Outflow Geometry Research Articles

Hot Gas Outflow Properties of the Starburst Galaxy NGC 4945

We analyze 330 ks of Chandra X-ray imaging and spectra of the nearby, edge-on starburst and Seyfert type 2 galaxy NGC 4945 to measure the hot gas properties along the galactic outflows. We extract and model spectra from 15 regions extending from −0.55 to +0.85 kpc above and below the galactic disk to determine the best-fit parameters and metal abundances. We find that the hot gas temperatures and number densities peak in the central regions and decrease along the outflows. These profiles are inconsistent with a spherical, adiabatically expanding wind model, suggesting the need to include mass loading and/or a nonspherical outflow geometry. We estimate the mass outflow rate of the hot wind to be 1.6 M ⊙ yr−1. Emission from charge exchange is detected in the northern outflow, and we estimate it contributes 12% to the emitted, broadband (0.5–7 keV) X-ray flux.

CON-quest

Context. Nuclear regions of ultraluminous and luminous infrared galaxies (U/LIRGs) are powered by starbursts and/or active galactic nuclei (AGNs). These regions are often obscured by extremely high columns of gas and dust. Molecular lines in the submillimeter windows have the potential to determine the physical conditions of these compact obscured nuclei (CONs). Aims. We aim to reveal the distributions of HCN and HCO+ emission in local U/LIRGs and investigate whether and how they are related to galaxy properties. Methods. Using the Atacama Large Millimeter/submillimeter Array (ALMA), we have conducted sensitive observations of the HCN J = 3−2 and HCO+J = 3−2 lines toward 23 U/LIRGs in the local Universe (z < 0.07) with a spatial resolution of ∼0.3″ (∼50−400 pc). Results. We detected both HCN and HCO+ in 21 galaxies, only HCN in one galaxy, and neither in one galaxy. The global HCN/HCO+ line ratios, averaged over scales of ∼0.5−4 kpc, range from 0.4 to 2.3, with an unweighted mean of 1.1. These line ratios appear to have no systematic trend with bolometric AGN luminosity or star formation rate. The line ratio varies with position and velocity within each galaxy, with an average interquartile range of 0.38 on a spaxel-by-spaxel basis. In eight out of ten galaxies known to have outflows and/or inflows, we found spatially and kinematically symmetric structures of high line ratios. These structures appear as a collimated bicone in two galaxies and as a thin spherical shell in six galaxies. Conclusions. Non-LTE analysis suggests that the high HCN/HCO+ line ratio in outflows is predominantly influenced by the abundance ratio. Chemical model calculations indicate that the enhancement of HCN abundance in outflows is likely due to high-temperature chemistry triggered by shock heating. These results imply that the HCN/HCO+ line ratio can aid in identifying the outflow geometry when the shock velocity of the outflows is sufficiently high to heat the gas.

Scaling relations for gamma-ray burst afterglow light curves and centroid motion independent of jet structure and dynamics

ABSTRACT Models for gamma-ray burst afterglow dynamics and synchrotron spectra are known to exhibit various scale invariances, owing to the scale-free nature of fluid dynamics and the power-law shape of synchrotron spectra. Since GRB 170817A, off-axis jet models including a lateral energy structure in the initial outflow geometry have gained in prominence. Here, we demonstrate how the scale invariance for arbitrary jet structure and dynamical stage can be expressed locally as a function of jet temporal light-curve slope. We provide afterglow flux expressions and demonstrate their use to quickly assess the physical implications of observations. We apply the scaling expressions to the Swift X-ray Telescope sample, which shows a spread in observed fluxes, binned by light-curve slope at time of observation, that increases with increasing light-curve slope. According to the scaling relations, this pattern is inconsistent with a large spread in environment densities if these were the dominant factor determining the variability of light curves. We further show how the late deep Newtonian afterglow stage remains scale-invariant but adds distinct spectral scaling regimes. Finally, we show that for given jet structure a universal curve can be constructed of the centroid offset, image size, and ellipticity (that can be measured using very large baseline interferometry) versus observer angle, in a manner independent of explosion energy and circumburst density. Our results apply to any synchrotron transient characterized by a release of energy in an external medium, including supernova remnants, kilonova afterglows, and soft gamma-repeater flares.

