Great Balls of FIRE I: The formation of star clusters across cosmic time
  in a Milky Way-mass galaxy
  The properties of young star clusters formed within a galaxy are thought to
vary in different interstellar medium (ISM) conditions, but the details of this
mapping from galactic to cluster scales are poorly understood due to the large
dynamic range involved in galaxy and star cluster formation. We introduce a new
method for modeling cluster formation in galaxy simulations: mapping giant
molecular clouds (GMCs) formed self-consistently in a FIRE-2 MHD galaxy
simulation onto a cluster population according to a GMC-scale cluster formation
model calibrated to higher-resolution simulations, obtaining detailed
properties of the galaxy's star clusters in mass, metallicity, space, and time.
We find $\sim 10\%$ of all stars formed in the galaxy originate in
gravitationally-bound clusters overall, and this fraction increases in regions
with elevated $\Sigma_{\rm gas}$ and $\Sigma_{\rm SFR}$, because such regions
host denser GMCs with higher star formation efficiency. These quantities vary
systematically over the history of the galaxy, driving variations in cluster
formation. The mass function of bound clusters varies -- no single
Schechter-like or power-law distribution applies at all times. In the most
extreme episodes, clusters as massive as $7\times 10^6 M_\odot$ form in
massive, dense clouds with high star formation efficiency. The initial
mass-radius relation of young star clusters is consistent with an
environmentally-dependent 3D density that increases with $\Sigma_{\rm gas}$ and
$\Sigma_{\rm SFR}$. The model does not reproduce the age and metallicity
statistics of old ($>11\rm Gyr$) globular clusters found in the Milky Way,
possibly because it forms stars more slowly at $z>3$.
