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Abstract Turbulent radiative mixing layers (TRMLs) are of fundamental importance to the transport of energy and momentum in multiphase, astrophysical fluids. We use measurements of the microscopic and macroscopic properties of these layers in high-resolutionAthenaKsimulations to investigate when their properties can be considered well resolved. In particular, we demonstrate that the previously noticed resolution independence of the total cooling, ${\dot{E}}_{\mathrm{cool}}$, in these simulations is due to a remarkable, and perhaps fortuitous, cancellation of the countervailing effects of numerical dissipation and numerical viscosity. This calls into question the degree to which we can trust the results of these experiments, as there is no physical picture that explains this cancellation. We also demonstrate that in order to correctly resolve the phase structure in these layers, important for accurate predictions of their observable properties, one must resolve the scale on which turbulent diffusion acts on timescales comparable to the cooling time. This “turbulent Field length,”λ_F,turb, is where the eddy turnover time is equal to the cooling time (t_eddy(λ_F,turb) = t_cool). We demonstrate that resolving this scale results in a converged phase structure and spatially resolved transitions in the gas phases. 

ABSTRACT Theoretical arguments and observations suggest that in massive haloes ($$>10^{12}\, {\rm M}_\odot$$), the circumgalactic medium (CGM) is dominated by a ‘hot’ phase with gas temperature near the virial temperature ($$T\approx T_{\rm vir}$$) and a quasi-hydrostatic pressure profile. Lower-mass haloes are however unlikely to be filled with a similar quasi-static hot phase, due to rapid radiative cooling. Using the FIRE (Feedback In Realistic Environment) cosmological zoom simulations, we demonstrate that the hot phase is indeed subdominant at inner radii ($$\lesssim 0.3 R_{\rm vir}$$) of $$\lesssim 10^{12}\, {\rm M}_\odot$$ haloes, and the inner CGM is instead filled with $$T\ll T_{\rm vir}$$ gas originating in outflows and inflows, with a turbulent velocity comparable to the halo virial velocity. The turbulent velocity thus exceeds the mass-weighted sound speed in the inner CGM, and the turbulence is supersonic. UV absorption features from such CGM trace the wide lognormal density distributions of the predominantly cool and turbulent volume-filling phase, in contrast with tracing localized cool ‘clouds’ embedded in a hot medium. We predict equivalent widths of $$W_\lambda \sim 2\lambda v_{\rm c}/c\sim 1$$Å for a broad range of strong UV and EUV transitions (Mg ii, C ii, C iv, Si ii–iv, O iii–v) in sightlines through inner CGM dominated by turbulent pressure of $$\lesssim L^\star$$ galaxies at redshifts $$0\le z\lesssim 2$$, where $$\lambda$$ is the transition wavelength, $$v_{\rm c}$$ is the circular velocity, and c is the speed of light. Comparison of our predictions with observational constraints suggests that star forming $$\lesssim$$ $$L^\star$$ and dwarf galaxies are generally dominated by turbulent pressure in their inner CGM, rather than by thermal pressure. The inner CGM surrounding these galaxies is thus qualitatively distinct from that around quenched galaxies and massive discs such as the Milky-Way and M31, in which thermal pressure likely dominates. 

Abstract Unraveling the multiphase structure of the diffuse interstellar medium as traced by neutral hydrogen (Hi) is essential to understanding the lifecycle of the Milky Way. However, Hiphase separation is a challenging and underconstrained problem. The neutral gas phase distribution is often inferred from the spectral line structure of Hiemission. In this work, we develop a data-driven phase-separation method that extracts Hiphase structure solely from the spatial morphology of Hiemission intensity structures. We combine scattering spectra (SS) statistics with a Gaussian-mixture variational autoencoder model to (1) derive an interpretable statistical model of different Hiphases from their multiscale morphological structures, and (2) we use this model to decompose the 2D channel maps of GALFA-Hiemission in diffuse high-latitude (∣b∣ > 30°) regions over narrow velocity channels (Δv = 3 km s⁻¹) into cold neutral medium (CNM), warm neutral medium (WNM), and noise components. We integrate our CNM map over velocity channels to compare it to an existing map produced by a spectrum-based method. We find that the two maps are highly correlated, but ours recovers more spatially coherent structures at small scales. Our work illustrates and quantifies a clear physical connection between the Himorphology and Hiphase structure, and it unlocks a new avenue for improving future phase-separation techniques by making use of both Hispectral and spatial information to decompose Hiin 3D position–position–velocity space. These results are consistent with a physical picture where processes that drive Hiphase transitions also shape the morphology of Higas, imprinting a sparse, filamentary CNM that forms out of a diffuse, extended WNM. 