A Hermite-type collocation mesh-free approach for simulating incompressible viscous fluid flows

In this work, a novel Hermite-type mesh-free model for incompressible viscous fluid flows is proposed, as a way, to overcome the numerical difficulties experienced by solving under weak or strong formulation the pure stream-function Navier–Stokes equation. In this model a dimensionless virtual domain is utilized, in order to make an accurate dimensionless shape-functions of the recently developed Hermite-type collocation Weighted Least Square approximation, by using dimensionless scale parameters. Then, a Jacobian transformation matrix is built to express the shape-functions spatial derivatives from the virtual to the considered domain. The resolution of the obtained nonlinear algebraic system is performed by the High Order Continuation Method. Numerical tests were investigated on incompressible viscous isothermal fluid flows in lid-driven cavity, backward-facing step, sudden expansions and around various obstacle shapes. They show the advantages of the proposed model compared to the reference Hermite-type collocation models used for pure stream-function equation and the weak Galerkin procedures as Finite Element Method and Element Free Galerkin method, in term of computation accuracy, number of degree of freedom, implementation simplicity and computation time. The numerical and experimental results of literature confirm the validity and capability of the proposed dimensionless model to simulate fluid flows in a expended-lengths inflow–outflow geometry and around obstacles of circular, inclined elliptical and Joukowski airfoil shapes.

The Dilated Veins Surrounding the Cord in Multiple Sclerosis suggest Elevated Pressure and Obstruction of the Glymphatic System

Recently, Clarke et al. published a study using spinal cord susceptibility weighted imaging in multiple sclerosis patients at 7T. They discovered dilated intradural extramedullary veins surrounding the cord. The purpose of this commentary is to point out some recent research by our group, which suggests this dilatation also occurs in the bridging cortical veins surrounding the brain. The dilatation indicates a focal elevation in the venous pressure secondary to impedance mismatching. Due to the shared outflow geometry, dilatation of the outflow veins will obstruct the glymphatic pathway of the spinal cord altering the immune response.

Spectral Hardness and Evolution of Swift Gamma-Ray Bursts and X-Ray Afterglows

Spectral hardness of gamma-ray bursts can be characterized by either hardness ratio or peak energy, and the inconsistency of these parameters can reflect spectral diversity and complexity. We have chosen 310 Swift bursts with well-measured X-ray afterglows to analyze the distributions of their peak energies and hardness ratios before and after absorption of neutral hydrogen. We categorize them into four types according to X-ray temporal patterns and calculate the hardness ratios of both γ-rays and X-rays. It is found that the hardness ratios of X-rays are systematically smaller than those of γ-rays. We also find that both γ-ray hardness ratios and peak energies of bursts of types I, II, and III are identically distributed and are significantly lower than the corresponding measurements of type IV bursts in statistics, which hints that the evolving spectra of type IV bursts are most probably affected by the geometry of outflows. This proves that the influence of neutral hydrogen absorption on the estimation of hardness ratio is negligible even though it can reduce the brightness of γ/X-rays somewhat. Furthermore, we verify that almost all the observed spectra of gamma-ray bursts can be described by a broken power-law function and confirm a magnetar origin for the bursts with internal X-ray plateaus.

Stars Bisected by Relativistic Blades

We consider the dynamics of an equatorial explosion powered by a millisecond magnetar formed from the core collapse of a massive star. We study whether these outflows—generated by a priori magneto-centrifugally driven, relativistic magnetar winds—might be powerful enough to produce an ultrarelativistic blade (“lamina”) that successfully carves its way through the dense stellar interior. We present high-resolution numerical special-relativistic hydrodynamic simulations of axisymmetric centrifugally driven explosions inside a star and follow the blast wave propagation just after breakout. We estimate the engine requirements to produce ultrarelativistic lamina jets and comment on the physicality of the parameters considered. We find that sufficiently collimated—half-opening angle θ r ≤ 0.°2—laminas successfully break out of a compact progenitor at ultrarelativistic velocities () and extreme isotropic energies (E k,iso ∼ 5 × 1052 erg) within a few percent of the typical spin-down period for a millisecond magnetar. The various phases of these ultrathin outflows, such as collimation shocks, Kelvin–Helmholtz instabilities, and lifetime, are discussed, and we speculate on the observational signatures echoed by this outflow geometry.

