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1. A new model for including galactic winds in simulations of galaxy formation – I. Introducing the Physically Evolved Winds (PhEW) model

   https://doi.org/10.1093/mnras/staa1978  

   Huang, Shuiyao ; Katz, Neal ; Scannapieco, Evan ; Cottle, J'Neil ; Davé, Romeel ; Weinberg, David H ; Peeples, Molly S ; Brüggen, Marcus ( September 2020 , Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT The propagation and evolution of cold galactic winds in galactic haloes is crucial to galaxy formation models. However, modelling of this process in hydrodynamic simulations of galaxy formation is oversimplified owing to a lack of numerical resolution and often neglects critical physical processes such as hydrodynamic instabilities and thermal conduction. We propose an analytic model, Physically Evolved Winds, that calculates the evolution of individual clouds moving supersonically through a uniform ambient medium. Our model reproduces predictions from very high resolution cloud-crushing simulations that include isotropic thermal conduction over a wide range of physical conditions. We discuss the implementation of this model into cosmological hydrodynamic simulations of galaxy formation as a subgrid prescription to model galactic winds more robustly both physically and numerically. 

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2. The origins of the circumgalactic medium in the FIRE simulations

   https://doi.org/10.1093/mnras/stz1773  

   Hafen, Zachary ; Faucher-Giguère, Claude-André ; Anglés-Alcázar, Daniel ; Stern, Jonathan ; Kereš, Dušan ; Hummels, Cameron ; Esmerian, Clarke ; Garrison-Kimmel, Shea ; El-Badry, Kareem ; Wetzel, Andrew ; et al ( June 2019 , Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We use a particle tracking analysis to study the origins of the circumgalactic medium (CGM), separating it into (1) accretion from the intergalactic medium (IGM), (2) wind from the central galaxy, and (3) gas ejected from other galaxies. Our sample consists of 21 FIRE-2 simulations, spanning the halo mass range Mh ∼ 1010–1012 M⊙, and we focus on z = 0.25 and z = 2. Owing to strong stellar feedback, only ∼L⋆ haloes retain a baryon mass $\gtrsim\! 50\hbox{ per cent}$ of their cosmic budget. Metals are more efficiently retained by haloes, with a retention fraction $\gtrsim\! 50\hbox{ per cent}$. Across all masses and redshifts analysed $\gtrsim \!60\hbox{ per cent}$ of the CGM mass originates as IGM accretion (some of which is associated with infalling haloes). Overall, the second most important contribution is wind from the central galaxy, though gas ejected or stripped from satellites can contribute a comparable mass in ∼L⋆ haloes. Gas can persist in the CGM for billions of years, resulting in well mixed-halo gas. Sightlines through the CGM are therefore likely to intersect gas of multiple origins. For low-redshift ∼L⋆ haloes, cool gas (T < 104.7 K) is distributed on average preferentially along the galaxy plane, however with strong halo-to-halo variability. The metallicity of IGM accretion is systematically lower than the metallicity of winds (typically by ≳1 dex), although CGM and IGM metallicities depend significantly on the treatment of subgrid metal diffusion. Our results highlight the multiple physical mechanisms that contribute to the CGM and will inform observational efforts to develop a cohesive picture. 

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3. Arkenstone – I. A novel method for robustly capturing high specific energy outflows in cosmological simulations

   https://doi.org/10.1093/mnras/stad3168  

   Smith, Matthew C. ; Fielding, Drummond B. ; Bryan, Greg L. ; Kim, Chang-Goo ; Ostriker, Eve C. ; Somerville, Rachel S. ; Stern, Jonathan ; Su, Kung-Yi ; Weinberger, Rainer ; Hu, Chia-Yu ; et al ( October 2023 , Monthly Notices of the Royal Astronomical Society)

