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Using high-resolution simulations with explicit treatment of stellar feedbackphysics based on the FIRE (Feedback in Realistic Environments) project, westudy how galaxy formation and the interstellar medium (ISM) are affected bymagnetic fields, anisotropic Spitzer-Braginskii conduction and viscosity, andsub-grid metal diffusion from unresolved turbulence. We consider controlledsimulations of isolated (non-cosmological) galaxies but also a limited set ofcosmological "zoom-in" simulations. Although simulations have shown significanteffects from these physics with weak or absent stellar feedback, the effectsare much weaker than those of stellar feedback when the latter is modeledexplicitly. The additional physics have no systematic effect on galactic starformation rates (SFRs) . In contrast, removing stellar feedback leads to SFRsbeing over-predicted by factors of $\sim 10 -100$. Without feedback, neithergalactic winds nor volume filling hot-phase gas exist, and discs tend torunaway collapse to ultra-thin scale-heights with unphysically dense clumpscongregating at the galactic center. With stellar feedback, a multi-phase,turbulent medium with galactic fountains and winds is established. At currentlyachievable resolutions and for the investigated halo mass range$10^{10}-10^{13} M_{\odot}$, the additional physics investigated here (MHD,conduction, viscosity, metal diffusion) have only weak ($\sim10\%$-level)effects on regulating SFR and altering the balance of phases, outflows, or theenergy in ISM turbulence, consistent with simple equipartition arguments. Weconclude that galactic star formation and the ISM are primarily governed by acombination of turbulence, gravitational instabilities, and feedback. We addthe caveat that AGN feedback is not included in the present work.

Feedback first: the surprisingly weak effects of magnetic fields, viscosity, conduction, and metal diffusion on galaxy formation