The growth and entrainment of cold gas in a hot wind
11 Oct 2018-Monthly Notices of the Royal Astronomical Society: Letters (Oxford University Press (OUP))-Vol. 480, Iss: 1
TL;DR: In this paper, the authors showed that cooling is often not efficient enough to prevent the destruction of cold gas, and identified regions of parameter space where the cooling efficiency of the mixed, ''warm'' gas is sufficiently large to contribute new comoving cold gas which can significantly exceed the original cold gas mass.
Abstract: Both absorption and emission line studies show that cold gas around galaxies is commonly outflowing at speeds of several hundred km$\,\textrm{s}^{-1}$. This observational fact poses a severe challenge to our theoretical models of galaxy evolution since most feedback mechanisms (e.g., supernovae feedback) accelerate hot gas, and the timescale it takes to accelerate a blob of cold gas via a hot wind is much larger than the time it takes to destroy the blob. We revisit this long-standing problem using three-dimensional hydrodynamical simulations with radiative cooling. Our results confirm previous findings, that cooling is often not efficient enough to prevent the destruction of cold gas. However, we also identify regions of parameter space where the cooling efficiency of the mixed, `warm' gas is sufficiently large to contribute new comoving cold gas which can significantly exceed the original cold gas mass. This happens whenever, $t_{\mathrm{cool, mix}}/t_{\mathrm{cc}} < 1$, where $t_{\mathrm{cool,mix}}$ is the cooling time of the mixed warm gas and $t_{\mathrm{cc}}$ is the cloud-crushing time. This criterion is always satisfied for a large enough cloud. Cooling `focuses' stripped material onto the tail where mixing takes place and new cold gas forms. A sufficiently large simulation domain is crucial to capturing this behavior.
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TL;DR: An overview of the basic physics of cool outflows, a comprehensive assessment of the observational techniques and diagnostic tools used to characterize them, a detailed description of the best-studied cases, and a more general discussion of the statistical properties of these outflows in the local and distant universe can be found in this article.
Abstract: Neutral-atomic and molecular outflows are a common occurrence in galaxies, near and far. They operate over the full extent of their galaxy hosts, from the innermost regions of galactic nuclei to the outermost reaches of galaxy halos. They carry a substantial amount of material that would otherwise have been used to form new stars. These cool outflows may have a profound impact on the evolution of their host galaxies and environments. This article provides an overview of the basic physics of cool outflows, a comprehensive assessment of the observational techniques and diagnostic tools used to characterize them, a detailed description of the best-studied cases, and a more general discussion of the statistical properties of these outflows in the local and distant universe. The remaining outstanding issues that have not yet been resolved are summarized at the end of the review to inspire new research directions.
117 citations
TL;DR: In this paper, the authors used three-dimensional hydrodynamic simulations of vertically stratified patches of galactic discs to study how the spatio-temporal clustering of supernovae (SNe) enhances the power of galactic winds.
Abstract: We use three-dimensional hydrodynamic simulations of vertically stratified patches of galactic discs to study how the spatio-temporal clustering of supernovae (SNe) enhances the power of galactic winds. SNe that are randomly distributed throughout a galactic disc drive inefficient galactic winds because most supernova remnants lose their energy radiatively before breaking out of the disc. Accounting for the fact that most star formation is clustered alleviates this problem. Super-bubbles driven by the combined effects of clustered SNe propagate rapidly enough to break out of galactic discs well before the clusters' SNe stop going off. The radiative losses post-breakout are reduced dramatically and a large fraction ($\gtrsim 0.2$) of the energy released by SNe vents into the halo powering a strong galactic wind. These energetic winds are capable of providing strong preventative feedback and eject substantial mass from the galaxy with outflow rates on the order of the star formation rate. The momentum flux in the wind is only of order that injected by the SNe, because the hot gas vents before doing significant work on the surroundings. We show that our conclusions hold for a range of galaxy properties, both in the local Universe (e.g., M82) and at high redshift (e.g., $z \sim 2$ star forming galaxies). We further show that if the efficiency of forming star clusters increases with increasing gas surface density, as suggested by theoretical arguments, the condition for star cluster-driven super-bubbles to break out of galactic discs corresponds to a threshold star formation rate surface density for the onset of galactic winds $\sim 0.03$ M$_\odot$ yr$^{-1}$ kpc$^{-2}$, of order that observed.
