The effects of magneto-hydrodynamics and cosmic rays on GMC lifetimes in FIRE-2 cosmological galaxy simulations

Giant Molecular Clouds (GMCs) are vast collections of dense, cold molecular gas and dust that are thought to be the chief reservoir of star formation. Despite their crucial role in the star formation process, the lifetime of GMCs are poorly constrained today. In this poster, we present measurement of the lifetimes of GMCs in cosmological simulations at z=0, using the Latte suite of FIRE-2 simulations of Milky Way (MW) mass galaxies. We analyze simulations that include magneto-hydrodynamics (MHD) and also model anisotropic viscosity and thermal conduction. While the inclusion of MHD has only weak (~10%) effects on galaxy-wide properties, such as the star formation or overall ISM turbulence, we find addition of anisotropic MHD pressure to be particularly important to the collapse and virialization of GMCs. We also examine MW-mass simulations that include cosmic rays (CRs), injected via supernovae shocks and propagated self-consistently through the magnetized ISM. We find the inclusion of CRs directly impacts GMCs and star-forming regions by driving non-thermal pressure gradients in the ISM. We track GMCs with total gas mass ≳105 M⊙ at high spatial (~1 pc), mass (7100 M⊙), and temporal (1 Myr) resolution. We compare our results to those from the default FIRE-2 physics presented in Benincasa et al., 2020. We find our simulated GMCs are consistent with the distribution of masses for massive GMCs in the MW and nearby galaxies, and we find GMC lifetimes to be short, with a small fraction of GMCs living longer than 20 Myr.