar initial mass function to cloud properties.