108 citations
Cites background from "The growth and entrainment of cold ..."
...In some cases, as these clouds are accelerated they grow due to the enhanced cooling of the hot medium in their wakes (Gronke & Oh 2018)....
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...This cold component may further out in the halo as the hot medium cools in the wake of the cold clouds (e.g., Thompson et al. 2016; Schneider et al. 2018; Gronke & Oh 2018)....
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TL;DR: In this article, the authors studied the role of radiative cooling in cloud acceleration and growth in a wide variety of scenarios, and found that cloud entrainment velocity is of order the cold gas sound speed, and growth is accompanied by cloud pulsations.
Abstract: The existence of fast moving, cold gas ubiquitously observed in galactic winds is theoretically puzzling, since the destruction time of cold gas is much smaller than its acceleration time In previous work, we showed that cold gas can accelerate to wind speeds and grow in mass if the radiative cooling time of mixed gas is shorter than the cloud destruction time Here, we study this process in much more detail, and find remarkably robust cloud acceleration and growth in a wide variety of scenarios Radiative cooling, rather than the Kelvin-Helmholtz instability, enables self-sustaining entrainment of hot gas onto the cloud via cooling-induced pressure gradients Indeed, growth peaks when the cloud is almost co-moving The entrainment velocity is of order the cold gas sound speed, and growth is accompanied by cloud pulsations Growth is also robust to the background wind and initial cloud geometry In an adiabatic Chevalier-Clegg type wind, for instance, the mass growth rate is constant Although growth rates are similar with magnetic fields, cloud morphology changes dramatically, with low density, magnetically supported filaments which have a small mass fraction but dominate by volume This could bias absorption line observations Cloud growth from entraining and cooling hot gas can potentially account for the cold gas content of the CGM It can also fuel star formation in the disk as cold gas recycled in a galactic fountain accretes and cools halo gas We speculate that galaxy-scale simulations should converge in cold gas mass once cloud column densities of ${\rm N} \sim 10^{18} \ {\rm cm^{-2}}$ are resolved
98 citations
Cites background or result from "The growth and entrainment of cold ..."
...The inequality tcc tcool,mix can be re-arranged in order to give a criterion that clouds exceed a critical cloud size r > rcl,crit (Gronke & Oh 2018) to escape destruction: rcl,crit ∼ vwindtcool,mix χ1/2 ≈ 2 pc T 5/2 cl,4Mw P3Λmix,−21.4 χ 100 ....
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...In Gronke & Oh (2018) (henceforth: Paper I), we revisited this classical ‘entrainment problem’, and found a quantitative criterion for cloud survival due to cooling....
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...In Gronke & Oh (2018), we showed that cold gas can accelerate to wind speeds and grow in mass if the radiative cooling time of mixed gas is shorter than the cloud destruction time....
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...We previously showed in Gronke & Oh (2018) that our results are robust to this; also see Fig....
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TL;DR: In this paper, the authors introduce the technique of Enhanced Halo Resolution (EHR), enabling more realistic physical modeling of the simulated CGM by consistently forcing gas refinement to smaller scales throughout the virial halo of a simulated galaxy.
Abstract: Traditional cosmological hydrodynamics simulations fail to spatially resolve the circumgalactic medium (CGM), the reservoir of tenuous gas surrounding a galaxy and extending to its virial radius. We introduce the technique of Enhanced Halo Resolution (EHR), enabling more realistic physical modeling of the simulated CGM by consistently forcing gas refinement to smaller scales throughout the virial halo of a simulated galaxy. We investigate the effects of EHR in the tempest simulations, a suite of enzo-based cosmological zoom simulations following the evolution of an L* galaxy, resolving spatial scales of 500 comoving pc out to 100 comoving kpc in galactocentric radius. Among its many effects, EHR (1) changes the thermal balance of the CGM, increasing its cool gas content and decreasing its warm/hot gas content; (2) preserves cool gas structures for longer periods; and (3) enables these cool clouds to exist at progressively smaller size scales. Observationally, this results in a boost in "low ions" like H i and a drop in "high ions" like O vi throughout the CGM. These effects of EHR do not converge in the tempest simulations, but extrapolating these trends suggests that the CGM is actually a mist consisting of ubiquitous, small, long-lived, cool clouds suspended in a medium at the halo virial temperature. We find that EHR produces the above effects by (1) better sampling the distribution of CGM phases, enabling runaway cooling in the dense, cool tail of the phase distribution; and (2) preventing cool gas clouds from artificially mixing with the ambient hot halo and evaporating.
