The Feedback In Realistic Environments (FIRE) project explores feedback in cosmological galaxy formation simulations. Previous FIRE simulations used an identical source code ("FIRE-1") for consistency. Motivated by the development of more accurate numerics -including hydrodynamic solvers, gravitational softening, and supernova coupling algorithms -and exploration of new physics (e.g. magnetic fields), we introduce "FIRE-2", an updated numerical implementation of FIRE physics for the GIZMO code. We run a suite of simulations and compare against FIRE-1: overall, FIRE-2 improvements do not qualitatively change galaxy-scale properties. We pursue an extensive study of numerics versus physics. Details of the star-formation algorithm, cooling physics, and chemistry have weak effects, provided that we include metal-line cooling and star formation occurs at higher-than-mean densities. We present new resolution criteria for high-resolution galaxy simulations. Most galaxy-scale properties are robust to numerics we test, provided: (1) Toomre masses are resolved; (2) feedback coupling ensures conservation, and (3) individual supernovae are time-resolved. Stellar masses and profiles are most robust to resolution, followed by metal abundances and morphologies, followed by properties of winds and circum-galactic media (CGM). Central (∼kpc) mass concentrations in massive (> L * ) galaxies are sensitive to numerics (via trapping/recycling of winds in hot halos). Multiple feedback mechanisms play key roles: supernovae regulate stellar masses/winds; stellar mass-loss fuels late star formation; radiative feedback suppresses accretion onto dwarfs and instantaneous star formation in disks. We provide all initial conditions and numerical algorithms used.

We present a suite of 3D multi-physics MHD simulations following star formation in isolated turbulent molecular gas disks ranging from 5 to 500 parsecs in radius. These simulations are designed to survey the range of surface densities between those typical of Milky Way GMCs (∼ 10 2 M pc −2 ) and extreme ULIRG environments (∼ 10 4 M pc −2 ) so as to map out the scaling of the cloud-scale star formation efficiency (SFE) between these two regimes. The simulations include prescriptions for supernova, stellar wind, and radiative feedback, which we find to be essential in determining both the instantaneous per-freefall ( f f ) and integrated ( int ) star formation efficiencies. In all simulations, the gas disks form stars until a critical stellar surface density has been reached and the remaining gas is blown out by stellar feedback. We find that surface density is a good predictor of int , as suggested by analytic force balance arguments from previous works. SFE eventually saturates to ∼ 1 at high surface density. We also find a proportional relationship between f f and int , implying that star formation is feedback-moderated even over very short time-scales in isolated clouds. These results have implications for star formation in galactic disks, the nature and fate of nuclear starbursts, and the formation of bound star clusters. The scaling of f f with surface density is not consistent with the notion that f f is always ∼ 1% on the scale of GMCs, but our predictions recover the ∼ 1% value for GMC parameters similar to those found in sprial galaxies, including our own.

Using a state-of-the-art cosmological simulation of merging proto-galaxies at high redshift from the FIRE project, with explicit treatments of star formation and stellar feedback in the interstellar medium, we investigate the formation of star clusters and examine one of the formation hypothesis of present-day metal-poor globular clusters. We find that frequent mergers in high-redshift proto-galaxies could provide a fertile environment to produce longlasting bound star clusters. The violent merger event disturbs the gravitational potential and pushes a large gas mass of 10 5−6 M ⊙ collectively to high density, at which point it rapidly turns into stars before stellar feedback can stop star formation. The high dynamic range of the reported simulation is critical in realizing such dense star-forming clouds with a small dynamical timescale, t ff 3 Myr, shorter than most stellar feedback timescales. Our simulation then allows us to trace how clusters could become virialized and tightly-bound to survive for up to ∼420 Myr till the end of the simulation. Because the cluster's tightly-bound core was formed in one short burst, and the nearby older stars originally grouped with the cluster tend to be preferentially removed, at the end of the simulation the cluster has a small age spread.

We report the formation of bound star clusters in a sample of high-resolution cosmological zoom-in simulations of z 5 galaxies from the FIRE project. We find that bound clusters preferentially form in high-pressure clouds with gas surface densities over 10 4 M pc −2 , where the cloud-scale star formation efficiency is near unity and young stars born in these regions are gravitationally bound at birth. These high-pressure clouds are compressed by feedback-driven winds and/or collisions of smaller clouds/gas streams in highly gas-rich, turbulent environments. The newly formed clusters follow a power-law mass function of dN/dM ∼ M −2 . The cluster formation efficiency is similar across galaxies with stellar masses of ∼ 10 7 -10 10 M at z 5. The age spread of cluster stars is typically a few Myrs and increases with cluster mass. The metallicity dispersion of cluster members is ∼ 0.08 dex in [Z/H] and does not depend on cluster mass significantly. Our findings support the scenario that present-day old globular clusters (GCs) were formed during relatively normal star formation in high-redshift galaxies. Simulations with a stricter/looser star formation model form a factor of a few more/fewer bound clusters per stellar mass formed, while the shape of the mass function is unchanged. Simulations with a lower local star formation efficiency form more stars in bound clusters. The simulated clusters are larger than observed GCs due to finite resolution. Our simulations are among the first cosmological simulations that form bound clusters self-consistently in a wide range of high-redshift galaxies.

Radiative feedback (RFB) from stars plays a key role in galaxies, but remains poorly-understood. We explore this using high-resolution, multi-frequency radiation-hydrodynamics (RHD) simulations from the Feedback In Realistic Environments (FIRE) project. We study ultra-faint dwarf through Milky Way mass scales, and explore a variety of RHD effects including H and He photo-ionization; photo-electric, Lyman Werner, Compton, and thermal dust heating; singlescattering (UV) and IR multiple-scattering radiation pressure (RP). We also compare fundamentally distinct numerical RHD algorithms: the ray-based LEBRON method (exact in optically-thin limits) and moments-based M1 method (exact in optically-thick limits). In all cases, the most important RFB channels on galaxy scales are photo-ionization heating and single-scattering RP: at all galaxy masses, most of the ionizing/far-UV luminosity from young stars (∼ 1/2 of the lifetime-integrated bolometric) is absorbed. In dwarfs, the most important effect is photo-ionization heating from the meta-galactic background suppressing accretion onto the galaxy. In MW-mass galaxies the meta-galactic background has negligible effects; but local photo-ionization and single-scattering RP both contribute significantly to regulating the galactic star formation efficiency and lowering central densities. Without some RFB (or some other "rapid" FB), resolved GMCs turn most of their mass into stars, making galaxies dominated by hyper-dense, bound star clusters. This also makes star formation more violent and "bursty" when SNe eventually explode in these hyper-clustered objects: thus, including RFB tends to "smooth" SFHs. These conclusions are robust to the numerical RHD method, but the M1 method produces somewhat stronger RFB effects. As in previous FIRE simulations, we show IR multiple-scattering is rare (contributing negligibly in low-metallicity dwarfs, and just ∼ 10% of the RP in massive galaxies): the majority of photon absorption occurs in "normal" GMCs with order-unity A V .