   Arkenstone is a new model for multiphase, stellar feedback-driven galactic winds designed for inclusion in coarse resolution cosmological simulations. In this first paper of a series, we describe the features that allow Arkenstone to properly treat high specific energy wind components and demonstrate them using idealized non-cosmological simulations of a galaxy with a realistic circumgalactic medium (CGM), using the arepo code. Hot, fast gas phases with low mass loadings are predicted to dominate the energy content of multiphase outflows. In order to treat the huge dynamic range of spatial scales involved in cosmological galaxy formation at feasible computational expense, cosmological volume simulations typically employ a Lagrangian code or else use adaptive mesh refinement with a quasi-Lagrangian refinement strategy. However, it is difficult to inject a high specific energy wind in a Lagrangian scheme without incurring artificial burstiness. Additionally, the low densities inherent to this type of flow result in poor spatial resolution. Arkenstone addresses these issues with a novel scheme for coupling energy into the transition region between the interstellar medium (ISM) and the CGM, while also providing refinement at the base of the wind. Without our improvements, we show that poor spatial resolution near the sonic point of a hot, fast outflow leads to an underestimation of gas acceleration as the wind propagates. We explore the different mechanisms by which low and high specific energy winds can regulate the star formation rate of galaxies. In future work, we will demonstrate other aspects of the Arkenstone model.

    

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4. The impact of wind scalings on stellar growth and the baryon cycle in cosmological simulations

   https://doi.org/10.1093/mnras/staa135  

   Huang, Shuiyao ; Katz, Neal ; Davé, Romeel ; Oppenheimer, Benjamin D ; Weinberg, David H ; Fardal, Mark ; Kollmeier, Juna A ; Peeples, Molly S ( March 2020 , Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT Many phenomenologically successful cosmological simulations employ kinetic winds to model galactic outflows. Yet systematic studies of how variations in kinetic wind scalings might alter observable galaxy properties are rare. Here we employ gadget-3 simulations to study how the baryon cycle, stellar mass function, and other galaxy and CGM predictions vary as a function of the assumed outflow speed and the scaling of the mass-loading factor with velocity dispersion. We design our fiducial model to reproduce the measured wind properties at 25 per cent of the virial radius from the Feedback In Realistic Environments simulations. We find that a strong dependence of η ∼ σ5 in low-mass haloes with $\sigma \lt 106\mathrm{\, km\, s^{-1}}$ is required to match the faint end of the stellar mass functions at $z$ > 1. In addition, faster winds significantly reduce wind recycling and heat more halo gas. Both effects result in less stellar mass growth in massive haloes and impact high ionization absorption in halo gas. We cannot simultaneously match the stellar content at $z$ = 2 and 0 within a single model, suggesting that an additional feedback source such as active galactic nucleus might be required in massive galaxies at lower redshifts, but the amount needed depends strongly on assumptions regarding the outflow properties. We run a 50 $\mathrm{Mpc}\, h^{-1}$, 2 × 5763 simulation with our fiducial parameters and show that it matches a range of star-forming galaxy properties at $z$ ∼ 0–2. 

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5. Characterizing mass, momentum, energy, and metal outflow rates of multiphase galactic winds in the FIRE-2 cosmological simulations

   https://doi.org/10.1093/mnras/stab2714  

   Pandya, Viraj ; Fielding, Drummond B ; Anglés-Alcázar, Daniel ; Somerville, Rachel S ; Bryan, Greg L ; Hayward, Christopher C ; Stern, Jonathan ; Kim, Chang-Goo ; Quataert, Eliot ; Forbes, John C ; et al ( October 2021 , Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We characterize mass, momentum, energy, and metal outflow rates of multiphase galactic winds in a suite of FIRE-2 cosmological ‘zoom-in’ simulations from the Feedback in Realistic Environments (FIRE) project. We analyse simulations of low-mass dwarfs, intermediate-mass dwarfs, Milky Way-mass haloes, and high-redshift massive haloes. Consistent with previous work, we find that dwarfs eject about 100 times more gas from their interstellar medium (ISM) than they form in stars, while this mass ‘loading factor’ drops below one in massive galaxies. Most of the mass is carried by the hot phase (>105 K) in massive haloes and the warm phase (103−105 K) in dwarfs; cold outflows (<103 K) are negligible except in high-redshift dwarfs. Energy, momentum, and metal loading factors from the ISM are of order unity in dwarfs and significantly lower in more massive haloes. Hot outflows have 2−5 × higher specific energy than needed to escape from the gravitational potential of dwarf haloes; indeed, in dwarfs, the mass, momentum, and metal outflow rates increase with radius whereas energy is roughly conserved, indicating swept up halo gas. Burst-averaged mass loading factors tend to be larger during more powerful star formation episodes and when the inner halo is not virialized, but we see effectively no trend with the dense ISM gas fraction. We discuss how our results can guide future controlled numerical experiments that aim to elucidate the key parameters governing galactic winds and the resulting associated preventative feedback. 

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