98 citations
Cites background from "The growth and entrainment of cold ..."
...Many of these simulations show evidence for increased cool gas content under high-resolution conditions (Gronke & Oh 2018) and by accounting for additional physical effects like magnetic fields (Ji et al. 2018; Liang & Remming 2018), thermal conduction (Armillotta et al. 2017), and hydrodynamic…...
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TL;DR: In this article, the authors study the effects of low-accretion black hole feedback in the IllustrisTNG model and demonstrate that the onset of lowaccretion feedback can lead to star formation quenching at stellar masses.
Abstract: Supermassive black holes (SMBHs) that reside at the centres of galaxies can inject vast amounts of energy into the surrounding gas and are thought to be a viable mechanism to quench star formation in massive galaxies. Here, we study the $10^{9-12.5}\, \mathrm{M_\odot }$ stellar mass central galaxy population of the IllustrisTNG simulation, specifically the TNG100 and TNG300 volumes at z = 0, and show how the three components – SMBH, galaxy, and circumgalactic medium (CGM) – are interconnected in their evolution. We find that gas entropy is a sensitive diagnostic of feedback injection. In particular, we demonstrate how the onset of the low-accretion black hole (BH) feedback mode, realized in the IllustrisTNG model as a kinetic, BH-driven wind, leads not only to star formation quenching at stellar masses $\gtrsim 10^{10.5}\, \mathrm{M_\odot }$ but also to a change in thermodynamic properties of the (non-star-forming) gas, both within the galaxy and beyond. The IllustrisTNG kinetic feedback from SMBHs increases the average gas entropy, within the galaxy and in the CGM, lengthening typical gas cooling times from $10\!-\!100\, \mathrm{Myr}$ to $1\!-\!10\, \mathrm{Gyr}$, effectively ceasing ongoing star formation and inhibiting radiative cooling and future gas accretion. In practice, the same active galactic nucleus (AGN) feedback channel is simultaneously ‘ejective’ and ‘preventative’ and leaves an imprint on the temperature, density, entropy, and cooling times also in the outer reaches of the gas halo, up to distances of several hundred kiloparsecs. In the IllustrisTNG model, a long-lasting quenching state can occur for a heterogeneous CGM, whereby the hot and dilute CGM gas of quiescent galaxies contains regions of low-entropy gas with short cooling times.
87 citations
Cites background from "The growth and entrainment of cold ..."
...The cold, low-entropy stripped gas joins the much hotter medium, and may even lead to cooling and entrainment of gas from the CGM (Marinacci et al. 2010; Gronke & Oh 2018, 2020)....
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References
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TL;DR: In this article, the cooling functions for a plasma slab are investigated under equilibrium and nonequilibrium conditions, over a range of 10 4 -10 85 K and for a variety of abundances.
Abstract: The cooling functions for a plasma slab are investigated under equilibrium and nonequilibrium conditions, over a range of 10 4 -10 85 K and for a range of abundances Radiative transfer and diffuse field are calculated in the isobaric nonequilibrium models using a one-dimensional cooling flow model, and the plasma is not assumed to be optically thin to all radiation Limiting cases of the plasma diffuse field coupling are calculated, and the resulting cooling functions are presented Some functions are terminated before reaching 10 4 K when the internal photoionization halts the cooling The functions represent a self-consistent set of curves covering a wide grid of temperature and metallicities using recently published atomic data and processes
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"The growth and entrainment of cold ..." refers methods in this paper
...We implement cooling using the Townsend (2009) algorithm with a 7 piece power-law fit to the Sutherland & Dopita (1993) cooling curve....
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TL;DR: Galactic winds are the primary mechanism by which energy and metals are recycled in galaxies and are deposited into the intergalactic medium New observations are revealing the ubiquity of this process, particularly at high redshift as discussed by the authors.