ABSTRACT We deploy the new Arkenstone galactic wind model in cosmological simulations for the first time, allowing us to robustly resolve the evolution and impact of high specific energy winds. In a (25 $$h^{-1}$$ Mpc)$^3$ box, we perform a set of numerical experiments that systematically vary the mass and energy loadings of such winds, finding that their energy content is the key parameter controlling the stellar to dark matter mass ratio. Increasing the mass loading, at fixed energy, actually results in mildly enhanced star formation, counter to prevailing wisdom, due to the wind becoming cooler. Of the simple parametrizations that we test, we find that an energy loading that scales inversely with halo mass best matches a wide range of observations and can do so with mass loadings drastically lower than those in most previous cosmological simulations. In this scenario, much less material is ejected from the interstellar medium. Instead, winds both heat gas in the circumgalactic medium, slowing infall onto the galaxy, and also drive shocks beyond the virial radius, decreasing the halo-scale accretion rate. We can also report that a much lower fraction of the available supernova energy is needed in preventative galaxy regulation than required by ejective wind feedback models such as IllustrisTNG. This is a Learning the Universe collaboration publication. 

Turbulent radiative mixing layers play an important role in many astrophysical contexts where cool (≲10⁴K) clouds interact with hot flows (e.g., galactic winds, high-velocity clouds, infalling satellites in halos and clusters). The fate of these clouds (as well as many of their observable properties) is dictated by the competition between turbulence and radiative cooling; however, turbulence in these multiphase flows remains poorly understood. We have investigated the emergent turbulence arising in the interaction between clouds and supersonic winds in hydrodynamicenzo-esimulations. In order to obtain robust results, we employed multiple metrics to characterize the turbulent velocity,v_turb. We find four primary results when cooling is sufficient for cloud survival. First,v_turbmanifests clear temperature dependence. Initially,v_turbroughly matches the scaling of sound speed on temperature. In gas hotter than the temperature where cooling peaks, this dependence weakens with time untilv_turbis constant. Second, the relative velocity between the cloud and wind initially drives rapid growth ofv_turb. As it drops (from entrainment),v_turbstarts to decay before it stabilizes at roughly half its maximum. At late times, cooling flows appear to support turbulence. Third, the magnitude ofv_turbscales with the ratio between the hot phase sound-crossing time and the minimum cooling time. Finally, we find tentative evidence for a length scale associated with resolving turbulence. Underresolving this scale may cause violent shattering and affect the cloud’s large-scale morphological properties. 

ABSTRACT Arkenstone is a new scheme that allows multiphase, stellar feedback-driven winds to be included in coarse resolution cosmological simulations. The evolution of galactic winds and their subsequent impact on the circumgalactic medium are altered by exchanges of mass, energy, momentum, and metals between their component phases. These exchanges are governed by complex, small-scale physical processes that cannot be resolved in cosmological simulations. In this second presentation paper, we describe Arkenstone’s novel cloud particle approach for modelling unresolvable cool clouds entrained in hot, fast winds. This general framework allows models of the cloud–wind interaction, derived from state-of-the-art high-resolution simulations, to be applied in a large-scale context. In this work, we adopt a cloud evolution model that captures simultaneous cloud mass loss to and gain from the ambient hot phase via turbulent mixing and radiative cooling, respectively. We demonstrate the scheme using non-cosmological idealized simulations of a galaxy with a realistic circumgalactic medium component, using the arepo code. We show that the ability of a high-specific energy wind component to perform preventative feedback may be limited by heavy loading of cool clouds coupled into it. We demonstrate that the diverging evolution of clouds of initially differing masses leads to a complex velocity field for the cool phase and a cloud mass function that varies both spatially and temporally in a non-trivial manner. These latter two phenomena can manifest in the simulation because of our choice of a Lagrangian discretization of the cloud population, in contrast to other proposed schemes. 

Abstract Metals in the diffuse, ionized gas at the boundary between the Milky Way’s interstellar medium (ISM) and circumgalactic medium, known as the disk–halo interface (DHI), are valuable tracers of the feedback processes that drive the Galactic fountain. However, metallicity measurements in this region are challenging due to obscuration by the Milky Way ISM and uncertain ionization corrections that affect the total hydrogen column density. In this work, we constrain ionization corrections to neutral hydrogen column densities using precisely measured electron column densities from the dispersion measures of pulsars that lie in the same globular clusters as UV-bright targets with high-resolution absorption spectroscopy. We address the blending of absorption lines with the ISM by jointly fitting Voigt profiles to all absorption components. We present our metallicity estimates for the DHI of the Milky Way based on detailed photoionization modeling of the absorption from ionized metal lines and ionization-corrected total hydrogen columns. Generally, the gas clouds show a large scatter in metallicity, ranging between 0.04 and 3.2Z_⊙, implying that the DHI consists of a mixture of gaseous structures having multiple origins. We estimate the inflow and outflow timescales of the DHI ionized clouds to be 6–35 Myr. We report the detection of an infalling cloud with supersolar metallicity that suggests a Galactic fountain mechanism, whereas at least one low-metallicity outflowing cloud (Z< 0.1Z_⊙) poses a challenge for Galactic fountain and feedback models. 