Outflow geometry for electrochemical desalination cells

Electrochemical desalination approaches continue to gain in application. Here, we introduce an electrochemical desalination system utilizing a novel flow geometry with symmetric, porous Ag/AgCl electrodes. The system takes advantage of advection to continuously deliver input solution to the surface of the electrodes and reduce depletion that limits throughput and energy efficiency of many desalination systems. We explore the system behavior using steady-state analytical and transient numerical models along with experimental characterization of a small (1 cm2 active area) cell. Desalination performance is explored in terms of degree of separation, throughput, charge efficiency, and energy usage. Optimal flowrate corresponding to minimum energy per mole of salt removed is found based on steady state models when advective separation and system resistive loss is balanced. As an example, for idealized flow, with 65% removal and 50% recovery from a 50 mM NaCl solution, system energy usage at optimum velocity is predicted to be 16.5x the thermodynamic minimum. A gradual tradeoff between energy efficiency and cell throughput exists, potentially allowing vastly higher superficial velocities (e.g., > 0.1 mm/s) and more compact systems than comparable techniques such as reverse osmosis. At example experimental operating conditions, the test cell provides 83% removal of NaCl from a 50 mM input solution at 0.32 V cell voltage with 50% recovery.

X-Ray Properties of NGC 253's Starburst-driven Outflow

We analyze image and spectral data from ≈365 ks of observations from the Chandra X-ray Observatory of the nearby, edge-on starburst galaxy NGC 253 to constrain properties of the hot phase of the outflow. We focus our analysis on the −1.1 to +0.63 kpc region of the outflow and define several regions for spectral extraction where we determine best-fit temperatures and metal abundances. We find that the temperatures and electron densities peak in the central ∼250 pc region of the outflow and decrease with distance. These temperature and density profiles are in disagreement with an adiabatic spherically expanding starburst wind model and suggest the presence of additional physics such as mass loading and nonspherical outflow geometry. Our derived temperatures and densities yield cooling times in the nuclear region of a few million years, which may imply that the hot gas can undergo bulk radiative cooling as it escapes along the minor axis. Our metal abundances of O, Ne, Mg, Si, S, and Fe all peak in the central region and decrease with distance along the outflow, with the exception of Ne, which maintains a flat distribution. The metal abundances indicate significant dilution outside of the starburst region. We also find estimates of the mass outflow rates, which are 2.8 M ⊙ yr−1 in the northern outflow and 3.2 M ⊙ yr−1 in the southern outflow. Additionally, we detect emission from charge exchange and find it makes a significant contribution (20%–42%) to the total broadband (0.5–7 keV) X-ray emission in the central and southern regions of the outflow.

First-Principles Theory of the Relativistic Magnetic Reconnection Rate in Astrophysical Pair Plasmas.

We develop a first-principles model for the relativistic magnetic reconnection rate in strongly magnetized pair plasmas. By considering the energy budget and required current density near the x-line, we analytically show that in the magnetically dominated relativistic regime, the x-line thermal pressure is significantly lower than the upstream magnetic pressure due to the extreme energy needed to sustain the current density, consistent with kinetic simulations. This causes the upstream magnetic field lines to collapse in, producing the open outflow geometry which enables fast reconnection. The result is important for understanding a wide range of extreme astrophysical environments, where fast reconnection has been evoked to explain observations such as transient flares and nonthermal particle signatures.

