SILCC – VII. Gas kinematics and multiphase outflows of the simulated ISM at high gas surface densities

Abstract

We present magnetohydrodynamic (MHD) simulations of the star-forming multiphase interstellar medium (ISM) in stratified galactic patches with gas surface densities Σgas = 10, 30, 50, and 100 M⊙ pc−2. The silcc project simulation framework accounts for non-equilibrium thermal and chemical processes in the warm and cold ISM. The sink-based star formation and feedback model includes stellar winds, hydrogen-ionising UV radiation, core-collapse supernovae, and cosmic ray (CR) injection and diffusion. The simulations follow the observed relation between Σgas and the star formation rate surface density ΣSFR. CRs qualitatively change the outflow phase structure. Without CRs, the outflows transition from a two-phase (warm and hot at 1 kpc) to a single-phase (hot at 2 kpc) structure. With CRs, the outflow always has three phases (cold, warm, and hot), dominated in mass by the warm phase. The impact of CRs on mass loading decreases for higher Σgas and the mass loading factors of the CR-supported outflows are of order unity independent of ΣSFR. Similar to observations, vertical velocity dispersions of the warm ionised medium (WIM) and the cold neutral medium (CNM) correlate with the star formation rate as $\sigma _\mathrm{z} \propto \Sigma _\mathrm{SFR}^a$, with a ∼ 0.20. In the absence of stellar feedback, we find no correlation. The velocity dispersion of the WIM is a factor ∼2.2 higher than that of the CNM, in agreement with local observations. For ΣSFR ≳ 1.5 × 10−2 M⊙ yr−1 kpc−2 the WIM motions become supersonic.

The Structure and Composition of Multiphase Galactic Winds in a Large Magellanic Cloud Mass Simulated Galaxy

“…The LMC-like galaxy model with ∼4 M e mass resolution is a challenging but critical step for a better understanding of the self-consistently driven multiphase outflows in a more massive, global galaxy setting. This provides a way to test results from local simulations that have even higher resolution and comparable physics but lack global geometry (e.g., Kim & Ostriker 2018;Kim et al 2020a;Rathjen et al 2023), but without imposing specific wind scaling relations as is necessary in lowerresolution, large-scale galaxy formation simulations (e.g., Vogelsberger et al 2013;Hirschmann et al 2014;Schaye et al 2015;Pillepich et al 2018;Nelson et al 2019). Previous studies at LMC-like or higher galaxy mass scales are limited to SNe implemented with momentum feedback or delayed cooling without fully resolving hot gas production (e.g., Hopkins et al 2012;Ford et al 2014;Christensen et al 2016;Hopkins et al 2018;Tollet 2019;Hopkins et al 2023).…”

“…Recently, Rathjen et al (2023) investigated the phase structure of the winds in the SILCC simulations. When only SNe, stellar winds, and photoionizing radiation are included, they find results for the pdfs for mass outflow rate and energy outflow rate in terms of outflow velocities (bulk mass outflow between 10 and 100 km s −1 ) and thermal state of the gas (warm gas with c s ∼ 10 km s −1 ) comparable to what we find.…”

“…It is unlikely that stellar winds would change this picture strongly given recent work on efficient cooling in stellar wind bubbles (e.g., Lancaster et al 2021aLancaster et al , 2021b. Rathjen et al (2023) investigated the impact of CR pressure, which leads to an additional cold-phase wind launched above the midplane (see also Girichidis et al 2016;Jacob et al 2018;Mao & Ostriker 2018;Girichidis et al 2022;Thomas et al 2023). It is difficult to assess the impact, however, since they exclusively selected the snapshots that have mass loading higher than 1 in their analysis, while we average over a full wind driving cycle.…”

“…In these simulations, phenomenological tuning is no longer adopted, and the properties of large-scale multiphase winds can be considered emergent predictions following from the physical processes operating on parsec scales. The solar neighborhood environment has been probed extensively in so-called "tall box" simulations, which have explored outflow launching mechanisms by SNe either in simulations with fixed SN rate (e.g., de Avillez & Berry 2001; Joung et al 2009;Hill et al 2012;Creasey et al 2013;Fielding et al 2018) or simulations where the SN rate follows the SFR with a characteristic time delay (e.g., Girichidis et al 2016Girichidis et al , 2018Kim & Ostriker 2018;Kannan et al 2019;Kim et al 2020aKim et al , 2020bLi et al 2020;Vijayan et al 2020;Hu et al 2021;Rathjen et al 2021;Hu et al 2022;Kim et al 2023;Rathjen et al 2023).…”

Cosmic-Ray Acceleration of Galactic Outflows in Multiphase Gas

“…In addition to gaining momentum from the hot wind, cool gas can in principle be accelerated by interactions with CRs. The dynamical role of CRs in driving galactic winds has been investigated in both analytic models (Dorfi & Breitschwerdt 2012;Recchia et al 2016;Mao & Ostriker 2018;Quataert et al 2022aQuataert et al , 2022bShimoda & Inutsuka 2022) and numerical simulations of isolated galaxies or cosmological zoom-ins (e.g., Uhlig et al 2012;Salem & Bryan 2014;Pakmor et al 2016;Ruszkowski et al 2017;Chan et al 2022;Girichidis et al 2022;Peschken et al 2023;Thomas et al 2023) and portions of ISM (e.g., Girichidis et al 2016Girichidis et al , 2018Simpson et al 2016;Farber et al 2018;Rathjen et al 2021Rathjen et al , 2023Tsung et al 2023). All these studies found that CR pressure gradients can drive galactic outflows; however, the efficiency of this process is strongly dependent on the CR transport prescription adopted in the model because this affects the CR pressure gradient and therefore the momentum transfer to thermal gas.…”