Abstract: Galactic winds are the primary mechanism by which energy and metals are recycled in galaxies and are deposited into the intergalactic medium New observations are revealing the ubiquity of this process, particularly at high redshift We describe the physics behind these winds, discuss the observational evidence for them in nearby star-forming and active galaxies and in the high-redshift universe, and consider the implications of energetic winds for the formation and evolution of galaxies and the intergalactic medium To inspire future research, we conclude with a set of observational and theoretical challenges
1,453 citations
TL;DR: Yt, an open source, community-developed astrophysical analysis and visualization toolkit, is presented and its methods for reading, handling, and visualizing data, including projections, multivariate volume rendering, multi-dimensional histograms, halo finding, light cone generation, and topologically connected isocontour identification are reported.
Abstract: The analysis of complex multiphysics astrophysical simulations presents a unique and rapidly growing set of challenges: reproducibility, parallelization, and vast increases in data size and complexity chief among them. In order to meet these challenges, and in order to open up new avenues for collaboration between users of multiple simulation platforms, we present yt (available at http://yt.enzotools.org/) an open source, community-developed astrophysical analysis and visualization toolkit. Analysis and visualization with yt are oriented around physically relevant quantities rather than quantities native to astrophysical simulation codes. While originally designed for handling Enzo's structure adaptive mesh refinement data, yt has been extended to work with several different simulation methods and simulation codes including Orion, RAMSES, and FLASH. We report on its methods for reading, handling, and visualizing data, including projections, multivariate volume rendering, multi-dimensional histograms, halo finding, light cone generation, and topologically connected isocontour identification. Furthermore, we discuss the underlying algorithms yt uses for processing and visualizing data, and its mechanisms for parallelization of analysis tasks.
1,238 citations
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...This research made use of yt (Turk et al. 2011)....
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TL;DR: In this paper is an open source, community-developed astrophysical analysis and visualization toolkit, which is oriented around physically relevant quantities rather than quantities native to astrophysical simulation codes, including Enzo's structure adaptive mesh refinement (AMR).
Abstract: The analysis of complex multiphysics astrophysical simulations presents a unique and rapidly growing set of challenges: reproducibility, parallelization, and vast increases in data size and complexity chief among them. In order to meet these challenges, and in order to open up new avenues for collaboration between users of multiple simulation platforms, we present yt (available at this http URL), an open source, community-developed astrophysical analysis and visualization toolkit. Analysis and visualization with yt are oriented around physically relevant quantities rather than quantities native to astrophysical simulation codes. While originally designed for handling Enzo's structure adaptive mesh refinement (AMR) data, yt has been extended to work with several different simulation methods and simulation codes including Orion, RAMSES, and FLASH. We report on its methods for reading, handling, and visualizing data, including projections, multivariate volume rendering, multi-dimensional histograms, halo finding, light cone generation and topologically-connected isocontour identification. Furthermore, we discuss the underlying algorithms yt uses for processing and visualizing data, and its mechanisms for parallelization of analysis tasks.
1,127 citations
TL;DR: Results from a test suite which includes problems in one-, two-, and three-dimensions for both hydrodynamics and MHD are given, not only to demonstrate the fidelity of the algorithms, but also to enable comparisons to other methods.
Abstract: A new code for astrophysical magnetohydrodynamics (MHD) is described. The code has been designed to be easily extensible for use with static and adaptive mesh refinement. It combines higher order Godunov methods with the constrained transport (CT) technique to enforce the divergence-free constraint on the magnetic field. Discretization is based on cell-centered volume averages for mass, momentum, and energy, and face-centered area averages for the magnetic field. Novel features of the algorithm include (1) a consistent framework for computing the time- and edge-averaged electric fields used by CT to evolve the magnetic field from the time- and area-averaged Godunov fluxes, (2) the extension to MHD of spatial reconstruction schemes that involve a dimensionally split time advance, and (3) the extension to MHD of two different dimensionally unsplit integration methods. Implementation of the algorithm in both C and FORTRAN95 is detailed, including strategies for parallelization using domain decomposition. Results from a test suite which includes problems in one-, two-, and three-dimensions for both hydrodynamics and MHD are given, not only to demonstrate the fidelity of the algorithms, but also to enable comparisons to other methods. The source code is freely available for download on the web.
1,096 citations
"The growth and entrainment of cold ..." refers methods in this paper
...We use Athena 4.0 (Stone et al. 2008) to solve the threedimensional hydrodynamical equations on a uniform, Cartesian grid....
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