Physics of ULIRGs with MUSE and ALMA: The PUMA project

We study molecular outflows in a sample of 25 nearby (z < 0.17, d < 750 Mpc) ultra-luminous infrared galaxy (ULIRG) systems (38 individual nuclei) as part of the Physics of ULIRGs with MUSE and ALMA (PUMA) survey, using ∼400 pc (0.1–1.0″ beam FWHM) resolution ALMA CO(2–1) observations. We used a spectro-astrometry analysis to identify high-velocity (> 300 km s−1) molecular gas disconnected from the galaxy rotation, which we attribute to outflows. In 77% of the 26 nuclei with log LIR/L⊙ > 11.8, we identified molecular outflows with an average vout = 490 km s−1, outflow masses 1 − 35 × 107 M⊙, mass outflow rates Ṁout = 6 − 300 M⊙ yr−1, mass-loading factors η = Ṁout/SFR = 0.1 − 1, and an average outflow mass escape fraction of 45 ± 6%. The majority of these outflows (18/20) are spatially resolved with radii of 0.2 − 0.9 kpc and have short dynamical times (tdyn = Rout/vout) in the range 0.5 − 2.8 Myr. The outflow detection rate is higher in nuclei dominated by starbursts (SBs, 14/15 = 93%) than in active galactic nuclei (AGN, 6/11 = 55%). Outflows perpendicular to the kinematic major axis are mainly found in interacting SBs. We also find that our sample does not follow the Ṁout versus AGN luminosity relation reported in previous works. In our analysis, we include a sample of nearby main-sequence galaxies (SFR = 0.3 − 17 M⊙ yr−1) with detected molecular outflows from the PHANGS-ALMA survey to increase the LIR dynamic range. Using these two samples, we find a correlation between the outflow velocity and the star-formation rate (SFR), as traced by LIR (vout ∝ SFR0.25±0.01), which is consistent with what was found for the atomic ionised and neutral phases. Using this correlation, and the relation between Mout/Rout and vout, we conclude that these outflows are likely momentum-driven. Finally, we compare the CO outflow velocities with the ones derived from the OH 119 μm doublet. In 76% of the targets, the outflow is detected in both CO and OH, while in three targets (18%) the outflow is only detected in CO, and in one target the outflow is detected in OH but not in CO. The difference between the OH and CO outflow velocities could be due to the far-IR background source required by the OH absorption which makes these observations more dependent on the specific outflow geometry.

Bright X-ray pulsars: how outflows influence beaming, pulsations and pulse phase lags

ABSTRACT Extreme accretion in X-ray pulsars (XRPs) results in radiation-driven outflows launched from the inner parts of the accretion disc. The outflows affect the apparent luminosity of the XRPs and their pulsations through the geometrical beaming. We model processes of geometrical beaming and pulse formation using Monte Carlo simulations. We confirm our earlier statement that strong amplification of luminosity due to the collimation of X-ray photons is inconsistent with a large pulsed fraction. Accounting for relativistic aberration due to possibly high outflow velocity (∼0.2c) does not affect this conclusion. We demonstrate that the beaming causes phase lags of pulsations. Within the opening angle of the accretion cavity formed by the outflows, phase lags tend to be sensitive to observers viewing angles. Variations in outflow geometry and corresponding changes of the phase lags might influence the detectability of pulsation in bright X-ray pulsars and ULXs. We speculate that the strong geometrical beaming is associated with large radiation pressure on the walls of accretion cavity due to multiple photons reflections. We expect that the mass-loss rate limits geometrical beaming: strong beaming becomes possible only under sufficiently large fractional mass-loss rate from the disc.

Revealing nature of GRB 210205A, ZTF21aaeyldq (AT2021any) and follow-up observations with the 4K$$\times$$4K CCD imager + 3.6m DOT

Optical follow-up observations of optical afterglows of gamma-ray bursts are crucial to probe the geometry of outflows, emission mechanisms, energetics and burst environments. We performed the follow-up observations of GRB 210205A and ZTF21aaeyldq (AT2021any) using the 3.6m Devasthal optical telescope (DOT) around one day after the burst to deeper limits due to the longitudinal advantage of the place. This paper presents our analysis of the two objects using data from other collaborative facilities, i.e., 2.2m Calar Alto Astronomical Observatory (CAHA) and other archival data. Our analysis suggests that GRB 210205A is a potential dark burst once compared with the X-ray afterglow data. Also, comparing results with other known and well-studied dark GRBs samples indicate that the reason for the optical darkness of GRB 210205A could either be intrinsic faintness or a high redshift event. Based on our analysis, we also found that ZTF21aaeyldq is the third known orphan afterglow with a measured redshift except for ZTF20aajnksq (AT2020blt) and ZTF19abvizsw (AT2019pim). The multiwavelength afterglow modeling of ZTF21aaeyldq using the afterglowpy package demands a forward shock model for an ISM-like ambient medium with a rather wider jet opening angle. We determine circumburst density of $$n_{0} = 0.87$$ cm $$^{-3}$$ , kinetic energy $$E_{k} = 3.80 \times 10^{52}$$ erg and the afterglow modeling also indicates that ZTF21aaeyldq is observed on-axis ( $$\theta _{\mathrm{obs}} < \theta _{\mathrm{core}}$$ ) and a gamma-ray counterpart was missed by GRBs satellites. Our results emphasize that the 3.6m DOT has a unique capability for deep follow-up observations of similar and other new transients for deeper observations as a part of time-domain astronomy in the future.

