But what about...: cosmic rays, magnetic fields, conduction, and viscosity in galaxy formation

Abstract

We present and study a large suite of high-resolution cosmological zoom-in simulations, using the FIRE-2 treatment of mechanical and radiative feedback from massive stars, together with explicit treatment of magnetic fields, anisotropic conduction and viscosity (accounting for saturation and limitation by plasma instabilities at high β), and cosmic rays (CRs) injected in supernovae shocks (including anisotropic diffusion, streaming, adiabatic, hadronic and Coulomb losses). We survey systems from ultrafaint dwarf ($M_{\ast }\sim 10^{4}\, \mathrm{M}_{\odot }$, $M_{\rm halo}\sim 10^{9}\, \mathrm{M}_{\odot }$) through Milky Way/Local Group (MW/LG) masses, systematically vary uncertain CR parameters (e.g. the diffusion coefficient κ and streaming velocity), and study a broad ensemble of galaxy properties [masses, star formation (SF) histories, mass profiles, phase structure, morphologies, etc.]. We confirm previous conclusions that magnetic fields, conduction, and viscosity on resolved ($\gtrsim 1\,$ pc) scales have only small effects on bulk galaxy properties. CRs have relatively weak effects on all galaxy properties studied in dwarfs ($M_{\ast } \ll 10^{10}\, \mathrm{M}_{\odot }$, $M_{\rm halo} \lesssim 10^{11}\, \mathrm{M}_{\odot }$), or at high redshifts (z ≳ 1–2), for any physically reasonable parameters. However, at higher masses ($M_{\rm halo} \gtrsim 10^{11}\, \mathrm{M}_{\odot }$) and z ≲ 1–2, CRs can suppress SF and stellar masses by factors ∼2–4, given reasonable injection efficiencies and relatively high effective diffusion coefficients $\kappa \gtrsim 3\times 10^{29}\, {\rm cm^{2}\, s^{-1}}$. At lower κ, CRs take too long to escape dense star-forming gas and lose their energy to collisional hadronic losses, producing negligible effects on galaxies and violating empirical constraints from spallation and γ-ray emission. At much higher κ CRs escape too efficiently to have appreciable effects even in the CGM. But around $\kappa \sim 3\times 10^{29}\, {\rm cm^{2}\, s^{-1}}$, CRs escape the galaxy and build up a CR-pressure-dominated halo which maintains approximate virial equilibrium and supports relatively dense, cool (T ≪ 106 K) gas that would otherwise rain on to the galaxy. CR ‘heating’ (from collisional and streaming losses) is never dominant.

Dynamical Equilibrium in the Molecular ISM in 28 Nearby Star-forming Galaxies

“…Observational evidence, including superthermal CO line widths and the size-line width relation observed within GMCs, suggest that turbulent motion dominates over thermal motion on physical scales comparable to cloud sizes (Larson 1981;Solomon et al 1987;Heyer & Brunt 2004; also see Heyer & Dame 2015). Numerical simulations of the star-forming ISM on galactic scales also find that the magnetic term is subdominant, typically reaching only50% of the kinetic term in the effective gas pressure Pakmor et al 2017;Su et al 2017;Hopkins et al 2020; also see observational evidence presented by Crutcher 1999;Falgarone et al 2008;Thompson et al 2019). Motivated by these findings, we assume in this work that turbulent motion represents the primary source of internal pressure in molecular gas, and treat all other contributions as subdominant.…”

Project AMIGA: The Circumgalactic Medium of Andromeda*

“…However, neither the SFR history nor the present-day SFR is similar. The "m12i" FIRE halo has a factor of 10-12 higher SFR (see Figure 3 in Hopkins et al 2020) than the present-day SFR of M31 of 0.5 M e yr −1 (e.g., Kang et al 2009). We compare Project AMIGA to FIRE-2 simulations with two different sets of physical ingredients.…”

“…In Section 4, we derive the empirical properties of the CGM of M31, including how the column densities and velocities vary with R and Φ, the covering factors of the ions and how they change with R, and the metal and baryon masses of the CGM of M31. In Section 5, we discuss the results derived in Section 4 and compare them to observations from the COS-Halos survey (Tumlinson et al 2013;Werk et al 2014) and to state-of-the-art cosmological zoom-ins from, in particular, the Feedback in Realistic Environments (FIRE; Hopkins et al 2020) and the Figuring Out Gas and Galaxies In Enzo (FOGGIE; Peeples et al 2019) simulation projects. In Section 6, we summarize our main conclusions.…”

The Impact of Cosmic Rays on Thermal Instability in the Circumgalactic Medium

“…Nonthermal cosmic-ray pressure supports cold gas, enabling it to cool isochorically (Sharma et al 2010;Kempski & Quataert 2020), and may explain the inferred low densities of cold CGM gas. Cosmicray pressure also counteracts gravity and may prevent cold halo gas from accreting onto the galaxy (Hopkins et al 2020a). In some transport approximations, streaming cosmic rays transfer energy to the thermal gas and provide a source of heating.…”