Abstract This paper presents a new framework for understanding the relationship between a galaxy and its circumgalactic medium (CGM). It focuses on howimbalancesbetween heating and cooling cause either expansion or contraction of the CGM. It does this by trackingallof the mass and energy associated with a halo’s baryons, including their gravitational potential energy, even if feedback has pushed some of those baryons beyond the halo’s virial radius. We show how a star-forming galaxy’s equilibrium state can be algebraically derived within the context of this framework, and we analyze how the equilibrium star formation rate depends on supernova feedback. We consider the consequences of varying the mass loading parameter ${\eta }_M\equiv {\dot{M}}_{\mathrm{wind}}/{\dot{M}}_{*}$relating a galaxy’s gas mass outflow rate ( ${\dot{M}}_{\mathrm{wind}}$) to its star formation rate ( ${\dot{M}}_{*}$) and obtain results that challenge common assumptions. In particular, we find that equilibrium star formation rates in low-mass galaxies are generally insensitive to mass loading, and when mass loading does matter, increasing it actually results inmorestar formation because more supernova energy is needed to resist atmospheric contraction. 

Abstract The scaling of galaxy properties with halo mass suggests that feedback loops regulate star formation, but there is no consensus yet about how those feedback loops work. To help clarify discussions of galaxy-scale feedback, Paper I presented a very simple model for supernova feedback that it called the minimalist regulator model. This follow-up paper interprets that model and discusses its implications. The model itself is an accounting system that tracks all of the mass and energy associated with a halo’s circumgalactic baryons—the central galaxy’s atmosphere. Algebraic solutions for the equilibrium states of that model reveal that star formation in low-mass halos self-regulates primarily by expanding the atmospheres of those halos, ultimately resulting in stellar masses that are insensitive to the mass-loading properties of galactic winds. What matters most is the proportion of supernova energy that couples with circumgalactic gas. However, supernova feedback alone fails to expand galactic atmospheres in higher-mass halos. According to the minimalist regulator model, an atmospheric contraction crisis ensues, which may be what triggers strong black hole feedback. The model also predicts that circumgalactic medium properties emerging from cosmological simulations should depend largely on the specific energy of the outflows they produce, and we interpret the qualitative properties of several numerical simulations in light of that prediction. 

Abstract We presentCloudFlex, an open-source tool for predicting absorption-line signatures of cool gas in galaxy halos with small-scale structure. Motivated by analyses of ∼10⁴K material in hydrodynamical simulations of turbulent, multiphase media, we model cool gas structures as complexes of cloudlets sampled from a power-law distribution of mass $\propto m_{\mathrm{cl}}^{-\alpha }$with velocities drawn from a turbulent velocity field. The user may specifyα, the lower limit of the cloudlet mass distribution ( $m_{\mathrm{cl},\min }$), and several other parameters that set the mass, size, and velocity distribution of the complex. This permits investigation of the relation between these parameters and absorption-line observables. As a proof-of-concept, we calculate the Mgiiλ2796 absorption induced by the cloudlets in background quasi-stellar object (QSO) spectra. We demonstrate that, at fixed metallicity, the covering fraction of sight lines with equivalent widthsW₂₇₉₆< 0.3 Å increases significantly with decreasing $m_{\mathrm{cl},\min }$, cloudlet number density (n_cl), and complex size. We then use this framework to predict the halo-scaleW₂₇₉₆distribution around ∼L^*galaxies. We show that the observed incidences ofW₂₇₉₆> 0.3 Å sight lines with impact parameters 10 kpc <R_⊥< 50 kpc in projected QSO–galaxy studies are consistent with our model over much of parameter space. However, they are underpredicted by models with $m_{\mathrm{cl},\min }\ge 100M_{\odot }$andn_cl≥ 0.03 cm⁻³, in keeping with a picture in which the inner cool circumgalactic medium (CGM) is dominated by numerous low-mass cloudlets (m_cl≲ 100M_⊙) with a volume filling factor ≲1%. When used to model absorption-line data sets built from multi-sight line and/or spatially extended background probes,CloudFlexenables detailed constraints on the size and velocity distributions of structures comprising the photoionized CGM.