Electrochemical Membranes in Outflow Geometry for Water Desalination

Electrochemical Membranes in Outflow Geometry for Water Desalination

Outflow Geometry for Electrochemical Desalination Cells

Outflow Geometry for Electrochemical Desalination Cells

Estimating outflow masses and velocities in merger simulations: Impact of r -process heating and neutrino cooling

The determination of the mass, composition, and geometry of matter outflows in black hole-neutron star and neutron star-neutron star binaries is crucial to current efforts to model kilonovae, and to understand the role of neutron star merger in r-process nucleosynthesis. In this manuscript, we review the simple criteria currently used in merger simulations to determine whether matter is unbound and what the asymptotic velocity of ejected material will be. We then show that properly accounting for both heating and cooling during r-process nucleosynthesis is important to accurately predict the mass and kinetic energy of the outflows. These processes are also likely to be crucial to predict the fallback timescale of any bound ejecta. We derive a model for the asymptotic veloicity of unbound matter and binding energy of bound matter that accounts for both of these effects and that can easily be implemented in merger simulations. We show, however, that the detailed velocity distribution and geometry of the outflows can currently only be captured by full 3D fluid simulations of the outflows, as non-local effect ignored by the simple criteria used in merger simulations cannot be safely neglected when modeling these effects. Finally, we propose the introduction of simple source terms in the fluid equations to approximately account for heating/cooling from r-process nucleosynthesis in future seconds-long 3D simulations of merger remnants, without the explicit inclusion of out-of-nuclear statistical equilibrium reactions in the simulations.

Mass-loading and non-spherical divergence in hot galactic winds: implications for X-ray observations

ABSTRACT Cool clouds are expected to be destroyed and incorporated into hot supernova-driven galactic winds. The mass-loading of a wind by the cool medium modifies the bulk velocity, temperature, density, entropy, and abundance profiles of the hot phase relative to an unmass-loaded outflow. We provide general equations and limits for this physics that can be used to infer the rate of cool gas entrainment from X-ray observations, accounting for non-spherical expansion. In general, mass-loading flattens the density and temperature profiles, decreases the velocity and increases the entropy if the Mach number is above a critical value. We first apply this model to a recent high-resolution galactic outflow simulation where the mass-loading can be directly inferred. We show that the temperature, entropy, and composition profiles are well matched, providing evidence that this physics sets the bulk hot gas profiles. We then model the diffuse X-ray emission from the local starburst M82. The non-spherical (more cylindrical) outflow geometry is directly taken from the observed X-ray surface brightness profile. These models imply a total mass-loading rate that is about equal to that injected in the starburst ≃ 10 M⊙ yr−1, and they predict an asymptotic hot wind velocity of $\sim 1000\, {\rm km \ s^{-1}}$, which is ∼1.5–2 times smaller than previous predictions. We also show how the observed entropy profile can be used to constrain the outflow velocity, making predictions for future missions like XRISM. We argue that the observed X-ray limb brightening may be explained by mass-loading at the outflow’s edges.

Ionized emission and absorption in a large sample of ultraluminous X-ray sources

ABSTRACT Most ultraluminous X-ray sources (ULXs) are thought to be powered by super-Eddington accretion on to stellar-mass compact objects. Accretors in this extreme regime are naturally expected to ionize copious amounts of plasma in their vicinity and launch powerful radiation-driven outflows from their discs. High spectral resolution X-ray observations [with reflection grating spectrometer (RGS) gratings onboard XMM–Newton] of a few ULXs with the best data sets indeed found complex line spectra and confirmed such extreme (0.1–0.3c) winds. However, a search for plasma signatures in a large ULX sample with a rigorous technique has never been performed, thereby preventing us from understanding their statistical properties such as the rate of occurrence, to constrain the outflow geometry, and its duty cycle. We developed a fast method for automated line detection in X-ray spectra and applied it to the full RGS ULX archive, rigorously quantifying the statistical significance of any candidate lines. Collecting the 135 most significant features detected in 89 observations of 19 objects, we created the first catalogue of spectral lines detected in soft X-ray ULX spectra. We found that the detected emission lines are concentrated around known rest-frame elemental transitions and thus originate from low-velocity material. The absorption lines instead avoid these transitions, suggesting they were imprinted by blueshifted outflows. Such winds therefore appear common among the ULX population. Additionally, we found that spectrally hard ULXs show fewer line detections than soft ULXs, indicating some difference in their accretion geometry and orientation, possibly causing overionization of plasma by the harder spectral energy distributions of harder ULXs.

Resolved Neutral Outflow from a Lensed Dusty Star-forming Galaxy at z = 2.09

Abstract We report the detection of a massive neutral gas outflow in the z = 2.09 gravitationally lensed dusty star-forming galaxy HATLAS J085358.9+015537 (G09v1.40), seen in absorption with the OH+(11−10) transition using spatially resolved (0.″5 × 0.″4) Atacama Large Millimeter/submillimeter Array (ALMA) observations. The blueshifted OH+ line is observed simultaneously with the CO(9−8) emission line and underlying dust continuum. These data are complemented by high-angular-resolution (0.″17 × 0.″13) ALMA observations of CH+(1−0) and underlying dust continuum, and Keck 2.2 μm imaging tracing the stellar emission. The neutral outflow, dust, dense molecular gas, and stars all show spatial offsets from each other. The total atomic gas mass of the observed outflow is 6.7 × 109 M ⊙, &gt;25% as massive as the gas mass of the galaxy. We find that a conical outflow geometry best describes the OH+ kinematics and morphology and derive deprojected outflow properties as functions of possible inclination (0.°38–64°). The neutral gas mass outflow rate is between 83 and 25,400 M ⊙ yr−1, exceeding the star formation rate (788 ± 300 M ⊙ yr−1) if the inclination is &gt;3.°6 (mass-loading factor = 0.3–4.7). Kinetic energy and momentum fluxes span (4.4–290) × 109 L ⊙ and (0.1–3.7) × 1037 dyne, respectively (energy-loading factor = 0.013–16), indicating that the feedback mechanisms required to drive the outflow depend on the inclination assumed. We derive a gas depletion time between 29 and 1 Myr, but find that the neutral outflow is likely to remain bound to the galaxy unless the inclination is small and may be reaccreted if additional feedback processes do not occur.

Blow-away in the Extreme Low-mass Starburst Galaxy Pox 186

Abstract Pox 186 is an exceptionally small dwarf starburst galaxy hosting a stellar mass of ∼105 M ⊙. Undetected in H i (M &lt; 106 M ⊙) from deep 21 cm observations and with an [O iii]/[O ii] (5007/3727) ratio of 18.3 ± 0.11, Pox 186 is a promising candidate Lyman continuum emitter. It may be a possible analog of low-mass reionization-era galaxies. We present a spatially resolved kinematic study of Pox 186 and identify two distinct ionized gas components: a broad one with σ &gt; 400 km s−1 and a narrow one with σ &lt; 30 km s−1. We find strikingly different morphologies between the two components and direct evidence of outflows as seen in the high-velocity gas. Possible physical mechanisms driving the creation of high-velocity gas seen in [O iii] are discussed, from outflow geometry to turbulent mixing between a hot (106 K) star-cluster wind and cooler (104 K) gas clouds. We find a modest mass-outflow rate of 0.022 M ⊙ yr−1 with a small mass-loading factor of 0.5, consistent with other low-mass galaxies. Finally, we compare the mass-loading factor of Pox 186 with extrapolations from numerical simulations and discuss possible reasons for the apparent discrepancy between them.